**3.5 Electron probe microanalyzer (EPMA)**

Element distribution analysis of L1, L2, L4, and L5 by EPMA is shown in **Figure 8**. After the ion exchange, the element distribution of N decreased obviously (by comparing red parts in these pictures), and this decrease is observed in both MgAl-LDHs (a, b) and ZnAl-LDHs (c, d). Furthermore, it is found that the moles of divalent metals are at least equal to or greater than that of the trivalent metals [34, 35], which is consistent with the results of chemical analysis.

#### **3.6 Specific surface area**

*Advanced Sorption Process Applications*

*XRD patterns of (a) L1, (b) L2, and (c) L3.*

**172**

**Table 2.**

**Figure 6.**

*XRD patterns of (a) L4 and (b) L5.*

**Figure 5.**

which may be due to the hydrogen bonding on the layer. Hydrogen bonding makes soft agglomeration occur on the surface of the LDHs, and after the chelating agent, anion replaces the nitrate ions between the layers, the hydrogen bonding between the hydroxyl groups is reduced, and the aggregation is weakened to a certain extent. The inhomogeneous surface of the adsorbent indicated that a large amount of metal salt attached to the surface of the hydrotalcite in an excessive state, its unique

*The basal spacing of L1, L2, L4, and L5 calculated from XRD by using Bragg's equation.*

Basal spacing 0.91 1.42 0.89 1.47 Gallery height 0.43 0.94 0.41 0.99

**L1 L2 L4 L5**

**Figure 9** has shown the specific surface area of the product. Specific surface area of L2 and L3 are bigger than that of L1, and that of L5 is bigger than L4, which may be attributed to intercalation of EDTA or EDDS. Specific surface area of L0 is bigger than that of L1; it is due to the difference of their particle size.

#### **3.7 Adsorption of Cu(II) or Pb(II) onto L1, L2, L3, and L0**

The adsorption capacities of Cu(II) or Pb(II) onto L1, L2, L3, and L0 are compared in **Figure 10**. The adsorption efficiency of Cu2+ was larger than that of Pb2+ for the same absorbent, which could be attributed to their stability constant (EDTA-Cu, 18.7; EDDS-Cu, 18.4; EDTA-Pb, 18.0; EDDS-Pb, 12.7) [12, 26]. That is to say, it can be considered that the large adsorption capacity is obtained when the stability constant of chelate-metal is high. By comparing among adsorbents used in this work, the order of the adsorption capacity is L2 > L3 > L0 > L1. The higher adsorption efficiency of L0 than L1 may be attributable to its high specific surface area.

#### **3.8 Adsorption of Cu(II), Pb(II), and Cd(II) onto L4 and L5**

In order to confirm the effect of the intercalation with chelate agents on the adsorption capacity of metals, the adsorption experiments of some metallic elements

**Figure 8.** *Element distribution analyzed by EPMA of (a) L1, (b) L2, (c) L4, and (d) L5.*

**Figure 9.** *Specific surface area of L0, L1, L2, L3, L4, and L5 by BET method.*

**175**

**Figure 13.**

**Figure 11.**

**Figure 12.**

*Adsorption of Cd(II) onto L4 and L5.*

*Adsorption of Cu(II) onto L4 and L5.*

*Adsorption of Pb(II) onto L4 and L5.*

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

onto L1 and L2 are compared. The adsorption of Cd(II), Cu(II), and Pb(II) onto these LDHs under the optimum condition are shown in **Figures 11**–**13**, respectively. Both LDHs were found to take up Cd(II), Cu(II), and Pb(II) from aqueous solutions, and the uptake was found to increase with time. The adsorption capacity of both LDHs for Cd(II), Cu(II), and Pb(II) increased rapidly during the initial stages, and thereafter it increased gradually. It is generally found that the time needed for L5 to reach equilibrium was shorter than that for L4. From the adsorption experiment, the improvement of adsorption capacity by intercalation was observed. On the other hand, the adsorption capacity of Cu(II) and Pb(II) at equilibrium was

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

**Figure 10.** *The adsorption capacity of Pb2+ and Cu2+ onto L1, L2, L3, and L0.*

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

onto L1 and L2 are compared. The adsorption of Cd(II), Cu(II), and Pb(II) onto these LDHs under the optimum condition are shown in **Figures 11**–**13**, respectively.

