**Zeolites Applications**

[19] Zhu HY, Lu GQ, Cool P, Vansant EF, Su BL, Gao X. Quantitative information on pore size distribution from the tangents of comparison plots. Langmuir. 2004;**20**:10115-10122.

[20] Stoeckli F, Lavanchy A, Hugi-Cleary D. Dubinin's theory: A versatile tool in adsorption science. In: Meunier FA, editor. Fundamentals of Adsorption VI. Amsterdam, The

[21] Astala R, Auerbach SM, Monson PA, Density Functional PA. Theory study of silica zeolite structures: Stabilities and mechanical properties of SOD, LTA, CHA, MOR, and MFI. The Journal of Physical Chemistry. B. 2004;**108**:9208-9215. DOI: 10.1021//jp0493733

Netherlands: Elsevier; 1998. pp. 75-80. oai:doc.rero.ch:20080509175439-GP

DOI: 10.1021/ja01145a126

90 Zeolites and Their Applications

**Chapter 6**

Provisional chapter

**Use of Synthetic and Natural Zeolites Tailored for As(V)**

DOI: 10.5772/intechopen.72614

Use of Synthetic and Natural Zeolites Tailored for As(V)

Arsenic in drinking water poses serious potential health risks in more than 30 countries with total affected population of around 100 million people. Natural and synthetic zeolites can be tailored in order to obtain improved sorption of As(V) making them a relatively cheap and efficient material for water remediation. The chapter is concentrated on the zeolitic materials for water remediation, and reports new findings regarding modification methods and comparison of such materials for the use in As(V) sorption applications. Methods of modification of zeolites are developed and explained. On the experimental and novel scale, using developed methods, 11 novel materials are synthesized and studied. Initial and modified materials are characterized by optical microscopy, SEM and EDX, as well as by metal content in those which are determined

Keywords: zeolites, water remediation, As(V), adsorption, sorbents, arsenic, environmental remediation, equilibrium, kinetics, heavy metals, metalloids

combustion are due to industrial and agricultural activities [7–9].

Groundwater contamination with arsenic compounds is an everyday problem for millions of people as water resources are crucial for use as drinking water, as well as in food production and agriculture [1–3]. Accumulation of arsenic in the body poses significant health risks and may lead to arsenicosis [3]. USA Environmental Protection Agency (EPA) in 2006 has diminished acceptable maximum contaminant level (MCL) for arsenic in drinking water from 50 down to 10 μg/L, and stricter norms have been set in headquarters of Peoples Republic of China, United Nations Health Organization, and European Union [4–6]. Arsenic contamination is both natural and due to anthropogenic sources. Weathering of rocks and minerals is a typical example of a natural process, while pesticides, metal waste, fertilizers and fossil fuel

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

**Sorption**

Sorption

Andrey E. Krauklis

Andrey E. Krauklis

Abstract

1. Introduction

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72614

using dissolution in acids and FAAS.

#### **Use of Synthetic and Natural Zeolites Tailored for As(V) Sorption** Use of Synthetic and Natural Zeolites Tailored for As(V) Sorption

DOI: 10.5772/intechopen.72614

#### Andrey E. Krauklis Andrey E. Krauklis

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72614

#### Abstract

Arsenic in drinking water poses serious potential health risks in more than 30 countries with total affected population of around 100 million people. Natural and synthetic zeolites can be tailored in order to obtain improved sorption of As(V) making them a relatively cheap and efficient material for water remediation. The chapter is concentrated on the zeolitic materials for water remediation, and reports new findings regarding modification methods and comparison of such materials for the use in As(V) sorption applications. Methods of modification of zeolites are developed and explained. On the experimental and novel scale, using developed methods, 11 novel materials are synthesized and studied. Initial and modified materials are characterized by optical microscopy, SEM and EDX, as well as by metal content in those which are determined using dissolution in acids and FAAS.

Keywords: zeolites, water remediation, As(V), adsorption, sorbents, arsenic, environmental remediation, equilibrium, kinetics, heavy metals, metalloids

### 1. Introduction

Groundwater contamination with arsenic compounds is an everyday problem for millions of people as water resources are crucial for use as drinking water, as well as in food production and agriculture [1–3]. Accumulation of arsenic in the body poses significant health risks and may lead to arsenicosis [3]. USA Environmental Protection Agency (EPA) in 2006 has diminished acceptable maximum contaminant level (MCL) for arsenic in drinking water from 50 down to 10 μg/L, and stricter norms have been set in headquarters of Peoples Republic of China, United Nations Health Organization, and European Union [4–6]. Arsenic contamination is both natural and due to anthropogenic sources. Weathering of rocks and minerals is a typical example of a natural process, while pesticides, metal waste, fertilizers and fossil fuel combustion are due to industrial and agricultural activities [7–9].

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Sorption is considered as one of the most reasonable water remediation techniques because of its lower costs [10–13]. Zeolites are crystalline, microporous, hydrated aluminosilicate minerals containing alkali or alkaline cations. These materials are often praised for their efficient sorptive properties. Zeolite surface modification is a possible route for substantially improving oxyanion sorption [12]. Zeolites generally possess high values of specific surface area, which can be larger than 700 m<sup>2</sup> /g [14]. This parameter is an extremely important one for adsorptive properties. Most of zeolitic water remediation technologies are based on cation exchange principles; however, there are cases when zeolites are used and useful for anion removal, such as cases of arsenate and arsenite sorption [15, 16]. Advantages of zeolitic sorbents are as follows: Generally, they are environmentally friendly, relatively cheap and can be treated for secondary use. Considering affinity of metalloids to interact with Fe and Mn-containing compounds [17], studies of As(V) sorption can be promoted in direction of using iron- and manganese-modified zeolites.

1.5. Manganese oxides

shown in Eqs. 1 and 2:

1.6. Sorption models

1.7. Langmuir model

Langmuir sorption model [31]:

As(V) is sorbed more easily onto solid surfaces than As(III), and thus oxidation of As(III) followed by adsorption is a potentially effective route for the removal of arsenic compounds [25, 26]. It has been reported that clinoptilolite modification with MnO2 significantly enhances sorption capacity of As(V) and is believed to be independent of pH [27]. MnO2 can oxidize As (III) to As(V) based on sources [24, 28], which is backed up with the following red-ox reactions,

¼ �0:56 e (1)

Use of Synthetic and Natural Zeolites Tailored for As(V) Sorption

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95

¼ þ1:23 e (2)

¼ þ0:67 e (3)

, (4)

(5)

HAsO2 <sup>þ</sup> 2H2O \$ H3AsO4 <sup>þ</sup> 2H<sup>þ</sup> <sup>þ</sup> 2e� <sup>φ</sup>�

MnO2 <sup>þ</sup> 4H<sup>þ</sup> <sup>þ</sup> 2e� \$ Mn<sup>2</sup><sup>þ</sup> <sup>þ</sup> 2H2O <sup>φ</sup>�

HAsO2 <sup>þ</sup> MnO2 <sup>þ</sup> 2H<sup>þ</sup> \$ H3AsO4 <sup>þ</sup> Mn<sup>2</sup><sup>þ</sup> <sup>φ</sup>�

shifted towards right of Eq. 3, in direction of As(V) as a product [24].

This is small enough in order to use it for modification of zeolites.

Radushkevich, Temkin and Redlich–Peterson models [18, 30].

The value of standard potential (ϕ� = +0.67 e) is positive, which indicates that equilibrium is

Mn(II), Mn(IV) and Mn(VII) ionic radii are 0.80–0.91 Å, 0.50–0.52 Å and 0.46 Å, respectively [24]. Mn-O bond in a permanganate ion is 1.629 Å, and Mn is in the tetrahedron's centre [29].

Langmuir and Freundlich isotherms are the most popular models in describing sorption in water environments; however, other models are also known and widely used such as Dubinin–

The model is based on assumption that each sorption active centre is equivalent, and it is energetically irrelevant whether adjacent sorption centres are empty or already occupied [20].

> qe <sup>¼</sup> qmKLCe 1 þ KLCe

where qe [mg/g] is equilibrium concentration in adsorbent which corresponds to initial concentration in solution C<sup>0</sup> [mg/L]; qm [mg/g] is maximum monolayer coverage capacity; Ce [mg/

> 1 qmKL

L] is equilibrium concentration in solution; KL [L/mg] is Langmuir constant.

Ce qe ¼ 1 qm Ce þ

Linearized form of Eq. 4 is represented with Eq. 5:

The summary red-ox reaction is represented by Eq. 3:

#### 1.1. Clinoptilolite

Clinoptilolite is a high silicate content heulandite group mineral (Na,K,Ca0.5,Sr0.5,Ba0.5,Mg0.5)6 [Al6Si30O72] 20H2O – HEU, often main component of natural zeolites. Its effective pore diameter is in range from about 4.5 up to 6.0 Å. Such clinoptilolite's properties as chemical stability in basic environments, thermostability and high sorption rate make it a useful material in chemical research and industry [15].

#### 1.2. Zeolite A

Zeolite A is one of the most widely used synthetic zeolites. It has a Linde Type A (LTA) structural type as defined by International Zeolite Association (IZA). Most commonly used in its sodium form (4A), its chemical formula in dehydrated form is [Na12Al12Si12O48]8 [18]. Potassium (3A) and calcium (5A) forms are also widely used. These zeolite names roughly represent pore opening diameters in ångstrøms (Å). It is possible to convert one form into another one via ion exchange [18].

#### 1.3. Zeolite X

Zeolites X contain 12 rings or pore openings with a diameter of 7.4 Å (8.1 Å when completely empty). Diameter of the central cavity is 13.7 Å [19].

#### 1.4. Iron oxides

Iron oxides and hydroxides are known for their use in wastewater treatment [20]. Natural iron oxides (without using zeolites as the host material) have affinity for arsenic compounds but show low sorption efficiency of treating aqueous solutions contaminated with As compounds (0.02–0.4 mg/g) due to low specific surface area [5, 21–23]. Thus it is important to introduce a host material with high specific surface area, i.e. zeolite, which can be modified with iron compounds, such as FeOOH. Fe(III) ionic radius is 0.67 Å [24], small enough in order to use it for modification of zeolites.

#### 1.5. Manganese oxides

Sorption is considered as one of the most reasonable water remediation techniques because of its lower costs [10–13]. Zeolites are crystalline, microporous, hydrated aluminosilicate minerals containing alkali or alkaline cations. These materials are often praised for their efficient sorptive properties. Zeolite surface modification is a possible route for substantially improving oxyanion sorption [12]. Zeolites generally possess high values of specific surface area, which

properties. Most of zeolitic water remediation technologies are based on cation exchange principles; however, there are cases when zeolites are used and useful for anion removal, such as cases of arsenate and arsenite sorption [15, 16]. Advantages of zeolitic sorbents are as follows: Generally, they are environmentally friendly, relatively cheap and can be treated for secondary use. Considering affinity of metalloids to interact with Fe and Mn-containing compounds [17], studies of As(V) sorption can be promoted in direction of using iron- and

Clinoptilolite is a high silicate content heulandite group mineral (Na,K,Ca0.5,Sr0.5,Ba0.5,Mg0.5)6 [Al6Si30O72] 20H2O – HEU, often main component of natural zeolites. Its effective pore diameter is in range from about 4.5 up to 6.0 Å. Such clinoptilolite's properties as chemical stability in basic environments, thermostability and high sorption rate make it a useful material in

Zeolite A is one of the most widely used synthetic zeolites. It has a Linde Type A (LTA) structural type as defined by International Zeolite Association (IZA). Most commonly used in its sodium form (4A), its chemical formula in dehydrated form is [Na12Al12Si12O48]8 [18]. Potassium (3A) and calcium (5A) forms are also widely used. These zeolite names roughly represent pore opening diameters in ångstrøms (Å). It is possible to convert one form into

Zeolites X contain 12 rings or pore openings with a diameter of 7.4 Å (8.1 Å when completely

Iron oxides and hydroxides are known for their use in wastewater treatment [20]. Natural iron oxides (without using zeolites as the host material) have affinity for arsenic compounds but show low sorption efficiency of treating aqueous solutions contaminated with As compounds (0.02–0.4 mg/g) due to low specific surface area [5, 21–23]. Thus it is important to introduce a host material with high specific surface area, i.e. zeolite, which can be modified with iron compounds, such as FeOOH. Fe(III) ionic radius is 0.67 Å [24], small enough in order to use it

/g [14]. This parameter is an extremely important one for adsorptive

can be larger than 700 m<sup>2</sup>

94 Zeolites and Their Applications

manganese-modified zeolites.

chemical research and industry [15].

another one via ion exchange [18].

empty). Diameter of the central cavity is 13.7 Å [19].

1.1. Clinoptilolite

1.2. Zeolite A

1.3. Zeolite X

1.4. Iron oxides

for modification of zeolites.

As(V) is sorbed more easily onto solid surfaces than As(III), and thus oxidation of As(III) followed by adsorption is a potentially effective route for the removal of arsenic compounds [25, 26]. It has been reported that clinoptilolite modification with MnO2 significantly enhances sorption capacity of As(V) and is believed to be independent of pH [27]. MnO2 can oxidize As (III) to As(V) based on sources [24, 28], which is backed up with the following red-ox reactions, shown in Eqs. 1 and 2:

$$\text{HAsO}\_2 + 2\text{H}\_2\text{O} \leftrightarrow \text{H}\_3\text{AsO}\_4 + 2\text{H}^+ + 2\text{e}^- \text{ (}\text{(}\text{\textdegree} = -0.56\text{ e}\text{)}\tag{1}$$

$$\text{MnO}\_2 + 4\text{H}^+ + 2\text{e}^- \leftrightarrow \text{Mn}^{2+} + 2\text{H}\_2\text{O} \quad \left(\text{q}^\circ = +1.23 \text{ e}\right) \tag{2}$$

The summary red-ox reaction is represented by Eq. 3:

$$\text{HAsO}\_2 + \text{MnO}\_2 + 2\text{H}^+ \leftrightarrow \text{H}\_3\text{AsO}\_4 + \text{Mn}^{2+} \quad \left(\text{ $\dot{\text{q}}^\circ$ } = +0.67\text{ e}\right) \tag{3}$$

The value of standard potential (ϕ� = +0.67 e) is positive, which indicates that equilibrium is shifted towards right of Eq. 3, in direction of As(V) as a product [24].

Mn(II), Mn(IV) and Mn(VII) ionic radii are 0.80–0.91 Å, 0.50–0.52 Å and 0.46 Å, respectively [24]. Mn-O bond in a permanganate ion is 1.629 Å, and Mn is in the tetrahedron's centre [29]. This is small enough in order to use it for modification of zeolites.

#### 1.6. Sorption models

Langmuir and Freundlich isotherms are the most popular models in describing sorption in water environments; however, other models are also known and widely used such as Dubinin– Radushkevich, Temkin and Redlich–Peterson models [18, 30].

#### 1.7. Langmuir model

The model is based on assumption that each sorption active centre is equivalent, and it is energetically irrelevant whether adjacent sorption centres are empty or already occupied [20]. Langmuir sorption model [31]:

$$q\_c = \frac{q\_m K\_L \mathbf{C}\_c}{1 + K\_L \mathbf{C}\_c} \, ^\prime \tag{4}$$

where qe [mg/g] is equilibrium concentration in adsorbent which corresponds to initial concentration in solution C<sup>0</sup> [mg/L]; qm [mg/g] is maximum monolayer coverage capacity; Ce [mg/ L] is equilibrium concentration in solution; KL [L/mg] is Langmuir constant.

Linearized form of Eq. 4 is represented with Eq. 5:

$$\frac{\mathbf{C}\_{\varepsilon}}{q\_{\varepsilon}} = \frac{1}{q\_{m}} \mathbf{C}\_{\varepsilon} + \frac{1}{q\_{m}K\_{L}} \tag{5}$$

In order to compare experimental data's fit with the model, linearized form is used, plotting data in coordinates Ce qe – Ce and obtaining determination coefficient R2 [30].

#### 1.8. Freundlich model

Freundlich model is based on assumption that sorption occurs on nonequivalent sorption centres, which is due to repulsion between the sorbed particles. It is assumed there is an infinite number of sorption centres [20]. Freundlich sorption model [32]:

$$q\_e = K\_F \overline{\mathbf{C}}\_e^{\perp} \tag{6}$$

qe <sup>¼</sup> RT bT

h i are parameters describing adsorbate-adsorbent interactions.

qe <sup>¼</sup> RT bT

data in coordinates qe – lnCe and obtaining determination coefficient R2 [30].

Linearized form of Eq. 12 is represented with Eq. 13 [30]:

Linearized form of Eq. 14 is represented with Eq. 15 [30, 34]:

Ce qe � 1

2.1. Slovakian natural clinoptilolite (Slov)

1.12. Purpose and aim of this work

ln KRP

Ce qe � 1 � �

1.11. Redlich-Peterson model

L mg � � <sup>1</sup>

in coordinates ln KRP

2. Materials

weight is 2200–2440 kg/m<sup>3</sup>

where αRP

bT J∙g mol∙L

where R [8.314 J/(mol∙K)] is universal gas constant; T [K] is temperature; KT [mg/L] and

lnKT þ

In order to compare experimental data's fit with the model, linearized form is used, plotting

The model is a hybrid between Langmuir and Freundlich models. When value of coefficient βRP is equal to 1, the model becomes equivalent with Langmuir model. Often this three-parameter model is able to explain experimental data more precisely. Redlich–Peterson sorption model [34]:

> qe <sup>¼</sup> KRPCe <sup>1</sup> <sup>þ</sup> <sup>α</sup>RPC<sup>β</sup>RP e

<sup>β</sup>RP � �, <sup>β</sup>RP [�] and KRP [L/g] are Redlich–Peterson isotherm parameters.

KRP is optimized by finding the closest fit to the model via the highest determination coefficient. In order to compare experimental data's fit with the model, linearized form is used, plotting data

The purpose of this work is to present zeolites as potential sorbents for As(V) sorption for water remediation as well as to present novel modification methods and materials. The aim of the study is to provide and interpret results of As(V) sorption onto raw and modified zeolites.

Clinoptilolite natural zeolite from Slovakian deposit Nižný Hrabovec was used. Its specific

; porosity 24–32%; clinoptilolitic content fluctuates in range from 86

� � – lnCe and obtaining determination coefficient R2 [30, 34].

RT bT

ln KTCe , (12)

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97

Use of Synthetic and Natural Zeolites Tailored for As(V) Sorption

lnCe (13)

, (14)

¼ βRPlnCe þ ln αRP (15)

where KF [mg1–1/n�L1/n/g] is Freundlich coefficient; n [�] is Freundlich constant.

Linearized form of Eq. 6 is represented with Eq. 7 [30]:

$$
\ln q\_{\epsilon} = \ln K\_{\mathbb{F}} + \frac{1}{n} \ln \mathbb{C}\_{\epsilon} \tag{7}
$$

In order to compare experimental data's fit with the model, linearized form is used, plotting data in coordinates lnqe – lnCe and obtaining determination coefficient R2 [30].

#### 1.9. Dubinin-Radushkevich model

Usually the model is used to differentiate between physisorption and chemisorption. Initially it was used to describe physisorption. Dubinin-Radushkevich sorption model [33]:

$$
\eta\_e = \eta\_s e^{\left(-\mathbb{K}\_{\rm DR} e\_{\rm DR}^2\right)}\tag{8}
$$

$$
\varepsilon\_{DR} = RTln\left(1 + \frac{1}{C\_e}\right) \tag{9}
$$

$$E = \frac{1}{\sqrt{2K\_{D\mathbb{R}}}},\tag{10}$$

where εDR [kJ/mol] is Dubinin–Radushkevich isotherm variable; E [kJ/mol] is mean free energy of adsorption; qs [mg/g] is theoretical Dubinin–Radushkevich saturation sorption capacity; KDR [mol2 /kJ2 ] is Dubinin–Radushkevich constant.

Linearized form of Eq. 8 is represented with Eq. 11 [30]:

$$\ln q\_e = \ln q\_s - K\_{DR} \varepsilon\_{DR}^2 \tag{11}$$

In order to compare experimental data's fit with the model, linearized form is used, plotting data in coordinates lnqe – ε<sup>2</sup> DR and obtaining determination coefficient R2 [30].

#### 1.10. Temkin model

The model is based on assumption that heat of adsorption (function of temperature) decreases linearly with increasing amount of sorbed particles. Temkin sorption model [33]:

Use of Synthetic and Natural Zeolites Tailored for As(V) Sorption http://dx.doi.org/10.5772/intechopen.72614 97

$$q\_e = \frac{RT}{b\_T} \ln K\_T \mathbf{C}\_e \tag{12}$$

where R [8.314 J/(mol∙K)] is universal gas constant; T [K] is temperature; KT [mg/L] and bT J∙g mol∙L h i are parameters describing adsorbate-adsorbent interactions.

Linearized form of Eq. 12 is represented with Eq. 13 [30]:

$$q\_e = \frac{RT}{b\_T} \ln K\_T + \frac{RT}{b\_T} \ln \mathbb{C}\_e \tag{13}$$

In order to compare experimental data's fit with the model, linearized form is used, plotting data in coordinates qe – lnCe and obtaining determination coefficient R2 [30].

#### 1.11. Redlich-Peterson model

In order to compare experimental data's fit with the model, linearized form is used, plotting

Freundlich model is based on assumption that sorption occurs on nonequivalent sorption centres, which is due to repulsion between the sorbed particles. It is assumed there is an

qe ¼ KFC

ln qe ¼ lnKF þ

In order to compare experimental data's fit with the model, linearized form is used, plotting

Usually the model is used to differentiate between physisorption and chemisorption. Initially

εDR ¼ RTln 1 þ

<sup>E</sup> <sup>¼</sup> <sup>1</sup>

] is Dubinin–Radushkevich constant.

Linearized form of Eq. 8 is represented with Eq. 11 [30]:

where εDR [kJ/mol] is Dubinin–Radushkevich isotherm variable; E [kJ/mol] is mean free energy of adsorption; qs [mg/g] is theoretical Dubinin–Radushkevich saturation sorption capacity;

ln qe <sup>¼</sup> ln qs � KDRε<sup>2</sup>

In order to compare experimental data's fit with the model, linearized form is used, plotting

The model is based on assumption that heat of adsorption (function of temperature) decreases

linearly with increasing amount of sorbed particles. Temkin sorption model [33]:

DR and obtaining determination coefficient R2 [30].

ffiffiffiffiffiffiffiffiffiffiffi 2KDR

1 n

> 1 n

1 Ce � �

<sup>e</sup> , (6)

ln Ce (7)

(9)

qe <sup>¼</sup> qse �KDRε<sup>2</sup> ð Þ DR (8)

p , (10)

DR (11)

infinite number of sorption centres [20]. Freundlich sorption model [32]:

Linearized form of Eq. 6 is represented with Eq. 7 [30]:

where KF [mg1–1/n�L1/n/g] is Freundlich coefficient; n [�] is Freundlich constant.

data in coordinates lnqe – lnCe and obtaining determination coefficient R2 [30].

it was used to describe physisorption. Dubinin-Radushkevich sorption model [33]:

– Ce and obtaining determination coefficient R2 [30].

data in coordinates Ce

96 Zeolites and Their Applications

1.8. Freundlich model

qe

1.9. Dubinin-Radushkevich model

KDR [mol2

/kJ2

data in coordinates lnqe – ε<sup>2</sup>

1.10. Temkin model

The model is a hybrid between Langmuir and Freundlich models. When value of coefficient βRP is equal to 1, the model becomes equivalent with Langmuir model. Often this three-parameter model is able to explain experimental data more precisely. Redlich–Peterson sorption model [34]:

$$q\_{\varepsilon} = \frac{K\_{RP} \mathbb{C}\_{\varepsilon}}{1 + \alpha\_{RP} \mathbb{C}\_{\varepsilon}^{\theta\_{RP}}},\tag{14}$$

where αRP L mg � � <sup>1</sup> <sup>β</sup>RP � �, <sup>β</sup>RP [�] and KRP [L/g] are Redlich–Peterson isotherm parameters.

Linearized form of Eq. 14 is represented with Eq. 15 [30, 34]:

$$\ln\left(K\_{RP}\frac{\mathbb{C}\_{\epsilon}}{q\_{\epsilon}}-1\right) = \beta\_{RP}\ln\mathbb{C}\_{\epsilon} + \ln\alpha\_{RP} \tag{15}$$

KRP is optimized by finding the closest fit to the model via the highest determination coefficient. In order to compare experimental data's fit with the model, linearized form is used, plotting data in coordinates ln KRP Ce qe � 1 � � – lnCe and obtaining determination coefficient R2 [30, 34].

#### 1.12. Purpose and aim of this work

The purpose of this work is to present zeolites as potential sorbents for As(V) sorption for water remediation as well as to present novel modification methods and materials. The aim of the study is to provide and interpret results of As(V) sorption onto raw and modified zeolites.

#### 2. Materials

#### 2.1. Slovakian natural clinoptilolite (Slov)

Clinoptilolite natural zeolite from Slovakian deposit Nižný Hrabovec was used. Its specific weight is 2200–2440 kg/m<sup>3</sup> ; porosity 24–32%; clinoptilolitic content fluctuates in range from 86 to 94%; according to manufacturer, it also contains cristobalite, clay mica, plagioclase, rutile, quartz; Si:Al ratio is in range 4.80–5.40; fraction of 1–2.5 mm was used [35].

3.1. First method

3.2. Second method

3.3. Third method

Slov with fourth method.

3.4. Fourth method

First method is based on the method described in source [40]. The basis for FeOOH-modified sorbent synthesis is the iron oxohydroxide precipitation on the raw material. Description of modification is the following: 0.25 mol FeCl36H2O is dissolved in 250 mL DI water, adding 250 mL 3 M NaOH solution and aged for 3 h. Obtained Fe(OH)3 precipitates are decanted. 100 g of raw material are mixed in the Fe(OH)3 dispersion. The mixture is then carefully mixed and filtered under vacuum and washed with 250 mL DI water. The filtered and washed material is then dried in air atmosphere for 1 h at room temperature and then dried in the

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Second method is based on the first method, modified based on the idea that the material is first soaked with a respective metal salt, followed by the reaction (FeOOH synthesis) inside the zeolite structure. The developed method is the following: 1 M FeCl36H2O solution is prepared and 100 g of zeolite are placed into it. The mixture is aged for 24 h. The mixture is then filtered, and without washing, 250 mL of 3 M NaOH solution are added to the soaked material and aged for 24 h. The mixture is filtered under vacuum and washed with 250 mL DI water. The filtered and washed material is dried in air atmosphere for 1 h at room temperature and

Third method is based on the first method, adapted to be applicable for efficient modification of powders (as in case of Fe-4A and Fe-5A described in another work [17]). The idea is to conduct a reaction in the wet FeOOH mass, simplifying and accelerating modification process. The developed method is the following: 0.25 mol FeCl36H2O is dissolved in 250 mL DI water, 250 mL of 3 M NaOH is added and the mixture is aged for at least 3 h. Synthesized Fe(OH)3 precipitates are decanted. The mass is filtered under vacuum, mixing in 100 g of raw material into the Fe(OH)3 precipitates, while carefully mixing. Porridge-consistency mixture is washed with 250 mL DI water. The filtered and washed material is then dried in air atmosphere for 2 h at room temper-

ature, and further dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 110C.

Materials obtained using this method: Fe-Ukr(3); in combination with other methods, FeMn-

Fourth method is based on methods described in sources [27, 41] which are natural zeolite modification methods with manganese oxides. Concentrations used are the same as described in source [27]. 100 g of zeolite is weighted in the beaker and dried in air atmosphere from initial moisture in air atmosphere in the oven for 1 h at 70C. 2.5 M MnCl2 solution and 10 M NaOH solution are prepared. 100 mL of prepared 2.5 M MnCl2 solution is added, while

subsequently dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 60C.

Materials obtained using this method: Fe-Ukr(2); Fe-13X; Fe-KHol.

oven Gallenkamp Plus II (London, UK) for 4 h at 60C. Materials obtained using this method: Fe-Ukr(1); Fe-Slov.

### 2.2. Ukrainian natural clinoptilolite (Ukr)

Clinoptilolite natural zeolite from Ukrainian Zakarpattian deposit Sokirnicke (ukr. Сокирницьке родовище). Specific weight 2370 kg/m3 ; porosity 44%; clinoptilolitic content 77%; fraction 1– 3 mm [36].

#### 2.3. Russian natural clinoptilolite (Khol)

Clinoptilolite natural zeolite from Russian Zabaykalsky Krai deposit Kholinskoe (rus. Холинское месторождение). Specific weight 1900–2800 kg/m3 , bulk density 1.02–1.20 g/cm3 . Porosity is in range of 20–23% [37]. Zeolite was crushed and sieved using vibro sieve FRITSCH analysette 3 SPARTAN (Germany; International), collecting fraction from 0.8 up to 1.4 mm.

### 2.4. Synthetic zeolite A (4A)

SYLOSIV A 4 was used (purchased from Grace Davison, USA), zeolite 4A in fine powder form (particle size 6–9 μm). According to manufacturer, this material is chemically stable in basic, neutral and weak acidic environments; specific surface area 800 m<sup>2</sup> /g; effective pore volume 0.25–0.30 cm3 /g; specific weight 1900–2300 kg/m<sup>3</sup> [38].

#### 2.5. Synthetic zeolite X (13X)

Zeolite 13X was used (purchased from Hong Kong Chemical Corp.). Bulk density 0.601 g/cm3 ; porosity 0.55%; fraction 4–5 mm.

#### 2.6. Reagents

All compounds used were of analytical grade (≥ 98%) and were used without further purification. Sodium hydroxide, potassium chloride and 65% nitric acid were obtained from Sigma-Aldrich (Riedel-de Haën, Germany). Iron(III) chloride hexahydrate, calcium chloride dihydrate and 30% hydrogen peroxide were obtained from Enola (Riga, Latvia). Manganese(II) chloride tetrahydrate was obtained from Firma Chempur (Piekary Śląskie, Poland). All aqueous solutions were prepared using high purity deionized water (10–15 MΩcm), produced via water purification system Millipore Elix 3 (Billerica, USA). Arsenate stock solution was prepared using disodium hydrogenarsenate heptahydrate Na2HAsO47H2O obtained from Alfa Aesar (Haverhill, USA).

### 3. Zeolite modification methods

Zeolites Ukr, Slov, Khol, 4A and 13X were modified using 6 different methods. Altogether, using these methods, 11 novel materials were synthesized and are described in this chapter. Additionally, FeOOH-modified zeolites A were obtained and described in the following source [17]. It should be also noted that another aluminosilicate, clay montmorillonite was modified using a similar approach and described in another work [39].

#### 3.1. First method

to 94%; according to manufacturer, it also contains cristobalite, clay mica, plagioclase, rutile,

Clinoptilolite natural zeolite from Ukrainian Zakarpattian deposit Sokirnicke (ukr. Сокирницьке

Clinoptilolite natural zeolite from Russian Zabaykalsky Krai deposit Kholinskoe (rus. Холинское

range of 20–23% [37]. Zeolite was crushed and sieved using vibro sieve FRITSCH analysette 3

SYLOSIV A 4 was used (purchased from Grace Davison, USA), zeolite 4A in fine powder form (particle size 6–9 μm). According to manufacturer, this material is chemically stable in basic,

Zeolite 13X was used (purchased from Hong Kong Chemical Corp.). Bulk density 0.601 g/cm3

All compounds used were of analytical grade (≥ 98%) and were used without further purification. Sodium hydroxide, potassium chloride and 65% nitric acid were obtained from Sigma-Aldrich (Riedel-de Haën, Germany). Iron(III) chloride hexahydrate, calcium chloride dihydrate and 30% hydrogen peroxide were obtained from Enola (Riga, Latvia). Manganese(II) chloride tetrahydrate was obtained from Firma Chempur (Piekary Śląskie, Poland). All aqueous solutions were prepared using high purity deionized water (10–15 MΩcm), produced via water purification system Millipore Elix 3 (Billerica, USA). Arsenate stock solution was prepared using disodium hydro-

genarsenate heptahydrate Na2HAsO47H2O obtained from Alfa Aesar (Haverhill, USA).

Zeolites Ukr, Slov, Khol, 4A and 13X were modified using 6 different methods. Altogether, using these methods, 11 novel materials were synthesized and are described in this chapter. Additionally, FeOOH-modified zeolites A were obtained and described in the following source [17]. It should be also noted that another aluminosilicate, clay montmorillonite was modified

; porosity 44%; clinoptilolitic content 77%; fraction 1–

, bulk density 1.02–1.20 g/cm3

. Porosity is in

;

/g; effective pore volume

quartz; Si:Al ratio is in range 4.80–5.40; fraction of 1–2.5 mm was used [35].

SPARTAN (Germany; International), collecting fraction from 0.8 up to 1.4 mm.

neutral and weak acidic environments; specific surface area 800 m<sup>2</sup>

/g; specific weight 1900–2300 kg/m<sup>3</sup> [38].

2.2. Ukrainian natural clinoptilolite (Ukr)

родовище). Specific weight 2370 kg/m3

2.3. Russian natural clinoptilolite (Khol)

2.4. Synthetic zeolite A (4A)

2.5. Synthetic zeolite X (13X)

porosity 0.55%; fraction 4–5 mm.

3. Zeolite modification methods

using a similar approach and described in another work [39].

месторождение). Specific weight 1900–2800 kg/m3

3 mm [36].

98 Zeolites and Their Applications

0.25–0.30 cm3

2.6. Reagents

First method is based on the method described in source [40]. The basis for FeOOH-modified sorbent synthesis is the iron oxohydroxide precipitation on the raw material. Description of modification is the following: 0.25 mol FeCl36H2O is dissolved in 250 mL DI water, adding 250 mL 3 M NaOH solution and aged for 3 h. Obtained Fe(OH)3 precipitates are decanted. 100 g of raw material are mixed in the Fe(OH)3 dispersion. The mixture is then carefully mixed and filtered under vacuum and washed with 250 mL DI water. The filtered and washed material is then dried in air atmosphere for 1 h at room temperature and then dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 60C.

Materials obtained using this method: Fe-Ukr(1); Fe-Slov.

#### 3.2. Second method

Second method is based on the first method, modified based on the idea that the material is first soaked with a respective metal salt, followed by the reaction (FeOOH synthesis) inside the zeolite structure. The developed method is the following: 1 M FeCl36H2O solution is prepared and 100 g of zeolite are placed into it. The mixture is aged for 24 h. The mixture is then filtered, and without washing, 250 mL of 3 M NaOH solution are added to the soaked material and aged for 24 h. The mixture is filtered under vacuum and washed with 250 mL DI water. The filtered and washed material is dried in air atmosphere for 1 h at room temperature and subsequently dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 60C.

Materials obtained using this method: Fe-Ukr(2); Fe-13X; Fe-KHol.

#### 3.3. Third method

Third method is based on the first method, adapted to be applicable for efficient modification of powders (as in case of Fe-4A and Fe-5A described in another work [17]). The idea is to conduct a reaction in the wet FeOOH mass, simplifying and accelerating modification process. The developed method is the following: 0.25 mol FeCl36H2O is dissolved in 250 mL DI water, 250 mL of 3 M NaOH is added and the mixture is aged for at least 3 h. Synthesized Fe(OH)3 precipitates are decanted. The mass is filtered under vacuum, mixing in 100 g of raw material into the Fe(OH)3 precipitates, while carefully mixing. Porridge-consistency mixture is washed with 250 mL DI water. The filtered and washed material is then dried in air atmosphere for 2 h at room temperature, and further dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 110C.

Materials obtained using this method: Fe-Ukr(3); in combination with other methods, FeMn-Slov with fourth method.

#### 3.4. Fourth method

Fourth method is based on methods described in sources [27, 41] which are natural zeolite modification methods with manganese oxides. Concentrations used are the same as described in source [27]. 100 g of zeolite is weighted in the beaker and dried in air atmosphere from initial moisture in air atmosphere in the oven for 1 h at 70C. 2.5 M MnCl2 solution and 10 M NaOH solution are prepared. 100 mL of prepared 2.5 M MnCl2 solution is added, while mixing, to the zeolite in the beaker. 1 mL of prepared 10 M NaOH solution is added and mixed. Solution is aged for 24 h. Without filtering, the mixture is placed in the oven for 3 h at 150�C. The result is densified zeolite granule/pellet mass, covered with precipitates. This mass is then placed into the crucibles. Crucibles with the obtained mass are placed in the muffle furnace and are held there for 5 h in air atmosphere at 550�C. Crucibles are then taken out and cooled down in air at room temperature. After cooling, modified zeolite is washed with 300 mL of DI water. Material is dried in air for 1 h at room temperature, and further dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 60�C.

4. Material characterization

stacking was Helicon Focus (Kharkiv, Ukraine).

electron regime with working voltage of 15 kV.

