**2.3 Adsorption**

Adsorption is a process in which solids are used for removing organic and inorganic substances from either gaseous or liquid solutions. The phenomena

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*A Brief Comparative Study on Removal of Toxic Dyes by Different Types of Clay*

**Removal process Advantages Disadvantages**

Photo-catalyst Low cost operational and

as energy

of azo dyes

Adsorption High adsorption capacity for all dyes.

economically feasible

By-products can be used

Operational cost is low and effective in removal

Ion exchange No loss of sorbents For disperse dyes not effective

Ozonation No sludge generation Operational cost is very high, half life

Some photo catalyst degrades into

Resources under aerobic conditions require more treatment and yield of methane and hydrogen sulphide

Provide suitable environment for growth of microorganisms and very

Low surface area and high cost of

Production of sludge and suitable for

toxic by-products.

is short (20 min)

slow process

some adsorbents.

treating low volume

driven adsorption are operative in most natural physical, biological, and chemical systems. The solid adsorbents widely used in the industries for the removal of these pollutants from industrial wastewater are diverse such as activated carbon, metal

Effective for dyes with high quality effluents

The adsorption process involves the separation of a substance from one phase by retaining it on the surface of another. The physical adsorption is mainly due to weak interactions such as van der Waals bonds and the electrostatic forces created between the adsorbate and the atoms which make up the surface of the adsorbent. The capacity of this process depends some parameters namely, adsorbent properties, adsorbate chemical properties, temperature, and pH of the medium. It should be noted that even if the adsorbents are available, they are still expensive and few of them are selective. Therefore, over the last decades the research has been redirected towards the search for other improved materials which will meet certain requirements such as regenerative capacity, easy availability, and cost effectiveness. Consequently, clays adsorbents have drawn attention to many researchers and characteristics as well as application of many such adsorbents are reported [19, 20]. However, clays adsorbents are discussed herein after. A summary of advantages and

Layered double hydroxides (LDHs) are intensively studied because of their high anionic exchange capacity [23], reuse, larger surface area, porosity, and fundamental properties [24]. They have advantages over commercially available adsorbents in terms of low cost, high adsorption properties, and non-toxicity. The use of LDH

disadvantages of some separation methods are presented in **Table 1**.

**3. Clay materials used as adsorbents for dye treatment**

**3.1 Layered double hydroxides (LDHs)**

*DOI: http://dx.doi.org/10.5772/intechopen.95755*

Anaerobic degradation

Aerobic degradation

Membrane filtration

*Separation techniques and their advantages and disadvantages [21, 22].*

*Chemical methods*

*Biological methods*

*Physico-chemical methods*

**Table 1.**

hydrides and synthetic resins.

*A Brief Comparative Study on Removal of Toxic Dyes by Different Types of Clay DOI: http://dx.doi.org/10.5772/intechopen.95755*


**Table 1.**

*Dyes and Pigments - Novel Applications and Waste Treatment*

capacities of the these clays.

**2.1 Membrane filtration**

**2. Removal methods for toxic dyes**

biological technologies in the use for the treatment of these polluted effluents namely, ion exchange, membrane separation, biological treatment and adsorption [5–10]. Physico-chemical processes such as ion exchange, electro dialysis and reverse osmosis are expensive, difficult to operate and require significant technologies. While in recent years, adsorption has continued to attract the attention of the researchers worldwide [11, 12] and appears to be an alternative, which has some advantages such as simple design, and ease of operation. Whereas, the biological treatment which is based on the microbial digestive metabolism, has the major drawback of the risk of microbiological contamination and a significant production of sludge, which poses problems of storage and handling [13, 14]. In the present work a comparative study between the capacities of raw, synthetic and modified clays for the removal of toxic dyes from aqueous solution has been given, with particular review of the main factors influencing the adsorption of dyes by clays such as pH of the solution, temperature and initial dye concentration on the adsorption

Currently, a number of different technologies and methods such as membrane separation, ion exchange, adsorption and biological methods are widely used for the removal of toxic anionic and cationic dyes from polluted water and wastewaters.

