**2. Experimental section**

#### **2.1 Materials and reagents**

Chemical reagents including Zn(NO3)2∙6H2O, Al(NO3)3∙9H2O, Mg(NO3)2∙6H2O, Cu(NO3)2∙6H2O, Pb(NO3)2, Na2H2EDTA∙2H2O, NaOH, HNO3 and Zn(II) Mg(II), and Al(II) standard solution were purchased from Kanto Chemical Co., Inc.; Cd(II) standard solutions were prepared by diluting a standard solution (1000 mg L<sup>−</sup><sup>1</sup> ); EDDS (35%) was purchased from Sigma Co., Ltd.; and all reagents used were of analytical grade. CO2 free water (>18.2 MΩ) which was treated as an ultrapure water system (RFU 424TA, Advantech Aquarius) was employed throughout the work. The pH meter (HORIBA F-72) was used for measurement of pH while adjusting the pH by using 0.01 or 0.1 mol L<sup>−</sup><sup>1</sup> NaOH aqueous solution and 0.01 or 0.1 mol L<sup>−</sup><sup>1</sup> HNO3 aqueous solution. All synthesis should be performed under a N2 atmosphere condition to avoid carbonate contamination.

#### **2.2 Synthesis of the adsorbents**

The synthesis of LDHs intercalated with EDTA or EDDS includes two steps: (1) the preparation of the precursor LDHs (L1 or L4) and (2) the anion exchange reaction of this compound with chelating agents [14]. All the synthesis was purged with N2 to avoid CO2 uptake from atmosphere.

• Synthesis of Precursor L1 and L4

L1 was prepared by dropping addition of 100 mL aqueous solution of 0.02 mol L<sup>−</sup><sup>1</sup> Zn(NO3)2∙6H2O and 0.01 mol L<sup>−</sup><sup>1</sup> Al (NO3)3∙9H2O to 100 mL NaOH/ NaNO3 solution. Then, the solutions were agitated at 70°C for 8 h by maintaining the pH, separated by centrifugation, and washed until neutral. L4 was also synthesized by using Mg (NO3)2∙6H2O and Al (NO3)3∙9H2O as the similar method [15, 16].

• Synthesis of L2, L3, and L5

L2 was synthesized as follows. Under a N2 atmosphere, 0.015 mol of EDTA or EDDS was added to the 150 mL of suspended solution of L1. Then, the mixing solutions were agitated at 70°C for 8 h under a certain pH degree, then separated by centrifugation, washed until neutral, and then dried at 60°C overnight [12, 17]. L5 was synthesized by L4 as the similar method for L2.

#### **2.3 Characterization of these adsorbents**

Elemental chemical analyses of C, H, and N in LDHs were carried out using an elemental analyzer instrument (JMC10, J-SCIENCE LAB CO., Ltd.). After dissolving the sample by HNO3, the amount of metallic ions in LDHs was obtained by ICP-MS (Agilent HP 4500, Thermo). Infrared spectra were obtained using the KBr disc method, with wavenumbers from 400 to 4000 cm<sup>−</sup><sup>1</sup> on a FT-IR (FTIR-4200, Jasco, Japan). XRD (X-ray powder diffraction) of LDHs samples were carried out on a RINT2500HR-PC (RIGAKU Corporation) using Cu *K*α radiation in the scanning range of 2–80°. N2 adsorption and desorption isotherms were employed to determine the specific surface area by the specific surface area analyzers (AUTOSORB-1, Quabtachrome Inc., USA). The surface morphology of LDHs was surveyed using scanning electron microscopy (SEM; JSM-5800, JEOL, Japan). The element distribution and the component analysis were also analyzed by electron probe micro analyzer (EPMA; 1600, Shimadzu Corporation).

#### **2.4 Adsorption experiments**

For obtaining the optimum conditions regarding the adsorption of heavy metal, the batch experiments were studied by varying pH, contact time, adsorbent dose, and initial concentration on the adsorption of heavy metal [18–20]. The adsorption experiments of Cu(II) and Pb(II) using L2 and L3 were carried out. A certain amount of L2 or L3 was contacted with 30 mL of an aqueous solution containing known initial each metal ion (nitrate salts) ranging from 0.1 to 2 g L<sup>−</sup><sup>1</sup> . Sorption experiments were conducted in the pH range of 2–6, contact time from 30 minutes to 6 h, temperature from 25 to 40°C, and adsorbent dosage 5–40 mg. The pH of each solution was adjusted using 0.1 mol L<sup>−</sup><sup>1</sup> NH4OH and 0.1 mol L<sup>−</sup><sup>1</sup> HNO3. The adsorption capacities of Cu(II) or Pb(II) on L1, L2, and L3 were compared with that of commercial LDHs: DHT-4A ([Mg4.5Al2(OH)13CO3⋅3.5H2O], Kyowa Chemical Industry Co., Ltd), which is abbreviated as L0 below.