4.1. Optical microscopy

4.2. SEM and EDX

4.3. FAAS

lated using Eqs. 17 and 18:

Modified and raw zeolites were characterized using optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX, EDS), by analysis of Fe and Mn content, which was performed by dissolution in acids followed by flame atomic absorption

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Optical microscopy allows to study the surface of the material and to evaluate homogeneity. Optical microscope Leica was used with a digital camera Leica DFC480, ocular Leica 10X/21B, which enables magnification from x7.1 up to x115. Additional lighting was used: Leica Fluorescent Ringlight and Leica CLS150X (Germany; International) [43]. Software used for obtaining images was Leica Application Suite v4.1.0. A square of 1 cm was cut out of millimetre paper, which was placed adjacent to the sample as a scale. In order to obtain qualitative images, 11 micrographs were used on average using focus stacking (pyramid) approach. Software used for

Materials were covered with a thin layer of gold in order to prevent charging due to electron beam. Gold sputtering was performed using Quorum Technologies Emitech K550X (Laughton, UK). Methods were performed using Tescan Mira/LMU (Brno, Czech Republic) in backscattered

Fe and Mn content was obtained using dissolution in acids followed by flame atomic absorption spectrometry (FAAS). 1 g of each material was weighed in a beaker, using analytical scales (� 0.1 mg). 25 mL 65% HNO3 and 5 mL 35% H2O2 were then added in each beaker. Beakers were placed in a thermostat Biosan MyLab Thermo-Block TDB 400 (Riga, Latvia) and heated up to 160�C. When half of the solution had evaporated, 25 ml 65% HNO3 was added while heating the sample. The solutions were allowed to cool down in air at room temperature, filtered into a graduated vessel and diluted with DI water to a total volume of 60 mL each. Furthermore, a blank sample was prepared for background correction. 10 mL of each solution were moved in test tubes, using pipettes. Prepared samples were analysed using PerkinElmer AAnalyst 200 (Waltham, USA) with flame atomization. Mn and Fe content measurements were performed using background correction in air–acetylene flame. Fe and Mn content in samples was calcu-

wFe ¼ CFe∙

wMn ¼ CMn∙

V

V

<sup>m</sup> (17)

<sup>m</sup> , (18)

spectrometry (FAAS). Bulk density of all sorbents was determined.

Materials obtained using this method: Mn-Slov(1); in combination with other methods, FeMn-Slov with third method, CaMn-Slov with sixth method.

#### 3.5. Fifth method

Fifth method is based on fourth method, but, while fourth method is rooted in Mn(II) oxidation at elevated temperature, this method is based on Mn(VII) reduction reaction using ethanol. The reaction as represented by Eq. 16 is conducted at room temperature [42]:

$$2\text{KMnO}\_4 + 3\text{C}\_2\text{H}\_5\text{OH} \to 2\text{MnO}\_2 + 3\text{CH}\_3\text{CHO} + 2\text{KOH} + 2\text{H}\_2\text{O} \tag{16}$$

This modification method is developed and chosen in order to perform modification of zeolites with MnO2 in softer conditions at room temperature. Concentration and volume of potassium permanganate solution is chosen based on KMnO4 solubility in water at room temperature (if limited by) and in order to introduce the same amount of Mn atoms as in fourth method (0.25 mol Mn). The amount of ethanol is chosen such so that C2H5OH and KMnO4 are in molar ratio 1:1 (stoichiometrically). 100 g of zeolite is weighted in the beaker. Then, 1.5 L 0.17 M KMnO4 solution is prepared and added into the beaker with the zeolite. The aging proceeds for 96 h in the solution. Then, 24.3 mL 60% C2H5OH is added to the mixture in order to reduce KMnO4. Acetaldehyde is obtained and characteristic smell can be felt. Modified zeolite is filtered under vacuum and washed with 300 mL DI water. Material is then dried in air atmosphere at room temperature for 1 h, and further dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 60�C.

Materials obtained using this method: Mn-Slov(2), Mn-4A.

#### 3.6. Sixth method

The sixth method is a Ca2+ ion-exchange reaction used in combination with previously described methods. It is based on a principle that one can tailor the effective micropore diameter in zeolites by exchanging the cations. Divalent Ca2+ cations are used to increase the effective pore opening [18]. 20 g of zeolite are placed in 100 mL glass vessels. 4.5 M CaCl2 solution is prepared and 50 mL is added to the material. The vessels are then placed on the orbital shaker Biosan Multifunctional Orbital Shaker PSU-20i (Riga Latvia), setting frequency to 150 rpm for 96 h. The mass is decanted and then washed with 100 mL DI water. Afterward, the material is dried for 2 h at room temperature in air atmosphere, and then subsequently dried at 110�C in the oven for 4 h.

Materials obtained using this method: in combination with other methods, CaMn-Slov with fourth method.

### 4. Material characterization

Modified and raw zeolites were characterized using optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX, EDS), by analysis of Fe and Mn content, which was performed by dissolution in acids followed by flame atomic absorption spectrometry (FAAS). Bulk density of all sorbents was determined.

#### 4.1. Optical microscopy

mixing, to the zeolite in the beaker. 1 mL of prepared 10 M NaOH solution is added and mixed. Solution is aged for 24 h. Without filtering, the mixture is placed in the oven for 3 h at 150�C. The result is densified zeolite granule/pellet mass, covered with precipitates. This mass is then placed into the crucibles. Crucibles with the obtained mass are placed in the muffle furnace and are held there for 5 h in air atmosphere at 550�C. Crucibles are then taken out and cooled down in air at room temperature. After cooling, modified zeolite is washed with 300 mL of DI water. Material is dried in air for 1 h at room temperature, and further dried in

Materials obtained using this method: Mn-Slov(1); in combination with other methods, FeMn-

Fifth method is based on fourth method, but, while fourth method is rooted in Mn(II) oxidation at elevated temperature, this method is based on Mn(VII) reduction reaction using ethanol. The

This modification method is developed and chosen in order to perform modification of zeolites with MnO2 in softer conditions at room temperature. Concentration and volume of potassium permanganate solution is chosen based on KMnO4 solubility in water at room temperature (if limited by) and in order to introduce the same amount of Mn atoms as in fourth method (0.25 mol Mn). The amount of ethanol is chosen such so that C2H5OH and KMnO4 are in molar ratio 1:1 (stoichiometrically). 100 g of zeolite is weighted in the beaker. Then, 1.5 L 0.17 M KMnO4 solution is prepared and added into the beaker with the zeolite. The aging proceeds for 96 h in the solution. Then, 24.3 mL 60% C2H5OH is added to the mixture in order to reduce KMnO4. Acetaldehyde is obtained and characteristic smell can be felt. Modified zeolite is filtered under vacuum and washed with 300 mL DI water. Material is then dried in air atmosphere at room temperature for

The sixth method is a Ca2+ ion-exchange reaction used in combination with previously described methods. It is based on a principle that one can tailor the effective micropore diameter in zeolites by exchanging the cations. Divalent Ca2+ cations are used to increase the effective pore opening [18]. 20 g of zeolite are placed in 100 mL glass vessels. 4.5 M CaCl2 solution is prepared and 50 mL is added to the material. The vessels are then placed on the orbital shaker Biosan Multifunctional Orbital Shaker PSU-20i (Riga Latvia), setting frequency to 150 rpm for 96 h. The mass is decanted and then washed with 100 mL DI water. Afterward, the material is dried for 2 h at room temperature in air atmosphere, and then subsequently dried at 110�C in the oven for 4 h. Materials obtained using this method: in combination with other methods, CaMn-Slov with

1 h, and further dried in the oven Gallenkamp Plus II (London, UK) for 4 h at 60�C.

Materials obtained using this method: Mn-Slov(2), Mn-4A.

2KMnO4 þ 3C2H5OH ! 2MnO2 þ 3CH3CHO þ 2KOH þ 2H2O (16)

the oven Gallenkamp Plus II (London, UK) for 4 h at 60�C.

reaction as represented by Eq. 16 is conducted at room temperature [42]:

Slov with third method, CaMn-Slov with sixth method.

3.5. Fifth method

100 Zeolites and Their Applications

3.6. Sixth method

fourth method.

Optical microscopy allows to study the surface of the material and to evaluate homogeneity. Optical microscope Leica was used with a digital camera Leica DFC480, ocular Leica 10X/21B, which enables magnification from x7.1 up to x115. Additional lighting was used: Leica Fluorescent Ringlight and Leica CLS150X (Germany; International) [43]. Software used for obtaining images was Leica Application Suite v4.1.0. A square of 1 cm was cut out of millimetre paper, which was placed adjacent to the sample as a scale. In order to obtain qualitative images, 11 micrographs were used on average using focus stacking (pyramid) approach. Software used for stacking was Helicon Focus (Kharkiv, Ukraine).

#### 4.2. SEM and EDX

Materials were covered with a thin layer of gold in order to prevent charging due to electron beam. Gold sputtering was performed using Quorum Technologies Emitech K550X (Laughton, UK). Methods were performed using Tescan Mira/LMU (Brno, Czech Republic) in backscattered electron regime with working voltage of 15 kV.

#### 4.3. FAAS

Fe and Mn content was obtained using dissolution in acids followed by flame atomic absorption spectrometry (FAAS). 1 g of each material was weighed in a beaker, using analytical scales (� 0.1 mg). 25 mL 65% HNO3 and 5 mL 35% H2O2 were then added in each beaker. Beakers were placed in a thermostat Biosan MyLab Thermo-Block TDB 400 (Riga, Latvia) and heated up to 160�C. When half of the solution had evaporated, 25 ml 65% HNO3 was added while heating the sample. The solutions were allowed to cool down in air at room temperature, filtered into a graduated vessel and diluted with DI water to a total volume of 60 mL each. Furthermore, a blank sample was prepared for background correction. 10 mL of each solution were moved in test tubes, using pipettes. Prepared samples were analysed using PerkinElmer AAnalyst 200 (Waltham, USA) with flame atomization. Mn and Fe content measurements were performed using background correction in air–acetylene flame. Fe and Mn content in samples was calculated using Eqs. 17 and 18:

$$w\_{\text{Fe}} = \mathbb{C}\_{\text{Fe}} \cdot \frac{V}{m} \tag{17}$$

$$
\Delta w\_{M\text{in}} = C\_{M\text{in}} \cdot \frac{V}{m'} \tag{18}
$$

where wFe and wMn are Fe and Mn mass fractions in materials, respectively [mas%]; CFe and CMn are Fe and Mn ion concentartions in solution, respectively [mg/L]; V is volume of the solution (0.060 L) [L]; m is mass of sorbent (1000 mg) [mg].

Material Bulk density ( <sup>g</sup>

Table 1. Material bulk density, form and homogeneity.

Table 2. Fe and Mn content of materials obtained using FAAS.

mL) Form Homogeneity

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4A 0.46 0.02 Powder Homogeneous Mn-4A 0.77 0.03 Powder Homogeneous 13X 0.75 0.03 Granules/Pellets Homogeneous Fe-13X 0.74 0.03 Granules/Pellets Homogeneous Ukr 1.04 0.04 Granules/Pellets Homogeneous Fe-Ukr(1) 1.05 0.04 Granules/Pellets Non-homogeneous Fe-Ukr(2) 1.01 0.04 Granules/Pellets Non-homogeneous Fe-Ukr(3) 1.00 0.04 Granules/Pellets Non-homogeneous Slov 0.91 0.04 Granules/Pellets Homogeneous Fe-Slov 0.92 0.04 Granules/Pellets Non-homogeneous Mn-Slov(1) 1.12 0.04 Granules/Pellets Homogeneous Mn-Slov(2) 0.97 0.04 Granules/Pellets Homogeneous FeMn-Slov 0.91 0.04 Granules/Pellets Homogeneous CaMn-Slov 1.11 0.04 Granules/Pellets Homogeneous Khol 0.90 0.04 Granules/Pellets Homogeneous Fe-Khol 0.96 0.04 Granules/Pellets Homogeneous

Material Fe content (mas%) Mn content (mas%)

4A 0.19 0.01 0.00 0.01 Mn-4A 0.03 0.00 0.57 0.02 13X 0.34 0.01 0.01 0.01 Fe-13X 2.92 0.11 0.02 0.00 Ukr 0.53 0.02 0.01 0.01 Fe-Ukr(1) 1.37 0.05 0.02 0.01 Fe-Ukr(2) 1.67 0.06 0.01 0.01 Fe-Ukr(3) 1.70 0.06 0.05 0.01 Slov 0.45 0.02 0.01 0.01 Fe-Slov 8.75 0.33 0.05 0.01 Mn-Slov(1) 0.47 0.02 4.58 0.17 Mn-Slov(2) 0.43 0.02 0.33 0.01 FeMn-Slov 19.22 0.73 2.80 0.10 CaMn-Slov 0.35 0.01 3.68 0.13 Khol 0.33 0.01 0.03 0.01 Fe-Khol 1.43 0.05 0.06 0.01

#### 4.4. Determination of bulk density

The method of bulk density determination is based on a standard method, which is described in literature [44–46]. An empty 250 mL graduated cylinder was weighted, using analytical scales Kern ALJ 220–4 ( 0.1 mg). Approximately, 100 g of powder or granules/pellets were placed in the graduated cylinder, while determining the total mass of material and the cylinder. Deducing cylinder's mass from the total, the mass of the material was obtained. The cylinder was gently hit on the flat surface until the material became compacted and the change in volume was not observable. The volume of the material was determined using the closest mark on the graduated cylinder. The bulk density was determined as the ratio of obtained mass and volume.

### 5. Sorption experiments

Sorption experiments were conducted using batch system. Na2HAsO4∙7H2O was used for preparing arsenic stock solutions at various concentrations (300, 200, 100, 50, 25, 10 and 5 mg/L). 0.5000 0.0001 g of each sorbent was weighed in every 100 mL glass vessel using analytical scales Kern ALJ 220–4 (Balingen, Germany). 30.00 0.05 mL of an As(V) solution was then added to every vessel with the adsorbent. Vessels were then shaken for 24 h at room temperature (23 1C) at 150 rpm using orbital shaker Biosan Multi-functional Orbital Shaker PSU-20i (Riga, Latvia) to ensure sorption equilibrium was achieved. Suspensions were filtered into 50 mL test tubes, and concentration of As(V) in the filtrate was then analysed using PerkinElmer AAnalyst 200 with flame atomization. Absorption was measured using background correction in N2O-C2H2 flame. A spectral line of 193.7 nm was used. FAAS spectrometer was calibrated with 1000 mg As/L standard solutions obtained from Scharlau (Barcelona, Spain) (As2O3 in 0.5 M HNO3). Each measurement was performed 3 times. In order to ensure arsenic analysis quality control, experiments were performed systematically; accurate As(V) concentration of respective stock solutions was measured 3 times and standard deviation was determined, which then was taken into account when describing sorption capacity of the materials.

### 6. Experimental results

#### 6.1. Homogeneity and bulk density

Homogeneity of materials was evaluated using optical microscope. All studied materials were homogeneous except Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov. Due to inhomogeneities, these materials were not further analysed with SEM and EDX. Obtained information about bulk density and homogeneity is summarized in Table 1.

#### 6.2. Metal ion content

Fe and Mn content by weight in all materials is summarized in Table 2 as an average of three repeated measurements with standard deviation. Material with highest Fe content is FeMn-Slov

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Table 1. Material bulk density, form and homogeneity.

where wFe and wMn are Fe and Mn mass fractions in materials, respectively [mas%]; CFe and CMn are Fe and Mn ion concentartions in solution, respectively [mg/L]; V is volume of the

The method of bulk density determination is based on a standard method, which is described in literature [44–46]. An empty 250 mL graduated cylinder was weighted, using analytical scales Kern ALJ 220–4 ( 0.1 mg). Approximately, 100 g of powder or granules/pellets were placed in the graduated cylinder, while determining the total mass of material and the cylinder. Deducing cylinder's mass from the total, the mass of the material was obtained. The cylinder was gently hit on the flat surface until the material became compacted and the change in volume was not observable. The volume of the material was determined using the closest mark on the graduated

Sorption experiments were conducted using batch system. Na2HAsO4∙7H2O was used for preparing arsenic stock solutions at various concentrations (300, 200, 100, 50, 25, 10 and 5 mg/L). 0.5000 0.0001 g of each sorbent was weighed in every 100 mL glass vessel using analytical scales Kern ALJ 220–4 (Balingen, Germany). 30.00 0.05 mL of an As(V) solution was then added to every vessel with the adsorbent. Vessels were then shaken for 24 h at room temperature (23 1C) at 150 rpm using orbital shaker Biosan Multi-functional Orbital Shaker PSU-20i (Riga, Latvia) to ensure sorption equilibrium was achieved. Suspensions were filtered into 50 mL test tubes, and concentration of As(V) in the filtrate was then analysed using PerkinElmer AAnalyst 200 with flame atomization. Absorption was measured using background correction in N2O-C2H2 flame. A spectral line of 193.7 nm was used. FAAS spectrometer was calibrated with 1000 mg As/L standard solutions obtained from Scharlau (Barcelona, Spain) (As2O3 in 0.5 M HNO3). Each measurement was performed 3 times. In order to ensure arsenic analysis quality control, experiments were performed systematically; accurate As(V) concentration of respective stock solutions was measured 3 times and standard deviation was determined, which then was

Homogeneity of materials was evaluated using optical microscope. All studied materials were homogeneous except Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov. Due to inhomogeneities, these materials were not further analysed with SEM and EDX. Obtained information about

Fe and Mn content by weight in all materials is summarized in Table 2 as an average of three repeated measurements with standard deviation. Material with highest Fe content is FeMn-Slov

cylinder. The bulk density was determined as the ratio of obtained mass and volume.

taken into account when describing sorption capacity of the materials.

bulk density and homogeneity is summarized in Table 1.

solution (0.060 L) [L]; m is mass of sorbent (1000 mg) [mg].

4.4. Determination of bulk density

102 Zeolites and Their Applications

5. Sorption experiments

6. Experimental results

6.2. Metal ion content

6.1. Homogeneity and bulk density


Table 2. Fe and Mn content of materials obtained using FAAS.

(19.22 0.73 mas%). Materials with highest Mn content are Mn-Slov(1) (4.58 0.17 mas%), CaMn-Slov (3.68 0.13 mas%) and FeMn-Slov (2.80 0.10 mas%).

### 6.3. Optical microscopy

All raw and modified materials were studied using optical microscopy. Optical micrographs were obtained at different magnifications: x7.1, x20 and x80; and x115 for some materials. Optical micrographs with magnification of x80 for all studied materials are summarized in Figure 1.

Optical micrographs (Figure 1) indicate that obtained materials FeMn-Slov, Fe-Khol, Fe-13X, Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov are modified with an iron compound. Furthermore, also in x80 magnification materials look homogeneous, except Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov, where iron compound covers only parts of the materials' surface. Optical micrographs (Figure 1) also indicate that obtained materials Mn-4A, Mn-Slov(1), Mn-Slov(2), FeMn-Slov and CaMn-Slov are modified with manganese compounds.

#### 6.4. Scanning electron microscopy

All raw and modified materials, except for nonhomogeneous Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov are studied with scanning electron microscopy (SEM). SEM micrographs are summarized in Figure 2.

SEM micrographs (Figure 2) indicate that obtained materials FeMn-Slov, Fe-Khol and Fe-13X are modified with an amorphous compound, which is also further proved using EDX (for more details, also see source [17]). SEM micrographs (Figure 2) indicate that obtained materials Mn-4A, Mn-Slov(1), Mn-Slov(2), FeMn-Slov and CaMn-Slov are modified with manganese compounds. In case of Mn-4A and Mn-Slov(2), manganese compound is amorphous, while for Mn-Slov(1), FeMn-Slov and CaMn-Slov a new crystalline phase (Mn8O10Cl3) is obtained (more details can be found in source [17]).

#### 6.5. Energy-dispersive X-ray spectroscopy

All raw and modified materials, except nonhomogeneous Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov were studied with energy-dispersive X-ray spectroscopy (EDX). EDX results are summarized in Table 3. Elements with probability ≥95% are shown as an average of 6 repeated measurements at different locations.

Analysing amorphous iron compounds with EDX, it was deduced that it consists of 65.28 3.91 mas% Fe and 34.72 2.08 mas% O. This result is in agreement with elemental content of FeOOH (62.85 mas% Fe, 36.01 mas% O and 1.14 mas% H).

Analysing manganese crystals with EDX, it was deduced that it consists of 54.58 3.27 mas % Mn, 30.27 1.82 mas% O and 15.15 0.91 mas% Cl. This result is in agreement with elemental content of Mn8O10Cl3 (62.26 mas% Mn, 22.67 mas% O, 15.07 mas% Cl). Elevated oxygen content can be explained with signal from zeolite oxygen and/or with other manganese oxide presence.

Figure 1. Optical micrographs with x80 magnification of all studied materials: (A) 4A; (B) Mn-4A; (C) Ukr; (D) Slov; (E) Mn-Slov(1); (F) Mn-Slov(2); (G) FeMn-Slov; (H) CaMn-Slov; (I) Khol; (J) Fe-Khol; (K) 13X; (L) Fe-13X; (M) Fe-Ukr(1); (N)

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Fe-Ukr(2); (O) Fe-Ukr(3); (P) Fe-Slov.

(19.22 0.73 mas%). Materials with highest Mn content are Mn-Slov(1) (4.58 0.17 mas%),

All raw and modified materials were studied using optical microscopy. Optical micrographs were obtained at different magnifications: x7.1, x20 and x80; and x115 for some materials. Optical micrographs with magnification of x80 for all studied materials are summarized in Figure 1.

Optical micrographs (Figure 1) indicate that obtained materials FeMn-Slov, Fe-Khol, Fe-13X, Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov are modified with an iron compound. Furthermore, also in x80 magnification materials look homogeneous, except Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov, where iron compound covers only parts of the materials' surface. Optical micrographs (Figure 1) also indicate that obtained materials Mn-4A, Mn-Slov(1), Mn-Slov(2),

All raw and modified materials, except for nonhomogeneous Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov are studied with scanning electron microscopy (SEM). SEM micrographs are

SEM micrographs (Figure 2) indicate that obtained materials FeMn-Slov, Fe-Khol and Fe-13X are modified with an amorphous compound, which is also further proved using EDX (for more details, also see source [17]). SEM micrographs (Figure 2) indicate that obtained materials Mn-4A, Mn-Slov(1), Mn-Slov(2), FeMn-Slov and CaMn-Slov are modified with manganese compounds. In case of Mn-4A and Mn-Slov(2), manganese compound is amorphous, while for Mn-Slov(1), FeMn-Slov and CaMn-Slov a new crystalline phase (Mn8O10Cl3) is

All raw and modified materials, except nonhomogeneous Fe-Ukr(1), Fe-Ukr(2), Fe-Ukr(3) and Fe-Slov were studied with energy-dispersive X-ray spectroscopy (EDX). EDX results are summarized in Table 3. Elements with probability ≥95% are shown as an average of 6 repeated

Analysing amorphous iron compounds with EDX, it was deduced that it consists of 65.28 3.91 mas% Fe and 34.72 2.08 mas% O. This result is in agreement with elemental content of FeOOH

Analysing manganese crystals with EDX, it was deduced that it consists of 54.58 3.27 mas % Mn, 30.27 1.82 mas% O and 15.15 0.91 mas% Cl. This result is in agreement with elemental content of Mn8O10Cl3 (62.26 mas% Mn, 22.67 mas% O, 15.07 mas% Cl). Elevated oxygen content can be explained with signal from zeolite oxygen and/or with other manga-

CaMn-Slov (3.68 0.13 mas%) and FeMn-Slov (2.80 0.10 mas%).

FeMn-Slov and CaMn-Slov are modified with manganese compounds.

6.3. Optical microscopy

104 Zeolites and Their Applications

6.4. Scanning electron microscopy

obtained (more details can be found in source [17]).

(62.85 mas% Fe, 36.01 mas% O and 1.14 mas% H).

6.5. Energy-dispersive X-ray spectroscopy

measurements at different locations.

nese oxide presence.

summarized in Figure 2.

Figure 1. Optical micrographs with x80 magnification of all studied materials: (A) 4A; (B) Mn-4A; (C) Ukr; (D) Slov; (E) Mn-Slov(1); (F) Mn-Slov(2); (G) FeMn-Slov; (H) CaMn-Slov; (I) Khol; (J) Fe-Khol; (K) 13X; (L) Fe-13X; (M) Fe-Ukr(1); (N) Fe-Ukr(2); (O) Fe-Ukr(3); (P) Fe-Slov.

6.6. As(V) sorption experiments

Table 3. EDX results (elements with probability ≥95%).

determination coefficients (R2

of Mn-4A and Mn-Slov(2).

cients of fit with the sorption models are reported in Table 4.

Material Elemental content [mas%]

inhomogeneity, leaving large areas of zeolite unmodified.

results represent average with standard deviation.

bottom: Slov, Ukr, Khol, A and X) are shown in Figure 3.

Experimental data is compared with sorption models: Langmuir, Freundlich, Dubinin– Radushkevich, Temkin and Redlich–Peterson isotherm linearized forms comparing obtained

Al Si O Fe Mn Cl Na Ca

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Ukr 6.43 0.39 35.94 2.20 52.10 3.19 2.80 0.17 — —— 2.74 0.17 Slov 5.27 0.32 31.70 1.94 59.94 3.67 1.50 0.09 — —— 1.58 0.10 Mn-Slov(1) 3.86 0.24 31.66 1.94 47.33 2.90 0.71 0.04 8.40 0.51 5.97 0.37 0.24 0.01 1.83 0.11 Mn-Slov(2) 4.05 0.25 22.51 1.38 59.66 3.65 1.37 0.08 10.64 0.65 — — 1.76 0.11 FeMn-Slov 3.16 0.17 19.38 1.05 43.28 2.34 6.31 0.34 11.10 0.60 6.15 1.04 9.24 0.50 1.38 0.07 CaMn-Slov 3.48 0.22 24.69 1.51 48.37 3.06 1.80 0.11 7.86 0.48 7.41 0.45 — 6.40 0.39 Khol 8.96 0.55 36.15 2.21 51.25 3.14 2.50 0.15 — — 0.42 0.03 0.73 0.04 Fe-Khol 5.20 0.31 17.43 1.05 64.08 3.80 7.38 0.57 — 1.53 0.09 4.28 0.26 0.10 0.01 13X 14.56 0.89 22.97 1.41 52.15 3.19 1.95 0.12 — — 7.95 0.48 0.41 0.03 Fe-13X 16.67 1.02 23.59 1.44 51.00 3.12 5.62 0.34 — — 2.91 0.18 0.20 0.01

4A 14.76 0.91 15.71 0.95 55.59 1.89 —— — 13.94 0.53 — Mn-4A 19.90 1.22 20.62 1.26 46.55 2.85 — 1.74 0.11 — 11.19 0.68 —

As(V) sorption on raw zeolites is most precisely characterized by Freundlich's sorption model in all cases. As(V) sorption on iron oxohydroxides indicates that for these materials physisorption is dominant [3]. As(V) on Fe(III)-modified clinoptilolite and synthetic zeolites is most precisely characterized by Langmuir model [17, 20], which is also consistent with experimental data in this work, except for Fe-Ukr(2). Inconsistency in case of Fe-Ukr(2) can be explained by material's

As(V) sorption on Mn-modified zeolites is most precisely characterized with Freundlich model in cases of Mn-Slov(1), FeMn-Slov and CaMn-Slov and with Redlich–Peterson model in cases

Obtained As(V) equilibrium sorption capacities and Langmuir monolayer coverage (where applicable) are systematized in Table 5. Improvement shows how much material's equilibrium sorption capacity (at the highest studied As(V) solution concentration of 300 mg As(V)/L) increased after modification. Each sorption experiment was performed 3 times and obtained

As(V) sorption isotherms for each respective material group (raw and modified, from top to

). As(V) sorption experimental results and determination coeffi-

Figure 2. SEM micrographs of studied materials: (A) 4A; (B) Mn-4A; (C) Ukr; (D) Slov; (E) Mn-Slov(1); (F) Mn-Slov(2); (G) FeMn-Slov; (H) CaMn-Slov; (I) Khol; (J) Fe-Khol; (K) 13X; (L) Fe-13X.


Table 3. EDX results (elements with probability ≥95%).

#### 6.6. As(V) sorption experiments

Figure 2. SEM micrographs of studied materials: (A) 4A; (B) Mn-4A; (C) Ukr; (D) Slov; (E) Mn-Slov(1); (F) Mn-Slov(2); (G)

FeMn-Slov; (H) CaMn-Slov; (I) Khol; (J) Fe-Khol; (K) 13X; (L) Fe-13X.

106 Zeolites and Their Applications

Experimental data is compared with sorption models: Langmuir, Freundlich, Dubinin– Radushkevich, Temkin and Redlich–Peterson isotherm linearized forms comparing obtained determination coefficients (R2 ). As(V) sorption experimental results and determination coefficients of fit with the sorption models are reported in Table 4.

As(V) sorption on raw zeolites is most precisely characterized by Freundlich's sorption model in all cases. As(V) sorption on iron oxohydroxides indicates that for these materials physisorption is dominant [3]. As(V) on Fe(III)-modified clinoptilolite and synthetic zeolites is most precisely characterized by Langmuir model [17, 20], which is also consistent with experimental data in this work, except for Fe-Ukr(2). Inconsistency in case of Fe-Ukr(2) can be explained by material's inhomogeneity, leaving large areas of zeolite unmodified.

As(V) sorption on Mn-modified zeolites is most precisely characterized with Freundlich model in cases of Mn-Slov(1), FeMn-Slov and CaMn-Slov and with Redlich–Peterson model in cases of Mn-4A and Mn-Slov(2).

Obtained As(V) equilibrium sorption capacities and Langmuir monolayer coverage (where applicable) are systematized in Table 5. Improvement shows how much material's equilibrium sorption capacity (at the highest studied As(V) solution concentration of 300 mg As(V)/L) increased after modification. Each sorption experiment was performed 3 times and obtained results represent average with standard deviation.

As(V) sorption isotherms for each respective material group (raw and modified, from top to bottom: Slov, Ukr, Khol, A and X) are shown in Figure 3.


Table 4. As(V) sorption data and comparison with sorption models. Values marked in bold signify a respective model with the highest fit.

#### 6.7. As(V) equilibrium sorption capacity correlation with Fe and Mn content

A correlation analysis was performed using obtained sorption data and Fe and Mn content of materials. The analysis was performed using Microsoft Excel Data Analysis toolpack (USA; International). Three different As(V) equilibrium concentrations were chosen – at initial As(V) solution concentration of 300, 100 and 5 mg/L. Correlation coefficients are summarized in Table 6. Correlation analysis was performed using all studied materials, as well as separate material groups (where applicable).

7. Conclusion

Table 5. As(V) equilibrium sorption capacities.

Material Equilibrium sorption capacity qe,300 [mg/g]

Equilibrium sorption capacity qe,100 [mg/g]

Equilibrium sorption capacity qe,5 [mg/g]

4A 0.15 0.01 0.06 0.00 0.01 0.00 — — 4.2 Mn-4A 0.39 0.02 0.30 0.02 0.13 0.01 0.40 2.6 43.0 Ukr 0.43 0.02 0.14 0.01 0.02 0.00 — — 6.6 Fe-Ukr(1) 1.08 0.06 0.96 0.07 0.17 0.02 1.19 2.5 57.2 Fe-Ukr(2) 0.87 0.05 0.40 0.03 0.05 0.01 — 2.0 17.4 Fe-Ukr(3) 0.92 0.05 0.83 0.04 0.24 0.01 0.93 2.1 80.2 Slov 0.36 0.02 0.14 0.01 0.02 0.00 — — 8.2 Fe-Slov 4.74 0.29 4.08 0.29 0.29 0.03 4.92 13.2 >98.0 Mn-Slov(1) 4.92 0.30 2.21 0.13 0.12 0.01 10.50 13.7 41.2 Mn-Slov(2) 0.66 0.04 0.41 0.02 0.13 0.01 0.71 1.8 44.3 FeMn-Slov 6.96 0.49 3.79 0.27 0.30 0.02 6.82 19.3 >98.0 CaMn-Slov 1.32 0.10 0.51 0.04 0.06 0.00 — 3.7 21.3 KHol 0.24 0.02 0.10 0.01 0.02 0.00 — — 7.9 Fe-KHol 1.25 0.06 1.18 0.06 0.30 0.02 1.28 5.2 >98.0 13X 0.30 0.02 0.12 0.01 0.01 0.00 — — 3.2 Fe-13X 1.05 0.05 0.42 0.02 0.08 0.00 — 3.5 26.0

Langmuir monolayer coverage qm [mg/g]

Improvement [times]

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Water remediation [%]

109

clinoptilolite natural zeolites.

Natural and synthetic zeolites were modified and tailored for As(V) sorption using novel methods. Based on optical microscopy, SEM, EDX and FAAS results, it was proved that zeolites were modified with amorphous Fe(III) oxohydroxide, amorphous Mn(IV) oxide and crystalline mixed oxidation state manganese oxide-chloride Mn8O10Cl3. FeOOH, and Mn8O10Cl3-modification improves As(V) sorption capacities of these aluminosilicates. Granulated material with the highest As(V) sorption capacity is Mn8O10Cl3-FeOOH-modified Slovakian clinoptilolite natural zeolite (6.96 0.49 mg/g). These materials are effective and potential As(V) sorbents, able to remove >98% As(V) in water environments. Sorption capacity of granulated materials (per unit volume) was improved up to 26.5 times. As(V) sorption on FeOOH-modified zeolites follows Langmuir model most precisely, while sorption on unmodified zeolites is described by Freundlich isotherm. In case of Mn8O10Cl3-modified and MnO2-modified zeolites, Freundlich and Redlich–Peterson models show the most precise fit, respectively. There is a strong and positive correlation between As(V) sorption capacity and Fe content among all zeolites. Among studied materials, Mn content showed a strong positive correlation only among Ukrainian

Correlation between material's As(V) equilibrium sorption capacity qe and Fe content is strong (correlation strength ≥0.67 by absolute value) for all groups and for all materials altogether at all studied concentrations.

Correlation between material's As(V) equilibrium sorption capacity qe and Mn content is strong (correlation strength ≥0.67 by absolute value) only for Ukrainian natural zeolites at initial As(V) solution's concentration of 5 and 100 mg/L. Medium correlation (correlation strength between 0.33 and 0.67 by absolute value) is observed for Ukrainian and Slovakian natural zeolites at initial As(V) solution's concentrations of 300 mg/L. For studied materials altogether at all concentrations and for Slovakian zeolite group at initial As(V) solution's concentration of 5 and 100 mg/L correlation is weak (correlation strength <0.33 by absolute value).


Table 5. As(V) equilibrium sorption capacities.

#### 7. Conclusion

6.7. As(V) equilibrium sorption capacity correlation with Fe and Mn content

Material Determination coefficient R2

4A 0.0586 0.8401 0.2180 0.7655 0.3729 Mn-4A 0.9911 0.9935 0.5360 0.9629 0.9990 Ukr 0.3874 0.9724 0.4559 0.7704 0.7768 Fe-Ukr(1) 0.9853 0.8693 0.6305 0.8840 0.9342 Fe-Ukr(2) 0.9062 0.9872 0.6655 0.8849 0.9635 Fe-Ukr(3) 0.9990 0.9342 0.8974 0.9452 0.9875 Slov 0.7607 0.9801 0.4944 0.8226 0.9314 Fe-Slov 0.9936 0.9299 0.8849 0.9404 0.8459 Mn-Slov(1) 0.9896 0.9932 0.6416 0.8834 0.7678 Mn-Slov(2) 0.9625 0.9838 0.4748 0.9015 0.9938 FeMn-Slov 0.9704 0.9722 0.7660 0.9174 0.3561 CaMn-Slov 0.5771 0.9750 0.5346 0.6840 0.8770 KHol 0.6213 0.9379 0.3429 0.7111 0.8379 Fe-KHol 0.9968 0.8968 0.9726 0.9599 0.8998 13X 0.5236 0.9610 0.5364 0.7113 0.6179 Fe-13X 0.7705 0.9720 0.4454 0.8207 0.9271

100 mg/L correlation is weak (correlation strength <0.33 by absolute value).

material groups (where applicable).

all studied concentrations.

with the highest fit.