Both LDHs were found to take up Cd(II), Cu(II), and Pb(II) from aqueous solutions, and the uptake was found to increase with time. The adsorption capacity of both LDHs for Cd(II), Cu(II), and Pb(II) increased rapidly during the initial stages, and thereafter it increased gradually. It is generally found that the time needed for L5 to reach equilibrium was shorter than that for L4. From the adsorption experiment, the improvement of adsorption capacity by intercalation was observed. On the other hand, the adsorption capacity of Cu(II) and Pb(II) at equilibrium was

**Figure 11.** *Adsorption of Cd(II) onto L4 and L5.*

*Advanced Sorption Process Applications*

**174**

**Figure 10.**

**Figure 9.**

**Figure 8.**

*Specific surface area of L0, L1, L2, L3, L4, and L5 by BET method.*

*Element distribution analyzed by EPMA of (a) L1, (b) L2, (c) L4, and (d) L5.*

*The adsorption capacity of Pb2+ and Cu2+ onto L1, L2, L3, and L0.*

**Figure 12.** *Adsorption of Cu(II) onto L4 and L5.*

**Figure 13.** *Adsorption of Pb(II) onto L4 and L5.*

higher than that of Cd(II). It is considered that heavy metal was removed by LDHs including two mechanisms: chemical precipitation and chelation [16]. In the first case, the hydroxyl anions compete with chelating agents for the precipitation of metal hydroxides at higher pH, and divalent ions are usually selectively dissolved. In the second case, the adsorption affinity is generally determined by the stability constant of the corresponding complex [36–38].

#### **3.9 Adsorption isotherms**

The adsorption isotherms for Cu(II) or Pb(II) were obtained under the optimum adsorption conditions (i.e., pH 6, contact time 120 minutes, temperature 25°C, and adsorbent dosage 10 mg). The adsorption isotherms of Cu(II) or Pb(II) onto L2 and L3 were analyzed using Langmuir and Freundlich equations and were shown in **Figures 14** and **15**, respectively. From **Figure 14**, the linear correlation coefficient (*R2* ) of L2 (Pb2+), L3 (Pb2+), L2 (Cu2+), and L3 (Cu2+) and other parameter for Langmuir and Freundlich isotherms model were shown in **Table 3**, respectively. That is, L2 and L3 synthesized in this work are well fitted by Freundlich adsorption isotherms models.

## **3.10 Kinetic model**

The kinetic isotherms for Cu(II) or Pb(II) were obtained under the optimum adsorption conditions (i.e., pH 6, concentration 200 ppm, temperature 25°C and

#### **Figure 14.** *The correlation of experimental data to Langmuir isotherm models.*

**177**

**Figure 16.**

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

**qmax (mg g<sup>−</sup><sup>1</sup> )**

L2(Pb2+) 0.979 5.60 × 10<sup>−</sup><sup>3</sup> 422 0.988 346 4.11 L3(Pb2+) 0.976 5.40 × 10<sup>−</sup><sup>3</sup> 330 0.983 199 4.17 L2(Cu2+) 0.994 1.90 × 10<sup>−</sup><sup>3</sup> 256 0.930 9.90 × 10<sup>−</sup><sup>3</sup> 1.87 L3(Cu2+) 0.993 2.10 × 10<sup>−</sup><sup>3</sup> 201 0.914 2.20 × 10<sup>−</sup><sup>3</sup> 1.89

**Langmuir Freundlich**

adsorbent dosage 10 mg). The parameters for two kinetic models of adsorption of Cu(II) or Pb(II) on L2 or L3 are presented in **Table 4** which showed that adsorption process followed pseudo-second-order rather than pseudo-first-order model. The second order kinetic models plot for the adsorption of Cu(II) or Pb(II) on L2 or L3 is shown in **Figure 16**. The experimentally calculated values of *q*e at various concentrations were in a good agreement with theoretical calculated values. Also, the

*Coefficient of Langmuir and Freundlich isotherms for Cu(II) and Pb(II) adsorption onto L2 or L3.*

nearly 1, indicated that pseudo-second-order kinetic model was better obeyed.