The adsorption experiments of Cd(II) were also carried out similar as the method below. The experiment using heavy metallic ions solution without the adsorbent was also performed to identify potential loss of heavy metallic ions during the process such as precipitation. To confirm the effect of intercalation with EDTA, the adsorption of Cu(II), Pb(II) Cd(II) onto L4 and L5 are also compared.

The suspension containing the adsorbent and each of the above metallic solution was filtered through a 0.10 μm membrane filter (Mixed Cellulose Ester 47 mm, Advantec MFS, Inc.) to remove each metallic ion that have been adsorbed into the adsorbent. Then, the concentration of Cu(II) or Pb(II) in the filtrate was determined with an atomic absorption spectrophotometer (AAS), and the concentration of Cd(II) in the filtrate was determined by inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) (SPS 1500, Seiko Instrument Inc).

#### **2.5 Data analysis**

For data analysis, various equilibrium, kinetic, and thermodynamic models (equations) were employed to interpret the data and establish the extent of adsorption. The metallic ions uptake by each adsorbent was calculated using the Eq. (1):

$$\mathbf{Q} = \frac{(\mathbf{C\_0 - C\_e})}{W} \cdot \mathbf{V} \quad \left[\mathfrak{lag} \cdot \mathbf{g}^{-1}\right] \tag{1}$$

**169**

[15, 16, 25].

*Adsorption of Heavy Metals on Layered Double Hydroxides (LDHs) Intercalated with Chelating…*

tial and equilibrium concentrations of metallic ions in a batch system, respectively

In adsorption processes, it is necessary and critical for the equilibrium isotherm studies to predict the behavior of pollutant adsorption onto the sorbent surfaces. Two common adsorption models, Langmuir and Freundlich isotherm models, were

The Langmuir adsorption model is based on the assumption that maximum adsorption corresponds to saturated monolayer of solute molecules on the adsor-

where *C*e is the concentration of metallic ions in batch system at equilibrium

lg*q*<sup>e</sup> = lg*K*<sup>F</sup> + (1/*n*) lg*C*<sup>e</sup> (3)

where *K*F and *1/n* indicate the adsorption capacity and the adsorption intensity of the system, respectively. The plots of *q*e versus *C*e in log scale can be plotted to determine values of 1*/n* and *K*F depicting the constants of Freundlich model. The greater the value of the *n*, the more favorable is the adsorption [15, 16, 23].

The kinetic data can be used to determine the time required for adsorption equilibrium and provide useful data to improve the efficiency of the adsorption model and develop predictive models [24, 25]. In this work, pseudo-first-order and pseudo-second-order models were applied for modeling the adsorption process. The

ln(*qe* − *qt*) = ln*qe* − *k*1*t* (4)

The linear form of the pseudo-second-order rate equation is given as follows:

*qt* <sup>=</sup> \_\_\_\_ <sup>1</sup> *k*2*qe* <sup>2</sup> <sup>+</sup> \_\_*<sup>t</sup>*

equilibrium, respectively, and *k*1 is the rate constant of the pseudo-first-order

) are the metal amount adsorbed at time at *t* (h) and

) is the pseudo-second-order rate constant of the adsorption

*qe* (5)

*q*max is the maximum adsorption capacity on the surface of adsorbent (mg g<sup>−</sup><sup>1</sup>

straight line (*Y = A + BX*) with slope of 1/*q*max, and the intercept is 1/(*K*<sup>L</sup> *q*max); The linearized Freundlich model isotherm is represented by the following

), *q*e is the amount of adsorption of metallic ions at equilibrium (mg g<sup>−</sup><sup>1</sup>

*<sup>q</sup>*max <sup>+</sup> \_\_\_\_\_\_ <sup>1</sup> *KL q*max

), *V* is the volume of the solution (L), and *W* is the weight of each adsorbent

), *Co* and *Ce* are the ini-

(2)

),

), and

). A plot of *C*e*/q*e versus *C*e gives a

where *Q* is the adsorption capacities at equilibrium (μg g<sup>−</sup><sup>1</sup>

applied to evaluate the adsorption data obtained in this study.