108 Zeolites and Their Applications

A correlation analysis was performed using obtained sorption data and Fe and Mn content of materials. The analysis was performed using Microsoft Excel Data Analysis toolpack (USA; International). Three different As(V) equilibrium concentrations were chosen – at initial As(V) solution concentration of 300, 100 and 5 mg/L. Correlation coefficients are summarized in Table 6. Correlation analysis was performed using all studied materials, as well as separate

Table 4. As(V) sorption data and comparison with sorption models. Values marked in bold signify a respective model

Langmuir Freundlich Dubinin–Radushkevich Temkin Redlich–Peterson

Correlation between material's As(V) equilibrium sorption capacity qe and Fe content is strong (correlation strength ≥0.67 by absolute value) for all groups and for all materials altogether at

Correlation between material's As(V) equilibrium sorption capacity qe and Mn content is strong (correlation strength ≥0.67 by absolute value) only for Ukrainian natural zeolites at initial As(V) solution's concentration of 5 and 100 mg/L. Medium correlation (correlation strength between 0.33 and 0.67 by absolute value) is observed for Ukrainian and Slovakian natural zeolites at initial As(V) solution's concentrations of 300 mg/L. For studied materials altogether at all concentrations and for Slovakian zeolite group at initial As(V) solution's concentration of 5 and Natural and synthetic zeolites were modified and tailored for As(V) sorption using novel methods. Based on optical microscopy, SEM, EDX and FAAS results, it was proved that zeolites were modified with amorphous Fe(III) oxohydroxide, amorphous Mn(IV) oxide and crystalline mixed oxidation state manganese oxide-chloride Mn8O10Cl3. FeOOH, and Mn8O10Cl3-modification improves As(V) sorption capacities of these aluminosilicates. Granulated material with the highest As(V) sorption capacity is Mn8O10Cl3-FeOOH-modified Slovakian clinoptilolite natural zeolite (6.96 0.49 mg/g). These materials are effective and potential As(V) sorbents, able to remove >98% As(V) in water environments. Sorption capacity of granulated materials (per unit volume) was improved up to 26.5 times. As(V) sorption on FeOOH-modified zeolites follows Langmuir model most precisely, while sorption on unmodified zeolites is described by Freundlich isotherm. In case of Mn8O10Cl3-modified and MnO2-modified zeolites, Freundlich and Redlich–Peterson models show the most precise fit, respectively. There is a strong and positive correlation between As(V) sorption capacity and Fe content among all zeolites. Among studied materials, Mn content showed a strong positive correlation only among Ukrainian clinoptilolite natural zeolites.

Acknowledgements

All studied materials

Ukrainian zeolites

Slovakian zeolites

Kristine Rugele and Ruta Ozola.

and Technology, Trondheim, Norway

\*Address all correspondence to: andrejs.krauklis@ntnu.no

Author details

References

ogy. 2008;99:1-7

Andrey E. Krauklis1,2,3\*

The author is thankful for an opportunity to perform this work in the laboratory of Department of Environmental Science of Latvian University. This would not be possible without Prof. Maris Klavins, to whom the author is deeply grateful. Acknowledgement also goes to the author's colleagues and teachers who have supported him during this work, Juris Burlakovs,

As(V) equilibrium sorption capacity Correlation with Fe content Correlation with Mn content

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qe, <sup>300</sup> 0.76 0.20 qe, <sup>100</sup> 0.84 0.17 qe, <sup>5</sup> 0.71 0.05

qe, <sup>300</sup> 0.83 0.50 qe, <sup>100</sup> 0.68 0.70 qe, <sup>5</sup> 0.67 0.94

qe, <sup>300</sup> 0.80 0.40 qe, <sup>100</sup> 0.80 0.09 qe, <sup>5</sup> 0.86 0.07

Table 6. As(V) equilibrium sorption capacities, Fe and Mn content correlation.

1 Department of Mechanical and Industrial Engineering, Norwegian University of Science

3 Institute of General Chemical Engineering, Riga Technical University, Riga, LV, Latvia

[1] Mukherjee A, Bhattacharya P, Savage K, Foster A, Bundschuh J. Distribution of geogenic arsenic in hydrologic systems: Controls and challenges. Journal of Contaminant Hydrol-

2 Department of Environmental Science, University of Latvia, Riga, LV, Latvia

Figure 3. As(V) sorption experimental data and constructed isotherms.


Table 6. As(V) equilibrium sorption capacities, Fe and Mn content correlation.

### Acknowledgements

The author is thankful for an opportunity to perform this work in the laboratory of Department of Environmental Science of Latvian University. This would not be possible without Prof. Maris Klavins, to whom the author is deeply grateful. Acknowledgement also goes to the author's colleagues and teachers who have supported him during this work, Juris Burlakovs, Kristine Rugele and Ruta Ozola.

### Author details

Andrey E. Krauklis1,2,3\*

\*Address all correspondence to: andrejs.krauklis@ntnu.no

1 Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, Trondheim, Norway


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Figure 3. As(V) sorption experimental data and constructed isotherms.

110 Zeolites and Their Applications

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[17] Krauklis A, Ozola R, Burlakovs J, Rugele K, Kirillov K, Trubaca-Boginska A, Rubenis K, Stepanova V, Klavins M. FeOOH and Mn8O10Cl3 modified zeolites for as(V) removal in aqueous medium. Journal of Chemical Technology and Biotechnology. 2017 Special Issue

South and East Asia, Technical Report No. 31303. 2005

[5] World Health Organization (http://www.who.int/en/)

modified peat sorbents. Open Chem. 2016;14:46-59

with ferric ion. Environmental Chemistry Letters. 2007;5:125-129

Marzocchi, Buenos Aires, Argentina. 1994

istry. 2014;25:56-66

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Chimica Acta. 2014;831:1-23

Hoboken, New Jersey. 2004

Chemosphere. 2005;60:319-325

Bulgaria: Stara Zagora; 2009

157-165

Ravenscroft.pdf)


**Chapter 7**

**Provisional chapter**

**Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for**

**Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for** 

Mixed matrix membranes (MMMs) could provide a solution to the permeability and selectivity trade-off in polymeric membranes and bridge the gap with inorganic membranes. MMM could offer the physicochemical stability of a ceramic material while ensuring the desired morphology with higher permeability, selectivity, hydrophilicity, fouling resistance, as well as greater thermal, mechanical, and chemical strength over a wider temperature and pH range. Zeolites are fascinating and versatile materials, vital for a wide range of industries due to their unique structure, greater mechanical strength, and chemical properties. This chapter focused on zeolite-MMM and characterized various zeolite-reinforced polymeric membrane types and applications. Several key rules in the synthesis procedures have been comprehensively discussed for the optimum interfacial morphology between the zeolites and polymers. Furthermore, the influence of the zeolite filler incorporation has been discussed and explored for a range of applications. This chapter provided a broad overview of the MMM's challenges and future improve-

**Keywords:** mixed matrix membrane, filler, zeolites, hydrophilicity, interfacial,

Both polymeric and ceramic membranes have been the center of interest for their tremendous contribution in water treatment industry. Despite their advantages, these synthetic membranes have limitations in terms of performance and durability. Over the years, researchers have been trying to combine the effective features of both, polymeric and ceramic, materials in one new material called mixed matrix membrane (MMM) or hybrid membrane. The sole purpose of

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.73824

**Sustainable Engineering**

**Sustainable Engineering**

http://dx.doi.org/10.5772/intechopen.73824

ment investigative directions.

morphology

**1. Introduction**

**Abstract**

Mahboobeh Maghami and Amira Abdelrasoul

Mahboobeh Maghami and Amira Abdelrasoul

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


#### **Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for Sustainable Engineering Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for Sustainable Engineering**

DOI: 10.5772/intechopen.73824

Mahboobeh Maghami and Amira Abdelrasoul Mahboobeh Maghami and Amira Abdelrasoul

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73824

#### **Abstract**

[36] http://www.dpzzz.com/ru/info/1/5.html Ukrainian natural zeolite (acc. 27.02.2016)

[38] Grace Davison – Materials & Packaging Technologies. SYLOSIV Molecular Sieve Powder

[39] Ozola R, Krauklis A, Leitietis M, Burlakovs J, Vircava I, Ansone-Bertina L, Bhatnagar A, Klavins M. FeOOH-modified clay sorbents for arsenic removal from aqueous solutions.

[40] Jankēvica M, Ansone L, Kļaviņš M. Material Science and Applied Chemistry. 2013;29:101-

[41] Wang S, Gao B, Li Y, Mosa A, Zimmerman AR, Ma LQ, Harris WG, Migliaccio KW.

[42] Mahadik KR. Concise Inorganic Pharmaceutical Chemistry. Pragati Books Pvt. Ltd; 2008

[44] The United States Pharmacopeial Convention. Bulk Density and Tapped Density of

[45] FMP Group (Australia) Pty. Ltd. Standart Test Method, Bulk Density/Volume Tapped.

[46] World Health Organization. Bulk density and tapped density of powders, Document

[43] http://www.leica-microsystems.com Leica Microscopes (acc. 27.05.2016)

[37] http://www.zeolite.spb.ru/nat\_zeo.htm Russian natural zeolite (acc. 17.05.2016)

Environmental. Technology and Innovation. 2016

Bioresource Technology. 2015;181:13-17

Powders, Stage 6 Harmonization. 2011

QAS/11.450 FINAL. 2012

Brochure (2008)

114 Zeolites and Their Applications

108

2009

Mixed matrix membranes (MMMs) could provide a solution to the permeability and selectivity trade-off in polymeric membranes and bridge the gap with inorganic membranes. MMM could offer the physicochemical stability of a ceramic material while ensuring the desired morphology with higher permeability, selectivity, hydrophilicity, fouling resistance, as well as greater thermal, mechanical, and chemical strength over a wider temperature and pH range. Zeolites are fascinating and versatile materials, vital for a wide range of industries due to their unique structure, greater mechanical strength, and chemical properties. This chapter focused on zeolite-MMM and characterized various zeolite-reinforced polymeric membrane types and applications. Several key rules in the synthesis procedures have been comprehensively discussed for the optimum interfacial morphology between the zeolites and polymers. Furthermore, the influence of the zeolite filler incorporation has been discussed and explored for a range of applications. This chapter provided a broad overview of the MMM's challenges and future improvement investigative directions.

**Keywords:** mixed matrix membrane, filler, zeolites, hydrophilicity, interfacial, morphology

### **1. Introduction**

Both polymeric and ceramic membranes have been the center of interest for their tremendous contribution in water treatment industry. Despite their advantages, these synthetic membranes have limitations in terms of performance and durability. Over the years, researchers have been trying to combine the effective features of both, polymeric and ceramic, materials in one new material called mixed matrix membrane (MMM) or hybrid membrane. The sole purpose of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

developing new materials has been to associate the advantageous characteristics of the two types of membranes boosting the overall process efficacy. Conventionally, objectives such as enhancement in permeability or selectivity, reduction in fouling and removal of specific contaminants have been attained either by combining two or more processes or by developing an integrated filtration process. However, material advancement in membrane technology and nanotechnology has made it possible to fine tune the process efficiency and have successfully paved the way for the synthesis of MMMs for different applications. Apart from the water purification applications, the advent of MMMs has revolutionized other areas also where separation or purification is of great significance. Some of these potential applications reported in literature include water purification, medical industry, catalytic, and gas separation. Nevertheless, MMMs have not yet crossed the lab-scale barrier because the MMM technology is still in a developmental phase and only a few lab-scale developments have been reported so far.

MMMs, depending on the type of the dispersed fillers in the polymer matrix, as presented in **Figure 1** [1]. This chapter will focus on inorganic filler-based MMM, especially zeolite-MMM.

The field of inorganic filler-based membrane is a promising type of membrane, which has been explored extensively over the recent years. In the polymeric matrix, the inorganic fillers attach themselves to support materials by covalent bonds, van der Waals forces, or hydrogen bonds. These inorganic fillers are prepared through processes such as sol gel, inert gas condensation, pulsed laser ablation, spark discharge generation, ion sputtering, spray pyrolysis, photothermal synthesis, thermal plasma synthesis, flame synthesis, low-temperature reactive synthesis, flame spray pyrolysis, mechanical alloying/milling, mechano-chemical synthesis, and electrodeposition. Currently, different types of inorganic fillers have been added to the polymeric phases. Some of

methods to incorporate inorganic fillers into membrane structure by blending with the solution or by attaching the fillers to the surface through different techniques [4]. Inorganic-based filler MMMs have been employed in water industry for the adsorptive removal of pollutants, disinfection and/ or microbial control, catalytic degradation, and desalination [13]. They also have potentials to provide both high gas superior selectivity and the desirable mechanical and economical properties. Researchers believe that a suitable combination of polymers and inorganic fillers should offer superior permeability and selectivity compared to simple materials. In this review, zeolite-MMM

Zeolites are microporous crystalline aluminosilicate materials with uniform pore and channel size, thus they are used in various fields such as catalysts in the petrochemical industry, ionexchangers, and absorbents for softening and purification of water [14–16]. Incorporation of zeolites into a polymer matrix has attracted great attention in membrane technology, due to several excellent advantages such as permeability improvement of the selective component, in addition to the enhancement of the thermal stability and the mechanical strength of a polymeric membrane, [17] and its molecular sieving property, thermal resistance and chemical stability [18–20]. On the other hand, zeolites are expensive. Limitation in both polymeric and zeolite offers the need to synthesize the novel polymer–zeolite MMM. The interaction of zeolites in the membrane matrix and its shape-selective catalytic properties could improve permeability and selectivity separations [21]. There have been numerous attempts to incorporate zeolite particles in polymer matrices for gas separation due to its superior separation and size

Rezakazemi et al. [24] studied the gas transport properties of zeolite-reinforced polydimethylsiloxane (PDMS) MMM. They evaluated the feasibility of this zeolite-MMM for hydrogen purifi-

assessed. The filler was dispersed homogenously in the matrix without any voids at the zeolite– polymer interface. It was confirmed that the homogenous incorporation of filler in the matrix resulted in higher gas permeability for the MMM, as compared with the polymeric membranes.

will be comprehensively studied, as a promising membrane for several applications.

exclusion and in water purification applications [22, 23].

cation and natural gas sweetening, and also the permeation rates of CH<sup>4</sup>

[10], carbon nanotubes [11], and silver [12]. There are two

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117

, H<sup>2</sup> , C<sup>3</sup> H8

, and CO<sup>2</sup>

were

**2.1. Inorganic filler-based MMMs**

these fillers are zeolite [8], silica [9], TiO<sup>2</sup>

*2.1.1. Zeolite-MMMs*

### **2. Types of MMMs**

MMMs can be defined as incorporation of dispersed nanomaterials such as zeolite, carbon molecular sieve, and carbon nanotubes incorporated in a continuous polymer phase. **Figure 1** presented a schematic of an ideal MMM structure including the dispersed phase and the polymer matrix [1].

MMM could offer the physicochemical stability of a ceramic material and the membrane forming ease of polymeric materials while promising the desired morphology with higher permeability, selectivity, higher hydrophilicity, high fouling resistance, high thermal, mechanical, and chemical strength over a wider temperature and pH range [2–7].These types of MMMs are named as inorganic filler-based MMMs, organic filler-based MMMs, biofiller-based MMMs, and hybrid filler-based

**Figure 1.** Schematic diagram of an ideal MMM structure [1].

MMMs, depending on the type of the dispersed fillers in the polymer matrix, as presented in **Figure 1** [1]. This chapter will focus on inorganic filler-based MMM, especially zeolite-MMM.

#### **2.1. Inorganic filler-based MMMs**

developing new materials has been to associate the advantageous characteristics of the two types of membranes boosting the overall process efficacy. Conventionally, objectives such as enhancement in permeability or selectivity, reduction in fouling and removal of specific contaminants have been attained either by combining two or more processes or by developing an integrated filtration process. However, material advancement in membrane technology and nanotechnology has made it possible to fine tune the process efficiency and have successfully paved the way for the synthesis of MMMs for different applications. Apart from the water purification applications, the advent of MMMs has revolutionized other areas also where separation or purification is of great significance. Some of these potential applications reported in literature include water purification, medical industry, catalytic, and gas separation. Nevertheless, MMMs have not yet crossed the lab-scale barrier because the MMM technology is still in a

developmental phase and only a few lab-scale developments have been reported so far.

MMMs can be defined as incorporation of dispersed nanomaterials such as zeolite, carbon molecular sieve, and carbon nanotubes incorporated in a continuous polymer phase. **Figure 1** presented a schematic of an ideal MMM structure including the dispersed phase and the

MMM could offer the physicochemical stability of a ceramic material and the membrane forming ease of polymeric materials while promising the desired morphology with higher permeability, selectivity, higher hydrophilicity, high fouling resistance, high thermal, mechanical, and chemical strength over a wider temperature and pH range [2–7].These types of MMMs are named as inorganic filler-based MMMs, organic filler-based MMMs, biofiller-based MMMs, and hybrid filler-based

**2. Types of MMMs**

116 Zeolites and Their Applications

polymer matrix [1].

**Figure 1.** Schematic diagram of an ideal MMM structure [1].

The field of inorganic filler-based membrane is a promising type of membrane, which has been explored extensively over the recent years. In the polymeric matrix, the inorganic fillers attach themselves to support materials by covalent bonds, van der Waals forces, or hydrogen bonds. These inorganic fillers are prepared through processes such as sol gel, inert gas condensation, pulsed laser ablation, spark discharge generation, ion sputtering, spray pyrolysis, photothermal synthesis, thermal plasma synthesis, flame synthesis, low-temperature reactive synthesis, flame spray pyrolysis, mechanical alloying/milling, mechano-chemical synthesis, and electrodeposition. Currently, different types of inorganic fillers have been added to the polymeric phases. Some of these fillers are zeolite [8], silica [9], TiO<sup>2</sup> [10], carbon nanotubes [11], and silver [12]. There are two methods to incorporate inorganic fillers into membrane structure by blending with the solution or by attaching the fillers to the surface through different techniques [4]. Inorganic-based filler MMMs have been employed in water industry for the adsorptive removal of pollutants, disinfection and/ or microbial control, catalytic degradation, and desalination [13]. They also have potentials to provide both high gas superior selectivity and the desirable mechanical and economical properties. Researchers believe that a suitable combination of polymers and inorganic fillers should offer superior permeability and selectivity compared to simple materials. In this review, zeolite-MMM will be comprehensively studied, as a promising membrane for several applications.

#### *2.1.1. Zeolite-MMMs*

Zeolites are microporous crystalline aluminosilicate materials with uniform pore and channel size, thus they are used in various fields such as catalysts in the petrochemical industry, ionexchangers, and absorbents for softening and purification of water [14–16]. Incorporation of zeolites into a polymer matrix has attracted great attention in membrane technology, due to several excellent advantages such as permeability improvement of the selective component, in addition to the enhancement of the thermal stability and the mechanical strength of a polymeric membrane, [17] and its molecular sieving property, thermal resistance and chemical stability [18–20]. On the other hand, zeolites are expensive. Limitation in both polymeric and zeolite offers the need to synthesize the novel polymer–zeolite MMM. The interaction of zeolites in the membrane matrix and its shape-selective catalytic properties could improve permeability and selectivity separations [21]. There have been numerous attempts to incorporate zeolite particles in polymer matrices for gas separation due to its superior separation and size exclusion and in water purification applications [22, 23].

Rezakazemi et al. [24] studied the gas transport properties of zeolite-reinforced polydimethylsiloxane (PDMS) MMM. They evaluated the feasibility of this zeolite-MMM for hydrogen purification and natural gas sweetening, and also the permeation rates of CH<sup>4</sup> , H<sup>2</sup> , C<sup>3</sup> H8 , and CO<sup>2</sup> were assessed. The filler was dispersed homogenously in the matrix without any voids at the zeolite– polymer interface. It was confirmed that the homogenous incorporation of filler in the matrix resulted in higher gas permeability for the MMM, as compared with the polymeric membranes.

**Figure 2.** Schematic cross-section of zeolite nanocomposite membrane (zeolite-MMM) [26].

Ciobanu et al. [25] reported that zeolite-polyurethane membranes demonstrated improved properties. The good interaction between the polymer and the zeolite at the interface was confirmed and the membrane swelling was reduced. Consequently, the water flux through membrane increased with increasing zeolite concentration.

Hoek [26] studied the formation of mixed matrix reverse osmosis membranes by the interfacial polymerization of thin film nanocomposite polysulfone supports impregnated with zeolites. **Figure 2** represents the cross-section image of zeolite nanocomposite reverse osmosis membrane, which is utilized for water purification through desalination process. It was found that increasing the zeolite nanofillers concentrations resulted in smoother, more hydrophilic, and more negatively charged MMM. As a consequence, the MMM membrane demonstrated high flux and a slight improvement in salt rejection compared to TFC membrane without zeolite nano-particles due to changes of membrane morphology.

channels that allow the solvent to pass through the membrane [34]. However, mechanical

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**Figure 3** represents various structures at the polymer/zeolite interface region [36]. **Figure 3a** demonstrates a homogenous blend of polymer and sieve, indicating an ideal interphase morphology. **Figure 3b** shows polymer chains rigidification due to the shrinkage stresses generated during solvent removal. **Figure 3c** confirmed poor compatibility between zeolite and polymer matrix morphology, due to the formation of voids at the interfacial region. **Figure 3d** indicates sealing surface pores of zeolites by the rigidified polymer chains. Overall, the interaction between polymer and zeolite is related to chemical nature of the polymer and sieve surfaces, and the stress encountered during material preparation, which are the critical factors

These features are a challenge and should be controlled or avoided for the synthesis of the targeted zeolite-MMM for several applications. The formation of relatively nonselective defects at the interface between the zeolite particles and the polymer medium will result in MMMs, which fail to demonstrate their performance [37]. Therefore, despite the good properties of

Several strategies have been offered to improve the polymer–zeolite interaction; hence to avoid nonselective voids. These methods are included incorporation of a plasticizer into

the polymer-zeolite membranes MMMs, they still face some challenges to overcome.

**4. Interfacial modification of zeolite-MMMs**

strength and rejection rate are also concerned by the channel density [35].

**Figure 3.** Illustration of various structures at the polymer/zeolite interface region [36].

to form the interphase.

### **3. Interfacial morphology of zeolite-MMMs**

To obtain the optimum interfacial morphology between the zeolites and polymer, several key roles should be considered. The first one is to promote the adhesion between polymer matrix and molecular sieve phases by modifying the zeolite surface with silane coupling agents [27–29]. The second one is to introduce low molecular weight materials to fill the voids between polymer and molecular sieve phases [30, 31]. The third one is to apply high processing temperatures close to glass transition temperature (Tg) of polymeric materials to maintain the polymer chain flexibility during the membrane formation [32]. The fourth one is to prime the surface of zeolites by polymer [33].

The polymer matrix plays an important role for permeability and the inorganic filler has a controlling factor for the selectivity of the separation process. As a result, interfacial compatibility between the two phases has profound impact on the separation performance for such membranes. The addition of inorganic fillers has key impacts on the interfacial void formation, aggregation, pore blockage of the morphology, and the transport phenomenon. Consequently, the impregnation of zeolites has a significant influence on the overall performance of the newly developed MMMs. The formation of these interfacial voids is attributed to two main phenomena, the interaction between the polymer phase and the filler and the stress exerted during preparation [1, 32]. The presence of interfacial voids creates additional

**Figure 3.** Illustration of various structures at the polymer/zeolite interface region [36].

Ciobanu et al. [25] reported that zeolite-polyurethane membranes demonstrated improved properties. The good interaction between the polymer and the zeolite at the interface was confirmed and the membrane swelling was reduced. Consequently, the water flux through

Hoek [26] studied the formation of mixed matrix reverse osmosis membranes by the interfacial polymerization of thin film nanocomposite polysulfone supports impregnated with zeolites. **Figure 2** represents the cross-section image of zeolite nanocomposite reverse osmosis membrane, which is utilized for water purification through desalination process. It was found that increasing the zeolite nanofillers concentrations resulted in smoother, more hydrophilic, and more negatively charged MMM. As a consequence, the MMM membrane demonstrated high flux and a slight improvement in salt rejection compared to TFC membrane without

To obtain the optimum interfacial morphology between the zeolites and polymer, several key roles should be considered. The first one is to promote the adhesion between polymer matrix and molecular sieve phases by modifying the zeolite surface with silane coupling agents [27–29]. The second one is to introduce low molecular weight materials to fill the voids between polymer and molecular sieve phases [30, 31]. The third one is to apply high processing temperatures close to glass transition temperature (Tg) of polymeric materials to maintain the polymer chain flexibility during the membrane formation [32]. The fourth one is to prime the surface of

The polymer matrix plays an important role for permeability and the inorganic filler has a controlling factor for the selectivity of the separation process. As a result, interfacial compatibility between the two phases has profound impact on the separation performance for such membranes. The addition of inorganic fillers has key impacts on the interfacial void formation, aggregation, pore blockage of the morphology, and the transport phenomenon. Consequently, the impregnation of zeolites has a significant influence on the overall performance of the newly developed MMMs. The formation of these interfacial voids is attributed to two main phenomena, the interaction between the polymer phase and the filler and the stress exerted during preparation [1, 32]. The presence of interfacial voids creates additional

membrane increased with increasing zeolite concentration.

**Figure 2.** Schematic cross-section of zeolite nanocomposite membrane (zeolite-MMM) [26].

zeolite nano-particles due to changes of membrane morphology.

**3. Interfacial morphology of zeolite-MMMs**

zeolites by polymer [33].

118 Zeolites and Their Applications

channels that allow the solvent to pass through the membrane [34]. However, mechanical strength and rejection rate are also concerned by the channel density [35].

**Figure 3** represents various structures at the polymer/zeolite interface region [36]. **Figure 3a** demonstrates a homogenous blend of polymer and sieve, indicating an ideal interphase morphology. **Figure 3b** shows polymer chains rigidification due to the shrinkage stresses generated during solvent removal. **Figure 3c** confirmed poor compatibility between zeolite and polymer matrix morphology, due to the formation of voids at the interfacial region. **Figure 3d** indicates sealing surface pores of zeolites by the rigidified polymer chains. Overall, the interaction between polymer and zeolite is related to chemical nature of the polymer and sieve surfaces, and the stress encountered during material preparation, which are the critical factors to form the interphase.

These features are a challenge and should be controlled or avoided for the synthesis of the targeted zeolite-MMM for several applications. The formation of relatively nonselective defects at the interface between the zeolite particles and the polymer medium will result in MMMs, which fail to demonstrate their performance [37]. Therefore, despite the good properties of the polymer-zeolite membranes MMMs, they still face some challenges to overcome.

### **4. Interfacial modification of zeolite-MMMs**

Several strategies have been offered to improve the polymer–zeolite interaction; hence to avoid nonselective voids. These methods are included incorporation of a plasticizer into the polymer solution that can decrease the polymer glass transition temperature (Tg) [32]. Consequently, polymer chain flexibility maintains during membrane preparation either by annealing the membranes above glass transition temperature of polymer [38, 39] or external surface of zeolites can be modified by coupling agents. The surface-initiated polymerization is a most frequent technique to improve the polymer−filler adhesion in polymer-zeolite MMMs [40]. Furthermore, adding the low molecular-weight (LMWAs) to the membrane formulation can act as a compatibilizer or a third component to prepare glassy polymer/ LMWAs blend membranes [31, 41], priming method can be also used to reduce the stress at the polymerparticle interface, and to minimize agglomeration of the particles. Consequently, the interfacial interaction between the two components will be improved through coating the surface of the filler particles with a dilute polymer dope [31], to minimize the zeolite-solvent/nonsolvent interaction, especially for the use of modified zeolite in asymmetric membranes [42]. Therefore, the obtained hydrophobic surface can suppress the zeolite particles from acting as nucleating agents. As a result, it will minimize the voids induced by the unfavorable interaction between polymer and zeolite particles.

#### **4.1. Interfacial modification with silane agents**

Silane coupling agents were commonly proposed to modify the zeolite surface in order to improve the compatibility of the inorganic filler with the polymeric matrix [43, 44]. It is known from literatures related to the silanation of zeolites that silane coupling agents have two types of reactive groups. First, the hydroxyl groups of zeolites, which could make hydrogen bonds with the amino silane agent [43]. Second, the organo functional group, such as amino and epoxy, which could be used to bond polymer chains to the zeolite. Therefore, improving adhesion between the zeolite and the bulk polymer phase in the membrane was achieved [42]. **Figure 4** shows a schematic silanation of zeolite surface with 3-aminopropyldimethylethoxysilane (APDMES) coupling agent [42].

permeability was dropped relatively, similarly, to the performance of pure polymeric mem-

In order to overcome this problem, other researchers such as Li et al [28] modified zeolite 3A, 4A, and 5A using 3-aminopropylmethyldiethoxysilane (APMDES) in toluene solvent. Hence, rigidification of polymer chain and partial pore blockage reduced through this modification process. As a result, they showed high improvement for both of the selectivity and permeabil-

blockage the zeolite pores. Therefore, in some cases, surface modification by the silane coupling agents was recommended to enhance interfacial adhesion but hardly improve permselectivity.

Adding low molecular weight additives (LMWAs) to the membrane formulation acts as a compatibilizer or a third component to improve the compatibility between zeolite and polymer matrix. The low molecular weight materials induce a hydrogen bond with hydroxyl and carbonyl moiety. In addition, the formation of hydrogen bond confirms its solubility in the solvent used to make the polymer dope solution. It should be noted that LMWMs should be solid at room temperature, in order to prevent their evaporation during membrane fabrication, and consequently losing their ability of forming interfacial voids [31]. Once hydrogen bonds are formed between polymer chains and LMWMs, the free volume of polymers decreases, which results in a decrease in their gas permeability whereas increase in their gas permselectivity. Yilmaz [49] reported mixed matrix membranes for the use in gas separation by blending polycarbonates (PC) with an additive p-nitroaniline (pNA) and incorporating zeolite 4A particles as filler. The permeability of all gases was measured using differential scanning calorimetry (DSC) analysis through PC/(pNA)/zeolite 4A membranes, which were lower than those through pure

than those MMMs containing zeolite without the modification and without major

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121

brane, due to the pore blockage of the ZSM-2 zeolite [48].

**Figure 4.** Schematic of the envisioned coupling reaction [42].

**4.2. Addition of low molecular weight materials (LMWMs)**

ity of CO<sup>2</sup>

Koros [45] indicated that the glass transition temperature of MMMs is influenced by silane modification. In other words, the Tg of the zeolite-MMMs increased with the increasing of silane concentration on the surface of the zeolite particles. As a result, the silane modification of zeolite affects the mechanical properties of continuous phase due to the formation of the hydrogen bonding between the zeolite particles and polymer matrix and the movement reduction of the polymer chains [29, 41].

Leo [46] investigates the effects of silane-grafting on the separation performance of MMM for gas permeation. The 3-aminopropyltrimethoxysilane (APMS) was added to modify SAPO-34 zeolite before the impregnation into the asymmetric polysulfone (PSf) MMMs through dry– wet phase inversion method. Both PSf/modified SAPO-34 membranes showed great enhancement in terms of selectivity and permeability, compared to the original PSf membrane. The increment of CO<sup>2</sup> selectivity and permeability was correlated to the diminishing of the interfacial voids, when SAPO-34 zeolite was modified using APMS in ethanol.

Pechar et al. [47] studied the use of 3-aminopropyltrimethoxysilane (APTMS) influence to modify ZSM-2 zeolite to synthesize polyimide MMMs. Although micrographs showed the absence of voids, however, the modified ZSM-2-MMMs performance for CO<sup>2</sup> selectivity and

**Figure 4.** Schematic of the envisioned coupling reaction [42].

the polymer solution that can decrease the polymer glass transition temperature (Tg) [32]. Consequently, polymer chain flexibility maintains during membrane preparation either by annealing the membranes above glass transition temperature of polymer [38, 39] or external surface of zeolites can be modified by coupling agents. The surface-initiated polymerization is a most frequent technique to improve the polymer−filler adhesion in polymer-zeolite MMMs [40]. Furthermore, adding the low molecular-weight (LMWAs) to the membrane formulation can act as a compatibilizer or a third component to prepare glassy polymer/ LMWAs blend membranes [31, 41], priming method can be also used to reduce the stress at the polymerparticle interface, and to minimize agglomeration of the particles. Consequently, the interfacial interaction between the two components will be improved through coating the surface of the filler particles with a dilute polymer dope [31], to minimize the zeolite-solvent/nonsolvent interaction, especially for the use of modified zeolite in asymmetric membranes [42]. Therefore, the obtained hydrophobic surface can suppress the zeolite particles from acting as nucleating agents. As a result, it will minimize the voids induced by the unfavorable interac-

Silane coupling agents were commonly proposed to modify the zeolite surface in order to improve the compatibility of the inorganic filler with the polymeric matrix [43, 44]. It is known from literatures related to the silanation of zeolites that silane coupling agents have two types of reactive groups. First, the hydroxyl groups of zeolites, which could make hydrogen bonds with the amino silane agent [43]. Second, the organo functional group, such as amino and epoxy, which could be used to bond polymer chains to the zeolite. Therefore, improving adhesion between the zeolite and the bulk polymer phase in the membrane was achieved [42]. **Figure 4** shows a schematic silanation of zeolite surface with 3-aminopropyldi-

Koros [45] indicated that the glass transition temperature of MMMs is influenced by silane modification. In other words, the Tg of the zeolite-MMMs increased with the increasing of silane concentration on the surface of the zeolite particles. As a result, the silane modification of zeolite affects the mechanical properties of continuous phase due to the formation of the hydrogen bonding between the zeolite particles and polymer matrix and the movement

Leo [46] investigates the effects of silane-grafting on the separation performance of MMM for gas permeation. The 3-aminopropyltrimethoxysilane (APMS) was added to modify SAPO-34 zeolite before the impregnation into the asymmetric polysulfone (PSf) MMMs through dry– wet phase inversion method. Both PSf/modified SAPO-34 membranes showed great enhancement in terms of selectivity and permeability, compared to the original PSf membrane. The

Pechar et al. [47] studied the use of 3-aminopropyltrimethoxysilane (APTMS) influence to modify ZSM-2 zeolite to synthesize polyimide MMMs. Although micrographs showed the

facial voids, when SAPO-34 zeolite was modified using APMS in ethanol.

absence of voids, however, the modified ZSM-2-MMMs performance for CO<sup>2</sup>

selectivity and permeability was correlated to the diminishing of the inter-

selectivity and

tion between polymer and zeolite particles.

120 Zeolites and Their Applications

**4.1. Interfacial modification with silane agents**

methylethoxysilane (APDMES) coupling agent [42].

reduction of the polymer chains [29, 41].

increment of CO<sup>2</sup>

permeability was dropped relatively, similarly, to the performance of pure polymeric membrane, due to the pore blockage of the ZSM-2 zeolite [48].

In order to overcome this problem, other researchers such as Li et al [28] modified zeolite 3A, 4A, and 5A using 3-aminopropylmethyldiethoxysilane (APMDES) in toluene solvent. Hence, rigidification of polymer chain and partial pore blockage reduced through this modification process. As a result, they showed high improvement for both of the selectivity and permeability of CO<sup>2</sup> than those MMMs containing zeolite without the modification and without major blockage the zeolite pores. Therefore, in some cases, surface modification by the silane coupling agents was recommended to enhance interfacial adhesion but hardly improve permselectivity.

#### **4.2. Addition of low molecular weight materials (LMWMs)**

Adding low molecular weight additives (LMWAs) to the membrane formulation acts as a compatibilizer or a third component to improve the compatibility between zeolite and polymer matrix. The low molecular weight materials induce a hydrogen bond with hydroxyl and carbonyl moiety. In addition, the formation of hydrogen bond confirms its solubility in the solvent used to make the polymer dope solution. It should be noted that LMWMs should be solid at room temperature, in order to prevent their evaporation during membrane fabrication, and consequently losing their ability of forming interfacial voids [31]. Once hydrogen bonds are formed between polymer chains and LMWMs, the free volume of polymers decreases, which results in a decrease in their gas permeability whereas increase in their gas permselectivity.

Yilmaz [49] reported mixed matrix membranes for the use in gas separation by blending polycarbonates (PC) with an additive p-nitroaniline (pNA) and incorporating zeolite 4A particles as filler. The permeability of all gases was measured using differential scanning calorimetry (DSC) analysis through PC/(pNA)/zeolite 4A membranes, which were lower than those through pure PC membrane. The incorporation of pNA was essential, since pNA acts as a facilitator for provision of better interaction between rigid, glassy polymer PC and zeolite 4A particles. Therefore, the incorporation of a molecular-weight additive with functional groups into zeolite-MMMs can be used as a tool to improve the structure and performance properties of the membranes.

method [45]. The agglomeration is considered as responsible for defects between polymer matrix and zeolite particle phases [53]. Since more agglomeration occurs in the polymer matrix when smaller particles are used, especially at high particle loadings, and large zeolite particles are used to form practical mixed matrix membranes. Therefore, zeolite particles were primed by increasing amount of polymer. It should be considered that polymer effectively coats the zeolite particles before adding remaining bulk polymer and mixing with the priming polymer [54]. The purpose of priming is to reduce stress at the polymer-particle interface, increase the compatibility between zeolite and polymer in MMMs, and to minimize agglomeration of zeolite particles [55, 56].

Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for Sustainable Engineering

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123

The advent of zeolite-MMMs enhances the separation or purification performance of the membranes significantly. The review represents various applications of zeolite-reinforced polymeric membranes. Some of these potential applications reported in literature include

Water treatment is increasingly important to remove water pollutants and solve water problems. Drinking water may compose of hazardous substances such as toxins and endocrine disrupting compound. Therefore, it would be urgent to invent more sustainable and reliable treatment process to remove water contaminations and to regulate the quality of drinking water. The development of cost-effective membranes is in a great need to effectively replace the conventional water treatment technologies to produce water that meet or exceed stringent standards. Recent studies have demonstrated that the zeolite-MMMs were applied to design reverse osmosis membrane to enhance the membrane properties such as permeability, selectivity, stability, surface area, or catalytic activity in water purification and separation processes [57, 58]. Nevertheless, there are only few studies performed on zeolite-MMMs for water treatment, it is determined that the size of zeolite was designed to match the expected polyimide active film thickness, thereby

For example, thin film nanocomposite (TFN) reverse osmosis (RO) membranes have been used by incorporating zeolite particles into the PA rejection layer. It has shown that the incorporation of zeolite in a PA layer could improve its water permeability without significant loss of salt rejection under high pressure during RO process [60]. Main reason for that is nanochannels of zeolites with great sub-nanometer pores in zeolite nanoparticles that behave as preferential flow channels for water molecules. The zeolite-PA-based TFN membranes are considered as superior separation performance for RO applications due to their enhanced

Tanga [61] provided an additional study to confirm that thin film nanocomposite membranes can significantly improve FO water flux significant with a relatively low zeolite loading due to both the surface and intrinsic separation properties of TFN membranes. Compared with TFC membrane, the TFN membrane is potentially more favorable during the application of

treating feed solutions with relative higher salinity water under AL-FS orientation.

water purification, gas separation, medical, catalytic, and biomedical applications.

providing a preferential flow path through the nanochannels of zeolites [26, 59].

**5. Applications of zeolite-MMM**

**5.1. Zeolite-MMM for water purification**

water permeability of active layer [61].

One of the examples of LMWMs is 2,4,6-triaminopyrimidine (TAP) that contained three primary amine groups, which are able to form hydrogen bonds with both hydroxyl and carbonyl groups [31]. Furthermore, it had been reported that the carbonyl groups of polyimides (PI) could interact with amine groups of urethanes through the hydrogen bond formation.

Park [31] used TAP to obtain the interfacial void-free PI membranes filled with zeolites. TAP enhanced the contact of zeolite particles with polyimide chains presumably by forming the hydrogen bonding. As a consequence, the void-free PI/zeolite 13X/TAP membrane showed the higher gas permeability for He, N<sup>2</sup> , O<sup>2</sup> , CO<sup>2</sup> , and CH<sup>4</sup> with little expense of selectivity compared to the PI/TAP membrane having the same PI/TAP ratio, while the PI/zeolite 4A/ TAP membrane showed the lower permeability but higher permselectivity. The difference between both membranes was influenced by the pore size of zeolites. In addition, the molecular sieving effect of zeolites seemed to take place when the kinetic diameter of gas penetrants approached the pore size of zeolites.

#### **4.3. Annealing**

One of the largest challenges in designing zeolite-MMMs is the poor contact between polymer and zeolite defects. Many efforts made to overcome to this problem associated with the zeolite-MMMs through the annealing of zeolite-MMMs above the glass transition temperature (Tg) [32]. In other words, Tg is considered as a qualitative estimation to compare the polymer chain rigidity of mixed matrix membranes at different zeolite types with simple polymer membrane and it also leads to better contact between zeolite and polymer chain [50]. Annealing process at temperature above the Tg results in the formation of stronger bond between polymer matrix and zeolite. Despite advantages of annealing in relaxing the stress imposed to the hollow fiber membrane, it results in higher packing density of polymer chains. Therefore, there are drawbacks associated with annealing. In addition, it did not lead to significant improvement in the morphology of the membranes. Annealing at high Tg formed sieve-in-a-cage morphology, which will be difficult to create a good contact between the polymer and the sieve [32]. In order to overcome to this disadvantage of annealing, incorporation of a plasticizer into the polymer solution can decrease the polymer Tg and thus maintain polymer chain mobility and flexibility during membrane fabrication [51]. Therefore, to develop membrane fabrication technology, a quench method after annealing membranes above Tg can be effective in gas separation process by forming frozen polymer chains quickly [52]. Therefore, it will have a higher free volume in the polymer matrix and subsequently higher gas permeability without the loss of selectivity.

#### **4.4. Priming method**

The dilute polymers are the same as the bulk polymers used for the preparation of MMMs. Coating the surface of the filler particles with a dilute polymer dope is known as the priming method [45]. The agglomeration is considered as responsible for defects between polymer matrix and zeolite particle phases [53]. Since more agglomeration occurs in the polymer matrix when smaller particles are used, especially at high particle loadings, and large zeolite particles are used to form practical mixed matrix membranes. Therefore, zeolite particles were primed by increasing amount of polymer. It should be considered that polymer effectively coats the zeolite particles before adding remaining bulk polymer and mixing with the priming polymer [54]. The purpose of priming is to reduce stress at the polymer-particle interface, increase the compatibility between zeolite and polymer in MMMs, and to minimize agglomeration of zeolite particles [55, 56].

## **5. Applications of zeolite-MMM**

PC membrane. The incorporation of pNA was essential, since pNA acts as a facilitator for provision of better interaction between rigid, glassy polymer PC and zeolite 4A particles. Therefore, the incorporation of a molecular-weight additive with functional groups into zeolite-MMMs can be used as a tool to improve the structure and performance properties of the membranes.

One of the examples of LMWMs is 2,4,6-triaminopyrimidine (TAP) that contained three primary amine groups, which are able to form hydrogen bonds with both hydroxyl and carbonyl groups [31]. Furthermore, it had been reported that the carbonyl groups of polyimides (PI)

Park [31] used TAP to obtain the interfacial void-free PI membranes filled with zeolites. TAP enhanced the contact of zeolite particles with polyimide chains presumably by forming the hydrogen bonding. As a consequence, the void-free PI/zeolite 13X/TAP membrane showed

compared to the PI/TAP membrane having the same PI/TAP ratio, while the PI/zeolite 4A/ TAP membrane showed the lower permeability but higher permselectivity. The difference between both membranes was influenced by the pore size of zeolites. In addition, the molecular sieving effect of zeolites seemed to take place when the kinetic diameter of gas penetrants

One of the largest challenges in designing zeolite-MMMs is the poor contact between polymer and zeolite defects. Many efforts made to overcome to this problem associated with the zeolite-MMMs through the annealing of zeolite-MMMs above the glass transition temperature (Tg) [32]. In other words, Tg is considered as a qualitative estimation to compare the polymer chain rigidity of mixed matrix membranes at different zeolite types with simple polymer membrane and it also leads to better contact between zeolite and polymer chain [50]. Annealing process at temperature above the Tg results in the formation of stronger bond between polymer matrix and zeolite. Despite advantages of annealing in relaxing the stress imposed to the hollow fiber membrane, it results in higher packing density of polymer chains. Therefore, there are drawbacks associated with annealing. In addition, it did not lead to significant improvement in the morphology of the membranes. Annealing at high Tg formed sieve-in-a-cage morphology, which will be difficult to create a good contact between the polymer and the sieve [32]. In order to overcome to this disadvantage of annealing, incorporation of a plasticizer into the polymer solution can decrease the polymer Tg and thus maintain polymer chain mobility and flexibility during membrane fabrication [51]. Therefore, to develop membrane fabrication technology, a quench method after annealing membranes above Tg can be effective in gas separation process by forming frozen polymer chains quickly [52]. Therefore, it will have a higher free volume in the polymer matrix and subsequently

The dilute polymers are the same as the bulk polymers used for the preparation of MMMs. Coating the surface of the filler particles with a dilute polymer dope is known as the priming

, and CH<sup>4</sup>

with little expense of selectivity

could interact with amine groups of urethanes through the hydrogen bond formation.

, O<sup>2</sup> , CO<sup>2</sup>

the higher gas permeability for He, N<sup>2</sup>

approached the pore size of zeolites.

higher gas permeability without the loss of selectivity.

**4.3. Annealing**

122 Zeolites and Their Applications

**4.4. Priming method**

The advent of zeolite-MMMs enhances the separation or purification performance of the membranes significantly. The review represents various applications of zeolite-reinforced polymeric membranes. Some of these potential applications reported in literature include water purification, gas separation, medical, catalytic, and biomedical applications.

#### **5.1. Zeolite-MMM for water purification**

Water treatment is increasingly important to remove water pollutants and solve water problems. Drinking water may compose of hazardous substances such as toxins and endocrine disrupting compound. Therefore, it would be urgent to invent more sustainable and reliable treatment process to remove water contaminations and to regulate the quality of drinking water. The development of cost-effective membranes is in a great need to effectively replace the conventional water treatment technologies to produce water that meet or exceed stringent standards.

Recent studies have demonstrated that the zeolite-MMMs were applied to design reverse osmosis membrane to enhance the membrane properties such as permeability, selectivity, stability, surface area, or catalytic activity in water purification and separation processes [57, 58]. Nevertheless, there are only few studies performed on zeolite-MMMs for water treatment, it is determined that the size of zeolite was designed to match the expected polyimide active film thickness, thereby providing a preferential flow path through the nanochannels of zeolites [26, 59].

For example, thin film nanocomposite (TFN) reverse osmosis (RO) membranes have been used by incorporating zeolite particles into the PA rejection layer. It has shown that the incorporation of zeolite in a PA layer could improve its water permeability without significant loss of salt rejection under high pressure during RO process [60]. Main reason for that is nanochannels of zeolites with great sub-nanometer pores in zeolite nanoparticles that behave as preferential flow channels for water molecules. The zeolite-PA-based TFN membranes are considered as superior separation performance for RO applications due to their enhanced water permeability of active layer [61].

Tanga [61] provided an additional study to confirm that thin film nanocomposite membranes can significantly improve FO water flux significant with a relatively low zeolite loading due to both the surface and intrinsic separation properties of TFN membranes. Compared with TFC membrane, the TFN membrane is potentially more favorable during the application of treating feed solutions with relative higher salinity water under AL-FS orientation.

Sridhar [62] studied reactive separation of lactic acid (LA) using a microporous hydrophobic H-beta zeolite/polyvinylidene fluoride (PVDF) mixed matrix membrane from aqueous streams. Experiments were conducted using a stirred cell assembly consisting of two bellshaped glass pipe reducers containing aqueous LA separated by the membrane from an organic solution of tri-noctylamine (TOA) carrier in alcoholic medium. The interfacial concentrations of species adjacent to the membrane in aqueous and organic chambers are influenced by mass transfer coefficients, the concentration of TOA in organic phase and the zeolite loading, and forward extraction rates. Overall, the mass transfer rates were improved with the zeolite addition, due to the kinetics of complex formation and diffusion. The continuous separation of LA by a membrane contactor could enhance the fermentation yield of the acid, which is inhibited by LA through deactivating of the lactate dehydrogenase enzyme of Lactobacillus bulgaricus microorganism used in the production of LA.

#### **5.2. Zeolite-MMM for gas separation**

Membrane technologies, such as pervaporation and gas separation, are recognized as highly promising approaches to reduce the energy consumption of industrial processes. Compared with polymeric membranes that show *Robeson upper bound* between selectivity and permeability, MMM are attracting research attention, due to their high permeability and selectivity.

It is known that the permeability of a gas through an MMM depends on several factors such as intrinsic properties of the filler and polymer, the filler loading, and the filler–polymer matrix interface, and the filler loading [58]. For designing a mixed matrix system for separating a certain gas pair, the molecular sieving phase must provide precise size and shape discrimination ability to distinguish the molecules. Moreover, zeolites with three-dimensional networks are generally preferred for gas separation since they offer less restricted diffusion paths. As a result, the attractive polymer matrix materials are generally glassy with relatively lower permeability and much higher selectivities. Indeed, addition of zeolites or another highly selective media would only improve the already industrially acceptable properties, if defects can be eliminated.

Pechar et al. [63] used silanated zeolite L filler modified with 3-aminopropyltriethoxysilane (APTES) and a glassy polyimide as polymer matrix for fabricating MMMs for gas separations. Both CO<sup>2</sup> selectivity and permeability of the modified MMM dropped relatively to the neat membrane, due to the blocked zeolite pores by APTES.

**5.3. Zeolite-MMM for catalysis**

matrix at different load [65].

reagents, thermal, mechanical, and chemical stability [67].

Recently, many reports demonstrated catalytic activity of polymer–zeolite MMM, because the interaction of materials in the membrane matrix and the shape-selective catalytic properties of zeolites can improve permselective separations. Membrane also functions as a separator in gas phase between different gaseous molecules. Thus, membrane should be permeable enough to give efficient separation. For liquid phase separation, metal organic complexes and inorganic filler such as zeolite have been used [66]. It is well presented mostly that polydimethylsiloxane (PDMS) is incorporated as a polymer matrix because of high permeability, affinity for

**Figure 5.** SEM image of a MIL-101/PSF MMMs shows a homogenous distribution of MOF particles in the polymer

Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for Sustainable Engineering

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Langhendries and Baron [68] studied the catalytic activity of zeolite-filled poly(dimethylsiloxane) polymer membranes. The incorporation of zeolite-encaged iron-phthalocyanine partial oxidation catalysts into a dense hydrophobic polymer membrane results in a substantial

Furthermore, metal–organic frameworks (MOFs), as porous fillers possessing molecular sieving properties, have been combined with polymers to give MMM with substantial enhanced separation performance of CO<sup>2</sup> /CH<sup>4</sup> for natural gas sweetening or CO<sup>2</sup> /N<sup>2</sup> in flue [64]. MOF-74 series recently have demonstrated superior CO<sup>2</sup> adsorption capacities, due to the presence of open metal sites. This finding positions the materials as a very promising candidate for CO<sup>2</sup> capture from flue gas. **Figure 5** represents MIL-101/PSF membranes at different MOF loadings. It was found that the CO<sup>2</sup> permeability increases from about 5 to over 35 barrer from pure PSF to 24 wt % MIL-101/PSF. The increase for CO<sup>2</sup> also raises the ideal selectivity for CO<sup>2</sup> /N<sup>2</sup> and CO<sup>2</sup> /CH<sup>4</sup> from about 20 to 25 [65].

Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for Sustainable Engineering http://dx.doi.org/10.5772/intechopen.73824 125

**Figure 5.** SEM image of a MIL-101/PSF MMMs shows a homogenous distribution of MOF particles in the polymer matrix at different load [65].

#### **5.3. Zeolite-MMM for catalysis**

Sridhar [62] studied reactive separation of lactic acid (LA) using a microporous hydrophobic H-beta zeolite/polyvinylidene fluoride (PVDF) mixed matrix membrane from aqueous streams. Experiments were conducted using a stirred cell assembly consisting of two bellshaped glass pipe reducers containing aqueous LA separated by the membrane from an organic solution of tri-noctylamine (TOA) carrier in alcoholic medium. The interfacial concentrations of species adjacent to the membrane in aqueous and organic chambers are influenced by mass transfer coefficients, the concentration of TOA in organic phase and the zeolite loading, and forward extraction rates. Overall, the mass transfer rates were improved with the zeolite addition, due to the kinetics of complex formation and diffusion. The continuous separation of LA by a membrane contactor could enhance the fermentation yield of the acid, which is inhibited by LA through deactivating of the lactate dehydrogenase enzyme of

Membrane technologies, such as pervaporation and gas separation, are recognized as highly promising approaches to reduce the energy consumption of industrial processes. Compared with polymeric membranes that show *Robeson upper bound* between selectivity and permeability, MMM are attracting research attention, due to their high permeability and selectivity.

It is known that the permeability of a gas through an MMM depends on several factors such as intrinsic properties of the filler and polymer, the filler loading, and the filler–polymer matrix interface, and the filler loading [58]. For designing a mixed matrix system for separating a certain gas pair, the molecular sieving phase must provide precise size and shape discrimination ability to distinguish the molecules. Moreover, zeolites with three-dimensional networks are generally preferred for gas separation since they offer less restricted diffusion paths. As a result, the attractive polymer matrix materials are generally glassy with relatively lower permeability and much higher selectivities. Indeed, addition of zeolites or another highly selective media would only improve the already industrially acceptable properties, if defects

Pechar et al. [63] used silanated zeolite L filler modified with 3-aminopropyltriethoxysilane (APTES) and a glassy polyimide as polymer matrix for fabricating MMMs for gas separations.

Furthermore, metal–organic frameworks (MOFs), as porous fillers possessing molecular sieving properties, have been combined with polymers to give MMM with substantial enhanced

open metal sites. This finding positions the materials as a very promising candidate for CO<sup>2</sup> capture from flue gas. **Figure 5** represents MIL-101/PSF membranes at different MOF load-

selectivity and permeability of the modified MMM dropped relatively to the neat

for natural gas sweetening or CO<sup>2</sup>

/N<sup>2</sup>

adsorption capacities, due to the presence of

also raises the ideal selectivity for

permeability increases from about 5 to over 35 barrer from

in flue [64]. MOF-74

Lactobacillus bulgaricus microorganism used in the production of LA.

**5.2. Zeolite-MMM for gas separation**

124 Zeolites and Their Applications

membrane, due to the blocked zeolite pores by APTES.

pure PSF to 24 wt % MIL-101/PSF. The increase for CO<sup>2</sup>

series recently have demonstrated superior CO<sup>2</sup>

/CH<sup>4</sup>

from about 20 to 25 [65].

can be eliminated.

separation performance of CO<sup>2</sup>

ings. It was found that the CO<sup>2</sup>

/CH<sup>4</sup>

and CO<sup>2</sup>

Both CO<sup>2</sup>

CO<sup>2</sup> /N<sup>2</sup> Recently, many reports demonstrated catalytic activity of polymer–zeolite MMM, because the interaction of materials in the membrane matrix and the shape-selective catalytic properties of zeolites can improve permselective separations. Membrane also functions as a separator in gas phase between different gaseous molecules. Thus, membrane should be permeable enough to give efficient separation. For liquid phase separation, metal organic complexes and inorganic filler such as zeolite have been used [66]. It is well presented mostly that polydimethylsiloxane (PDMS) is incorporated as a polymer matrix because of high permeability, affinity for reagents, thermal, mechanical, and chemical stability [67].

Langhendries and Baron [68] studied the catalytic activity of zeolite-filled poly(dimethylsiloxane) polymer membranes. The incorporation of zeolite-encaged iron-phthalocyanine partial oxidation catalysts into a dense hydrophobic polymer membrane results in a substantial improvement in catalyst performance. Both mathematical model and kinetics determined exact concentrations in polymer and catalyst, and subsequently, the resulting catalyst activity and selectivity. Their results also indicate that hydrophobic poly-(dimethylsiloxane) is an attractive polymer for the incorporation of the hydrophilic zeolite-encaged iron-phthalocyanine catalyst. As a result, diffusion through composite catalytic membranes can be predicted using the mass transfer coefficients of pure zeolite and pure polymer material, and a tortuosity factor based on the zeolite loading as a catalyst.

inorganic particle concentration, phase separation, control of morphology and structural defects. Moreover, some zeolite-MMMs for water purification application is considered a potential hazard to humans and environmental, which also needs to more study to determine the hazardous character of these nanoparticles and mechanism of nanoparticles embedded

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127

One of most difficulties associated with membrane technology is fouling for a long time. Although, several strategies such as incorporation of antifouling nanoparticles, and surface modification have been used to overcome this problem, intensive investigations are needed to stop regeneration of microbial colonies on membrane surface and to reduce the leaching of filler. The next generation MMM should be developed with producing nano-size fillers without aggregation to improve their separation properties for membrane industry especially MMMs. There are several reasons to produce nano-size fillers, especially zeolite fillers such as more polymer/particle interfacial area and enhanced polymer–filler interface contact by smaller particles. The potential of incorporating fillers such as zeolite particles has not been attained up to the expectation of zeolite-MMMs performance, due to the smaller sizes, homogeneous distribution, agglomeration, price, availability, compatibility with polymer interface,

Despite many novel MMMs and fillers are being investigated so far but their performances are restricted due to limited synthesis processes. Previously process fails to demonstrate their performance due to the formation of relatively nonselective defects at the interface between the zeolite particles and the polymer medium on laboratory scale. Therefore, other major issue related to MMM is the interface defects that can lead to isolating zeolite fillers from the transport processes. Therefore, new techniques to achieve a perfect interface between inorganic fillers and polymers in membranes without compromising performance and scaling up

these novel membranes under industrially relevant conditions is greatly needed [72].

intrinsic properties of these fillers and its interaction of the polymeric matrix.

operating conditions.

In addition, many of these novel MMMs reported so far have been only tested on a laboratory scale and need further research to use commercially in industry. It is required to produce novel materials that can have high selectivity as well as nano-sized fillers with incredibly small sizes. There are limitations on developing novel materials due to high prices or expensive synthesis processes. The molecular dynamic simulations (MD) of mixed matrix materials could be effective approach to predict diffusive performance of MMM, especially zeolite-MMMs, and to provide experimental guidelines for tuning the membrane permeability at the molecular level without high costs. Although there are previously predicted models for predicting the processes contributing to membrane separations, however, studies in MMMs showed inadequate suitable models. Therefore, MD will be essential and effective to predict the morphology and

Last but not least factor, is changing and membrane morphology could change properties of membranes, and subsequently will influence the membrane performance. Therefore, improving membrane performance in real conditions such as high temperature, high pressure, and with incorporating of a plasticizer into the polymer solution would be possible and essential in order to provide better thermally and chemically zeolite-MMMs at different

membrane fouling in industrially water purification in the future.

their relation with water chemistry, better interfacial contact, and stability.

Another study, Jia and Peinemann [69] investigated the incorporation of polydimethylsiloxane (PDMS) into a polymer matrix and silicalite-1, a hydrophobic zeolite in order to study the permeation of various gases. In their study, only a couple of very high zeolite loadings were investigated, and they indicated that zeolite played the role of a molecular sieve in the membrane by facilitating the permeation of smaller molecules while it prevents the permeation of larger ones.

#### **5.4. Zeolite-MMM for biomedical application**

Combination of polymer materials with zeolite particles has been attracted attention not only due to enhanced mechanical and thermal properties, but also because of antibacterial properties. Polymer hosting can provide the enhanced antibacterial activity. There are three methods such as, production of reactive oxygen species (ROS) MMM, mixed matrix membrane direct damage to cell membrane; uptake of ions from mixed matrix membrane followed by DNA replication; and disruption of adenosine triphosphate (ATP) production [70].

Siddiq [71] studied the antibacterial effects of polysulfone/polyimide (PSf/PI) mixed matrix membranes fabricated by incorporation of modified zeolite (MZ) particles through solution casting method. The antibacterial property of fabricated zeolite-MMMs against Gram-negative bacteria (*Klebsiella pneumonia*, *Salmonella typhi*) and gram positive bacteria (*Staphylococcus aureus*, *Bacillus subtilis*) were also investigated. The MMM showed good antibacterial activity and a highest activity by PSf/PI/MZ mixed matrix membrane. Therefore, the combination effect of polysulfone/polyimide and modified zeolite sufficiently increased the antibacterial effect of mixed matrix membranes.

### **6. Challenges and future prospects**

Recently, novel zeolite-MMMs have attracted great attention in membrane technology, due to the excellent advantages such as improvement in the permeability, selectivity, thermal stability, mechanical strength of a polymeric membrane. Furthermore, the recent developments demonstrated that gas separation as well as water treatment has significantly benefited from membrane technology so far and advancements in these areas are still in progress in order of their wider use can become a reality. However, the comprehensive understanding of organic–inorganic interfaces is in a great need. Zeolite-MMMs performance suffers from defects caused by poor contact at the molecular sieve/polymer interface, the complexity of the synthesis process, high cost, identification of compatible inorganic particles, agglomeration, inorganic particle concentration, phase separation, control of morphology and structural defects. Moreover, some zeolite-MMMs for water purification application is considered a potential hazard to humans and environmental, which also needs to more study to determine the hazardous character of these nanoparticles and mechanism of nanoparticles embedded membrane fouling in industrially water purification in the future.

improvement in catalyst performance. Both mathematical model and kinetics determined exact concentrations in polymer and catalyst, and subsequently, the resulting catalyst activity and selectivity. Their results also indicate that hydrophobic poly-(dimethylsiloxane) is an attractive polymer for the incorporation of the hydrophilic zeolite-encaged iron-phthalocyanine catalyst. As a result, diffusion through composite catalytic membranes can be predicted using the mass transfer coefficients of pure zeolite and pure polymer material, and a tortuos-

Another study, Jia and Peinemann [69] investigated the incorporation of polydimethylsiloxane (PDMS) into a polymer matrix and silicalite-1, a hydrophobic zeolite in order to study the permeation of various gases. In their study, only a couple of very high zeolite loadings were investigated, and they indicated that zeolite played the role of a molecular sieve in the membrane by facilitating the permeation of smaller molecules while it prevents the permeation of

Combination of polymer materials with zeolite particles has been attracted attention not only due to enhanced mechanical and thermal properties, but also because of antibacterial properties. Polymer hosting can provide the enhanced antibacterial activity. There are three methods such as, production of reactive oxygen species (ROS) MMM, mixed matrix membrane direct damage to cell membrane; uptake of ions from mixed matrix membrane followed by

Siddiq [71] studied the antibacterial effects of polysulfone/polyimide (PSf/PI) mixed matrix membranes fabricated by incorporation of modified zeolite (MZ) particles through solution casting method. The antibacterial property of fabricated zeolite-MMMs against Gram-negative bacteria (*Klebsiella pneumonia*, *Salmonella typhi*) and gram positive bacteria (*Staphylococcus aureus*, *Bacillus subtilis*) were also investigated. The MMM showed good antibacterial activity and a highest activity by PSf/PI/MZ mixed matrix membrane. Therefore, the combination effect of polysulfone/polyimide and modified zeolite sufficiently increased the antibacterial

Recently, novel zeolite-MMMs have attracted great attention in membrane technology, due to the excellent advantages such as improvement in the permeability, selectivity, thermal stability, mechanical strength of a polymeric membrane. Furthermore, the recent developments demonstrated that gas separation as well as water treatment has significantly benefited from membrane technology so far and advancements in these areas are still in progress in order of their wider use can become a reality. However, the comprehensive understanding of organic–inorganic interfaces is in a great need. Zeolite-MMMs performance suffers from defects caused by poor contact at the molecular sieve/polymer interface, the complexity of the synthesis process, high cost, identification of compatible inorganic particles, agglomeration,

DNA replication; and disruption of adenosine triphosphate (ATP) production [70].

ity factor based on the zeolite loading as a catalyst.

**5.4. Zeolite-MMM for biomedical application**

effect of mixed matrix membranes.

**6. Challenges and future prospects**

larger ones.

126 Zeolites and Their Applications

One of most difficulties associated with membrane technology is fouling for a long time. Although, several strategies such as incorporation of antifouling nanoparticles, and surface modification have been used to overcome this problem, intensive investigations are needed to stop regeneration of microbial colonies on membrane surface and to reduce the leaching of filler. The next generation MMM should be developed with producing nano-size fillers without aggregation to improve their separation properties for membrane industry especially MMMs. There are several reasons to produce nano-size fillers, especially zeolite fillers such as more polymer/particle interfacial area and enhanced polymer–filler interface contact by smaller particles. The potential of incorporating fillers such as zeolite particles has not been attained up to the expectation of zeolite-MMMs performance, due to the smaller sizes, homogeneous distribution, agglomeration, price, availability, compatibility with polymer interface, their relation with water chemistry, better interfacial contact, and stability.

Despite many novel MMMs and fillers are being investigated so far but their performances are restricted due to limited synthesis processes. Previously process fails to demonstrate their performance due to the formation of relatively nonselective defects at the interface between the zeolite particles and the polymer medium on laboratory scale. Therefore, other major issue related to MMM is the interface defects that can lead to isolating zeolite fillers from the transport processes. Therefore, new techniques to achieve a perfect interface between inorganic fillers and polymers in membranes without compromising performance and scaling up these novel membranes under industrially relevant conditions is greatly needed [72].

In addition, many of these novel MMMs reported so far have been only tested on a laboratory scale and need further research to use commercially in industry. It is required to produce novel materials that can have high selectivity as well as nano-sized fillers with incredibly small sizes. There are limitations on developing novel materials due to high prices or expensive synthesis processes. The molecular dynamic simulations (MD) of mixed matrix materials could be effective approach to predict diffusive performance of MMM, especially zeolite-MMMs, and to provide experimental guidelines for tuning the membrane permeability at the molecular level without high costs. Although there are previously predicted models for predicting the processes contributing to membrane separations, however, studies in MMMs showed inadequate suitable models. Therefore, MD will be essential and effective to predict the morphology and intrinsic properties of these fillers and its interaction of the polymeric matrix.

Last but not least factor, is changing and membrane morphology could change properties of membranes, and subsequently will influence the membrane performance. Therefore, improving membrane performance in real conditions such as high temperature, high pressure, and with incorporating of a plasticizer into the polymer solution would be possible and essential in order to provide better thermally and chemically zeolite-MMMs at different operating conditions.

Although there is development success of the synthesis and the application of MMMs impregnated with zeolites for gas, water separation, and other applications, however, the mechanisms behind these phenomena require intensive investigations for more advanced MMM technology.

**Author details**

**References**

Mahboobeh Maghami and Amira Abdelrasoul\*

Saskatoon, Saskatchewan, Canada

Technology. 2010;**75**:229

Science. 2007;**297**:236

2007;**288**:231

Membrane Science. 2007;**300**:13

ing! Membrane Technology. 1999;**19**:6

\*Address all correspondence to: amira.abdelrasoul@usask.ca

Department of Chemical and Biological Engineering, University of Saskatchewan,

[1] Aroon MA, Ismail AF, Matsuura T, Montazer-Rahmati MM. Performance studies of mixed matrix membranes for gas separation: A review. Separation and Purification

Zeolite Mixed Matrix Membranes (Zeolite-MMMs) for Sustainable Engineering

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[2] Mahajan R, Koros W, Thundyil M. Mixed matrix membranes: Important and challeng-

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[4] Celik E, Park H, Choi H, Choi H. Carbon nanotube blended polyethersulfone mem-

[5] Peng F, Hu C, Jiang Z. Novel ploy (vinyl alcohol)/carbon nanotube hybrid membranes for pervaporation separation of benzene/cyclohexane mixtures. Journal of Membrane

[6] Peng F, Pan F, Sun H, Lu L, Jiang Z. Novel nanocomposite pervaporation membranes composed of poly (vinyl alcohol) and chitosan-wrapped carbon nanotube. Journal of

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the morphologies and properties of PSf UF membrane. Journal of Membrane Science.

nanoparticles with gold Nanocrystals for tunable green-to-red Upconversion emissions.

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[11] Ge L, Zhu Z, Li F, Liu S, Wang L, Tang X, Rudolph V. Modification of NaYF<sup>4</sup>

to polysulfone based UF

fillers on

:Yb,Er@SiO<sup>2</sup>

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[8] Genné I, Kuypers S, Leysen R. Effect of the addition of ZrO<sup>2</sup>

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branes for fouling control in water treatment. Water Research. 2011;**45**:274

### **7. Conclusion**

Mixed matrix membranes with zeolite fillers has attracted a lot of attention in membrane technology research due to its excellent advantages, such as high permeability and improved selectivity. Zeolite-MMMs could be considered an ideal candidate for purification industry since it combines the properties of polymeric matric and zeolite inorganic fillers. Application and fabrication techniques of zeolite reinforced polymeric membranes have been comprehensively reviewed in this article with the aim of optimizing interfacial interaction between the zeolite and the polymeric matrix. Compatibility between zeolite and polymer matrix can be improved with a number of methods, such as: by applying high processing temperature during membrane formation, the silane modification, and priming on the particle's surface, annealing that can relax the stress imposed to hollow fiber and result in higher packing density of polymer chains, and the introduction of a LMWA agent between the polymer matrix and inorganic particles.

There have been numerous attempts to incorporate zeolite particles in polymer matrices in water purification applications and for gas separation due to its superior separation properties and size exclusion. Applications of zeolite-MMMs were re-evaluated for a variety of industrial processes, including water purification, medical industry, catalytic, and gas separation. However, despite its advantages, there are still issues and difficulties associated with zeolite-MMMs that have restricted their wider applications.

It can be concluded that the advancements in the application and fabrication of zeolite-MMM needs further intensive investigations. Future research should be conducted with the aim of developing new techniques that provide better understanding of zeolite incorporation into polymer structures. New materials should also be considered as a way of reducing the fouling concerns. Additional study is necessary for an improved understanding of the basic transport mechanism occurring through the MMMs. The next generation MMMs must be developed with nano-size fillers and without aggregation so as to improve their separation properties severely needed in the membrane industry. Some results indicate that the nanosize zeolite particles incorporated in MMMs offer better performance in comparison with micron size particles. New additives and modification agents should be produced to improve adhesion between polymer and inorganic fillers. In conclusion, despite of all the identified problems, MMM technology with zeolites could be considered a strong candidate for modern purification industry due to the remarkable properties of polymeric and inorganic zeolite materials.

### **Acknowledgements**

The authors would like to acknowledge the Department of Chemical and Biological Engineering at the University of Saskatchewan for the support provided.

### **Author details**

Although there is development success of the synthesis and the application of MMMs impregnated with zeolites for gas, water separation, and other applications, however, the mechanisms behind these phenomena require intensive investigations for more advanced MMM technology.

Mixed matrix membranes with zeolite fillers has attracted a lot of attention in membrane technology research due to its excellent advantages, such as high permeability and improved selectivity. Zeolite-MMMs could be considered an ideal candidate for purification industry since it combines the properties of polymeric matric and zeolite inorganic fillers. Application and fabrication techniques of zeolite reinforced polymeric membranes have been comprehensively reviewed in this article with the aim of optimizing interfacial interaction between the zeolite and the polymeric matrix. Compatibility between zeolite and polymer matrix can be improved with a number of methods, such as: by applying high processing temperature during membrane formation, the silane modification, and priming on the particle's surface, annealing that can relax the stress imposed to hollow fiber and result in higher packing density of polymer chains, and

the introduction of a LMWA agent between the polymer matrix and inorganic particles.

zeolite-MMMs that have restricted their wider applications.

There have been numerous attempts to incorporate zeolite particles in polymer matrices in water purification applications and for gas separation due to its superior separation properties and size exclusion. Applications of zeolite-MMMs were re-evaluated for a variety of industrial processes, including water purification, medical industry, catalytic, and gas separation. However, despite its advantages, there are still issues and difficulties associated with

It can be concluded that the advancements in the application and fabrication of zeolite-MMM needs further intensive investigations. Future research should be conducted with the aim of developing new techniques that provide better understanding of zeolite incorporation into polymer structures. New materials should also be considered as a way of reducing the fouling concerns. Additional study is necessary for an improved understanding of the basic transport mechanism occurring through the MMMs. The next generation MMMs must be developed with nano-size fillers and without aggregation so as to improve their separation properties severely needed in the membrane industry. Some results indicate that the nanosize zeolite particles incorporated in MMMs offer better performance in comparison with micron size particles. New additives and modification agents should be produced to improve adhesion between polymer and inorganic fillers. In conclusion, despite of all the identified problems, MMM technology with zeolites could be considered a strong candidate for modern purification industry due to the remarkable properties of polymeric and inorganic zeolite materials.

The authors would like to acknowledge the Department of Chemical and Biological Engi-

neering at the University of Saskatchewan for the support provided.