Membrane separation is a pressure driven process. Pressure-focused processes are generally divided into four overlapping classes of increased selectivity: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and hyperfiltration or reverse osmosis (RO). Microfiltration can be used to remove bacteria and suspended solids with pore sizes from 0.1 to microns. Whereas, Ultrafiltration eliminates colloids, viruses, and some proteins by pores from 0.0003 to 0.1 microns. Nanofiltration is based on physical rejection based on molecular size and charge. The pore sizes are between 0.001 and 0.003 microns [15]. Reverse osmosis has a pore size approximately 0.0005 microns and can be used for desalination. High pressures are needed to makepass water through the membrane from a concentrated solution to dilute. Shih [16] has studied the elimination of dyes on membrane and explored the parameters that could influence the efficiency of toxic dyes removal by membrane technologies such as parameters source, membrane type and membrane

Ion exchange has been widely used to remove dyes due to its many advantages, such as high processing capacity, speed and increasing the efficiency of dye retention [17]. Ion exchange resin, either natural or resin solid synthetic, has the specific ability to exchange its cations with dyes in wastewater. Among materials most used in the ion exchange process, synthetic resins: are commonly preferred because they

Adsorption is a process in which solids are used for removing organic and inorganic substances from either gaseous or liquid solutions. The phenomena

are effective in virtually removing dyes in solution [18].

**154**

process.

**2.2 Ion exchange**

**2.3 Adsorption**

*Separation techniques and their advantages and disadvantages [21, 22].*

driven adsorption are operative in most natural physical, biological, and chemical systems. The solid adsorbents widely used in the industries for the removal of these pollutants from industrial wastewater are diverse such as activated carbon, metal hydrides and synthetic resins.

The adsorption process involves the separation of a substance from one phase by retaining it on the surface of another. The physical adsorption is mainly due to weak interactions such as van der Waals bonds and the electrostatic forces created between the adsorbate and the atoms which make up the surface of the adsorbent. The capacity of this process depends some parameters namely, adsorbent properties, adsorbate chemical properties, temperature, and pH of the medium. It should be noted that even if the adsorbents are available, they are still expensive and few of them are selective. Therefore, over the last decades the research has been redirected towards the search for other improved materials which will meet certain requirements such as regenerative capacity, easy availability, and cost effectiveness. Consequently, clays adsorbents have drawn attention to many researchers and characteristics as well as application of many such adsorbents are reported [19, 20]. However, clays adsorbents are discussed herein after. A summary of advantages and disadvantages of some separation methods are presented in **Table 1**.

#### **3. Clay materials used as adsorbents for dye treatment**

#### **3.1 Layered double hydroxides (LDHs)**

Layered double hydroxides (LDHs) are intensively studied because of their high anionic exchange capacity [23], reuse, larger surface area, porosity, and fundamental properties [24]. They have advantages over commercially available adsorbents in terms of low cost, high adsorption properties, and non-toxicity. The use of LDH

**Figure 1.** *A schematic representation of the LDH structure [28, 29].*

could bring significant economic and environmental benefits to the wastewater treatment industries. Previous works [25] have proven their usefulness as adsorbents for the removal of some organic and inorganic pollutants from polluted water and wastewater. Among the different types of LDH material, a species similar to hydrotalcite, a compound consisting of a double compound of MgAl-LDH hydroxides with carbonate as interlayer anions, is commonly used for various applications [26, 27]. A schematic representation of the general structure of the LDH structure is given in **Figure 1**.

The effectiveness of these compounds in the treatment of polluted water and in particular organic pollutants such as textile dyes has already been demonstrated by various research teams [30–33]. Studies that have been done by Elmoubarki *et al*. [34] demonstrate the effectiveness of Layered double hydroxides based on Mg and Fe for the removal of methyl orange. Shan et al. [35] has studied the trapping of Congo red on carbonated HDL Mg (II) /Al (III), this study showed that the materials are more efficient, and have a very high yield of Congo red elimination. **Table 2** shows some adsorbents of the lamellaires types, used for the removal of some toxic dyes from aqueous solutions.