*R<sup>2</sup> q***<sup>e</sup>**

*The kinetic fit parameters for Cu(II) and Pb(II) adsorbed on L2 or L3.*

*The correlation of experimental data to pseudo-second-order models.*

**(mg g<sup>−</sup><sup>1</sup> )**

**3.11 Comparison of the adsorption capacities of the materials with other sorbents**

The comparison of maximum adsorption capacity of these LDHs for Cu(II) in a present study with that of another adsorbents in previous literatures [39] are

L2(Pb2+) 228 0.990 217 5.48 0.997 276 2.75 L3(Pb2+) 169 0.940 169 5.27 0.993 229 1.25 L2(Cu2+) 71 0.983 78.5 4.35 0.991 111 0.104 L3(Cu2+) 59 0.983 54.8 4.09 0.995 91.9 0.910

) for the pseudo-second-order kinetic model was

*R<sup>2</sup> q***<sup>e</sup>**

**(mg g<sup>−</sup><sup>1</sup> )**

*K***<sup>2</sup> (g mg<sup>−</sup><sup>1</sup>**

 **h<sup>−</sup><sup>1</sup> )**

**Pseudo-first-order Pseudo-second-order**

*K***1 (h<sup>−</sup><sup>1</sup> )** **R2 KF**

**(mg1–1/n g−<sup>1</sup>**

 **L<sup>−</sup><sup>1</sup> )** **n**

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

**R2 KL (L<sup>−</sup><sup>1</sup> mg<sup>−</sup><sup>1</sup> )**

**Sample/T (298 K)**

**Table 3.**

**Sample/T (298 K)**

**Table 4.**

values of correlation coefficients (*R*<sup>2</sup>

*q***e EXP (mg g<sup>−</sup><sup>1</sup> )**

**Figure 15.** *The correlation of experimental data to Freundlich isotherms models.*

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


**Table 3.**

*Advanced Sorption Process Applications*

**3.9 Adsorption isotherms**

**3.10 Kinetic model**

constant of the corresponding complex [36–38].

higher than that of Cd(II). It is considered that heavy metal was removed by LDHs including two mechanisms: chemical precipitation and chelation [16]. In the first case, the hydroxyl anions compete with chelating agents for the precipitation of metal hydroxides at higher pH, and divalent ions are usually selectively dissolved. In the second case, the adsorption affinity is generally determined by the stability

The adsorption isotherms for Cu(II) or Pb(II) were obtained under the optimum adsorption conditions (i.e., pH 6, contact time 120 minutes, temperature 25°C, and adsorbent dosage 10 mg). The adsorption isotherms of Cu(II) or Pb(II) onto L2 and L3 were analyzed using Langmuir and Freundlich equations and were shown in **Figures 14**

L3 (Pb2+), L2 (Cu2+), and L3 (Cu2+) and other parameter for Langmuir and Freundlich isotherms model were shown in **Table 3**, respectively. That is, L2 and L3 synthesized in

The kinetic isotherms for Cu(II) or Pb(II) were obtained under the optimum adsorption conditions (i.e., pH 6, concentration 200 ppm, temperature 25°C and

) of L2 (Pb2+),

and **15**, respectively. From **Figure 14**, the linear correlation coefficient (*R2*

this work are well fitted by Freundlich adsorption isotherms models.

**176**

**Figure 15.**

**Figure 14.**

*The correlation of experimental data to Langmuir isotherm models.*

*The correlation of experimental data to Freundlich isotherms models.*

*Coefficient of Langmuir and Freundlich isotherms for Cu(II) and Pb(II) adsorption onto L2 or L3.*

adsorbent dosage 10 mg). The parameters for two kinetic models of adsorption of Cu(II) or Pb(II) on L2 or L3 are presented in **Table 4** which showed that adsorption process followed pseudo-second-order rather than pseudo-first-order model.

The second order kinetic models plot for the adsorption of Cu(II) or Pb(II) on L2 or L3 is shown in **Figure 16**. The experimentally calculated values of *q*e at various concentrations were in a good agreement with theoretical calculated values. Also, the values of correlation coefficients (*R*<sup>2</sup> ) for the pseudo-second-order kinetic model was nearly 1, indicated that pseudo-second-order kinetic model was better obeyed.