**7. Conclusion**

128 Zeolites and Their Applications

**Acknowledgements**

Mahboobeh Maghami and Amira Abdelrasoul\*

\*Address all correspondence to: amira.abdelrasoul@usask.ca

Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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**Chapter 8**

**Provisional chapter**

**Characterizations and Industrial Applications for**

**Characterizations and Industrial Applications for** 

Iván Ayoseth Chulines Domínguez,

Iván Ayoseth Chulines Domínguez,

http://dx.doi.org/10.5772/intechopen.74953

Cristóbal Patiño-Carachure

**Abstract**

zeolite

**1. Introduction**

Cristóbal Patiño-Carachure

Youness Abdellaoui, Mohamed Abatal and

Youness Abdellaoui, Mohamed Abatal and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Cement and Concrete Incorporated Natural Zeolite**

Zeolites have been widely used in various industries, leading to a high commercial value; this is mainly due to the wide diversity of naturally occurring species and the ability to synthesize new types. In the cement and concrete industry, natural zeolite is a popular natural pozzolanic material in some regions of the world owing to their economic, environmental, and technical advantages, among others, used pozzolanic materials. Many works have reported the use of natural zeolite as substituent material for cement in mortar and concrete. Generally, the use of natural zeolite can overcome environmental and economic problems associated with the use of high quantity of cement; furthermore, it is shown a strength enhancement and durability improvement properties of cement and concrete composites. In this context, this chapter strives to review the application of natural zeolite as pozzolan in cement and concrete composites, its characteristic, its proper incorporation, and each of the influencing parameters. In addition, the elaboration methods, textural and mechanical characterization, and applications of these composites will be treated. **Keywords:** cement substitution, concrete, hydration, pozzolanic material, natural

The number of zeolites is constantly increasing; currently, the commission of structures of the International Zeolite Association (IZA) recognizes 232 unique structures that have been approved and have been assigned three letters of code [1]. The chemistry of zeolite has been

**Cement and Concrete Incorporated Natural Zeolite**

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.74953


#### **Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural Zeolite Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural Zeolite**

DOI: 10.5772/intechopen.74953

Iván Ayoseth Chulines Domínguez, Youness Abdellaoui, Mohamed Abatal and Cristóbal Patiño-Carachure Iván Ayoseth Chulines Domínguez, Youness Abdellaoui, Mohamed Abatal and Cristóbal Patiño-Carachure

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74953

#### **Abstract**

[67] Gu Y, Emin C, Remigy JC, Favier I, Gomez M, Noble RD, Lahitte JF. Hybrid catalytic membranes: Tunable and versatile materials for fine chemistry application. Materials

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Today Proceedings. 2016;**3**:419

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of Membrane Science. 2004;**228**:227

branes. Journal of Membrane Science. 1998;**141**:265

branes. Journal of Membrane Science. 1991;**57**:289

tion. Chinese Journal of Polymer Science. 2018;**36**:65

Zeolites have been widely used in various industries, leading to a high commercial value; this is mainly due to the wide diversity of naturally occurring species and the ability to synthesize new types. In the cement and concrete industry, natural zeolite is a popular natural pozzolanic material in some regions of the world owing to their economic, environmental, and technical advantages, among others, used pozzolanic materials. Many works have reported the use of natural zeolite as substituent material for cement in mortar and concrete. Generally, the use of natural zeolite can overcome environmental and economic problems associated with the use of high quantity of cement; furthermore, it is shown a strength enhancement and durability improvement properties of cement and concrete composites. In this context, this chapter strives to review the application of natural zeolite as pozzolan in cement and concrete composites, its characteristic, its proper incorporation, and each of the influencing parameters. In addition, the elaboration methods, textural and mechanical characterization, and applications of these composites will be treated.

**Keywords:** cement substitution, concrete, hydration, pozzolanic material, natural zeolite

#### **1. Introduction**

The number of zeolites is constantly increasing; currently, the commission of structures of the International Zeolite Association (IZA) recognizes 232 unique structures that have been approved and have been assigned three letters of code [1]. The chemistry of zeolite has been

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

a subject of significant interest due to the ion exchange properties of zeolite, crystallinity, thermal stability, and well-defined molecular size closed structures [2]. The code ASTM-618 (American Society for Testing Materials) defines pozzolans as siliceous or aluminum-siliceous materials, which by themselves have little to no cementing value, but when they are finely divided and in the presence of water and calcium hydroxide (Cal), they react chemically at room temperature to form cementitious agents [3].

cation exchange; dissolution and/or breakdown of the zeolitic structure; possible formation of transient gel phases; and precipitation of hydrated calcium silicates and aluminates [12].

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

There are different methods of work for the evaluation of pozzolanic activity. In general, these methods are classified as indirect or direct, depending on the parameter to be studied [13].

over time, as the pozzolanic reaction proceeds, using analytical methods such as X-ray diffrac-

The Frattini test is a commonly used direct method involving chemical titration to determine

test pozzolan. This method has been used to measure the pozzolanic activity of metakaolin

The saturated lime method is a simplified version of the Frattini test, in which pozzolan is

and water. The amount of lime set by the pozzolan is determined by measuring the residual

Indirect test methods measure a physical property of a test sample, which indicates the degree of pozzolanic activity. This may involve the measurement of properties such as compressive strength, electrical conductivity, or heat evolution by conduction calorimetry. The results of an indirect pozzolanic activity test are often corroborated by direct tests to confirm that poz-

Zeolites naturally occur from volcanic origin and belong to the family of hydrated aluminosilicates. Their microporous structures can accommodate a wide variety of cations, which compensate the negative charge created by the substitution of Si by Al. Therefore, natural zeolites appear as cation exchangers in many applications, thanks to this property. Some of the main applications of zeolites in this area include the selective treatment of wastewater, extraction of ammonia, odor control, extraction of heavy metals from nuclear, mining and industrial wastes, soil conditioning for agricultural use, and even as an additive for animal feedstock [23].

Among all identified zeolites, clinoptilolite is the most abundant natural zeolite. In the structure of the zeolite, there are three relatively independent components: the aluminosilicate system, the interchangeable cations, and the zeolitic water. The simplified empirical formula of zeolites is:

Al2 Si1−*<sup>x</sup>* O2

ture of aluminosilicates is the most conserved and stable component and defines the type of structure water molecules that can be presented in voids of large cavities and joined between ion systems and ions interchangeable through aqueous bridges. Water can also serve as a

.*y* H2

, and Ba2+. Among them, Na+

⁄*n n*+

The channels of the natural zeolites are predominantly occupied by Na+

tion (XRD), thermogravimetric analysis (TGA), or assessment chemistry.

[14], catalytic cracking residues [15], fly ash [16], and zeolites [17].

mixed with a saturated solution of lime (slaked lime, Ca(OH)2

and its subsequent reduction in abundance

http://dx.doi.org/10.5772/intechopen.74953

137

) instead of Portland cement

O (1)

, K+

, Ca2+, and H2

, Ca2+, and Mg2+ can be

O,

represents Alkali and alkaline earth metals. The struc-

, K+

dissolved in a solution containing Portland cement and the

Direct methods monitor the presence of Ca(OH)2

the concentrations of Ca2+ and OH<sup>−</sup>

zolanic reactions are occurring [19–22].

M*<sup>x</sup>*

where *x* can vary from 0 to 0.5 and M*<sup>n</sup>*<sup>+</sup>

bridge between interchangeable cations.

as well as traces of Mg2+, Ti4+, Pd2+, K+

dissolved calcium [18].

**2.2. Ion exchange**

Concrete made from Portland cement is the most widely used construction material today, mainly due to its cost-benefit ratio, in terms of compressive strength. Its manufacture involves the release of approximately 900 kg of carbon dioxide per 1000 kg of produced cements. Likewise, this amount of cement demands the use of 1693 kg of water and 4798 MJ in energy resources [4]. This is the importance of reducing the environmental impact that the manufacture of the same produces. The use of mineral additions in the production of concrete is a technologically possible alternative. However, its use requires high standards of assurance and quality assurance, similar to those of cement [5].

The extent of the benefits provided by the use of blended cements increases with increasing content of additives in blended Portland cements. However, the content of additives in blended Portland cements, especially for natural pozzolans, is limited by some factors, such as an increase in water requirement and a decrease in the rate of strength development of the cementitious systems. It has been found that the blended cements containing high volume (55% by weight) of natural pozzolans (volcanic tuff) possess lower 28-day compressive strength when compared to the reference Portland cement, although they show similar strength values at 91 days of age [6, 7]. Therefore, the production of high-volume natural pozzolan blended cements, which are able to compete against ordinary Portland cement, requires natural pozzolans exhibiting significantly high-strength activity.

In this chapter, we intend to review the literature available on these topics, addressing structural, mineralogical, and morphological properties, in the case of zeolites, and manufacturing processes, characterization, evaluation, and application, in the case of cement. As well each of the parameters influences the formation of cementing pastes, such as hydration, porosity, transport properties, durability, and carbonation, among others.

### **2. Properties of natural zeolite**

#### **2.1. Pozzolanic activity surface area**

Pozzolans are materials with an amorphous and aluminous siliceous or siliceous content, which react with calcium hydroxide in the presence of water to form cementitious hydration products [8]. Among the most common natural pozzolanic materials, such as fly ash and silica fume, is zeolite, which is used in some regions of the world, due to its lower cost and accessibility [9, 10]. Zeolites generally show pozzolanic activity due to their structural characteristics, and their use as additions in cements provides additional technical advantages to construction materials [11].

The pozzolanic properties of natural zeolites are determined by their high sorption ability, ion exchange potential, and specific structure. Their action involves various steps, including cation exchange; dissolution and/or breakdown of the zeolitic structure; possible formation of transient gel phases; and precipitation of hydrated calcium silicates and aluminates [12].

There are different methods of work for the evaluation of pozzolanic activity. In general, these methods are classified as indirect or direct, depending on the parameter to be studied [13]. Direct methods monitor the presence of Ca(OH)2 and its subsequent reduction in abundance over time, as the pozzolanic reaction proceeds, using analytical methods such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), or assessment chemistry.

The Frattini test is a commonly used direct method involving chemical titration to determine the concentrations of Ca2+ and OH<sup>−</sup> dissolved in a solution containing Portland cement and the test pozzolan. This method has been used to measure the pozzolanic activity of metakaolin [14], catalytic cracking residues [15], fly ash [16], and zeolites [17].

The saturated lime method is a simplified version of the Frattini test, in which pozzolan is mixed with a saturated solution of lime (slaked lime, Ca(OH)2 ) instead of Portland cement and water. The amount of lime set by the pozzolan is determined by measuring the residual dissolved calcium [18].

Indirect test methods measure a physical property of a test sample, which indicates the degree of pozzolanic activity. This may involve the measurement of properties such as compressive strength, electrical conductivity, or heat evolution by conduction calorimetry. The results of an indirect pozzolanic activity test are often corroborated by direct tests to confirm that pozzolanic reactions are occurring [19–22].

#### **2.2. Ion exchange**

a subject of significant interest due to the ion exchange properties of zeolite, crystallinity, thermal stability, and well-defined molecular size closed structures [2]. The code ASTM-618 (American Society for Testing Materials) defines pozzolans as siliceous or aluminum-siliceous materials, which by themselves have little to no cementing value, but when they are finely divided and in the presence of water and calcium hydroxide (Cal), they react chemically at

Concrete made from Portland cement is the most widely used construction material today, mainly due to its cost-benefit ratio, in terms of compressive strength. Its manufacture involves the release of approximately 900 kg of carbon dioxide per 1000 kg of produced cements. Likewise, this amount of cement demands the use of 1693 kg of water and 4798 MJ in energy resources [4]. This is the importance of reducing the environmental impact that the manufacture of the same produces. The use of mineral additions in the production of concrete is a technologically possible alternative. However, its use requires high standards of assurance and

The extent of the benefits provided by the use of blended cements increases with increasing content of additives in blended Portland cements. However, the content of additives in blended Portland cements, especially for natural pozzolans, is limited by some factors, such as an increase in water requirement and a decrease in the rate of strength development of the cementitious systems. It has been found that the blended cements containing high volume (55% by weight) of natural pozzolans (volcanic tuff) possess lower 28-day compressive strength when compared to the reference Portland cement, although they show similar strength values at 91 days of age [6, 7]. Therefore, the production of high-volume natural pozzolan blended cements, which are able to compete against ordinary Portland cement, requires

In this chapter, we intend to review the literature available on these topics, addressing structural, mineralogical, and morphological properties, in the case of zeolites, and manufacturing processes, characterization, evaluation, and application, in the case of cement. As well each of the parameters influences the formation of cementing pastes, such as hydration, porosity,

Pozzolans are materials with an amorphous and aluminous siliceous or siliceous content, which react with calcium hydroxide in the presence of water to form cementitious hydration products [8]. Among the most common natural pozzolanic materials, such as fly ash and silica fume, is zeolite, which is used in some regions of the world, due to its lower cost and accessibility [9, 10]. Zeolites generally show pozzolanic activity due to their structural characteristics, and their use as additions in cements provides additional technical advantages to construction materials [11]. The pozzolanic properties of natural zeolites are determined by their high sorption ability, ion exchange potential, and specific structure. Their action involves various steps, including

room temperature to form cementitious agents [3].

136 Zeolites and Their Applications

quality assurance, similar to those of cement [5].

natural pozzolans exhibiting significantly high-strength activity.

transport properties, durability, and carbonation, among others.

**2. Properties of natural zeolite**

**2.1. Pozzolanic activity surface area**

Zeolites naturally occur from volcanic origin and belong to the family of hydrated aluminosilicates. Their microporous structures can accommodate a wide variety of cations, which compensate the negative charge created by the substitution of Si by Al. Therefore, natural zeolites appear as cation exchangers in many applications, thanks to this property. Some of the main applications of zeolites in this area include the selective treatment of wastewater, extraction of ammonia, odor control, extraction of heavy metals from nuclear, mining and industrial wastes, soil conditioning for agricultural use, and even as an additive for animal feedstock [23].

Among all identified zeolites, clinoptilolite is the most abundant natural zeolite. In the structure of the zeolite, there are three relatively independent components: the aluminosilicate system, the interchangeable cations, and the zeolitic water. The simplified empirical formula of zeolites is:

$$\mathbf{M}\_{\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft}\mathbf{H}\_{\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedbl}\end{bmatrix}\tag{1}$$

where *x* can vary from 0 to 0.5 and M*<sup>n</sup>*<sup>+</sup> represents Alkali and alkaline earth metals. The structure of aluminosilicates is the most conserved and stable component and defines the type of structure water molecules that can be presented in voids of large cavities and joined between ion systems and ions interchangeable through aqueous bridges. Water can also serve as a bridge between interchangeable cations.

The channels of the natural zeolites are predominantly occupied by Na+ , K+ , Ca2+, and H2 O, as well as traces of Mg2+, Ti4+, Pd2+, K+ , and Ba2+. Among them, Na+ , K+ , Ca2+, and Mg2+ can be exchanged with NH4 + ions. The type and density of interchangeable cations influence the stability of the cavities and the thermal behavior of a zeolite [24–27].

Due to the nature of the cation exchange, natural zeolites show a high performance in the adsorption of cations in aqueous solution such as ammonium and heavy metals. However, zeolites show varied ion selectivity and competitive adsorption for multicomponent system [27]. In addition, these materials are not good adsorbents for the adsorption of anionic and organic ions. Surface modification with cationic surfactant can change the surface charge of the natural zeolite, making them applicable for adsorption of anions and organics. Most zeolites when heated give off water continuously rather than in separate stages at certain temperatures. The dehydrated zeolite can then reabsorb the original amount of water when exposed to water vapor. Recent investigation has shown that some zeolites such as phillipsite and gismondite lose and gain water in a stepwise manner. All-silica zeolites are chemically and hydrothermally more stable than aluminum containing ones and are therefore preferred for membrane application, including for dehydration, even though these types of membranes

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

http://dx.doi.org/10.5772/intechopen.74953

139

The adsorption characteristics of any zeolite depend on the detailed chemical/structural composition of the adsorbent. The Si/Al ratio, the type of cation, the number, and the location are particularly influential in the adsorption. These properties can be modified by various chemical treatments to improve the separation efficiency of the natural raw zeolite. Acid/base treatment and impregnation with surfactant by ion exchange are commonly used to change the hydrophilic/hydrophobic properties for the adsorption of various ions or organic [27].

**3. Effects of natural zeolite on the properties of cement and concrete** 

mixed cement compounds with lower water-cementitious material (w/cm) ratio [41].

containing zeolite exceeds the strength of concrete without zeolite.

In general, the higher the cement replacement by natural zeolite has the lower the compressive strength. However, the percentages of reduction in resistance generally decrease with increasing age in cement. This behavior may be related to the pozzolanic activity of the natural zeolite. In terms of compressive strength, it appears that natural zeolite performs better in

In term of compressive strength, it seems that natural zeolite performs better in blended cement composites with lower w/cm ratios. Ahmadi and Shekarchi [42] showed that the concretes containing natural zeolite with a w/cm ratio of 0.40 displayed higher compressive strength than the control mixture at the ages of 3, 7, 28, and 90 days, whereas contrary results were obtained for the concretes with a w/cm ratio of 0.5 in the present study. Markiv et al. [43] showed, recently, that the substitution of cement by zeolite resulted in some reduction in strength until 90 days of hardening, but after 180 days, compressive strength of concretes

Uzal and Turanlı [44] reported the similar compressive strength of mortar with 55% zeolite in the binder to that of 100% Portland cement mortar, but this could only be achieved using superplasticizers. Karakurt and Topçu [45] found 30% replacement of Portland cement by zeolite in their blended cement mortars as optimum; the compressive strength was similar to the reference mortar. Valipour et al. [46] observed a fast decrease in the compressive strength

are hydrophobic [40].

**composites**

**3.1. Mechanical properties**

#### **2.3. Expansion at elevated temperatures**

Concrete is more durable against elevated temperature and fire effects than many other construction materials. Although ordinary concrete is considered to have a satisfactory fire resistance, it can lose 40–60% of its original strength upon exposure to 500°C [28]. Bilim [29] reported mortars containing zeolite show generally better performance to high temperatures about 900°C.

Negative thermal expansion (NTE) is an unusual phenomenon in which materials shrink in volume when heated (or expand when cooled). It has only been observed in a small number of solids, including some metallic oxides, cyanide metal, polymers, and zeolites [30]. It is important to understand how zeolites behave as a function of temperature. In recent years, research has been carried out on NTE in purely siliceous zeolitic structures [31, 32] and with a little less attention [31, 33] to aluminum-containing zeolites, in which it is known that structures and properties are particularly sensitive to extra-structural charge balancing cations and host molecules in the pores.

Water is an important guest molecule in the pore system of natural and synthetic zeolites. Adsorbent and catalytic properties of zeolites are also strongly affected by their water content. Cations seek the most energy-stable positions, and these positions play an important role in the catalytic activity.

This subject was investigated by Ilić and Wettstein [34], who examined three distinct temperature ranges for volumetric thermal expansion. Associates with the dehydration of coordinated water molecules and with transverse vibrations of bridging oxygen atoms result in the reduction in the bonding angle.

#### **2.4. Desorption of water at low humidities**

As already mentioned, zeolite is a porous solid with a large capacity to house water molecules and can be used as a pozzolanic aggregate. The efficiency of porous aggregates as an internal curing agent in concrete depends on its water absorption and desorption characteristics. The desorption behavior of zeolite and other porous aggregates depends on the structure of their pores and mainly on the porosity size distribution. In general, a thick pore structure will lead to better desorption behavior [35–37].

Ghourchian et al. [38] were conducted a study of the performance of the porous aggregates and concluded that the water desorption of the zeolites is closely related to their microstructure. For a proper desorption, a thick-pored structure is needed with a high proportion of well-interconnected pores. While, in the case of zeolites, despite having a high-water adsorption capacity, they have a fine pore structure, which leads to retaining the water absorbed.

#### **2.5. Re-adsorption of water at high humidities**

Zeolites are selective adsorbents for the removal of carbon dioxide, water vapor, and other impurities from the mixtures. The impurities adversely affect the capacity of the adsorbents used for the separation or purification. Water is a strongly adsorbed component in zeolite [39].

Due to the nature of the cation exchange, natural zeolites show a high performance in the adsorption of cations in aqueous solution such as ammonium and heavy metals. However, zeolites show varied ion selectivity and competitive adsorption for multicomponent system [27]. In addition, these materials are not good adsorbents for the adsorption of anionic and organic ions. Surface modification with cationic surfactant can change the surface charge of the natural zeolite, making them applicable for adsorption of anions and organics. Most zeolites when heated give off water continuously rather than in separate stages at certain temperatures. The dehydrated zeolite can then reabsorb the original amount of water when exposed to water vapor. Recent investigation has shown that some zeolites such as phillipsite and gismondite lose and gain water in a stepwise manner. All-silica zeolites are chemically and hydrothermally more stable than aluminum containing ones and are therefore preferred for membrane application, including for dehydration, even though these types of membranes are hydrophobic [40].

The adsorption characteristics of any zeolite depend on the detailed chemical/structural composition of the adsorbent. The Si/Al ratio, the type of cation, the number, and the location are particularly influential in the adsorption. These properties can be modified by various chemical treatments to improve the separation efficiency of the natural raw zeolite. Acid/base treatment and impregnation with surfactant by ion exchange are commonly used to change the hydrophilic/hydrophobic properties for the adsorption of various ions or organic [27].

### **3. Effects of natural zeolite on the properties of cement and concrete composites**

### **3.1. Mechanical properties**

exchanged with NH4

138 Zeolites and Their Applications

host molecules in the pores.

reduction in the bonding angle.

**2.4. Desorption of water at low humidities**

to better desorption behavior [35–37].

**2.5. Re-adsorption of water at high humidities**

the catalytic activity.

+

**2.3. Expansion at elevated temperatures**

ity of the cavities and the thermal behavior of a zeolite [24–27].

ions. The type and density of interchangeable cations influence the stabil-

Concrete is more durable against elevated temperature and fire effects than many other construction materials. Although ordinary concrete is considered to have a satisfactory fire resistance, it can lose 40–60% of its original strength upon exposure to 500°C [28]. Bilim [29] reported mortars

Negative thermal expansion (NTE) is an unusual phenomenon in which materials shrink in volume when heated (or expand when cooled). It has only been observed in a small number of solids, including some metallic oxides, cyanide metal, polymers, and zeolites [30]. It is important to understand how zeolites behave as a function of temperature. In recent years, research has been carried out on NTE in purely siliceous zeolitic structures [31, 32] and with a little less attention [31, 33] to aluminum-containing zeolites, in which it is known that structures and properties are particularly sensitive to extra-structural charge balancing cations and

Water is an important guest molecule in the pore system of natural and synthetic zeolites. Adsorbent and catalytic properties of zeolites are also strongly affected by their water content. Cations seek the most energy-stable positions, and these positions play an important role in

This subject was investigated by Ilić and Wettstein [34], who examined three distinct temperature ranges for volumetric thermal expansion. Associates with the dehydration of coordinated water molecules and with transverse vibrations of bridging oxygen atoms result in the

As already mentioned, zeolite is a porous solid with a large capacity to house water molecules and can be used as a pozzolanic aggregate. The efficiency of porous aggregates as an internal curing agent in concrete depends on its water absorption and desorption characteristics. The desorption behavior of zeolite and other porous aggregates depends on the structure of their pores and mainly on the porosity size distribution. In general, a thick pore structure will lead

Ghourchian et al. [38] were conducted a study of the performance of the porous aggregates and concluded that the water desorption of the zeolites is closely related to their microstructure. For a proper desorption, a thick-pored structure is needed with a high proportion of well-interconnected pores. While, in the case of zeolites, despite having a high-water adsorption capacity, they have a fine pore structure, which leads to retaining the water absorbed.

Zeolites are selective adsorbents for the removal of carbon dioxide, water vapor, and other impurities from the mixtures. The impurities adversely affect the capacity of the adsorbents used for the separation or purification. Water is a strongly adsorbed component in zeolite [39].

containing zeolite show generally better performance to high temperatures about 900°C.

In general, the higher the cement replacement by natural zeolite has the lower the compressive strength. However, the percentages of reduction in resistance generally decrease with increasing age in cement. This behavior may be related to the pozzolanic activity of the natural zeolite. In terms of compressive strength, it appears that natural zeolite performs better in mixed cement compounds with lower water-cementitious material (w/cm) ratio [41].

In term of compressive strength, it seems that natural zeolite performs better in blended cement composites with lower w/cm ratios. Ahmadi and Shekarchi [42] showed that the concretes containing natural zeolite with a w/cm ratio of 0.40 displayed higher compressive strength than the control mixture at the ages of 3, 7, 28, and 90 days, whereas contrary results were obtained for the concretes with a w/cm ratio of 0.5 in the present study. Markiv et al. [43] showed, recently, that the substitution of cement by zeolite resulted in some reduction in strength until 90 days of hardening, but after 180 days, compressive strength of concretes containing zeolite exceeds the strength of concrete without zeolite.

Uzal and Turanlı [44] reported the similar compressive strength of mortar with 55% zeolite in the binder to that of 100% Portland cement mortar, but this could only be achieved using superplasticizers. Karakurt and Topçu [45] found 30% replacement of Portland cement by zeolite in their blended cement mortars as optimum; the compressive strength was similar to the reference mortar. Valipour et al. [46] observed a fast decrease in the compressive strength of concrete with the increasing amount of zeolite (10–30% of the mass of Portland cement) in the mix, even with an increasing superplasticizer dosage.

It has been shown that the use of pozzolanic additives for cements increases their resistance

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

reduction in the presence of capillary pores in the matrix. In fact, it hinders the penetration of aggressive media [53]. Concrete can be attacked by acids both internally and externally. The existence of different kinds of acid in the environment around the concrete causes a great reduction in the pH of the concrete, and the reaction between the acids and the hydrated and unhydrated cement finally leads to the deterioration of the concrete. The primary effect of any

Małolepszy and Grabowska [55] carried out a study dedicated to studying the sulfate resistance of a cementing paste with zeolitic addition. They confirmed the beneficial effect of the zeolitic additive for cement mortars because those containing zeolite did not show visible damage in the surface in an aggressive solution of sulfate, while in the mortar that did not possess it, I present surface microcracks pronounced. Exfoliation of corners and colors

The contraction is a phenomenon in which the concrete reduces its volume with time. The internal and external drying of the concrete is the main factor that causes the contraction. Internal drying, also known as self-desiccation, is caused by the consumption of water in the hydrating cement paste and the resulting creation of interfacial menisci between the pore fluid and the vapor in progressively smaller pores [56]. The creation of meniscus leads to the accumulation of capillary pressure that puts the solids under compression and causes a macroscopic contraction, called autogenous contraction [56, 57]. External drying takes place due to the evaporation of water from the surface of the concrete to the ambient air or due to the migration of water to adjacent members. The evaporation of moisture from the surface of the fresh concrete can cause cracking of the plastic shrinkage [58], and a greater moisture loss of

Generally, the durability properties of concrete improved by partial replacement of cement with natural zeolite. However, the concrete that contains 15% of natural zeolite achieves a suitable drying shrinkage. The latter does not have a satisfactory performance in the acid

The pores can have an effect on the properties of the material in different ways. The compressive strength is primarily related to the total porosity, the size of pores and their distribution, the size and form of the biggest pores, and the relation between the pores. Shrinking is the function of energy exchange on the surface of pore walls. Durability depends on freeze-thaw resistance and is controlled by the volume of air entrained in the pores and spaces between

Water absorption and freeze-thaw resistance of hardened cement paste depend on the size of pores and capillaries, their type and distribution, and the closing of the pores. Closed and small

, and the

141

http://dx.doi.org/10.5772/intechopen.74953

to corrosion, due to the high impermeability, the decrease in the content of Ca(OH)2

type of acid attack on concrete is the dissolution of the cement paste matrix [54].

changes (yellow and gray incursions on the walls of the samples).

the hardened concrete can cause shrinkage by drying [59].

*3.4.2. Drying shrinkage*

environment [41].

the pores [60].

*3.4.3. Freeze and thaw resistance*

#### **3.2. Alkali-silica reaction**

It is generally recognized that the addition of pozzolan reduces the calcium hydroxide content in cement paste and improves the permeability of concrete [47]. The most important concerns in the design of durable concrete are the alkali-silica reaction (ASR) and sulfate attack.

ASR causes the premature deterioration in concrete. Alkali hydroxides present in the concrete pore solution react with amorphous or poorly crystalline silica phases in aggregates, forming a gel that imbibes water and expands [48]. The expansive pressure generated by the hydrated alkali silicate has been widely believed to induce cracking and deterioration of concrete. However, this notion may not be necessarily correct. Concrete is a porous material, and the hydrated alkali silicate is rheologically a fluid material that can slowly diffuse into the pores and preexisting cracks to lose its expansive pressure. The diffused alkali silicate has been proposed to generate an expansive pressure by reacting with Ca2+ ions [49].

#### **3.3. Transport properties**

The transport properties of concrete with the addition of zeolite have been studied by Ahmadi and Shekarchi [42] and Najimi et al. [41], who found a significant reduction in the penetration of water and chlorides, in the concrete with natural zeolite. On the other hand, Valipour et al. [50] reported that water sorptivity and gas permeability increase with the increase of zeolite in the mixture. Similar results were obtained for the oxygen permeability of Ahmadi and Shekarchi [42] but only for the dose of zeolite greater than 10%.

The liquid water transport parameters increase with the increasing addition of zeolite in the mixed binder. This is due to the pore distribution, that is, the volume of capillary pores, which is the most important factor, in the capacity of a porous medium to transport water in liquid form. In studies conducted by Vejmelková et al. [51], it was found that, for lower zeolitic contents, up to 20%, the values of water absorption coefficient and apparent moisture diffusivity were still acceptable. However, for high levels of cement replacement, the acceleration of water transport is so high that it could present a concrete durability risk.

Ahmadi et al. [52] reported that natural zeolitic addition in concrete results in better water absorption, water penetration, and electrical resistivity, and the ternary mixtures containing natural zeolite with silica fume or fly ash perform best in water permeability and chloride penetration tests.

#### **3.4. Durability properties**

#### *3.4.1. Sulfate and acid attack resistance*

The durability of a concrete is a determining feature for its use, and this is due to the different corrosive environments to which it is exposed, including marine construction or hydraulic engineering. One of the important factors of this characteristic of concrete is the type of cement. It has been shown that the use of pozzolanic additives for cements increases their resistance to corrosion, due to the high impermeability, the decrease in the content of Ca(OH)2 , and the reduction in the presence of capillary pores in the matrix. In fact, it hinders the penetration of aggressive media [53]. Concrete can be attacked by acids both internally and externally. The existence of different kinds of acid in the environment around the concrete causes a great reduction in the pH of the concrete, and the reaction between the acids and the hydrated and unhydrated cement finally leads to the deterioration of the concrete. The primary effect of any type of acid attack on concrete is the dissolution of the cement paste matrix [54].

Małolepszy and Grabowska [55] carried out a study dedicated to studying the sulfate resistance of a cementing paste with zeolitic addition. They confirmed the beneficial effect of the zeolitic additive for cement mortars because those containing zeolite did not show visible damage in the surface in an aggressive solution of sulfate, while in the mortar that did not possess it, I present surface microcracks pronounced. Exfoliation of corners and colors changes (yellow and gray incursions on the walls of the samples).

#### *3.4.2. Drying shrinkage*

of concrete with the increasing amount of zeolite (10–30% of the mass of Portland cement) in

It is generally recognized that the addition of pozzolan reduces the calcium hydroxide content in cement paste and improves the permeability of concrete [47]. The most important concerns in the design of durable concrete are the alkali-silica reaction (ASR) and sulfate attack. ASR causes the premature deterioration in concrete. Alkali hydroxides present in the concrete pore solution react with amorphous or poorly crystalline silica phases in aggregates, forming a gel that imbibes water and expands [48]. The expansive pressure generated by the hydrated alkali silicate has been widely believed to induce cracking and deterioration of concrete. However, this notion may not be necessarily correct. Concrete is a porous material, and the hydrated alkali silicate is rheologically a fluid material that can slowly diffuse into the pores and preexisting cracks to lose its expansive pressure. The diffused alkali silicate has

been proposed to generate an expansive pressure by reacting with Ca2+ ions [49].

Shekarchi [42] but only for the dose of zeolite greater than 10%.

transport is so high that it could present a concrete durability risk.

The transport properties of concrete with the addition of zeolite have been studied by Ahmadi and Shekarchi [42] and Najimi et al. [41], who found a significant reduction in the penetration of water and chlorides, in the concrete with natural zeolite. On the other hand, Valipour et al. [50] reported that water sorptivity and gas permeability increase with the increase of zeolite in the mixture. Similar results were obtained for the oxygen permeability of Ahmadi and

The liquid water transport parameters increase with the increasing addition of zeolite in the mixed binder. This is due to the pore distribution, that is, the volume of capillary pores, which is the most important factor, in the capacity of a porous medium to transport water in liquid form. In studies conducted by Vejmelková et al. [51], it was found that, for lower zeolitic contents, up to 20%, the values of water absorption coefficient and apparent moisture diffusivity were still acceptable. However, for high levels of cement replacement, the acceleration of water

Ahmadi et al. [52] reported that natural zeolitic addition in concrete results in better water absorption, water penetration, and electrical resistivity, and the ternary mixtures containing natural zeolite with silica fume or fly ash perform best in water permeability and chloride

The durability of a concrete is a determining feature for its use, and this is due to the different corrosive environments to which it is exposed, including marine construction or hydraulic engineering. One of the important factors of this characteristic of concrete is the type of cement.

the mix, even with an increasing superplasticizer dosage.

**3.2. Alkali-silica reaction**

140 Zeolites and Their Applications

**3.3. Transport properties**

penetration tests.

**3.4. Durability properties**

*3.4.1. Sulfate and acid attack resistance*

The contraction is a phenomenon in which the concrete reduces its volume with time. The internal and external drying of the concrete is the main factor that causes the contraction. Internal drying, also known as self-desiccation, is caused by the consumption of water in the hydrating cement paste and the resulting creation of interfacial menisci between the pore fluid and the vapor in progressively smaller pores [56]. The creation of meniscus leads to the accumulation of capillary pressure that puts the solids under compression and causes a macroscopic contraction, called autogenous contraction [56, 57]. External drying takes place due to the evaporation of water from the surface of the concrete to the ambient air or due to the migration of water to adjacent members. The evaporation of moisture from the surface of the fresh concrete can cause cracking of the plastic shrinkage [58], and a greater moisture loss of the hardened concrete can cause shrinkage by drying [59].

Generally, the durability properties of concrete improved by partial replacement of cement with natural zeolite. However, the concrete that contains 15% of natural zeolite achieves a suitable drying shrinkage. The latter does not have a satisfactory performance in the acid environment [41].

#### *3.4.3. Freeze and thaw resistance*

The pores can have an effect on the properties of the material in different ways. The compressive strength is primarily related to the total porosity, the size of pores and their distribution, the size and form of the biggest pores, and the relation between the pores. Shrinking is the function of energy exchange on the surface of pore walls. Durability depends on freeze-thaw resistance and is controlled by the volume of air entrained in the pores and spaces between the pores [60].

Water absorption and freeze-thaw resistance of hardened cement paste depend on the size of pores and capillaries, their type and distribution, and the closing of the pores. Closed and small pores are not filled with water completely. Pores that are not filled up with water are called reserve pores. In freezing conditions, some water from fully filled pores may move to these reserve pores and thus create a space for ice expansion. The distance between filled and unfilled pores must be small, so that the freezing water would move from filled to unfilled pores [60].

Capillary pores in hardened cement paste are formed through the evaporation of excessive water used in producing the cement paste. Usually, the cement paste is made using more water, and it is necessary for chemical reactions that occur during the setting of concrete. According to A. K. Kallipi, capillary pores are open and easily fill with water. The destructive effect during freezing depends on the amount of water in hardened cement paste. Presumably, the bigger amount and size of the pores reduce the freeze-thaw resistance of concrete. Furthermore, it is important to mention that the concrete prepared with saturated and dry recycled aggregates exhibits poorer freeze-thaw resistance, whereas better results obtained from the concrete made with the semi-saturated aggregates [61].

#### *3.4.4. Carbonation*

When a cementitious paste begins to harden with an air cure, and after the first minute of hydration, it is subjected to the action of carbon dioxide ions (CO <sup>3</sup><sup>−</sup> 2 ) contained in the air, which reacts with Ca 2+ ions of the portlandite, ettringite, and the silica gel of calcium in the form of Ca-carbonates (CaCO <sup>3</sup> ). This carbonation of the hydrate products is provided in the following schemes [62]:

$$\text{Ca(OH)}\_{2} + \text{CO}\_{2} \rightarrow \text{CaCO}\_{3} + \text{H}\_{2}\text{O} \tag{2}$$

Lilkov et al. [62] have studied the early hydration of the cement, mixed with additives of natural zeolite (clinoptilolite), and they concluded that the process of carbonation on the surface of the cement paste takes place directly between the calcium ions of the solution and the carbon dioxide of the air without the formation of portlandite and ettringite. The depth of the carbon-

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

The main causes of reinforcing steel corrosion are reacted with various aggressive agents, such as atmospheric carbon dioxide and chloride ions, and chemical attack throughout the service life of the concrete [68]. In ordinary Portland cement, these harmful effects can be reduced by substitute pozzolans [69]. Under a corrosive environment, concrete properties can

Steel rebars are protected against corrosion by both chemical and physical mechanisms. The chemical protection is provided by the concrete high pH (12–13), which promotes the formation of a passive film on the steel surface. On the other hand, concrete acts as a physical barrier, hindering the access of aggressive agents. However, oxygen, water, chlorides, and/ or carbon dioxide can be transported through concrete, reaching the rebars and inducing the corrosion attack. The chloride ions, when above a threshold value, provoke a local breakdown of the passive film and pitting corrosion. Carbon dioxide and its hydrolysis products react

Ahmadi and Shekarchi [42] found a positive effect of zeolite in cement mortar on the resistance to alkali-silica reaction. Janotka and Krajči [71] reported an improvement of sulfate corrosion resistance of zeolite-containing concrete. Similar effects of zeolite on alkali-silica reaction and sulfate resistance were observed by Karakurt and Topçu [45]. On the other hand, Najimi et al. [41] reported a significant strength reduction of zeolitic concrete after exposure to sulfuric-acid environment, that is, ~20% after 356 days when compared with ~5% for reference

Besides eco-friendly, concrete should be sustainable and durable due to its use in infrastructure applications, which are mostly in aggressive environments, such as harsh marine environments with highly possible chloride-induced corrosion. Valipour et al. [72] reported that natural zeolite from a durability point of view in harsh marine environments could be a good option for replacement of cement even comparing with metakaolin and silica fume, which would be beneficial even from environmentally friendly point of view. Because concrete containing 20% replacement level in splash exposure showed chloride diffusion resistance even better than metakaolin with 5% replacement level and silica fume with 5 and 7.5% replace-

Several studies have indicated that lowering the w/b ratio and adding different types of pozzolanic materials to the mix can improve the compressive strength, durability, and permeability of concrete. Lowering the w/b ratio reduces the porosity, which thereby reduces chloride

with the alkaline species present in concrete, leading to pH values as low as 9 [70].

the formed layer, where the crystallite size of the calcite is reduced overtime days.

and the rate of diffusion through

http://dx.doi.org/10.5772/intechopen.74953

143

ate layer formed depends on the contact time with the CO2

be improved by using pozzolans such as zeolite and diatomite [68].