*Ce qe* <sup>=</sup> \_\_\_\_ *Ce*

bent surface [21, 22]. Langmuir model is given by Eq. (2):

*K*L is the equilibrium adsorption constant (L mg<sup>−</sup><sup>1</sup>

pseudo-first-order model is expressed as the Eq. (4):

\_\_\_

*DOI: http://dx.doi.org/10.5772/intechopen.80865*

(mg L<sup>−</sup><sup>1</sup>

(mg L<sup>−</sup><sup>1</sup>

equation:

**2.7 Kinetic model**

where *q*t and *q*e (μg g<sup>−</sup><sup>1</sup>

).

\_\_*<sup>t</sup>*

h<sup>−</sup><sup>1</sup>

adsorption (h<sup>−</sup><sup>1</sup>

where *k*2 (g μg<sup>−</sup><sup>1</sup>

(g) [15, 16].

**2.6 Adsorption isotherms**

*Adsorption of Heavy Metals on Layered Double Hydroxides (LDHs) Intercalated with Chelating… DOI: http://dx.doi.org/10.5772/intechopen.80865*

where *Q* is the adsorption capacities at equilibrium (μg g<sup>−</sup><sup>1</sup> ), *Co* and *Ce* are the initial and equilibrium concentrations of metallic ions in a batch system, respectively (mg L<sup>−</sup><sup>1</sup> ), *V* is the volume of the solution (L), and *W* is the weight of each adsorbent (g) [15, 16].

#### **2.6 Adsorption isotherms**

*Advanced Sorption Process Applications*

was synthesized by L4 as the similar method for L2.

disc method, with wavenumbers from 400 to 4000 cm<sup>−</sup><sup>1</sup>

analyzer (EPMA; 1600, Shimadzu Corporation).

each solution was adjusted using 0.1 mol L<sup>−</sup><sup>1</sup>

Industry Co., Ltd), which is abbreviated as L0 below.

*<sup>Q</sup>* <sup>=</sup> (*C*<sup>0</sup> <sup>−</sup> *Ce*) \_\_\_\_\_\_\_ *<sup>W</sup>* <sup>∙</sup> *<sup>V</sup>* [μg <sup>∙</sup> <sup>g</sup>−1

**2.4 Adsorption experiments**

**2.3 Characterization of these adsorbents**

centrifugation, washed until neutral, and then dried at 60°C overnight [12, 17]. L5

Elemental chemical analyses of C, H, and N in LDHs were carried out using an elemental analyzer instrument (JMC10, J-SCIENCE LAB CO., Ltd.). After dissolving the sample by HNO3, the amount of metallic ions in LDHs was obtained by ICP-MS (Agilent HP 4500, Thermo). Infrared spectra were obtained using the KBr

Jasco, Japan). XRD (X-ray powder diffraction) of LDHs samples were carried out on a RINT2500HR-PC (RIGAKU Corporation) using Cu *K*α radiation in the scanning range of 2–80°. N2 adsorption and desorption isotherms were employed to determine the specific surface area by the specific surface area analyzers (AUTOSORB-1, Quabtachrome Inc., USA). The surface morphology of LDHs was surveyed using scanning electron microscopy (SEM; JSM-5800, JEOL, Japan). The element distribution and the component analysis were also analyzed by electron probe micro

For obtaining the optimum conditions regarding the adsorption of heavy metal, the batch experiments were studied by varying pH, contact time, adsorbent dose, and initial concentration on the adsorption of heavy metal [18–20]. The adsorption experiments of Cu(II) and Pb(II) using L2 and L3 were carried out. A certain amount of L2 or L3 was contacted with 30 mL of an aqueous solution containing

experiments were conducted in the pH range of 2–6, contact time from 30 minutes to 6 h, temperature from 25 to 40°C, and adsorbent dosage 5–40 mg. The pH of

adsorption capacities of Cu(II) or Pb(II) on L1, L2, and L3 were compared with that of commercial LDHs: DHT-4A ([Mg4.5Al2(OH)13CO3⋅3.5H2O], Kyowa Chemical