## **3.11 Comparison of the adsorption capacities of the materials with other sorbents**


The comparison of maximum adsorption capacity of these LDHs for Cu(II) in a present study with that of another adsorbents in previous literatures [39] are

#### **Table 4.**

*The kinetic fit parameters for Cu(II) and Pb(II) adsorbed on L2 or L3.*

**Figure 16.**

*The correlation of experimental data to pseudo-second-order models.*


#### **Table 5.**

*Comparison of the adsorption capacities of LDHs in other literature Cu2+.*


#### **Table 6.**

*Comparison of the adsorption capacities of LDHs in other literature Pb2+.*

presented in **Table 5**. Moreover, **Table 6** shows the comparison of adsorption capacity of Pb(II) by other adsorbents reported in the literature. As seen in **Tables 5** and **6**, the adsorption capacity of these LDHs for Cu(II) and Pb(II) in this work is on a level with that of another adsorbents in previous works.

## **4. Conclusions**

In present study, LDHs intercalated with chelating agents have been extensively examined and applied for adsorption of aqueous containing heavy metals and REEs. The following five kinds of compounds were synthesized (MgAl-NO3 (L1), MgAl-EDTA (L2) and MgAl-EDDS (L3), ZnAl-NO3 (L4), ZnAl-EDTA (L5)). These

**179**

waters.

**Acknowledgements**

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

1.In present study, the precursor LDHs (ZnAl-NO3 and MgAl-NO3) were intercalated with the chelating agent EDTA (ethylenediaminetetraacetic acid) and EDDS (N, N′-1, 2-Ethanediylbis-1-Aspartic Acid) by anion exchange. The obtained material was characterized and used for the removal of heavy metallic ions and REEs removal from aqueous solutions. The result from FT-IR etc.

2.LDHs synthesized in this work were very effective for removing heavy metallic ions from water solutions. Higher adsorption efficiency is obtained by intercalating chelating agent (i.e., EDTA or EDDS) into LDHs. It is considered that the adsorption capacity of metallic ions onto LDHs is based on the stability constant of metal-chelating agents. For example, the adsorption efficiency of

suggests that the intercalation into LDHs is performed successfully.

Cu(III) was higher than that of Pb(III) for the same absorbent.

3.Adsorption isotherms of adsorption data were studied at varying initial

concentration of metallic ions under optimized conditions of contact time and the dosage of adsorbents in this work. The adsorption experimental data of heavy metallic ions onto LDHs were well fitted by the Freundlich adsorption isotherms model. The results suggest that LDHs synthesized in this work could be suitable as sorbent materials for the adsorption and removal of heavy metal

4.The pseudo-first-order kinetic and pseudo-second-order models were applied to test the experimental data and explain the kinetics of the LDHs adsorption process. The comparison of evaluated correlation coefficients suggested that the pseudo-second-order model is most suitable for describing the adsorption processes. The confirmation of this model implies that the rate-limiting step in this adsorption system may be controlled by chemical process. Also, the concentrations of both adsorbent and adsorbate are associated with the rate

From this work, it was quantitatively clarified that LDHs could be an efficient adsorbent for heavy metal. It is a very significant information from the viewpoint of environmental protection and can be used for treating industrial waste waters including pollutants and thus a promising option for the treatment of contaminated

The present work was partially supported by a Grant-in-Aid for Scientific

Program(C), no. 16K00599) and a fund for the promotion of Niigata University

Research from the Japan Society for the Promotion of Science (Research

five kinds of synthesized samples are characterized by some instruments and the adsorption capacities of LDHs intercalated with chelating agents for Cu(III), Pb(III), and Cd(III), and REEs ions were investigated by batch experiments. Influence of various condition including pH, adsorbents dose, concentration of metallic ions, adsorption time, and temperature on the removal of metallic ions was evaluated. The Langmuir and Freundlich models were used for the mathematical description of the adsorption isotherms. The suitability of the kinetic model for the adsorption processes is also discussed. The following matters were suggested from

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

ions from aqueous solutions.

determining step of the adsorption process.

the experimental results:

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

five kinds of synthesized samples are characterized by some instruments and the adsorption capacities of LDHs intercalated with chelating agents for Cu(III), Pb(III), and Cd(III), and REEs ions were investigated by batch experiments. Influence of various condition including pH, adsorbents dose, concentration of metallic ions, adsorption time, and temperature on the removal of metallic ions was evaluated. The Langmuir and Freundlich models were used for the mathematical description of the adsorption isotherms. The suitability of the kinetic model for the adsorption processes is also discussed. The following matters were suggested from the experimental results:


From this work, it was quantitatively clarified that LDHs could be an efficient adsorbent for heavy metal. It is a very significant information from the viewpoint of environmental protection and can be used for treating industrial waste waters including pollutants and thus a promising option for the treatment of contaminated waters.