*3.4.5. Corrosion of steel reinforcement*

Portland-cement concrete.

ment level.

*3.4.6. Chloride-induced corrosion*

$$\text{\textbullet\text{CaO}} \cdot 2\text{SiO}\_2 \cdot 3\text{H}\_2\text{O} + 3\text{CO}\_2 \rightarrow \text{\textbullet\text{CaCO}}\_3 + 2\text{SiO}\_2 + 3\text{H}\_2\text{O},\tag{3}$$

$$\begin{aligned} \text{3CaO} \cdot \text{Al}\_2\text{O}\_3 \cdot \text{3CaSO}\_4 \cdot \text{32H}\_2\text{O} + \text{3CO}\_2 &\rightarrow \text{3CaCO}\_3 + \text{3(CaSO}\_4 \cdot 2\text{H}\_2\text{O})\\ \star \text{Al}\_2\text{O} \cdot \text{xH}\_2\text{O} + \text{(26-x)H}\_2\text{O} &\end{aligned} \tag{4}$$

The air-curing conditions normally increase carbonation and cause incomplete hydration, self-neutralization, and drying shrinkage. These effects are most probably caused by decreasing the capacity for retaining sufficient water during the hydration and pozzolanic reactions. The depth of the carbonate layer formed depends on the contact time with C O2 and its concentration in the surrounding environment as well as the diffusion coefficient of the hardened cement paste [63–65]. As the air-curing conditions are more important for long-term reactions than for short-term ones, the C2S phase, which usually reacts after 21 days, is the most readily affected cement phase. The volume changes, which accompany the carbonation processes, lead to the filling of empty pore volumes with Ca carbonates and densify the structure of the hardened cement paste. Groves et al. [66], the microstructure of hardened pastes of C3 S and a smoke mixture of C3 S/silica by TEM, before and after partial carbonation in a pure CO2 atmosphere, concluded that calcium carbonate forms mainly in the outer product regions such as microcrystalline vaterite or calcite, which leads to a substantial level of carbonation of pulp in a day with a little additional carbonation in the coming days. The depth of penetration of Ca CO3 in the cement matrix depends on the time of contact of carbon dioxide and its concentration in the medium and on the coefficient of diffusion [67].

Lilkov et al. [62] have studied the early hydration of the cement, mixed with additives of natural zeolite (clinoptilolite), and they concluded that the process of carbonation on the surface of the cement paste takes place directly between the calcium ions of the solution and the carbon dioxide of the air without the formation of portlandite and ettringite. The depth of the carbonate layer formed depends on the contact time with the CO2 and the rate of diffusion through the formed layer, where the crystallite size of the calcite is reduced overtime days.

#### *3.4.5. Corrosion of steel reinforcement*

pores are not filled with water completely. Pores that are not filled up with water are called reserve pores. In freezing conditions, some water from fully filled pores may move to these reserve pores and thus create a space for ice expansion. The distance between filled and unfilled pores must be small, so that the freezing water would move from filled to unfilled pores [60]. Capillary pores in hardened cement paste are formed through the evaporation of excessive water used in producing the cement paste. Usually, the cement paste is made using more water, and it is necessary for chemical reactions that occur during the setting of concrete. According to A. K. Kallipi, capillary pores are open and easily fill with water. The destructive effect during freezing depends on the amount of water in hardened cement paste. Presumably, the bigger amount and size of the pores reduce the freeze-thaw resistance of concrete. Furthermore, it is important to mention that the concrete prepared with saturated and dry recycled aggregates exhibits poorer freeze-thaw resistance, whereas better results

When a cementitious paste begins to harden with an air cure, and after the first minute of hydra-

). This carbonation of the hydrate products is provided in the following schemes [62]:

The air-curing conditions normally increase carbonation and cause incomplete hydration, self-neutralization, and drying shrinkage. These effects are most probably caused by decreasing the capacity for retaining sufficient water during the hydration and pozzolanic reactions.

centration in the surrounding environment as well as the diffusion coefficient of the hardened cement paste [63–65]. As the air-curing conditions are more important for long-term reactions than for short-term ones, the C2S phase, which usually reacts after 21 days, is the most readily affected cement phase. The volume changes, which accompany the carbonation processes, lead to the filling of empty pore volumes with Ca carbonates and densify the structure of the hardened cement paste. Groves et al. [66], the microstructure of hardened pastes of C3 S and a smoke mixture of C3 S/silica by TEM, before and after partial carbonation in a pure CO2

sphere, concluded that calcium carbonate forms mainly in the outer product regions such as microcrystalline vaterite or calcite, which leads to a substantial level of carbonation of pulp in a day with a little additional carbonation in the coming days. The depth of penetration of Ca

in the cement matrix depends on the time of contact of carbon dioxide and its concentra-

The depth of the carbonate layer formed depends on the contact time with C O2

ions of the portlandite, ettringite, and the silica gel of calcium in the form of Ca-carbonates

<sup>2</sup> + CO2 → CaCO3 + H2

O + 3CO2 → 3CaCO3 + 2Si O2 + 3H2

O + 3CO2 → 3CaC O3 + 3(CaS O4 ∙ 2 H2 O)

O (4)

2

) contained in the air, which reacts

O, (2)

O, (3)

and its con-

atmo-

obtained from the concrete made with the semi-saturated aggregates [61].

tion, it is subjected to the action of carbon dioxide ions (CO <sup>3</sup><sup>−</sup>

O3 ∙ 3CaSO4 ∙ 32 H2

O + (26 − *x*) H2

tion in the medium and on the coefficient of diffusion [67].

Ca (OH)

3CaO ∙ 2Si O2 ∙ 3 H2

3CaO ∙ Al2

O ∙ *x* H2

*3.4.4. Carbonation*

142 Zeolites and Their Applications

with Ca 2+

+ Al2

CO3

(CaCO <sup>3</sup>

The main causes of reinforcing steel corrosion are reacted with various aggressive agents, such as atmospheric carbon dioxide and chloride ions, and chemical attack throughout the service life of the concrete [68]. In ordinary Portland cement, these harmful effects can be reduced by substitute pozzolans [69]. Under a corrosive environment, concrete properties can be improved by using pozzolans such as zeolite and diatomite [68].

Steel rebars are protected against corrosion by both chemical and physical mechanisms. The chemical protection is provided by the concrete high pH (12–13), which promotes the formation of a passive film on the steel surface. On the other hand, concrete acts as a physical barrier, hindering the access of aggressive agents. However, oxygen, water, chlorides, and/ or carbon dioxide can be transported through concrete, reaching the rebars and inducing the corrosion attack. The chloride ions, when above a threshold value, provoke a local breakdown of the passive film and pitting corrosion. Carbon dioxide and its hydrolysis products react with the alkaline species present in concrete, leading to pH values as low as 9 [70].

Ahmadi and Shekarchi [42] found a positive effect of zeolite in cement mortar on the resistance to alkali-silica reaction. Janotka and Krajči [71] reported an improvement of sulfate corrosion resistance of zeolite-containing concrete. Similar effects of zeolite on alkali-silica reaction and sulfate resistance were observed by Karakurt and Topçu [45]. On the other hand, Najimi et al. [41] reported a significant strength reduction of zeolitic concrete after exposure to sulfuric-acid environment, that is, ~20% after 356 days when compared with ~5% for reference Portland-cement concrete.

#### *3.4.6. Chloride-induced corrosion*

Besides eco-friendly, concrete should be sustainable and durable due to its use in infrastructure applications, which are mostly in aggressive environments, such as harsh marine environments with highly possible chloride-induced corrosion. Valipour et al. [72] reported that natural zeolite from a durability point of view in harsh marine environments could be a good option for replacement of cement even comparing with metakaolin and silica fume, which would be beneficial even from environmentally friendly point of view. Because concrete containing 20% replacement level in splash exposure showed chloride diffusion resistance even better than metakaolin with 5% replacement level and silica fume with 5 and 7.5% replacement level.

Several studies have indicated that lowering the w/b ratio and adding different types of pozzolanic materials to the mix can improve the compressive strength, durability, and permeability of concrete. Lowering the w/b ratio reduces the porosity, which thereby reduces chloride ingress during the exposure period by as much as 25% [46]. Moreover, pozzolanic materials are being used widely as mineral admixtures to enhance the mechanical properties of concrete and thereby improve the concrete's microstructure. These admixtures, either natural or artificial, reduce the Ca(OH)<sup>2</sup> content produced during the cement hydration process and instead form C─S─H gel through the secondary reactions. This process retards the hydration process, significantly reducing the porosity and permeability of the concrete [73–75].

In the previous studies concerning the use of zeolites in concrete production, one of the most frequent topics was their pozzolanic activity as a fundamental condition for their utilization as supplementary cementitious materials (SCMs). Perraki et al. [77] reported a good pozzo-

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

Ahmadi and Shekarchi [42] found out that the pozzolanic activity of zeolite was lower than

Many studies have promoted the use of the zeolite-bearing tuffs as SCMs due to their positive influence on the long-term compressive strength and durability. Nevertheless, the variability in tuff's mineralogical and physical properties results in limited understanding of pozzolanic

When zeolite is added to the cement, at a level of around 10%, some characteristics are modified, such as, an increase in compressive strength [43, 85], a decrease in pore size [86, 87], and

Many authors have talked about the appropriate amount of zeolite that must be incorporated into the cement, so that it improves its properties or it can maintain them. Najimi et al. [41] found out that incorporation of 15% natural zeolite in the blended binder improved compressive strength of concrete, but for concrete with 30% zeolitic content, they observed a 25% strength decrease even with adding a superplasticizer, which was not used in the reference mix; they concluded that concretes incorporating zeolite are characterized by the reduction in the heat of hydration and consequently of thermal cracking; and they also improved durability properties such as chloride ion penetration, corrosion rate, drying shrinkage, and water penetration. Ahmadi and Shekarchi [42] also reported that the incorporation of natural zeolite as a mineral admixture in concrete enhanced its durability properties. However, various types, structures, and purities of natural zeolites influence concrete strength and durability in different ways and can lead in some cases to contradictory results in experimental studies and observed an increase in compressive strength for up to 20% of natural zeolite used as Portland cement replacement, but this was achieved with an increasing amount of superplasticizer in the mixes containing zeolites. Uzal and Turanli [44] reported the similar compressive strength of mortar with 55% zeolite in the binder to that of 100% Portland cement mortar, but once again, this could only be achieved using superplasticizers; they describe that a lime reactivity of the clinoptilolite zeolite is comparable to silica fume, higher than fly ash and a non-zeolitic natural pozzolan. Therefore, calcium hydroxide as a cement hydration

hydrosilicates. Karakurt and Topçu [88] found 30% replacement of Portland cement by zeolite in their blended cement mortars as optimum; the compressive strength was similar to the reference mortar. Valipour et al. [50] observed a fast decrease in the compressive strength of concrete with the increasing amount of zeolite (10–30% of the mass of Portland cement) in the mix, even with an increasing superplasticizer dosage. Investigations of mechanical properties of concretes show that the substitution of cement by zeolite resulted in some reduction in strength until 90 days of hardening, but after 180 days, compressive strength of concretes containing zeolite exceeds the strength of concrete without zeolite. Introduction of zeolite and chemical admixtures in concrete permits the modifications of the phase composition of

per 1 g of zeolite according to the Chapelle test.

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145

and Al2

O3

to form calcium

lanic reactivity of zeolite, 0.555 g of Ca(OH)2

an increase in corrosion resistance to the acid solution [55, 88].

product combines with natural zeolite consisting of reactive SiO2

silica fume but higher than fly ash.

activity of natural zeolites.

**4.2. Proper incorporation**

Valipour et al. [76] found that the partial replacement of cement by 10 and 20% natural zeolite drives a higher compressive strength at 28 days, while 30% zeolitic replacement decrease the compressive strength when compared with conventional concrete. Moreover, the use of natural zeolite to improve the durability of concrete in aggressive environments, such as the Persian Gulf, results in a concrete with appreciably low chloride permeability.

#### *3.4.7. Heat of hydration*

Nowadays, there is a discussion concerning the role of natural zeolitic effects on the blended cement hydration during the time. During the early stage of hydration, the effects of zeolite are related to the critical role played by their structure and the large surface area of the particles determining the cation ability in the pore solution and the collateral effect on stimulation of Portland cement hydration due to the low reaction degree of the zeolite [77]. The later stage is the proper pozzolanic reaction between the CH liberated during the hydration of cement and the soluble SiO2 and Al2 O3 present in bulk zeolite occur after 2 weeks producing compounds with cementing properties [78, 79]. Some zeolites are thermally activated [80]. Özen et al. [10], Cornejo et al. [81], and García de Lomas et al. [82] studied the early-age hydration heat of Portland cement blended with a spent zeolite catalyst in an amount of up to 35% of the mass of cement, but the composition of the applied catalysts was significantly different from the natural zeolite. Caputo et al. [83] analyzed the hydration heat development in natural zeolite samples mixed with portlandite in a 1:1 ratio.

Furthermore, Tydlitát et al. [84] concluded that natural zeolite did not react during the early age, but it causes the acceleration of cement phase's hydration. Hence, the early effects of zeolitic addition depend on their physical and chemical characteristics, and it also depends on the Portland cement composition.

### **4. Application of natural zeolite as pozzolan in cement and concrete composites**

#### **4.1. Characteristic**

Supplementary cementitious materials (SCMs) are natural or by-product materials, which react with Ca(OH)2 , (CH), and form hydraulic compounds, such as hydrated calcium silicate hydrate (C─S─H) and calcium aluminate hydrate (C─A─H) [1]. Natural zeolite belongs to the group of natural SCM, whose pozzolanic activity depends on several factors (chemical and mineralogical composition, particle size distribution, specific surface area, cation-exchange capacity, Si/Al ratio of the zeolite framework, etc.) [77]. Each of these factors provides unique characteristics to each cement mix.

In the previous studies concerning the use of zeolites in concrete production, one of the most frequent topics was their pozzolanic activity as a fundamental condition for their utilization as supplementary cementitious materials (SCMs). Perraki et al. [77] reported a good pozzolanic reactivity of zeolite, 0.555 g of Ca(OH)2 per 1 g of zeolite according to the Chapelle test. Ahmadi and Shekarchi [42] found out that the pozzolanic activity of zeolite was lower than silica fume but higher than fly ash.

Many studies have promoted the use of the zeolite-bearing tuffs as SCMs due to their positive influence on the long-term compressive strength and durability. Nevertheless, the variability in tuff's mineralogical and physical properties results in limited understanding of pozzolanic activity of natural zeolites.

When zeolite is added to the cement, at a level of around 10%, some characteristics are modified, such as, an increase in compressive strength [43, 85], a decrease in pore size [86, 87], and an increase in corrosion resistance to the acid solution [55, 88].

### **4.2. Proper incorporation**

ingress during the exposure period by as much as 25% [46]. Moreover, pozzolanic materials are being used widely as mineral admixtures to enhance the mechanical properties of concrete and thereby improve the concrete's microstructure. These admixtures, either natural

instead form C─S─H gel through the secondary reactions. This process retards the hydration

Valipour et al. [76] found that the partial replacement of cement by 10 and 20% natural zeolite drives a higher compressive strength at 28 days, while 30% zeolitic replacement decrease the compressive strength when compared with conventional concrete. Moreover, the use of natural zeolite to improve the durability of concrete in aggressive environments, such as the

Nowadays, there is a discussion concerning the role of natural zeolitic effects on the blended cement hydration during the time. During the early stage of hydration, the effects of zeolite are related to the critical role played by their structure and the large surface area of the particles determining the cation ability in the pore solution and the collateral effect on stimulation of Portland cement hydration due to the low reaction degree of the zeolite [77]. The later stage is the proper pozzolanic reaction between the CH liberated during the hydration of cement

pounds with cementing properties [78, 79]. Some zeolites are thermally activated [80]. Özen et al. [10], Cornejo et al. [81], and García de Lomas et al. [82] studied the early-age hydration heat of Portland cement blended with a spent zeolite catalyst in an amount of up to 35% of the mass of cement, but the composition of the applied catalysts was significantly different from the natural zeolite. Caputo et al. [83] analyzed the hydration heat development in natural

Furthermore, Tydlitát et al. [84] concluded that natural zeolite did not react during the early age, but it causes the acceleration of cement phase's hydration. Hence, the early effects of zeolitic addition depend on their physical and chemical characteristics, and it also depends

Supplementary cementitious materials (SCMs) are natural or by-product materials, which

hydrate (C─S─H) and calcium aluminate hydrate (C─A─H) [1]. Natural zeolite belongs to the group of natural SCM, whose pozzolanic activity depends on several factors (chemical and mineralogical composition, particle size distribution, specific surface area, cation-exchange capacity, Si/Al ratio of the zeolite framework, etc.) [77]. Each of these factors provides unique

, (CH), and form hydraulic compounds, such as hydrated calcium silicate

**4. Application of natural zeolite as pozzolan in cement and concrete** 

process, significantly reducing the porosity and permeability of the concrete [73–75].

Persian Gulf, results in a concrete with appreciably low chloride permeability.

content produced during the cement hydration process and

present in bulk zeolite occur after 2 weeks producing com-

or artificial, reduce the Ca(OH)<sup>2</sup>

144 Zeolites and Their Applications

*3.4.7. Heat of hydration*

and the soluble SiO2

**composites**

**4.1. Characteristic**

react with Ca(OH)2

and Al2

zeolite samples mixed with portlandite in a 1:1 ratio.

on the Portland cement composition.

characteristics to each cement mix.

O3

Many authors have talked about the appropriate amount of zeolite that must be incorporated into the cement, so that it improves its properties or it can maintain them. Najimi et al. [41] found out that incorporation of 15% natural zeolite in the blended binder improved compressive strength of concrete, but for concrete with 30% zeolitic content, they observed a 25% strength decrease even with adding a superplasticizer, which was not used in the reference mix; they concluded that concretes incorporating zeolite are characterized by the reduction in the heat of hydration and consequently of thermal cracking; and they also improved durability properties such as chloride ion penetration, corrosion rate, drying shrinkage, and water penetration. Ahmadi and Shekarchi [42] also reported that the incorporation of natural zeolite as a mineral admixture in concrete enhanced its durability properties. However, various types, structures, and purities of natural zeolites influence concrete strength and durability in different ways and can lead in some cases to contradictory results in experimental studies and observed an increase in compressive strength for up to 20% of natural zeolite used as Portland cement replacement, but this was achieved with an increasing amount of superplasticizer in the mixes containing zeolites. Uzal and Turanli [44] reported the similar compressive strength of mortar with 55% zeolite in the binder to that of 100% Portland cement mortar, but once again, this could only be achieved using superplasticizers; they describe that a lime reactivity of the clinoptilolite zeolite is comparable to silica fume, higher than fly ash and a non-zeolitic natural pozzolan. Therefore, calcium hydroxide as a cement hydration product combines with natural zeolite consisting of reactive SiO2 and Al2 O3 to form calcium hydrosilicates. Karakurt and Topçu [88] found 30% replacement of Portland cement by zeolite in their blended cement mortars as optimum; the compressive strength was similar to the reference mortar. Valipour et al. [50] observed a fast decrease in the compressive strength of concrete with the increasing amount of zeolite (10–30% of the mass of Portland cement) in the mix, even with an increasing superplasticizer dosage. Investigations of mechanical properties of concretes show that the substitution of cement by zeolite resulted in some reduction in strength until 90 days of hardening, but after 180 days, compressive strength of concretes containing zeolite exceeds the strength of concrete without zeolite. Introduction of zeolite and chemical admixtures in concrete permits the modifications of the phase composition of cement hydration products with the formation of an extra amount of calcium hydrosilicates, hydrogelenite, and ettringite [43].

not exceed 30% [62, 91, 92] because this process is more evident in the early hydration phase, but it reduces with the passage of time. It is necessary to consider that the hydration of the cement is perhaps the most important aspect because it will derive almost all the processes addressed in this article, besides being a determining factor in the cementing process of the mixture. Several researchers [84, 93, 94] concluded that the addition of zeolite to the cement paste greatly increases the early stage of hydration, and therefore, it is not recommended to exceed 40% of pozzolanic addition in the mixture. In this way, operational parameters that contribute to the development of a composite mix with the best features and operational performance can be established.

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

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147

It is not enough to establish limits in the substitution of cement with pozzolans because the structure of the zeolite also plays a very important role in the interaction with the mixture; as already mentioned, there is a great variety of zeolites, each with its respective family and structure [95], and the treatment that each one must receive before its incorporation into the cementing pastes can be very different. First, it must be understood that the Si/Al ratio will define the homogeneity of the mixture and the setting process [96] with a higher percentage of alumina in their structure that will tend to have a weaker and slower setting, while those that have a very large pore diameter will facilitate the carbonation of the mixtures. There are many ways to incorporate zeolite to cement pastes, but they have in common the amorphization of the same, whether, by mechanical, chemical, or thermal methods, the main objective is to generate a correct balance between the Portland cement and the percentage of Si/Al content in the zeolite. The dealumination is one of the most widespread methods for the control of the percentage Si/ Al present in the zeolite and encompasses the aforementioned methods. In 1968, McDaniel and Maher prepared the ultra-stable Y zeolite by combining procedures for the exchange of sodium ions with ammonium and hydrothermal treatments at elevated temperatures (T ≥ 600°C). Under the conditions of "deep bed," these treatments caused the removal of structural Al with the consequent decrease of the cell parameter. On the contrary, when the hydrothermal treatment was carried out in a "shallow bed," the protonated form of the zeolite was obtained without causing dealumination. It has been possible to dealuminate up to 98% Y zeolite by hydrothermal treat-

In 1968, Kerr developed a method to dealuminate Y zeolites at low temperature (around 100°C) by using a hydrolyzing and complexing agent of aluminum such as EDTA [97] with this method; however, you cannot dealuminate above 70% without drastic losses of crystallinity.

lites in a single step, quickly and with percentages of Al removal that could reach values above

Finally, Skeels and Breck reported a procedure in which ammonium hexafluorosilicate is used as a deadening agent, under mild conditions at low temperatures. It is a quick and simple method, but the superior level of dealumination, as with EDTA, is limited to 70% to

The textural characterization of the zeolite-cement compound consists of measuring the pore size present in the element; the analysis of this porosity has as an objective to measure the surface of the pores and the volume of the same. It should be mentioned that the compound

at high temperatures was reported to dealuminate Y-type zeo-

ments without an appreciable loss of crystallinity.

**4.5. Textural and mechanical characterization**

Subsequently, the use of SiCl4

avoid crystalline collapse.

90% [98].

#### **4.3. Influencing parameters**

Najimi et al. [41] concluded that concretes incorporating zeolite are characterized by the reduction of the heat of hydration and consequently of thermal cracking and improved durability properties such as chloride ion penetration, corrosion rate, drying shrinkage, and water penetration. Sabet et al. [89] and Ahmadi and Shekarchi [42] also reported that the incorporation of natural zeolite as a mineral admixture in concrete enhanced its durability properties. However, various types, structures, and purities of natural zeolites influence concrete strength and durability in different ways and can lead in some cases to contradictory results in experimental studies.

Uzal and Turanli [44] reported that the type of major cation was found to be one of the probable factors governing the pozzolanic activity of clinoptilolite zeolites by affecting their degree of solubility in alkaline conditions. Experimentally demonstrated that pastes of blended cements containing a large amount of clinoptilolite tuff contain less amount of pores >50 nm when compared with Portland cement paste, which is beneficial in terms of mechanical strength and impermeability of the pastes.

#### **4.4. Elaboration methods**

The methods of preparation of cement mixtures with zeolite vary according to the type of study and parameter that is intended to know, and this is due to the constant search to improve their properties. However, there are regulations that help to delimit the use and management of pozzolans, this, if it is intended that the study has a commercial impact, or that meets the standards established for the development of mortars (**Table 1**).

Several authors have investigated how to improve the mechanical properties of cement with zeolite, and its resistance to compression is specific. Valipour et al. [50] and Chan [90] were agreed that, to improve this characteristic, cement mixtures should not exceed 45% of zeolitic addition. While if what is desired is to avoid the carbonation process of the cement, the additions should


**Table 1.** Standard specifications by ASTM.

not exceed 30% [62, 91, 92] because this process is more evident in the early hydration phase, but it reduces with the passage of time. It is necessary to consider that the hydration of the cement is perhaps the most important aspect because it will derive almost all the processes addressed in this article, besides being a determining factor in the cementing process of the mixture. Several researchers [84, 93, 94] concluded that the addition of zeolite to the cement paste greatly increases the early stage of hydration, and therefore, it is not recommended to exceed 40% of pozzolanic addition in the mixture. In this way, operational parameters that contribute to the development of a composite mix with the best features and operational performance can be established.

It is not enough to establish limits in the substitution of cement with pozzolans because the structure of the zeolite also plays a very important role in the interaction with the mixture; as already mentioned, there is a great variety of zeolites, each with its respective family and structure [95], and the treatment that each one must receive before its incorporation into the cementing pastes can be very different. First, it must be understood that the Si/Al ratio will define the homogeneity of the mixture and the setting process [96] with a higher percentage of alumina in their structure that will tend to have a weaker and slower setting, while those that have a very large pore diameter will facilitate the carbonation of the mixtures. There are many ways to incorporate zeolite to cement pastes, but they have in common the amorphization of the same, whether, by mechanical, chemical, or thermal methods, the main objective is to generate a correct balance between the Portland cement and the percentage of Si/Al content in the zeolite.

The dealumination is one of the most widespread methods for the control of the percentage Si/ Al present in the zeolite and encompasses the aforementioned methods. In 1968, McDaniel and Maher prepared the ultra-stable Y zeolite by combining procedures for the exchange of sodium ions with ammonium and hydrothermal treatments at elevated temperatures (T ≥ 600°C). Under the conditions of "deep bed," these treatments caused the removal of structural Al with the consequent decrease of the cell parameter. On the contrary, when the hydrothermal treatment was carried out in a "shallow bed," the protonated form of the zeolite was obtained without causing dealumination. It has been possible to dealuminate up to 98% Y zeolite by hydrothermal treatments without an appreciable loss of crystallinity.

In 1968, Kerr developed a method to dealuminate Y zeolites at low temperature (around 100°C) by using a hydrolyzing and complexing agent of aluminum such as EDTA [97] with this method; however, you cannot dealuminate above 70% without drastic losses of crystallinity.

Subsequently, the use of SiCl4 at high temperatures was reported to dealuminate Y-type zeolites in a single step, quickly and with percentages of Al removal that could reach values above 90% [98].

Finally, Skeels and Breck reported a procedure in which ammonium hexafluorosilicate is used as a deadening agent, under mild conditions at low temperatures. It is a quick and simple method, but the superior level of dealumination, as with EDTA, is limited to 70% to avoid crystalline collapse.

#### **4.5. Textural and mechanical characterization**

**Name Scope**

hydrogelenite, and ettringite [43].

**4.3. Influencing parameters**

146 Zeolites and Their Applications

in experimental studies.

**4.4. Elaboration methods**

strength and impermeability of the pastes.

ASTM C-618 Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete

cement hydration products with the formation of an extra amount of calcium hydrosilicates,

Najimi et al. [41] concluded that concretes incorporating zeolite are characterized by the reduction of the heat of hydration and consequently of thermal cracking and improved durability properties such as chloride ion penetration, corrosion rate, drying shrinkage, and water penetration. Sabet et al. [89] and Ahmadi and Shekarchi [42] also reported that the incorporation of natural zeolite as a mineral admixture in concrete enhanced its durability properties. However, various types, structures, and purities of natural zeolites influence concrete strength and durability in different ways and can lead in some cases to contradictory results

Uzal and Turanli [44] reported that the type of major cation was found to be one of the probable factors governing the pozzolanic activity of clinoptilolite zeolites by affecting their degree of solubility in alkaline conditions. Experimentally demonstrated that pastes of blended cements containing a large amount of clinoptilolite tuff contain less amount of pores >50 nm when compared with Portland cement paste, which is beneficial in terms of mechanical

The methods of preparation of cement mixtures with zeolite vary according to the type of study and parameter that is intended to know, and this is due to the constant search to improve their properties. However, there are regulations that help to delimit the use and management of pozzolans, this, if it is intended that the study has a commercial impact, or

Several authors have investigated how to improve the mechanical properties of cement with zeolite, and its resistance to compression is specific. Valipour et al. [50] and Chan [90] were agreed that, to improve this characteristic, cement mixtures should not exceed 45% of zeolitic addition. While if what is desired is to avoid the carbonation process of the cement, the additions should

ASTM C-311 Standard test methods for sampling and testing fly ash or natural pozzolans for use in Portland-

ASTM C-1202 Standard test method for electrical indication of concrete's ability to resist chloride ion

ASTM C-876 Standard test method for corrosion potentials of uncoated reinforcing steel in concrete

ASTM C-191 Standard test methods for time of setting of hydraulic cement by the Vicat needle

that meets the standards established for the development of mortars (**Table 1**).

ASTM C-33 Standard specification for concrete aggregates

cement concrete

ASTM C150 Standard specification for Portland cement

penetration

**Table 1.** Standard specifications by ASTM.

The textural characterization of the zeolite-cement compound consists of measuring the pore size present in the element; the analysis of this porosity has as an objective to measure the surface of the pores and the volume of the same. It should be mentioned that the compound may have microporosity or mesoporosity; as the study by Franus et al. [99], there are several ways to calculate the volume, distribution, and surface area of the pores among which are the general isotherm equation based on the combination of a modified Kelvin equation and a statistic thickness of the adsorbed film and the multilayer adsorption theory [100].

**Author details**

Campeche, Mexico

**References**

Iván Ayoseth Chulines Domínguez1

\*Address all correspondence to: mabatal@pampano.unacar.mx

Chemistry, Croydon. 2017. DOI: 10.1039

Cristóbal Patiño-Carachure1

10.1006/jcis.2001.7961

conbuildmat.2010.04.052

S0008-8846(03)00173-X

conbuildmat.2005.01.013

DOI: 10.1617/s11527-010-9689-2

2014;**5**:1-10

, Youness Abdellaoui2

Characterizations and Industrial Applications for Cement and Concrete Incorporated Natural…

1 Facultad de Ingeniería, Universidad Autónoma del Carmen, Ciudad del Carmen,

2 Universidad Autónoma de Yucatán Facultad de Ingeniería, Mérida, Yucatán, Mexico

[1] Gebbink BK. Zeolites in Catalysis Series Editors. 2nd ed. Croydon: Royal Society of

[2] Doula M, Ioannou A, Dimirkou A. Copper adsorption and Si, Al, Ca, Mg, and Na release from Clinoptilolite. Journal of Colloid and Interface Science. 2002;**245**:237-250. DOI:

[3] Miranda I, Burgos D, Castro S. Technical study of impact of hydraulic concrete mixer for the replacement part of cement by zeolite. Iberoamerican Journal of Project Management.

[4] Villa C, Pecina ET, Torres R, Gómez L. Geopolymer synthesis using alkaline activation of natural zeolite. Construction and Building Materials. 2010;**24**:2084-2090. DOI: 10.1016/j.

[5] Ortiz LR, Gutiérrez AE, Lara ÁVG. Behavior of the resistence test to compression of cement P 35 for different additions of zeolite. 2016;**5**:13-29 http://revistas.unica.cu/uciencia

[6] Turanli L, Uzal B, Bektas F. Effect of material characteristics on the properties of blended cements containing high volumes of natural pozzolans. Cement and Concrete Research.

[7] Uzal B, Turanli L. Studies on blended cements containing a high volume of natural pozzolans. Cement and Concrete Research. 2003;**33**:1777-1781. DOI: 10.1016/

[8] Walker R, Pavía S. Physical properties and reactivity of pozzolans, and their influence on the properties of lime-pozzolan pastes. Materials and Structures. 2011;**44**:1139-1150.

[9] Feng N-Q, Peng G-F. Applications of natural zeolite to construction and building materials in China. Construction and Building Materials. 2005;**19**:579-584. DOI: 10.1016/j.

2004;**34**:2277-2282. DOI: 10.1016/j.cemconres.2004.04.011

, Mohamed Abatal1

\* and

149

http://dx.doi.org/10.5772/intechopen.74953

The mechanical characterization performed on cement pastes with zeolitic content does not differ from the typical mechanical compression tests (performed to see the maximum load before causing a fracture), which are carried out by means of a universal machine. However, both the size of the specimens and the laboratory conditions (temperature, pressure, and percentage of aggregates) depend on the author and the results that he intends to obtain. There is a lot of research on the mechanical state of the test tubes [41, 51, 99, 101], and it is important to mention that in most of them, the state of operation is evaluated after having been subjected to adverse media, such as chloride attack or exposure to acid media.

#### **4.6. Special applications of these composites**

At present, due to the many investigations that exist on cement with zeolitic addition, we can find large areas of opportunity for its application. However, its use as a cement for construction is the most researched application [9, 41, 93, 96], and this is obvious when the main function of this compound is to work as an alternative to ordinary cement (usually the Portland type) in the cement industry building. Although there are other applications of the same scope and not so obvious, which have not been given the same attention, among them we can find its use for the stabilization of sandy soils prior to the construction of roads [102, 103], agent for the reduction in pollutants and environmental conditions [76, 104], and its application in cementing operations carried out on the high seas and oil platforms [46, 105].

### **5. Concluding remarks**

The partial replacement of cement is a fact that every day becomes more important; it is an activity that is already carried out and that although there are few cement producers that commercialize with these compounds, it is a great step for the reduction in emissions of CO2 . Many authors have contributed and are still investigating the properties that this mixture provides us in order to improve them and to be able to develop a mixed cement paste that is capable of equalizing the effectiveness of Portland cement, which although it is close to it, in the case of zeolite as a pozzolan, it has not yet been achieved that this represents 50% of the mixture. Fortunately, with the growing research and development of new methods for the synthesis and management of these minerals, achieving a balance between the costs of production and the effectiveness of the product can be a reality. It should be mentioned that the use of zeolite as a pozzolan is not a whim, and although it competes with other pozzolans that can deliver a better mechanical performance such as silica fume and volcanic ash, the possible production savings due to availability and improvement from other areas such as resistance to carbonation, transport properties, hydration of the mixture, and absorption of contaminants are concepts that are worth investigating and investing for their development and improvement.

### **Author details**

may have microporosity or mesoporosity; as the study by Franus et al. [99], there are several ways to calculate the volume, distribution, and surface area of the pores among which are the general isotherm equation based on the combination of a modified Kelvin equation and a

The mechanical characterization performed on cement pastes with zeolitic content does not differ from the typical mechanical compression tests (performed to see the maximum load before causing a fracture), which are carried out by means of a universal machine. However, both the size of the specimens and the laboratory conditions (temperature, pressure, and percentage of aggregates) depend on the author and the results that he intends to obtain. There is a lot of research on the mechanical state of the test tubes [41, 51, 99, 101], and it is important to mention that in most of them, the state of operation is evaluated after having been subjected

At present, due to the many investigations that exist on cement with zeolitic addition, we can find large areas of opportunity for its application. However, its use as a cement for construction is the most researched application [9, 41, 93, 96], and this is obvious when the main function of this compound is to work as an alternative to ordinary cement (usually the Portland type) in the cement industry building. Although there are other applications of the same scope and not so obvious, which have not been given the same attention, among them we can find its use for the stabilization of sandy soils prior to the construction of roads [102, 103], agent for the reduction in pollutants and environmental conditions [76, 104], and its application in

The partial replacement of cement is a fact that every day becomes more important; it is an activity that is already carried out and that although there are few cement producers that com-

authors have contributed and are still investigating the properties that this mixture provides us in order to improve them and to be able to develop a mixed cement paste that is capable of equalizing the effectiveness of Portland cement, which although it is close to it, in the case of zeolite as a pozzolan, it has not yet been achieved that this represents 50% of the mixture. Fortunately, with the growing research and development of new methods for the synthesis and management of these minerals, achieving a balance between the costs of production and the effectiveness of the product can be a reality. It should be mentioned that the use of zeolite as a pozzolan is not a whim, and although it competes with other pozzolans that can deliver a better mechanical performance such as silica fume and volcanic ash, the possible production savings due to availability and improvement from other areas such as resistance to carbonation, transport properties, hydration of the mixture, and absorption of contaminants are concepts that are worth investigating and investing for their development and improvement.