#### **3.2 Kaolinite**

The kaolinite group is comprised of trioctahedral minerals such as, chrysotile, antigorite, cronstedite, and chamosite, dioctahedral minerals such as kaolinite, halloysite, dickite and nacrite. It is white and soft clay, composed primarily of the mineral kaolinite, a hydrated aluminum silicate. Commonly, the kaolinite structure group is known to be composed of silicate sheets (Si2O5) linked to aluminum oxide/ hydroxide layers (Al2(OH)4) called gibbsitelayers [51]. Additionally, the primary structural unit of this group is a layer composed of one octahedral sheet condensed with one tetrahedral sheet. About the dioctahedral minerals, the octahedral sites are occupied by aluminum, while those of trioctahedral minerals are occupied by magnesium and iron. Kaolinite and halloysite are single layer structures. Furthermore, kaolinite, nacrite and dickite occur as plates; halloysite, which can have a single

**157**

environmentally [54, 55].

*Adsorbents used for the removal of dyes.*

**3.3 Bentonite**

**Table 2.**

*A Brief Comparative Study on Removal of Toxic Dyes by Different Types of Clay*

**Adsorbent Dye Reference** NiFe-LDH Methyl orange [36] ZnMgAl–CO3 Methyl orange [37] MgNiAl Methyl orange [38] MgAl- LDH Methyl orange [39] rGO/Ni/MMO Methyl orange [40] ZnO/CuO/ɣ-Al2O3 Methyl orange [41] Fe3O4/ZnCr-LDH Methyl orange [42] NiAl-LDH Congo red [43] Mg–Al–LDH Congo red [44] CaAl-NO3 Congo red [45] Ni/Fe-CO3 Congo red [46] Mg/FeCO3 Congo red [47] Mg - Al - Cl Congo red [48] Zn-Fe–LDH Methylene blue [49] Mn-Fe–LDH Methylene blue [49] Mg-Fe–LDH Methylene blue [49] GOaerogels/MgAl Methylene blue [49] Zn- Fe -CO3 Indigo Carmine [50] Zn-Cr -CO3 Indigo Carmine [50] Zn-Mn -CO3 Indigo Carmine [50] Zn-Al -CO3 Indigo Carmine [50]

layer of water between its sheets, occurs in a tubular form. It consists of feldspar and muscovite formed by the alteration of [51, 52], and is a layered silicate mineral composed of a tetrahedral sheet, bonded through oxygen atoms to an octahedral sheet of alumina, which are layered silicate minerals composed of one tetrahedral sheet, linked through the oxygen atoms to one octahedral sheet of alumina octahedra. Nacrite, kaolinite and dickite exist as plates, halloysite occurs in a tubular form, have a single layer of water between its sheets. Rocks having large amount of kaolinite are referred to as kaolin or china clay [53]. Kaolinite contains heterogeneous surface charge is a well-known fact. It is believed that its basal surface has a constant structural charge which is attributed to isomorphs substitutions of Si4+ by Al3+. The charge on the edges is due to protonation or deprotonation of surface hydroxyl groups and so it depends on pH of solution. Adsorption can occur on flat exposed planes of silica and alumina sheets. It is least reactive clay. Kaolin has no side effects, no health problems till the fine dust particle is controlled, thus it is safe

The most common group of clay used in water treatment is bentonite. It is a lowcost, effective and eco-friendly adsorbent, and it is commonly impure clay consisting mostly of montmorillonite, although some may consist of the rare clay minerals

*DOI: http://dx.doi.org/10.5772/intechopen.95755*


*A Brief Comparative Study on Removal of Toxic Dyes by Different Types of Clay DOI: http://dx.doi.org/10.5772/intechopen.95755*

#### **Table 2.**

*Dyes and Pigments - Novel Applications and Waste Treatment*

*A schematic representation of the LDH structure [28, 29].*

could bring significant economic and environmental benefits to the wastewater treatment industries. Previous works [25] have proven their usefulness as adsorbents for the removal of some organic and inorganic pollutants from polluted water and wastewater. Among the different types of LDH material, a species similar to hydrotalcite, a compound consisting of a double compound of MgAl-LDH hydroxides with carbonate as interlayer anions, is commonly used for various applications [26, 27]. A schematic representation of the general structure of the LDH structure