The adsorption experiments of Cd(II) were also carried out similar as the method below. The experiment using heavy metallic ions solution without the adsorbent was also performed to identify potential loss of heavy metallic ions during the process such as precipitation. To confirm the effect of intercalation with EDTA, the adsorption of Cu(II), Pb(II) Cd(II) onto L4 and L5 are also compared. The suspension containing the adsorbent and each of the above metallic solution was filtered through a 0.10 μm membrane filter (Mixed Cellulose Ester 47 mm, Advantec MFS, Inc.) to remove each metallic ion that have been adsorbed into the adsorbent. Then, the concentration of Cu(II) or Pb(II) in the filtrate was determined with an atomic absorption spectrophotometer (AAS), and the concentration of Cd(II) in the filtrate was determined by inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) (SPS 1500, Seiko Instrument Inc).

For data analysis, various equilibrium, kinetic, and thermodynamic models (equations) were employed to interpret the data and establish the extent of adsorption. The metallic ions uptake by each adsorbent was calculated using the Eq. (1):

NH4OH and 0.1 mol L<sup>−</sup><sup>1</sup>

known initial each metal ion (nitrate salts) ranging from 0.1 to 2 g L<sup>−</sup><sup>1</sup>

on a FT-IR (FTIR-4200,

. Sorption

HNO3. The

] (1)

**168**

**2.5 Data analysis**

In adsorption processes, it is necessary and critical for the equilibrium isotherm studies to predict the behavior of pollutant adsorption onto the sorbent surfaces. Two common adsorption models, Langmuir and Freundlich isotherm models, were applied to evaluate the adsorption data obtained in this study.

The Langmuir adsorption model is based on the assumption that maximum adsorption corresponds to saturated monolayer of solute molecules on the adsorbent surface [21, 22]. Langmuir model is given by Eq. (2):

$$\frac{C\_e}{q\_e} = \frac{C\_e}{q\_{\text{max}}} + \frac{1}{K\_L q\_{\text{max}}} \tag{2}$$

where *C*e is the concentration of metallic ions in batch system at equilibrium (mg L<sup>−</sup><sup>1</sup> ), *q*e is the amount of adsorption of metallic ions at equilibrium (mg g<sup>−</sup><sup>1</sup> ), *q*max is the maximum adsorption capacity on the surface of adsorbent (mg g<sup>−</sup><sup>1</sup> ), and *K*L is the equilibrium adsorption constant (L mg<sup>−</sup><sup>1</sup> ). A plot of *C*e*/q*e versus *C*e gives a straight line (*Y = A + BX*) with slope of 1/*q*max, and the intercept is 1/(*K*<sup>L</sup> *q*max);

The linearized Freundlich model isotherm is represented by the following equation:

$$\lg q\_{\rm e} = \lg K\_{\rm F} + (1/n) \lg C\_{\rm e} \tag{3}$$

where *K*F and *1/n* indicate the adsorption capacity and the adsorption intensity of the system, respectively. The plots of *q*e versus *C*e in log scale can be plotted to determine values of 1*/n* and *K*F depicting the constants of Freundlich model. The greater the value of the *n*, the more favorable is the adsorption [15, 16, 23].

#### **2.7 Kinetic model**

The kinetic data can be used to determine the time required for adsorption equilibrium and provide useful data to improve the efficiency of the adsorption model and develop predictive models [24, 25]. In this work, pseudo-first-order and pseudo-second-order models were applied for modeling the adsorption process. The pseudo-first-order model is expressed as the Eq. (4):

$$
\ln \left( q\_{\varepsilon} - q\_{t} \right) \quad = \begin{array}{c} \ln q\_{\varepsilon} - k \not\downarrow \end{array} \tag{4}
$$

where *q*t and *q*e (μg g<sup>−</sup><sup>1</sup> ) are the metal amount adsorbed at time at *t* (h) and equilibrium, respectively, and *k*1 is the rate constant of the pseudo-first-order adsorption (h<sup>−</sup><sup>1</sup> ).

The linear form of the pseudo-second-order rate equation is given as follows:

$$\frac{t}{q\_1} = \frac{1}{k\_2 q\_e^{-2}} + \frac{t}{q\_e} \tag{5}$$

where *k*2 (g μg<sup>−</sup><sup>1</sup> h<sup>−</sup><sup>1</sup> ) is the pseudo-second-order rate constant of the adsorption [15, 16, 25].