. Many

mercialize with these compounds, it is a great step for the reduction in emissions of CO2

statistic thickness of the adsorbed film and the multilayer adsorption theory [100].

to adverse media, such as chloride attack or exposure to acid media.

cementing operations carried out on the high seas and oil platforms [46, 105].

**4.6. Special applications of these composites**

**5. Concluding remarks**

148 Zeolites and Their Applications

Iván Ayoseth Chulines Domínguez1 , Youness Abdellaoui2 , Mohamed Abatal1 \* and Cristóbal Patiño-Carachure1

\*Address all correspondence to: mabatal@pampano.unacar.mx

1 Facultad de Ingeniería, Universidad Autónoma del Carmen, Ciudad del Carmen, Campeche, Mexico

2 Universidad Autónoma de Yucatán Facultad de Ingeniería, Mérida, Yucatán, Mexico

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**Chapter 9**

**Provisional chapter**

**Potential Desalination of Coal Seam Gas Coproduced**

**Potential Desalination of Coal Seam Gas Coproduced** 

Natural clinoptilolite type zeolite was examined for its potential to treat coal seam gas (CSG) water and remove sodium ions to lower sodium adsorption ratio and reduce pH.The effectiveness of unmodified and modified natural zeolite is due to their regular composition and open porous structure, high exchange capacity, mechanical strength and stability, and consistency in particle size. The effects of acid modification in bringing about changes to the physicochemical properties of clinoptilolite underpin the effectiveness of this material for treatment of CSG saline water. The sodium adsorption capacity of acid-modified zeolite increases up to three times greater than that of the unmodified zeolite. The atomic percentage and binding energies of the chemical elements comprising zeolite are changed significantly following the acid modification. The Si/Al atomic ratio increases with increasing sulfuric acid concentration. Dealumination is the main reason for the increase in the surface charge and cation exchange capacity of clinoptilolite after acid modification. It is due to the increased defects in the crystal structure/lattice, which result in increasing numbers of charge vacancies. Sodium-rich synthetic zeolites 4A and Na─Y after acid modification are also examined by following the dissolution of the first-order fast kinetics and recrystallization processes which can be homogeneous or heterogeneous.

**Keywords:** CSG, acid treatment, SAR, clinoptilolite, surface potential, XPS

Zeolite is a cost-effective material that has been investigated for its potential use as adsorbents because of its large volumes of internal space [1–3]. It belongs to the group of natural silicate minerals which have high sorption capacity and selectivity resulting from high porosity and sieving properties. The capability of zeolite to exchange cations is one of their most useful

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.73613

**Water Using Zeolite**

**Water Using Zeolite**

Xiaoyu Wang and Anh V. Nguyen

Xiaoyu Wang and Anh V. Nguyen

http://dx.doi.org/10.5772/intechopen.73613

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Potential Desalination of Coal Seam Gas Coproduced Water Using Zeolite Potential Desalination of Coal Seam Gas Coproduced Water Using Zeolite**

DOI: 10.5772/intechopen.73613

Xiaoyu Wang and Anh V. Nguyen Xiaoyu Wang and Anh V. Nguyen

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73613

#### **Abstract**

Natural clinoptilolite type zeolite was examined for its potential to treat coal seam gas (CSG) water and remove sodium ions to lower sodium adsorption ratio and reduce pH.The effectiveness of unmodified and modified natural zeolite is due to their regular composition and open porous structure, high exchange capacity, mechanical strength and stability, and consistency in particle size. The effects of acid modification in bringing about changes to the physicochemical properties of clinoptilolite underpin the effectiveness of this material for treatment of CSG saline water. The sodium adsorption capacity of acid-modified zeolite increases up to three times greater than that of the unmodified zeolite. The atomic percentage and binding energies of the chemical elements comprising zeolite are changed significantly following the acid modification. The Si/Al atomic ratio increases with increasing sulfuric acid concentration. Dealumination is the main reason for the increase in the surface charge and cation exchange capacity of clinoptilolite after acid modification. It is due to the increased defects in the crystal structure/lattice, which result in increasing numbers of charge vacancies. Sodium-rich synthetic zeolites 4A and Na─Y after acid modification are also examined by following the dissolution of the first-order fast kinetics and recrystallization processes which can be homogeneous or heterogeneous.

**Keywords:** CSG, acid treatment, SAR, clinoptilolite, surface potential, XPS

#### **1. Introduction**

Zeolite is a cost-effective material that has been investigated for its potential use as adsorbents because of its large volumes of internal space [1–3]. It belongs to the group of natural silicate minerals which have high sorption capacity and selectivity resulting from high porosity and sieving properties. The capability of zeolite to exchange cations is one of their most useful

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

properties as it has been used to remove heavy metals, such as Cd2+, Cu2+, Ni2+, and Pb2+. Of the natural zeolites, clinoptilolite is regarded as the most useful adsorbent. It is also the most researched of all zeolites. Clinoptilolite has a cage-like structure with pores and channels running through the crystal. The cage consisting of SiO4 and AlO<sup>4</sup> tetrahedral joined by shared oxygen atoms carries a net negative charge which is balanced by the presence of exchangeable cations—especially calcium, sodium, magnesium, potassium, and iron. These ions can be readily replaced by other cations that can easily interact with the negative zeolite framework, such as heavy metals [4–6] and ammonium ions [7, 8]. Due to its very high cationic exchange capacity, clinoptilolite is a potential adsorbent for removing cations from an aqueous solution, for example, in desalination, drinking water purification [9–14] and processing coal seam gas (CSG) coproduced water [15].

dealuminate the zeolite structure by applying protons to attack and weaken Al─O bonds, thereby causing skeletal vacancies and defects. They can enlarge the pore mouth of the zeolite and increase the surface area and adsorption ability. Acid treatment is a simple and economical option for increasing the adsorption capacity of natural zeolites that improve

In this regard, the applicability of the water treatment method for CSG water using natural

between 12 and 25, and pH value is about 7) and improved treatments to increase the adsorp-

The chemical composition of the zeolite (Zeolite Australia Pty Ltd) is shown in **Table 1**. Zeolite

tions (0.001, 0.1, 1, and 2 M) in order to increase the adsorption capacity of the sample. The acid treatment included mixing the natural zeolite sample with acid solutions (1:1 weight ratio) and drying of the washed treated zeolite overnight at 80°C. The adsorption experiments were carried out at 10, 20, and 30% solid ratios (w/w) separately for 6 h using an orbital shaker at 200 rpm at room temperature. Primary exchangeable cations and effective cation exchange

natural zeolite was about 119 meq/100 g, as for cations released from zeolite sample during

The analysis by X-ray diffraction (XRD) showed that the main compositions were clinopti-

SO4

that can clog the zeolite pores and slow the exchange rates [23–26]. In addition, SEM images showed that the modified zeolite surface contained more cracks and small openings compared to the natural zeolite. These details prove the structural changes on the zeolite surfaces

SO4

SO4

tion ability of the natural zeolite by acid modification is reviewed in this chapter.

**2.1. Characterization of zeolite before and after acid treatment**

treatment was carried out with different acids (H<sup>2</sup>

capacity (CEC) for the natural zeolite sample are Na+

SO4

NH4+ exchange [22]. The CEC of 0.1 M H<sup>2</sup>

sample. With increasing concentration of H2

based on the adsorption calculation.

samples increased from 9.8 to 15.7 m2

after the acid treatment (**Figure 1**).

**Table 1.** Chemical composition of zeolite [15].

lolite and quartz and 0.1 M H<sup>2</sup>

**2. The effect of zeolite modification by acids on sodium adsorption** 

content less than 200 ppm, sodium adsorption ratio (SAR) value

Potential Desalination of Coal Seam Gas Coproduced Water Using Zeolite

http://dx.doi.org/10.5772/intechopen.73613

161

, HCl, and HAc) at various concentra-

. The total CEC for

, Mg2+, Ca2+, and K+

acid modification did not change the phases of the zeolite

/g. The change was attributed to removing fine particles

acid-modified zeolite was about 300 meq/100 g

from 0 to 2 M, the BET surface area of zeolite

the viability of zeolite on CSG water treatment.

zeolite (to reduce the Na+

**ratio of coal seam gas water**

CSG can be considered as conventional natural gas that is adsorbed into coal seams in deep underground [16]. The CSG extraction discharges a large amount of water (several megaliters per day). For example, 12.5 giga litres of CSG water was produced and discharged by CSG industry in Queensland, Australia, in 2007 [17]. However, high salt content of the CSG water can potentially harm the environment. Therefore, it cannot be released directly to land or other water resources. Sodium concentration must be reduced to less than 200 ppm for irrigation. Management of CSG water is an environmental challenge. Desalination but with cost-effective technologies is in demand to allow the CSG water usage for beneficial purposes, including irrigation use [18].

Current CSG water management and treatment technologies include infiltration ponds, shallow or deep aquifer injection, reverse osmosis, ion exchange, and subsurface drip irrigation [16]. Unfortunately, these treatments are either rather time-consuming or very expensive. For example, using reinjection to clean up an aquifer with dissolved volatile organic compounds over an area of 3 km2 needs around 50 years [18]. In addition, reverse osmosis (RO) is effective in removing dissolved salts but produces a significant amount of brines that also need to be addressed and dealt with high capital costs than most other strategies. The additional effort and costs and high capital costs make it difficult for RO to be a cost-effective management option in a large geographical area where raw water is spread over. Hence, a better way of CSG water treatment is urgently needed.

Sodium adsorption ratio (SAR) is an important parameter to analyses the effective removal of sodium when dealing with CSG water managed irrigation [16]. Being calculated as the ratio of Na+ to Ca2+ and Mg2+ in the soil, SAR describes the tendency for Na<sup>+</sup> to be adsorbed at soil ionexchange sites at the expense of other ions (in this case calcium and magnesium ions). Soils with an excess of Na+ , compared to of Ca2+ and Mg2+, remain in a dispersed condition, almost impermeable to rain or irrigation water. In general, the higher the SAR, the less suitable the water is for irrigation. Irrigation using water with high SAR may require soil amendments to prevent long-term damage to the soil.

To increase adsorption capacity, natural zeolites can be modified by sole or combined treatments such as chemical attacks (alkali, acids, and salts of alkaline metals) and heating. Different methods are used to prepare zeolites with specific properties for different applications. For example, acid treatment has been shown to improve the adsorption ability and enlarge the pore system of different synthetic zeolites [19–21]. Acid treatment can dealuminate the zeolite structure by applying protons to attack and weaken Al─O bonds, thereby causing skeletal vacancies and defects. They can enlarge the pore mouth of the zeolite and increase the surface area and adsorption ability. Acid treatment is a simple and economical option for increasing the adsorption capacity of natural zeolites that improve the viability of zeolite on CSG water treatment.

In this regard, the applicability of the water treatment method for CSG water using natural zeolite (to reduce the Na+ content less than 200 ppm, sodium adsorption ratio (SAR) value between 12 and 25, and pH value is about 7) and improved treatments to increase the adsorption ability of the natural zeolite by acid modification is reviewed in this chapter.

### **2. The effect of zeolite modification by acids on sodium adsorption ratio of coal seam gas water**

#### **2.1. Characterization of zeolite before and after acid treatment**

properties as it has been used to remove heavy metals, such as Cd2+, Cu2+, Ni2+, and Pb2+. Of the natural zeolites, clinoptilolite is regarded as the most useful adsorbent. It is also the most researched of all zeolites. Clinoptilolite has a cage-like structure with pores and channels run-

oxygen atoms carries a net negative charge which is balanced by the presence of exchangeable cations—especially calcium, sodium, magnesium, potassium, and iron. These ions can be readily replaced by other cations that can easily interact with the negative zeolite framework, such as heavy metals [4–6] and ammonium ions [7, 8]. Due to its very high cationic exchange capacity, clinoptilolite is a potential adsorbent for removing cations from an aqueous solution, for example, in desalination, drinking water purification [9–14] and processing coal seam

CSG can be considered as conventional natural gas that is adsorbed into coal seams in deep underground [16]. The CSG extraction discharges a large amount of water (several megaliters per day). For example, 12.5 giga litres of CSG water was produced and discharged by CSG industry in Queensland, Australia, in 2007 [17]. However, high salt content of the CSG water can potentially harm the environment. Therefore, it cannot be released directly to land or other water resources. Sodium concentration must be reduced to less than 200 ppm for irrigation. Management of CSG water is an environmental challenge. Desalination but with cost-effective technologies is in demand to allow the CSG water usage for beneficial purposes, including irrigation use [18].

Current CSG water management and treatment technologies include infiltration ponds, shallow or deep aquifer injection, reverse osmosis, ion exchange, and subsurface drip irrigation [16]. Unfortunately, these treatments are either rather time-consuming or very expensive. For example, using reinjection to clean up an aquifer with dissolved volatile organic compounds

in removing dissolved salts but produces a significant amount of brines that also need to be addressed and dealt with high capital costs than most other strategies. The additional effort and costs and high capital costs make it difficult for RO to be a cost-effective management option in a large geographical area where raw water is spread over. Hence, a better way of

Sodium adsorption ratio (SAR) is an important parameter to analyses the effective removal of sodium when dealing with CSG water managed irrigation [16]. Being calculated as the ratio of

exchange sites at the expense of other ions (in this case calcium and magnesium ions). Soils

impermeable to rain or irrigation water. In general, the higher the SAR, the less suitable the water is for irrigation. Irrigation using water with high SAR may require soil amendments to

To increase adsorption capacity, natural zeolites can be modified by sole or combined treatments such as chemical attacks (alkali, acids, and salts of alkaline metals) and heating. Different methods are used to prepare zeolites with specific properties for different applications. For example, acid treatment has been shown to improve the adsorption ability and enlarge the pore system of different synthetic zeolites [19–21]. Acid treatment can

to Ca2+ and Mg2+ in the soil, SAR describes the tendency for Na<sup>+</sup>

needs around 50 years [18]. In addition, reverse osmosis (RO) is effective

, compared to of Ca2+ and Mg2+, remain in a dispersed condition, almost

and AlO<sup>4</sup>

tetrahedral joined by shared

to be adsorbed at soil ion-

ning through the crystal. The cage consisting of SiO4

gas (CSG) coproduced water [15].

160 Zeolites and Their Applications

over an area of 3 km2

with an excess of Na+

Na+

CSG water treatment is urgently needed.

prevent long-term damage to the soil.

The chemical composition of the zeolite (Zeolite Australia Pty Ltd) is shown in **Table 1**. Zeolite treatment was carried out with different acids (H<sup>2</sup> SO4 , HCl, and HAc) at various concentrations (0.001, 0.1, 1, and 2 M) in order to increase the adsorption capacity of the sample. The acid treatment included mixing the natural zeolite sample with acid solutions (1:1 weight ratio) and drying of the washed treated zeolite overnight at 80°C. The adsorption experiments were carried out at 10, 20, and 30% solid ratios (w/w) separately for 6 h using an orbital shaker at 200 rpm at room temperature. Primary exchangeable cations and effective cation exchange capacity (CEC) for the natural zeolite sample are Na+ , Mg2+, Ca2+, and K+ . The total CEC for natural zeolite was about 119 meq/100 g, as for cations released from zeolite sample during NH4+ exchange [22]. The CEC of 0.1 M H<sup>2</sup> SO4 acid-modified zeolite was about 300 meq/100 g based on the adsorption calculation.

The analysis by X-ray diffraction (XRD) showed that the main compositions were clinoptilolite and quartz and 0.1 M H<sup>2</sup> SO4 acid modification did not change the phases of the zeolite sample. With increasing concentration of H2 SO4 from 0 to 2 M, the BET surface area of zeolite samples increased from 9.8 to 15.7 m2 /g. The change was attributed to removing fine particles that can clog the zeolite pores and slow the exchange rates [23–26]. In addition, SEM images showed that the modified zeolite surface contained more cracks and small openings compared to the natural zeolite. These details prove the structural changes on the zeolite surfaces after the acid treatment (**Figure 1**).


**Table 1.** Chemical composition of zeolite [15].

The surface potential analysis by electrophoresis (with −38 μm size fractions) was carried out for natural and modified zeolite samples (**Figure 3**). Substitutions of Al3+ ions for Si4+ in the clinoptilolite lattice and the broken Si─O─Si bonds at the particle surface during the grinding process were the reasons for the surface charge negative [24, 25, 28]. With the pH value chang-

As per the chemical analysis of the CSG water, its pH value is 8.4, SAR 94, concentration Na<sup>+</sup>

(weight of zeolite per weight of solution) are shown in **Figure 4A**. A slight decrease in

concentrations increased correspondingly. This is the direct result of the ion exchange of sodium with calcium and magnesium which are exchanged and entered the bulk

SO4

modified zeolite samples [15].

concentration was seen with increasing the solid ratio, while the Mg2+ and Ca2+

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

], [Ca2+], and [Mg2+] are the concentrations in solution and meq is the milliequiva-

removal using natural zeolite as a function of solid concentration

(1)

163

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ [Ca2+](meq) <sup>+</sup> [Mg2+ ](meq) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ 2

 could easily become exchangeable cations. These results agree with the literature data [6, 24]. For each pH value tested, the zeta potential of acid-modified zeolite was always around 9 mV more negative than natural zeolite. It indicates that acid modification weakens the Si─O bond. When there is a cation exchange environment exists, ion exchange can occur easily in

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ing, H+

where [Na+

lent weight.

the Na+

The results for Na+

acid-modified zeolite surface than natural zeolite.

SAR <sup>=</sup> [Na+](meq)

**Figure 3.** Zeta potential profiles of natural and 0.1 M H<sup>2</sup>

**2.2. CSG water treatment using untreated and acid-treated zeolite**

563 ppm, Mg2+ less than 1 ppm, and Ca2+ 2 ppm. SAR is defined as follows:

√

**Figure 1.** SEM images of (A) natural zeolite and (B) modified zeolite with 0.1 M H<sup>2</sup> SO4 [15].

The DTA/TG tests showed that the natural and acid-modified zeolite samples had the same trend of weight loss. This indicates that both natural and acid-modified zeolite samples have just physically lost moisture content [27]. DTG analysis (**Figure 2**) also showed the surface water desorption decreased with increasing acid concentration at 200°C. Interestingly, for the zeolite modified by 2 M H<sup>2</sup> SO4 solution, the surface water desorption at around 100°C was much reduced. At a higher temperature (350°C), there was no such an evidence of loss of hydrated water occurred with natural and 0.1 M H<sup>2</sup> SO4 acid-modified zeolites. This suggests that acid with high concentration can destroy the pore structure of the zeolite. Particle size analysis showed that the particle size was less than 200 μm, with a mean of about 50 μm.

**Figure 2.** DTA/TG results of the natural and modified zeolite samples with 0.1 M H<sup>2</sup> SO4 (NZ, natural zeolite; MZ\_0.1, modified zeolite 0.1 M; MZ\_2, modified zeolite 2 M) [15].

The surface potential analysis by electrophoresis (with −38 μm size fractions) was carried out for natural and modified zeolite samples (**Figure 3**). Substitutions of Al3+ ions for Si4+ in the clinoptilolite lattice and the broken Si─O─Si bonds at the particle surface during the grinding process were the reasons for the surface charge negative [24, 25, 28]. With the pH value changing, H+ could easily become exchangeable cations. These results agree with the literature data [6, 24]. For each pH value tested, the zeta potential of acid-modified zeolite was always around 9 mV more negative than natural zeolite. It indicates that acid modification weakens the Si─O bond. When there is a cation exchange environment exists, ion exchange can occur easily in acid-modified zeolite surface than natural zeolite.

#### **2.2. CSG water treatment using untreated and acid-treated zeolite**

As per the chemical analysis of the CSG water, its pH value is 8.4, SAR 94, concentration Na<sup>+</sup> 563 ppm, Mg2+ less than 1 ppm, and Ca2+ 2 ppm. SAR is defined as follows:

 $\stackrel{\text{I}}{563\text{ ppm}}$   $\text{Mg}^{2+}$  less than  $\stackrel{\text{I}}{1}\text{ ppm}$ , and  $\text{Ca}^{2+}$   $\text{2 ppm}$ ,  $\text{SAR}$  is defined as follows: 
$$\text{SAR} = \frac{\text{[Na}^{\cdot}\text{]}\text{mq}\text{(m}\text{q})}{\sqrt{\frac{[\text{Ca}^{2+}\text{(m}\text{q})\*[\text{Mg}^{2+}\text{]}]\text{(m}\text{q}]}{2}}}} \tag{1}$$

where [Na+ ], [Ca2+], and [Mg2+] are the concentrations in solution and meq is the milliequivalent weight.

The results for Na+ removal using natural zeolite as a function of solid concentration (weight of zeolite per weight of solution) are shown in **Figure 4A**. A slight decrease in the Na+ concentration was seen with increasing the solid ratio, while the Mg2+ and Ca2+ concentrations increased correspondingly. This is the direct result of the ion exchange of sodium with calcium and magnesium which are exchanged and entered the bulk

**Figure 3.** Zeta potential profiles of natural and 0.1 M H<sup>2</sup> SO4 modified zeolite samples [15].

**Figure 2.** DTA/TG results of the natural and modified zeolite samples with 0.1 M H<sup>2</sup>

The DTA/TG tests showed that the natural and acid-modified zeolite samples had the same trend of weight loss. This indicates that both natural and acid-modified zeolite samples have just physically lost moisture content [27]. DTG analysis (**Figure 2**) also showed the surface water desorption decreased with increasing acid concentration at 200°C. Interestingly, for the

much reduced. At a higher temperature (350°C), there was no such an evidence of loss of

that acid with high concentration can destroy the pore structure of the zeolite. Particle size analysis showed that the particle size was less than 200 μm, with a mean of about 50 μm.

SO4

solution, the surface water desorption at around 100°C was

SO4 [15].

acid-modified zeolites. This suggests

modified zeolite 0.1 M; MZ\_2, modified zeolite 2 M) [15].

zeolite modified by 2 M H<sup>2</sup>

162 Zeolites and Their Applications

SO4

**Figure 1.** SEM images of (A) natural zeolite and (B) modified zeolite with 0.1 M H<sup>2</sup>

hydrated water occurred with natural and 0.1 M H<sup>2</sup>

SO4

(NZ, natural zeolite; MZ\_0.1,

**Figure 4.** (A) Ion concentrations in CSG water after treatment with natural zeolite. (B) SAR values of CSG water after treatment with natural zeolite [15].

aqueous phase. Because of the decrease in sodium concentration and increase in calcium and magnesium concentrations, the value of SAR decreases with the solid ratio as shown in **Figure 4B**, which is expected. While the SAR value is in the acceptable range between 12 and 25, Na+ concentration remains high, making the treated water unacceptable for practical purposes.

Ca2+ ions exchanged during the adsorption process. These results suggested that 0.1 M acid

Based on the optimum acid concentration, systematic adsorption tests were carried out with

modified zeolite was considerably increased. For example, the modified zeolite reduced Na<sup>+</sup> concentrations to approximately 180 ppm at 30% solid ratio that is acceptable for irrigation.

was no significant difference on the SAR values for the samples due to how much Mg2+ and Ca2+ ions released from the zeolite, which could be affected by ions releasing in modification

represent the initial and residual concentrations in Na+

amount of zeolite used (g), *V* is the volume of the solution (L), A is the BET surface area

that the modification process increased the sodium adsorption capacity of zeolite up to four times. The modified zeolite had a larger surface area than the natural zeolite. The DTA/TG, XRD, and particle size distribution results for both samples also indicated that there was no

removal by the

adsorption significantly. There

by the acid-modified zeolite as com-

*mA* (2)

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165

). The results in **Figure 6** evidently indicate

in the batch tests was calcu-

(mg/L), *m* is the

zeolite sample modified at 0.1 M with different acids (**Figure 5**, right). The Na+

adsorption density by zeolite samples versus solid ratio [15].

concentration was optimal for the modification process.

However, zeolite modified by HAc did not change the Na<sup>+</sup>

**Figure 6** shows an increase in adsorption capacity of Na+

<sup>Γ</sup> <sup>=</sup> (*Ci* <sup>−</sup> *Cr*) <sup>×</sup> *<sup>V</sup>* \_\_\_\_\_\_\_\_\_

/g), and Γ is the adsorption density (mg/m<sup>2</sup>

structural difference between two samples.

pared with the natural zeolite. The adsorption density (Γ) of Na<sup>+</sup>

and washing process before adsorption tests.

lated by the following formula:

and *Cr*

where *Ci*

**Figure 6.** Na+

(m2

**Figure 5** (left panel) shows the effect of acid modification of zeolite particles on the adsorption of Na+ ions. The tests were conducted with different concentrations of H<sup>2</sup> SO4 to establish the most suitable acid concentration. The adsorption tests were carried out using an orbital shaker at 200 rpm at the solid ratio of 30% for 6 h. The Na<sup>+</sup> concentration of the CSG water after treatment with the modified zeolite decreases with the increasing acid concentration: The Na<sup>+</sup> adsorption capacity of zeolite can be significantly improved by acid modification. The SAR values with the modified zeolite were slightly higher than natural zeolite due to the release of Mg2+ and Ca2+ ions during the washing and acid modification process and sequentially lessening Mg2+ and

**Figure 5.** Left: Na<sup>+</sup> concentration and SAR in CSG water after treatment with 30% solid ratio zeolite (natural and modified by H<sup>2</sup> SO4 ) [15]. Right: Na<sup>+</sup> removal by the modified zeolite as a function of acid type and solid concentration at room temperature [15].

**Figure 6.** Na+ adsorption density by zeolite samples versus solid ratio [15].

aqueous phase. Because of the decrease in sodium concentration and increase in calcium and magnesium concentrations, the value of SAR decreases with the solid ratio as shown in **Figure 4B**, which is expected. While the SAR value is in the acceptable range

**Figure 4.** (A) Ion concentrations in CSG water after treatment with natural zeolite. (B) SAR values of CSG water after

**Figure 5** (left panel) shows the effect of acid modification of zeolite particles on the adsorption of

suitable acid concentration. The adsorption tests were carried out using an orbital shaker at

capacity of zeolite can be significantly improved by acid modification. The SAR values with the modified zeolite were slightly higher than natural zeolite due to the release of Mg2+ and Ca2+ ions during the washing and acid modification process and sequentially lessening Mg2+ and

concentration and SAR in CSG water after treatment with 30% solid ratio zeolite (natural and

removal by the modified zeolite as a function of acid type and solid concentration

ions. The tests were conducted with different concentrations of H<sup>2</sup>

with the modified zeolite decreases with the increasing acid concentration: The Na<sup>+</sup>

concentration remains high, making the treated water unaccept-

SO4

concentration of the CSG water after treatment

to establish the most

adsorption

between 12 and 25, Na+

treatment with natural zeolite [15].

164 Zeolites and Their Applications

Na+

**Figure 5.** Left: Na<sup>+</sup>

at room temperature [15].

SO4

) [15]. Right: Na<sup>+</sup>

modified by H<sup>2</sup>

able for practical purposes.

200 rpm at the solid ratio of 30% for 6 h. The Na<sup>+</sup>

Ca2+ ions exchanged during the adsorption process. These results suggested that 0.1 M acid concentration was optimal for the modification process.

Based on the optimum acid concentration, systematic adsorption tests were carried out with zeolite sample modified at 0.1 M with different acids (**Figure 5**, right). The Na+ removal by the modified zeolite was considerably increased. For example, the modified zeolite reduced Na<sup>+</sup> concentrations to approximately 180 ppm at 30% solid ratio that is acceptable for irrigation. However, zeolite modified by HAc did not change the Na<sup>+</sup> adsorption significantly. There was no significant difference on the SAR values for the samples due to how much Mg2+ and Ca2+ ions released from the zeolite, which could be affected by ions releasing in modification and washing process before adsorption tests.

**Figure 6** shows an increase in adsorption capacity of Na+ by the acid-modified zeolite as compared with the natural zeolite. The adsorption density (Γ) of Na<sup>+</sup> in the batch tests was calculated by the following formula:

$$
\Gamma = \frac{(\mathbf{C}\_i - \mathbf{C}\_i) \times V}{mA} \tag{2}
$$

where *Ci* and *Cr* represent the initial and residual concentrations in Na+ (mg/L), *m* is the amount of zeolite used (g), *V* is the volume of the solution (L), A is the BET surface area (m2 /g), and Γ is the adsorption density (mg/m<sup>2</sup> ). The results in **Figure 6** evidently indicate that the modification process increased the sodium adsorption capacity of zeolite up to four times. The modified zeolite had a larger surface area than the natural zeolite. The DTA/TG, XRD, and particle size distribution results for both samples also indicated that there was no structural difference between two samples.

### **3. X-ray photoelectron spectroscopy (XPS) investigation of sulfuric acid modification of natural zeolite**

XPS analysis of the zeolite structural composition of zeolite samples shows the presence of the O1s, C1s, Na1s, Ca2p, Si2p, Al2p, and K2p photoelectron lines [29]. The variations of the atomic concentrations are significant as shown in **Figure 7**. With increasing sulfuric acid concentration from 0 to 5 M, the atomic proportions of each element change differently. When the acid concentration increases from 0 to 1 M and beyond, the atomic % of Na decreases from 0.6% to 0. Changes in the atomic % of Ca and Mg follow a similar trend, i.e., after treating by 1 M acid, their atomic proportion is either near zero or cannot be detected. That suggests most of the cations which are not part of the main structure and can be easily removed by acid modification. However, K cannot be totally removed after acid treatment, i.e., although the change in its atomic percentage fluctuates, the overall trend is decreasing. The proportion of Al decreases with increasing the acid concentration from 0 to 2 M and then slightly increases by 5 M acid treatment. This may due to the significant decrease in the atomic proportion of the other cations, especially at high acid concentrations. The atomic percentage of O increases from 52% (unmodified) to 60.9% after treating with 0.1 M acid and then does not change much with higher acid concentrations as Si. The atomic proportions of Al, O, and Si do not change significantly as they are the building elements of zeolite structure.

the acid solutions during the modification process. K ions as an exception (unaffected by the acid modification) remain in zeolite structure at relatively constant levels. **Figure 9** shows highresolution scan results for Si, Al, and O which cannot be easily detected in the survey scan.

**Figure 8.** The effect of increasing acid concentration on Si/Al ratio (A) and [Si2p-Al2p] binding energy separations (B)

High-resolution scans for Si2p, Al2p, O1s, K2p, and Ca2p of natural and sulfuric acid modification show different extents of BE increase for each element. For the building elements of zeolite,

can be observed by analyzing K2p3/2 peaks. Because of Ca removal, the progressive shifting patterns show the relative covalency-ionicity of the chemical bonds within the zeolite structure [30]. For natural zeolite studied here, the easy removal of Na from the zeolite surface after acid modification may be due to the high ionicity of the Na─O bond. Likewise, Ca─O and Mg─O ionic bonds are similarly strong, and the atomic proportion can also reduce significantly.

**Figure 9.** XPS high-resolution spectra of natural and acid-modified zeolite for Si2p, Al2p, O1s, K2p, and Ca2p [29].

and Ca2+. The small increase in K2p BE

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167

such as Si, Al, and O, the BE shifts are greater than K<sup>+</sup>

(trend line included to illustrate the relationship) [29].

These XPS results support the sodium adsorption ratio studies. **Figure 7** shows a significant decrease in the atomic % of Ca and Mg after 0.01 M acid modification. This data agrees with the hypothesis that Ca and Mg are removed from the zeolite surface after the modification (0–1 M acid). **Figure 8** shows the trend of Si/Al ratio on the zeolite surface and [Si2p-Al2p] BE separation. With increasing the acid concentration, the Si/Al ratio increases from 2.99 to 4.92. However, the [Si2p-Al2p] BE separation increases to 29 eV when acid concentration increases from 0.01 to 0.1 M. It remains unchanged up to 2 M and finally decreases to 28.5 eV after 5 M acid modification. The increase in Si/Al atomic ratio also indicates the dealumination process occurred within the zeolite structure on the surface. Na, Mg, and Ca ions can be released into

**Figure 7.** Effect of acid concentration on elemental atomic proportions revealed by XPS [29].

**3. X-ray photoelectron spectroscopy (XPS) investigation of sulfuric** 

significantly as they are the building elements of zeolite structure.

**Figure 7.** Effect of acid concentration on elemental atomic proportions revealed by XPS [29].

XPS analysis of the zeolite structural composition of zeolite samples shows the presence of the O1s, C1s, Na1s, Ca2p, Si2p, Al2p, and K2p photoelectron lines [29]. The variations of the atomic concentrations are significant as shown in **Figure 7**. With increasing sulfuric acid concentration from 0 to 5 M, the atomic proportions of each element change differently. When the acid concentration increases from 0 to 1 M and beyond, the atomic % of Na decreases from 0.6% to 0. Changes in the atomic % of Ca and Mg follow a similar trend, i.e., after treating by 1 M acid, their atomic proportion is either near zero or cannot be detected. That suggests most of the cations which are not part of the main structure and can be easily removed by acid modification. However, K cannot be totally removed after acid treatment, i.e., although the change in its atomic percentage fluctuates, the overall trend is decreasing. The proportion of Al decreases with increasing the acid concentration from 0 to 2 M and then slightly increases by 5 M acid treatment. This may due to the significant decrease in the atomic proportion of the other cations, especially at high acid concentrations. The atomic percentage of O increases from 52% (unmodified) to 60.9% after treating with 0.1 M acid and then does not change much with higher acid concentrations as Si. The atomic proportions of Al, O, and Si do not change

These XPS results support the sodium adsorption ratio studies. **Figure 7** shows a significant decrease in the atomic % of Ca and Mg after 0.01 M acid modification. This data agrees with the hypothesis that Ca and Mg are removed from the zeolite surface after the modification (0–1 M acid). **Figure 8** shows the trend of Si/Al ratio on the zeolite surface and [Si2p-Al2p] BE separation. With increasing the acid concentration, the Si/Al ratio increases from 2.99 to 4.92. However, the [Si2p-Al2p] BE separation increases to 29 eV when acid concentration increases from 0.01 to 0.1 M. It remains unchanged up to 2 M and finally decreases to 28.5 eV after 5 M acid modification. The increase in Si/Al atomic ratio also indicates the dealumination process occurred within the zeolite structure on the surface. Na, Mg, and Ca ions can be released into

**acid modification of natural zeolite**

166 Zeolites and Their Applications

**Figure 8.** The effect of increasing acid concentration on Si/Al ratio (A) and [Si2p-Al2p] binding energy separations (B) (trend line included to illustrate the relationship) [29].

the acid solutions during the modification process. K ions as an exception (unaffected by the acid modification) remain in zeolite structure at relatively constant levels. **Figure 9** shows highresolution scan results for Si, Al, and O which cannot be easily detected in the survey scan.

High-resolution scans for Si2p, Al2p, O1s, K2p, and Ca2p of natural and sulfuric acid modification show different extents of BE increase for each element. For the building elements of zeolite, such as Si, Al, and O, the BE shifts are greater than K<sup>+</sup> and Ca2+. The small increase in K2p BE can be observed by analyzing K2p3/2 peaks. Because of Ca removal, the progressive shifting patterns show the relative covalency-ionicity of the chemical bonds within the zeolite structure [30]. For natural zeolite studied here, the easy removal of Na from the zeolite surface after acid modification may be due to the high ionicity of the Na─O bond. Likewise, Ca─O and Mg─O ionic bonds are similarly strong, and the atomic proportion can also reduce significantly.

**Figure 9.** XPS high-resolution spectra of natural and acid-modified zeolite for Si2p, Al2p, O1s, K2p, and Ca2p [29].

## **4. Characterization of electrokinetic properties of clinoptilolite before and after activation by sulfuric acid for treating CSG water**

The change in pH of zeolite suspensions versus time can be an important parameter for the surface dissociation as well as the environmental and industrial applications. **Figure 10** shows a rapid increase in the pH value of the suspensions from pH = 5.6 of DI water (before adding natural zeolite) to 8.02 within the first 2 min. The increase in pH is due to the rapid adsorption of H<sup>+</sup> from solution. After the first 2 min, a very slow pH decrease to 7.13 is observed for 2 h. It shows the adsorption of H+ in water onto the negative surface charge of zeolite. Therefore, the H+ acts as a potential determining ion (PDI) in the electrical double layer to provide electroneutrality for the first 2 min. Besides, the H exchange with some of the cations in the lattice of zeolite structure is also the reason for the consumption of H+ in the suspension [31]. Zeolite tends to neutralize the aqueous medium acting the H+ desorption from the solution onto the surface of particles.

the surface of zeolite particles and exposed to water after grinding as a result of the chemi-

Since the cation exchange capacity (CEC) of zeolite mainly results from the permanently negative surface charge, surface (zeta) potential of acid-activated zeolite is an interesting parameter to be examined. As shown in **Figure 11**, the zeta potential of natural and acid-modified zeolite became more negative with the pH value increasing from 2 to 10. However, the value of zeta potential is more negative with increasing the acid concentration. In the first place, zeta potential became more negative after being modified by sulfuric acid with the concentration up to 0.1 M. With the acid concentration increased further, generally the zeta potential became less negative. The change in zeta potential versus pH was not significant at 5 M of sulfuric acid. In the neutral pH environment (pH 7), the most negative charge can be detected on the

Investigation of electrical double layer (EDL) of zeolite structure via zeta potential measure-

the inner Helmholtz plane (IHP) of the Stern layer on zeolite surface as a charge reversal.

and diffuse layer. Dealumination occurred during the acid activation process of natural zeolite [31]. It is assumed as the main reason for increasing surface charge and cation exchange capacity of zeolite surface due to the increasing of defects in zeolite crystal structure and lat-

tice. It leads to the increase in charge vacancies. Thus, it can be assumed that the H<sup>+</sup>

in IHP increased due to more zeolite crystal defects appeared after dealumination.