The effectiveness of these compounds in the treatment of polluted water and in particular organic pollutants such as textile dyes has already been demonstrated by various research teams [30–33]. Studies that have been done by Elmoubarki *et al*. [34] demonstrate the effectiveness of Layered double hydroxides based on Mg and Fe for the removal of methyl orange. Shan et al. [35] has studied the trapping of Congo red on carbonated HDL Mg (II) /Al (III), this study showed that the materials are more efficient, and have a very high yield of Congo red elimination. **Table 2** shows some adsorbents of the lamellaires types, used for the removal of some toxic

The kaolinite group is comprised of trioctahedral minerals such as, chrysotile, antigorite, cronstedite, and chamosite, dioctahedral minerals such as kaolinite, halloysite, dickite and nacrite. It is white and soft clay, composed primarily of the mineral kaolinite, a hydrated aluminum silicate. Commonly, the kaolinite structure group is known to be composed of silicate sheets (Si2O5) linked to aluminum oxide/ hydroxide layers (Al2(OH)4) called gibbsitelayers [51]. Additionally, the primary structural unit of this group is a layer composed of one octahedral sheet condensed with one tetrahedral sheet. About the dioctahedral minerals, the octahedral sites are occupied by aluminum, while those of trioctahedral minerals are occupied by magnesium and iron. Kaolinite and halloysite are single layer structures. Furthermore, kaolinite, nacrite and dickite occur as plates; halloysite, which can have a single

**156**

is given in **Figure 1**.

**Figure 1.**

dyes from aqueous solutions.

**3.2 Kaolinite**

*Adsorbents used for the removal of dyes.*

layer of water between its sheets, occurs in a tubular form. It consists of feldspar and muscovite formed by the alteration of [51, 52], and is a layered silicate mineral composed of a tetrahedral sheet, bonded through oxygen atoms to an octahedral sheet of alumina, which are layered silicate minerals composed of one tetrahedral sheet, linked through the oxygen atoms to one octahedral sheet of alumina octahedra. Nacrite, kaolinite and dickite exist as plates, halloysite occurs in a tubular form, have a single layer of water between its sheets. Rocks having large amount of kaolinite are referred to as kaolin or china clay [53]. Kaolinite contains heterogeneous surface charge is a well-known fact. It is believed that its basal surface has a constant structural charge which is attributed to isomorphs substitutions of Si4+ by Al3+. The charge on the edges is due to protonation or deprotonation of surface hydroxyl groups and so it depends on pH of solution. Adsorption can occur on flat exposed planes of silica and alumina sheets. It is least reactive clay. Kaolin has no side effects, no health problems till the fine dust particle is controlled, thus it is safe environmentally [54, 55].

#### **3.3 Bentonite**

The most common group of clay used in water treatment is bentonite. It is a lowcost, effective and eco-friendly adsorbent, and it is commonly impure clay consisting mostly of montmorillonite, although some may consist of the rare clay minerals such as, nontronite saponite, and beidellite. Montmorillonite structure is a layer of gypsum site sandwiched between two sheets of silica to form the structural unit [56, 57]. The substitutes are found mainly in the octahedral layer (Mg2+, Fe2+) and to a lesser extent in the silicate layer. The clay mineral group is mainly composed of a hydroxyl-aluminosilicate framework. As well as, the crystal structures of the clay minerals are composed of a combination of silica tetrahedral sheets and alumino octahedral. Apart of the trivalent Al3+ is substituted by Mg2+ or Fe2+ ions in some cases. In such cases, substitution is accompanied by the addition of alkaline metals like Na<sup>+</sup> and K+ or alkaline earth metals like Mg2+ and Ca2+ to provide charge balance [57]. Stockmeyer et al. (1991) [58] have investigated the adsorption of some organic compounds from aqueous solutions by using organophilic bentonites. Phenol, diethyl ketone, nitroethane, aniline ethoxy acetic acid, maleic acid and hexadecyl pyridinium bromide were investigated as test organic compounds. The used organophilic bentonites vary in the degree of their total cation exchange capacity exchanged by organic counter ions [58].