**Figure 11.** Surface (zeta) potential of natural and acid-modified zeolites vs. pH [32].

plays an important role as a counterion in EDL; it adsorbs onto

Potential Desalination of Coal Seam Gas Coproduced Water Using Zeolite

was adsorbed by the outer Helmholtz plane (OHP), between the Stern layer

O into the suspension.

ions from ─SiOH groups

169

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adsorption

cal bonds breaking, and/or (ii) the desorption or transfer of the H<sup>+</sup>

of H2

surface of zeolite modified by 0.1 M sulfuric acid.

inside the lattice to free OH−

ments demonstrated that H+

Meanwhile, OH−

In the acidic medium, the initial pH of water was changed to 2 by 0.1 M HCl addition and then zeolite particles into the solution. The suspension pH increased to 2.4 in first 2 min and attained the equilibrium of pH 2.9 for 120 min. Al3+ from the octahedral sheets moves into the acid solution. They were conducive to ion exchange. Indeed, if the pH was adjusted to a lower value, dealumination process occurred throughout the measurements.

In basic medium, the initial pH of water was changed to 11.5 by 0.1 M NaOH, and then the zeolite particles were added into the solution. The suspension pH decreased to 11.27 in 2 min and equilibrated to pH = 10.5 in 120 min. The decrease in the pH value may be due to (i) the adsorption of OH<sup>−</sup> in the suspension onto the positive charge sites, which were presented on

**Figure 10.** Transient pH profiles of suspensions of natural zeolite sample [32].

the surface of zeolite particles and exposed to water after grinding as a result of the chemical bonds breaking, and/or (ii) the desorption or transfer of the H<sup>+</sup> ions from ─SiOH groups inside the lattice to free OH− of H2 O into the suspension.

**4. Characterization of electrokinetic properties of clinoptilolite before and after activation by sulfuric acid for treating CSG water**

the adsorption of H+

168 Zeolites and Their Applications

adsorption of OH<sup>−</sup>

is also the reason for the consumption of H+

the aqueous medium acting the H+

The change in pH of zeolite suspensions versus time can be an important parameter for the surface dissociation as well as the environmental and industrial applications. **Figure 10** shows a rapid increase in the pH value of the suspensions from pH = 5.6 of DI water (before adding natural zeolite) to 8.02 within the first 2 min. The increase in pH is due to the rapid adsorption of H<sup>+</sup> from solution. After the first 2 min, a very slow pH decrease to 7.13 is observed for 2 h. It shows

as a potential determining ion (PDI) in the electrical double layer to provide electroneutrality for the first 2 min. Besides, the H exchange with some of the cations in the lattice of zeolite structure

In the acidic medium, the initial pH of water was changed to 2 by 0.1 M HCl addition and then zeolite particles into the solution. The suspension pH increased to 2.4 in first 2 min and attained the equilibrium of pH 2.9 for 120 min. Al3+ from the octahedral sheets moves into the acid solution. They were conducive to ion exchange. Indeed, if the pH was adjusted to a lower

In basic medium, the initial pH of water was changed to 11.5 by 0.1 M NaOH, and then the zeolite particles were added into the solution. The suspension pH decreased to 11.27 in 2 min and equilibrated to pH = 10.5 in 120 min. The decrease in the pH value may be due to (i) the

value, dealumination process occurred throughout the measurements.

**Figure 10.** Transient pH profiles of suspensions of natural zeolite sample [32].

in water onto the negative surface charge of zeolite. Therefore, the H+

in the suspension onto the positive charge sites, which were presented on

in the suspension [31]. Zeolite tends to neutralize

desorption from the solution onto the surface of particles.

acts

Since the cation exchange capacity (CEC) of zeolite mainly results from the permanently negative surface charge, surface (zeta) potential of acid-activated zeolite is an interesting parameter to be examined. As shown in **Figure 11**, the zeta potential of natural and acid-modified zeolite became more negative with the pH value increasing from 2 to 10. However, the value of zeta potential is more negative with increasing the acid concentration. In the first place, zeta potential became more negative after being modified by sulfuric acid with the concentration up to 0.1 M. With the acid concentration increased further, generally the zeta potential became less negative. The change in zeta potential versus pH was not significant at 5 M of sulfuric acid. In the neutral pH environment (pH 7), the most negative charge can be detected on the surface of zeolite modified by 0.1 M sulfuric acid.

Investigation of electrical double layer (EDL) of zeolite structure via zeta potential measurements demonstrated that H+ plays an important role as a counterion in EDL; it adsorbs onto the inner Helmholtz plane (IHP) of the Stern layer on zeolite surface as a charge reversal. Meanwhile, OH− was adsorbed by the outer Helmholtz plane (OHP), between the Stern layer and diffuse layer. Dealumination occurred during the acid activation process of natural zeolite [31]. It is assumed as the main reason for increasing surface charge and cation exchange capacity of zeolite surface due to the increasing of defects in zeolite crystal structure and lattice. It leads to the increase in charge vacancies. Thus, it can be assumed that the H<sup>+</sup> adsorption in IHP increased due to more zeolite crystal defects appeared after dealumination.

**Figure 11.** Surface (zeta) potential of natural and acid-modified zeolites vs. pH [32].

## **5. Effect of sulfuric acid modification on surface and particle properties of 4A and Na**─**Y synthetic zeolites**

4A and Na─Y are sodium-rich synthetic zeolites, which are well-known crystalline microporous materials and widely used as solid acid catalysts and molecular sieves. Both of them have three-dimensional structures which are composed of [SiO4 ]4− and [AlO<sup>4</sup> ]5− tetrahedra. All of the tetrahedra are interconnected with shared corners to form channels of molecular dimensions. Each AlO<sup>4</sup> tetrahedron bears a net charge which is neutralized by the additional positive charge from the non-framework Na+ which is located within the channel. Because of the purity of crystalline products and the uniformity of particle sizes, synthetic zeolites are used commercially more often than natural zeolites [33]. Application of the acid modification to synthetic zeolites could provide a useful reference for understanding into improving their adsorption or ion-exchange properties, given that the effects of sulfuric acid modification of zeolite 4A and Na─Y are unknown.

the Na─Y fine particles were observed to remain in a suspending state and swell significantly and then formed a diffuse solid-liquid interface. The small Na─Y particles dissolved fully at higher acid concentrations (0.5, 1, 2, and 3 M). A significant amount of acid was consumed in this process, leaving less amount of acid that was insufficient to cause the larger particles to swell to a state of suspension. That can be the reason for observing a clear solid-liquid interface. With the acid concentration increased to 5 M, not only the fine Na─Y particles were dissolved, but also a proportion of the larger particles underwent a partial dissolution, which led to swelling and suspension in solutions as being observed. Thus, the solid-liquid interface

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As increasing the acid concentration to 0.5 M, the 4A molecular sieve could be completely dissolved and formed a gel. It was hard to observe the particle dissolution as occurred in Na─Y system. It indicates Na─Y has a strong acid resistance and is consequently more stable than

**Figure 12** shows the experimental data of solid mass of the Na─Y samples left/gained over time in the suspensions after standing for 6, 24, 48, and 70 h after treatment with sulfuric acid.

*y* = *a* + *b* exp(−*kx*) (3)

where *y* is the solid mass (obtained by filtration) of the collected Na─Y sample (grams), *x* is the acid concentration used in the treatment process (mole/L), and *a*, *b*, and *k* are the fitting

As per **Figure 12A**, the residual mass of Na─Y particles was rapidly decreased with increasing acid concentration from 0 to 0.5 M. The dissolution kinetics was fast. For concentrations

**Figure 12A** shows that less than 6 h is required for dissolution of the acid-soluble component to reach completion. It was concluded that the acid-soluble component of Na─Y was

concentration on mass of solid of Na─Y zeolite suspension and (B) its constant values

higher than 0.5 M, the residual mass of Na─Y particles inclined to constant values.

 **concentration on solid mass in Na**─**Y zeolite suspensions**

developed a diffuse appearance, like that seen at the 0.1 M acid concentrations.

The data can be well fitted by the following equation of the first-order kinetics:

4A, possibly attributable to the higher Si/Al ratio in the structure.

parameters, with grams and L/mol as their units, respectively.

**5.2. Impact of H<sup>2</sup>**

**Figure 12.** (A) Effect of H<sup>2</sup>

versus standing time [34].

SO4

**SO<sup>4</sup>**

#### **5.1. Stability and resistance of 4A and Na**─**Y zeolites against H<sup>2</sup> SO<sup>4</sup> modification**

After 3-day acid modification, 4A zeolite particles were observed to become totally settled in 0 and 0.1 M H<sup>2</sup> SO4 solutions and partly settled in 0.2 and 0.3 M H<sup>2</sup> SO4 solutions. However, they completely dissolved to form a gel in the 0.5 and 1 M H<sup>2</sup> SO4 solutions. Low concentrations of sulfuric acid, such as 0.2 and 0.3 M, cannot dissolve the zeolite particles completely but are strong enough to break bigger 4A particles into smaller ones.

After mixing Na─Y powder with various concentrations of H2 SO4 solutions, cloudy white suspensions were generated. Substantial settling occurred for the first day and continued to a lesser extent over the subsequent 2 days. **Table 2** describes the appearance of the suspensions after 3-day settlement.

The Na─Y powder can also be dissolved in sulfuric acid to a certain extent. The dissolution of small Na─Y particles is limited with a low concentration of sulfuric acid (0.1 M). Some of


**Table 2.** Behavior of the Na─Y suspensions after 3-day standing [34].

the Na─Y fine particles were observed to remain in a suspending state and swell significantly and then formed a diffuse solid-liquid interface. The small Na─Y particles dissolved fully at higher acid concentrations (0.5, 1, 2, and 3 M). A significant amount of acid was consumed in this process, leaving less amount of acid that was insufficient to cause the larger particles to swell to a state of suspension. That can be the reason for observing a clear solid-liquid interface. With the acid concentration increased to 5 M, not only the fine Na─Y particles were dissolved, but also a proportion of the larger particles underwent a partial dissolution, which led to swelling and suspension in solutions as being observed. Thus, the solid-liquid interface developed a diffuse appearance, like that seen at the 0.1 M acid concentrations.

As increasing the acid concentration to 0.5 M, the 4A molecular sieve could be completely dissolved and formed a gel. It was hard to observe the particle dissolution as occurred in Na─Y system. It indicates Na─Y has a strong acid resistance and is consequently more stable than 4A, possibly attributable to the higher Si/Al ratio in the structure.

#### **5.2. Impact of H<sup>2</sup> SO<sup>4</sup> concentration on solid mass in Na**─**Y zeolite suspensions**

**Figure 12** shows the experimental data of solid mass of the Na─Y samples left/gained over time in the suspensions after standing for 6, 24, 48, and 70 h after treatment with sulfuric acid. The data can be well fitted by the following equation of the first-order kinetics:

$$y = a + b \exp(-kx) \tag{3}$$

where *y* is the solid mass (obtained by filtration) of the collected Na─Y sample (grams), *x* is the acid concentration used in the treatment process (mole/L), and *a*, *b*, and *k* are the fitting parameters, with grams and L/mol as their units, respectively.

As per **Figure 12A**, the residual mass of Na─Y particles was rapidly decreased with increasing acid concentration from 0 to 0.5 M. The dissolution kinetics was fast. For concentrations higher than 0.5 M, the residual mass of Na─Y particles inclined to constant values.

**Figure 12A** shows that less than 6 h is required for dissolution of the acid-soluble component to reach completion. It was concluded that the acid-soluble component of Na─Y was

**Figure 12.** (A) Effect of H<sup>2</sup> SO4 concentration on mass of solid of Na─Y zeolite suspension and (B) its constant values versus standing time [34].

**Table 2.** Behavior of the Na─Y suspensions after 3-day standing [34].

**5. Effect of sulfuric acid modification on surface and particle** 

4A and Na─Y are sodium-rich synthetic zeolites, which are well-known crystalline microporous materials and widely used as solid acid catalysts and molecular sieves. Both of them

All of the tetrahedra are interconnected with shared corners to form channels of molecular

the purity of crystalline products and the uniformity of particle sizes, synthetic zeolites are used commercially more often than natural zeolites [33]. Application of the acid modification to synthetic zeolites could provide a useful reference for understanding into improving their adsorption or ion-exchange properties, given that the effects of sulfuric acid modification of

After 3-day acid modification, 4A zeolite particles were observed to become totally settled in 0

sulfuric acid, such as 0.2 and 0.3 M, cannot dissolve the zeolite particles completely but are

suspensions were generated. Substantial settling occurred for the first day and continued to a lesser extent over the subsequent 2 days. **Table 2** describes the appearance of the suspensions

The Na─Y powder can also be dissolved in sulfuric acid to a certain extent. The dissolution of small Na─Y particles is limited with a low concentration of sulfuric acid (0.1 M). Some of

solutions and partly settled in 0.2 and 0.3 M H<sup>2</sup>

tetrahedron bears a net charge which is neutralized by the additional

SO4

]4− and [AlO<sup>4</sup>

which is located within the channel. Because of

**SO<sup>4</sup>**

SO4

SO4

 **modification**

solutions. Low concentrations of

solutions. However, they

solutions, cloudy white

]5− tetrahedra.

**properties of 4A and Na**─**Y synthetic zeolites**

positive charge from the non-framework Na+

zeolite 4A and Na─Y are unknown.

SO4

after 3-day settlement.

dimensions. Each AlO<sup>4</sup>

170 Zeolites and Their Applications

and 0.1 M H<sup>2</sup>

have three-dimensional structures which are composed of [SiO4

**5.1. Stability and resistance of 4A and Na**─**Y zeolites against H<sup>2</sup>**

completely dissolved to form a gel in the 0.5 and 1 M H<sup>2</sup>

strong enough to break bigger 4A particles into smaller ones.

After mixing Na─Y powder with various concentrations of H2

dissolved by sulfuric acid, after that specific component has been totally dissolved, and the residual mass did not change with increasing acid concentration. It was difficult to identify the precise nature of this dissolution by the mass loss or XRD analysis. However, it likely represented Na and Al losses based on the known structure of Na─Y. The final solid mass of the acid-treated sample increased with increasing standing time (**Figure 12B**). Therefore, it was argued that with the standing time increasing, the dissolved component partly recrystallized, leading to the mass increase. Combining with the XPS analysis shows that it is the Al content that has been reduced significantly after acid modification, as discussed in a later section. Also, the components of Na─Y which were dissolved in sulfuric acid included both Al and non-Al contents. The non-Al content was recrystallized.

incorporated in structures such as Si─O─Si and non-bridging oxygen (NBO) comprising structures such as Si─O─M (where M represents other elements such as Na, Al, etc.) and H from water [34]. There are two possible reasons for the increase in BE of Si (and O): (1) the placement of Na and Al by H and (2) partly or full destruction of the surface structure dissolved in acid

 was detected by XRD. Similar to the case of interaction with H, the BE of both Si and O can readily increase because the electronegativity of Si is higher than Al and Na. The deconvolution of XPS high-resolution spectra of O1s [33] also demonstrates that the NBO of the Na─Y surface can be lost. It supports the argument that the surface structure of Na─Y has been dissolved and re-formed into a Si─O─Si structure after being modified by 0.5 M sulfuric acid.

Analyzing the BE shifts with the Si/Al ratio determined from survey scans, we can see after sulfuric acid modification that the BE of O and Si increases with increasing Si/Al ratio, and even at a high Si/Al ratio (Si/Al = 8.18), the BE shifts are still significant [34]. However, the BE of Al is almost unchanged in comparison with that of Si and O, no matter what acid concentration has been used or how much Al has been removed from the Na─Y structure. It may indicate that the Al─O bond is too strongly ionic in order to be affected by Na removal from

**Figure 14** shows the effect of acid treatment on the change in Na─Y zeolite particle size. The particle size of the original Na─Y was distributed around two peaks, i.e., 4.5 μm (fine particle peak, volume in 4.21%) and 1124.7 μm (coarse particle peak, volume in 1.49%). After being washed with Milli-Q water, the fine particle peak did not change but its volume % reduced to 3.38%, while the coarse particle peak changed to 447.7 μm, and its volume decreased to 1.14%. The big particles of original Na─Y powder were formed by aggregation of smaller particles via the van der Waals force and electrostatic attractions. The large particle size fraction of the Milli-Q water-washed sample has a wider range of particle size distribution than the original Na─Y. However, it converges on the lower particle size region. The total number of particles increased because of the lager particle separation; thus, the relative volume % of the fine particles decreased. The newly created small particles did not affect the size distribution of

After 0.1 M acid modification, no particles larger than 1000 μm remained in solutions, and the volume % of coarse particles increased to 1.61%. The particles might be further separated, and the Na─Y structure partly dissolved in 0.1 M acid due to the dealumination process. Another type of stable silicon aluminum oxide with a particle size distributed at 399.1 μm was formed

There was no significant change of fine particle distribution after 0.5 M sulfuric acid modification. However, the peak position of coarse particle distribution has shifted to a smaller particle size region. Combined analysis of XRD and XPS revealed neither the Na─Y crystal phases nor Al was present in this sample [34]. It agrees with the experimental results showing that the structure of Na─Y was totally de-aluminized with increasing concentration of sulfuric acid to a certain level. However, the remaining Si-O structures still remained, with a size

. The presence of the recrystallized amorphous

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173

Potential Desalination of Coal Seam Gas Coproduced Water Using Zeolite

solution and recrystallized into amorphous SiO2

the structure and H replacement.

by the partly de-aluminized Na─Y.

distribution peaked at 251.8 μm.

**5.4. The effect of acid treatment on the particle size of Na**─**Y**

original fine particles due to the small proportion of larger particles.

SiO2

XRD analysis showed the crystal structure collapsed after acid modification [34]. Comparing the untreated Na─Y to that exposed to water only, the crystal phases did not change after contacting Milli-Q water. However, the XRD results for the acid treatments showed a significant energy intensity reduction of all the crystal peaks at 0.1 M, indicating most of the crystal phases were destroyed at 0.1 M. The XRD results for 0.5–5 M acid treatments showed the same line shapes which indicated that the crystal peaks were no longer present and a transformation to noncrystalline SiO2 took place. Therefore, the crystal structure was totally changed into a noncrystalline one after treating with sulfuric acid of >0.1 M.

#### **5.3. XPS analysis of the effect of acid treatment on Na**─**Y zeolite**

The results of XPS survey scans showed two different significant changes in atomic proportions of the Na─Y zeolite elements [34]: Na and Al decreased from 7.98% to 0 as the acid concentration increases from 0 to 0.5 M, but O and Si increased until the acid concentration reaches 0.5 M as well and then remained almost constant to 5 M. Sulfuric acid modifications on Na─Y zeolite could totally remove Na and Al from the surface, such that the main structural elements remaining were only Si and O. Together with the XRD results, the XPS data confirmed that Na and Al have been removed not only on the particle surface but also from the internal structure.

**Figure 13** shows the detailed results of XPS high-resolution scans for the Si2p, O1s, and Al2p regions of unmodified and modified Na─Y. The BE of Si2p initially decreased by 0.10 eV after washing by Milli-Q water and then increased after 0.1 and 0.5 M sulfuric acid modification. The BE of O 1s increased with increasing acid concentration, but no BE shift in Al2p was observed up to 0.1 M acid modification. No Al component was detected after being modified by 0.5 M and higher concentrations of acid. The change in O1s BE energy is explained by the interactions between the two known O bonds in the structure of Na─Y [35] (i.e., bridging oxygen

**Figure 13.** XPS high-resolution spectra of original and sulfuric acid-treated Na─Y for Si2p, O1s, and Al2p [34].

incorporated in structures such as Si─O─Si and non-bridging oxygen (NBO) comprising structures such as Si─O─M (where M represents other elements such as Na, Al, etc.) and H from water [34]. There are two possible reasons for the increase in BE of Si (and O): (1) the placement of Na and Al by H and (2) partly or full destruction of the surface structure dissolved in acid solution and recrystallized into amorphous SiO2 . The presence of the recrystallized amorphous SiO2 was detected by XRD. Similar to the case of interaction with H, the BE of both Si and O can readily increase because the electronegativity of Si is higher than Al and Na. The deconvolution of XPS high-resolution spectra of O1s [33] also demonstrates that the NBO of the Na─Y surface can be lost. It supports the argument that the surface structure of Na─Y has been dissolved and re-formed into a Si─O─Si structure after being modified by 0.5 M sulfuric acid.

Analyzing the BE shifts with the Si/Al ratio determined from survey scans, we can see after sulfuric acid modification that the BE of O and Si increases with increasing Si/Al ratio, and even at a high Si/Al ratio (Si/Al = 8.18), the BE shifts are still significant [34]. However, the BE of Al is almost unchanged in comparison with that of Si and O, no matter what acid concentration has been used or how much Al has been removed from the Na─Y structure. It may indicate that the Al─O bond is too strongly ionic in order to be affected by Na removal from the structure and H replacement.

#### **5.4. The effect of acid treatment on the particle size of Na**─**Y**

dissolved by sulfuric acid, after that specific component has been totally dissolved, and the residual mass did not change with increasing acid concentration. It was difficult to identify the precise nature of this dissolution by the mass loss or XRD analysis. However, it likely represented Na and Al losses based on the known structure of Na─Y. The final solid mass of the acid-treated sample increased with increasing standing time (**Figure 12B**). Therefore, it was argued that with the standing time increasing, the dissolved component partly recrystallized, leading to the mass increase. Combining with the XPS analysis shows that it is the Al content that has been reduced significantly after acid modification, as discussed in a later section. Also, the components of Na─Y which were dissolved in sulfuric acid included both Al and

XRD analysis showed the crystal structure collapsed after acid modification [34]. Comparing the untreated Na─Y to that exposed to water only, the crystal phases did not change after contacting Milli-Q water. However, the XRD results for the acid treatments showed a significant energy intensity reduction of all the crystal peaks at 0.1 M, indicating most of the crystal phases were destroyed at 0.1 M. The XRD results for 0.5–5 M acid treatments showed the same line shapes which indicated that the crystal peaks were no longer present and a transfor-

The results of XPS survey scans showed two different significant changes in atomic proportions of the Na─Y zeolite elements [34]: Na and Al decreased from 7.98% to 0 as the acid concentration increases from 0 to 0.5 M, but O and Si increased until the acid concentration reaches 0.5 M as well and then remained almost constant to 5 M. Sulfuric acid modifications on Na─Y zeolite could totally remove Na and Al from the surface, such that the main structural elements remaining were only Si and O. Together with the XRD results, the XPS data confirmed that Na and Al have been removed not only on the particle surface but also from the internal structure. **Figure 13** shows the detailed results of XPS high-resolution scans for the Si2p, O1s, and Al2p regions of unmodified and modified Na─Y. The BE of Si2p initially decreased by 0.10 eV after washing by Milli-Q water and then increased after 0.1 and 0.5 M sulfuric acid modification. The BE of O 1s increased with increasing acid concentration, but no BE shift in Al2p was observed up to 0.1 M acid modification. No Al component was detected after being modified by 0.5 M and higher concentrations of acid. The change in O1s BE energy is explained by the interactions between the two known O bonds in the structure of Na─Y [35] (i.e., bridging oxygen

**Figure 13.** XPS high-resolution spectra of original and sulfuric acid-treated Na─Y for Si2p, O1s, and Al2p [34].

took place. Therefore, the crystal structure was totally changed

non-Al contents. The non-Al content was recrystallized.

into a noncrystalline one after treating with sulfuric acid of >0.1 M.

**5.3. XPS analysis of the effect of acid treatment on Na**─**Y zeolite**

mation to noncrystalline SiO2

172 Zeolites and Their Applications

**Figure 14** shows the effect of acid treatment on the change in Na─Y zeolite particle size. The particle size of the original Na─Y was distributed around two peaks, i.e., 4.5 μm (fine particle peak, volume in 4.21%) and 1124.7 μm (coarse particle peak, volume in 1.49%). After being washed with Milli-Q water, the fine particle peak did not change but its volume % reduced to 3.38%, while the coarse particle peak changed to 447.7 μm, and its volume decreased to 1.14%. The big particles of original Na─Y powder were formed by aggregation of smaller particles via the van der Waals force and electrostatic attractions. The large particle size fraction of the Milli-Q water-washed sample has a wider range of particle size distribution than the original Na─Y. However, it converges on the lower particle size region. The total number of particles increased because of the lager particle separation; thus, the relative volume % of the fine particles decreased. The newly created small particles did not affect the size distribution of original fine particles due to the small proportion of larger particles.

After 0.1 M acid modification, no particles larger than 1000 μm remained in solutions, and the volume % of coarse particles increased to 1.61%. The particles might be further separated, and the Na─Y structure partly dissolved in 0.1 M acid due to the dealumination process. Another type of stable silicon aluminum oxide with a particle size distributed at 399.1 μm was formed by the partly de-aluminized Na─Y.

There was no significant change of fine particle distribution after 0.5 M sulfuric acid modification. However, the peak position of coarse particle distribution has shifted to a smaller particle size region. Combined analysis of XRD and XPS revealed neither the Na─Y crystal phases nor Al was present in this sample [34]. It agrees with the experimental results showing that the structure of Na─Y was totally de-aluminized with increasing concentration of sulfuric acid to a certain level. However, the remaining Si-O structures still remained, with a size distribution peaked at 251.8 μm.

have been studied in unchanged and acid-activated forms in regard to surface composition, surface binding energy, and surface charge properties. The CSG water treatment study by applying the natural and acid-activated zeolites has been completed. The possibility of reducing the sodium concentration and SAR of CSG water using natural zeolite has been

Potential Desalination of Coal Seam Gas Coproduced Water Using Zeolite

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175

Natural zeolite containing mainly clinoptilolite can be modified and activated by different concentrations of sulfuric acid. The XPS analysis of the natural zeolite and acid-modified zeolite shows significant changes in the surface properties of natural zeolite because of acid modification. Increasing acid concentration from 0 to 5 M can increase the Si/Al ratio on the zeolite surface from 2.99 up to 4.92. Cations within the zeolite atomic structure exchange with hydrogen ion, weakening the zeolite surface structure. The structural bonds become relatively covalent after acid modification and can be indicated by the BE increase of each main element. High-resolution spectral analysis also shows that the covalent nature of the remaining bonds within the structure can be increased by the ionic bond breakage inside the modified zeolite. The surface properties of clinoptilolite-type natural zeolite can be affected by sulfuric treatment which causes zeolite surface charge more pH-independent by increasing the acid concentration. Based on the results obtained from these studies, dealumination can be the main reason for the increase in surface charge of the zeolite. The hydrogen ions adsorb onto the negatively charged surface sites, reducing the surface potential and charge density. The adsorbed hydrogen ions can be very useful for neutralizing bicarbonate ions in

Examining the dissolution and recrystallization of the zeolite particles, particle size measurement, XRD, and XPS have successfully been applied for investigating the changes in structure and properties of Na-rich synthetic 4A and Na─Y zeolites by modifying with H2

at room temperature. The XRD analysis shows the acid modification can cause structural damage, where sodium cations can be removed and dealumination occurs as dissolution progress takes place, thereby the main tetrahedral structure is affected. The Si/Al atomic ratio increases from 2.94 at 0 M to 8.18 at 0.1 M, and a significant binding energy (BE) shift of Si and O can be observed even at a high Si/Al ratio. A relatively low acid concentration (lower than 0.3 M) can be used in the modification of 4A zeolite (low Si/Al ratio) because higher acid concentrations dissolve and can destroy it completely. Na─Y zeolite (high Si/

5 M. For both 4A and Na─Y zeolites, the acid modification produces dissolution (of both Si and Al) of the first-order fast kinetics and then recrystallization comes about. Gel generation after the acid dissolution can be homogenous or heterogeneous. The constant mass of solid, which left in solution after a long time formed by the recrystallization, increases with time. However, neither Al nor Na participates in the recrystallization process, only the dissolved Si gradually recrystallizes with the standing time goes on once the solubility of Na─Y achieves a threshold value. The Al and Na sites of Na─Y zeolite structure can be completely removed when the acid concentration reaches 0.5 M, which is observed as the threshold value for the Na─Y zeolite. The peak particle size of the size distribution which can characterize the change of the Na─Y crystal phase before and after acid modification is found to vary from

Al ratio) has stronger acid resistivity than 4A zeolite and can be treated with H<sup>2</sup>

SO4

SO4

up to

demonstrated.

CSG water, thereby reducing its pH.

1124.7 to 399.4 μm.

**Figure 14.** Particle size distribution of original and acid-treated Na─Y [34].

Comparing with 0.5 M acid modification, the volume % of fine particles increased greatly after treating with 5 M acid. The distribution of the coarse particles disappeared, but the fine particle size distribution remained the same. Thus, coarse particles dissolved to fine particles by 5 M acid modification, and the newly generated fine particles displayed a similar size distribution as the original fine particles. As per both XRD and XPS analyses for the 5 M acid-modified sample, there were no crystal phases, and Al was present in the sample, and the Si atomic ratio was slightly lower than for the case of the 0.5 M modification. Therefore, after the 5 M acid modification, not only was the Na─Y structure but also the remaining Si─O structure was fully destroyed by dealumination. It is the reason for the disappearance of the coarse particles. The dissolved Si and O recrystallized and formed an amorphous SiO2 with a particle size distribution peaked at 5.0 μm.

As there was a constant peak of fine particle size at 4.5–5.0 μm in each sample, this particle size did not characterize the particles of the crystal phase of original Na─Y. On the contrary, the coarse particles with changing size from 1124.7 to 399.1 μm when the acid concentration increased from 0 to 0.1 M could be considered as the characteristic peak of the silica-alumina structure in Na─Y crystal phase. Even if there was a coarse particle peak at 251.8 μm detected for the 0.5 M acid-modified sample, these particles would not be considered as representative of the Na─Y crystal phase.

### **6. Conclusions**

The major objective of this chapter is to investigate the potential use of natural and acidactivated zeolites (clinoptilolite) for CSG water treatment. Both natural and synthetic zeolites have been studied in unchanged and acid-activated forms in regard to surface composition, surface binding energy, and surface charge properties. The CSG water treatment study by applying the natural and acid-activated zeolites has been completed. The possibility of reducing the sodium concentration and SAR of CSG water using natural zeolite has been demonstrated.

Natural zeolite containing mainly clinoptilolite can be modified and activated by different concentrations of sulfuric acid. The XPS analysis of the natural zeolite and acid-modified zeolite shows significant changes in the surface properties of natural zeolite because of acid modification. Increasing acid concentration from 0 to 5 M can increase the Si/Al ratio on the zeolite surface from 2.99 up to 4.92. Cations within the zeolite atomic structure exchange with hydrogen ion, weakening the zeolite surface structure. The structural bonds become relatively covalent after acid modification and can be indicated by the BE increase of each main element. High-resolution spectral analysis also shows that the covalent nature of the remaining bonds within the structure can be increased by the ionic bond breakage inside the modified zeolite. The surface properties of clinoptilolite-type natural zeolite can be affected by sulfuric treatment which causes zeolite surface charge more pH-independent by increasing the acid concentration. Based on the results obtained from these studies, dealumination can be the main reason for the increase in surface charge of the zeolite. The hydrogen ions adsorb onto the negatively charged surface sites, reducing the surface potential and charge density. The adsorbed hydrogen ions can be very useful for neutralizing bicarbonate ions in CSG water, thereby reducing its pH.

Comparing with 0.5 M acid modification, the volume % of fine particles increased greatly after treating with 5 M acid. The distribution of the coarse particles disappeared, but the fine particle size distribution remained the same. Thus, coarse particles dissolved to fine particles by 5 M acid modification, and the newly generated fine particles displayed a similar size distribution as the original fine particles. As per both XRD and XPS analyses for the 5 M acid-modified sample, there were no crystal phases, and Al was present in the sample, and the Si atomic ratio was slightly lower than for the case of the 0.5 M modification. Therefore, after the 5 M acid modification, not only was the Na─Y structure but also the remaining Si─O structure was fully destroyed by dealumination. It is the reason for the disappearance of the

coarse particles. The dissolved Si and O recrystallized and formed an amorphous SiO2

As there was a constant peak of fine particle size at 4.5–5.0 μm in each sample, this particle size did not characterize the particles of the crystal phase of original Na─Y. On the contrary, the coarse particles with changing size from 1124.7 to 399.1 μm when the acid concentration increased from 0 to 0.1 M could be considered as the characteristic peak of the silica-alumina structure in Na─Y crystal phase. Even if there was a coarse particle peak at 251.8 μm detected for the 0.5 M acid-modified sample, these particles would not be considered as representative

The major objective of this chapter is to investigate the potential use of natural and acidactivated zeolites (clinoptilolite) for CSG water treatment. Both natural and synthetic zeolites

particle size distribution peaked at 5.0 μm.

**Figure 14.** Particle size distribution of original and acid-treated Na─Y [34].

of the Na─Y crystal phase.

**6. Conclusions**

174 Zeolites and Their Applications

with a

Examining the dissolution and recrystallization of the zeolite particles, particle size measurement, XRD, and XPS have successfully been applied for investigating the changes in structure and properties of Na-rich synthetic 4A and Na─Y zeolites by modifying with H2 SO4 at room temperature. The XRD analysis shows the acid modification can cause structural damage, where sodium cations can be removed and dealumination occurs as dissolution progress takes place, thereby the main tetrahedral structure is affected. The Si/Al atomic ratio increases from 2.94 at 0 M to 8.18 at 0.1 M, and a significant binding energy (BE) shift of Si and O can be observed even at a high Si/Al ratio. A relatively low acid concentration (lower than 0.3 M) can be used in the modification of 4A zeolite (low Si/Al ratio) because higher acid concentrations dissolve and can destroy it completely. Na─Y zeolite (high Si/ Al ratio) has stronger acid resistivity than 4A zeolite and can be treated with H<sup>2</sup> SO4 up to 5 M. For both 4A and Na─Y zeolites, the acid modification produces dissolution (of both Si and Al) of the first-order fast kinetics and then recrystallization comes about. Gel generation after the acid dissolution can be homogenous or heterogeneous. The constant mass of solid, which left in solution after a long time formed by the recrystallization, increases with time. However, neither Al nor Na participates in the recrystallization process, only the dissolved Si gradually recrystallizes with the standing time goes on once the solubility of Na─Y achieves a threshold value. The Al and Na sites of Na─Y zeolite structure can be completely removed when the acid concentration reaches 0.5 M, which is observed as the threshold value for the Na─Y zeolite. The peak particle size of the size distribution which can characterize the change of the Na─Y crystal phase before and after acid modification is found to vary from 1124.7 to 399.4 μm.

## **Acknowledgements**

The authors gratefully acknowledge the facilities and the scientific and the technical assistance of the Australian Microscopy & Microanalysis Research facility at the Center for Microscopy and Microanalysis, The University of Queensland. We thank Professor Duong D. Do, Dr. Marc A. Hampton, and Dr. Chris A. Plackowski from The University of Queensland; Prof. Kunping Wang from Chongqing University, China; and Dr. Orhan Ozdemir from Istanbul University, Turkey, for providing useful feedback on this manuscript.

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### **Conflict of interest**

There is no conflict of interest.

### **Author details**

Xiaoyu Wang\* and Anh V. Nguyen

\*Address all correspondence to: xiaoyu.wang@uq.edu.au

School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia

### **References**


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**Acknowledgements**

176 Zeolites and Their Applications

**Conflict of interest**

**Author details**

**References**

There is no conflict of interest.

Xiaoyu Wang\* and Anh V. Nguyen

\*Address all correspondence to: xiaoyu.wang@uq.edu.au

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The authors gratefully acknowledge the facilities and the scientific and the technical assistance of the Australian Microscopy & Microanalysis Research facility at the Center for Microscopy and Microanalysis, The University of Queensland. We thank Professor Duong D. Do, Dr. Marc A. Hampton, and Dr. Chris A. Plackowski from The University of Queensland; Prof. Kunping Wang from Chongqing University, China; and Dr. Orhan Ozdemir from Istanbul

School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia

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