**1. Introduction**

Recently, environmental pollution is one of the most serious problems in the world due to its deep effect on the future of human beings. Then the investigation for resolving the problem of the environmental pollution in the world began to draw major public attention [1, 2]. It is well known that the major forms of environmental pollution include air pollution, water pollution, soil pollution, and so on. Among them, water pollution is the most serious due to its liquidity which may bring other pollutions. Surface water pollution and groundwater contamination are some of the environmental problems today. One of the cases of environmental pollution is due to heavy metal contaminants such as copper, lead, cadmium, chromium, arsenic, zinc, etc. Also, heavy metals are concerned because of their strong toxicity even at

low concentrations. Based on the type of mining, the kinds and the concentrations of metal ions are many and varied. Heavy metal ions have high toxicity and poor biodegradability for plants and animals at higher concentrations [3, 4].

In recent years, clay minerals have been aroused increasing interest as adsorbents by virtue of their properties, which make them attractive materials for adsorbing heavy metal ions. Their abundance in nature, low cost, and good cation adsorptive properties, a result of their negatively charged layers and high specific surface areas, make them suitable for adsorption of metal ions [5, 6]. LDH used in this paper are the antitypes of clay minerals. Layered double hydroxides (LDHs) are lamellar ionic compounds containing a positively charged layer and exchangeable anions in the interlayer. They consist of brucite-like layers and are represented by the general formula [*M*1−*<sup>x</sup> II Mx III* (*OH*)2] (*A<sup>n</sup>*<sup>−</sup> )*x*/*<sup>n</sup>* <sup>∙</sup> *mH*2*<sup>O</sup>*, where cationic *MII* and *MIII* are divalent and trivalent metals and occupy the octahedral holes in the brucite-like layer. *An*<sup>−</sup> is the interlayer exchangeable anions, which is located in the hydrate layered galleries, and *x* is the layer charge density x <sup>=</sup> [*MII*]/([*MIII*] <sup>+</sup> [*MIII*]) [7–9]. The layered structure of LDHs is shown in **Figure 1**.

The ethylenediaminetetraacetic acid (EDTA) is a chelating agent widely used in industry and agriculture. It forms strong complexes with the ratio 1:1 between heavy metal ions and ligand. The EDDS (N, N′-1, 2-Ethanediylbis-1-Aspartic Acid) is also a chelating agent, which may offer a biodegradable alternative to EDTA and is currently used on a large scale in numerous applications [10, 11]. The structure of this two chelating agents were shown in **Figure 2**.

Considering the structure of LDHs, it is suggested that these compounds can be intercalated with different polydentate ligands. Recently, the study using LDHs modified with chelating agents as the potential adsorbents of heavy metals from aqueous solution has been reported [12, 13]. The aim of this work is at first to synthesize and to characterize LDHs intercalated with EDTA or EDDS and to study the uptake of heavy metals (Cu2+, Pb2+, Cd2+) by these hybrid compounds. The following five kinds of compounds synthesized in this work are ZnAl-NO3 (L1), ZnAl-EDTA (L2), MgAl-NO3 (L3), MgAl-EDTA (L4), and MgAl-EDDS (L5). To confirm the effect of intercalation with EDTA, the adsorption of metallic ions onto L1 and L2 is also compared. This study investigated the adsorption ability of LDHS as adsorbent for Pb, Cu, and Cd from aqueous solution. Finally, the further

**167**

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

developments of LDHs as useful adsorbent with the future of application in the

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>

HNO3 aqueous solution. All synthesis should be performed under a N2

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

L1 was prepared by dropping addition of 100 mL aqueous solution of

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].

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

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

);

NaOH aqueous solution and 0.01 or

Al (NO3)3∙9H2O to 100 mL NaOH/

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

environmental chemistry are mentioned.

adjusting the pH by using 0.01 or 0.1 mol L<sup>−</sup><sup>1</sup>

N2 to avoid CO2 uptake from atmosphere.

• Synthesis of Precursor L1 and L4

• Synthesis of L2, L3, and L5

**2.2 Synthesis of the adsorbents**

atmosphere condition to avoid carbonate contamination.

Zn(NO3)2∙6H2O and 0.01 mol L<sup>−</sup><sup>1</sup>

**2. Experimental section**

*The structure of EDTA and EDDS.*

**2.1 Materials and reagents**

0.1 mol L<sup>−</sup><sup>1</sup>

**Figure 2.**

0.02 mol L<sup>−</sup><sup>1</sup>

**Figure 1.** *The layered structure of LDHs.*

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

**Figure 2.** *The structure of EDTA and EDDS.*

*Advanced Sorption Process Applications*

the general formula [*M*1−*<sup>x</sup>*

*II Mx*

this two chelating agents were shown in **Figure 2**.

structure of LDHs is shown in **Figure 1**.

*III* (*OH*)2] (*A<sup>n</sup>*<sup>−</sup>

low concentrations. Based on the type of mining, the kinds and the concentrations of metal ions are many and varied. Heavy metal ions have high toxicity and poor

In recent years, clay minerals have been aroused increasing interest as adsorbents by virtue of their properties, which make them attractive materials for adsorbing heavy metal ions. Their abundance in nature, low cost, and good cation adsorptive properties, a result of their negatively charged layers and high specific surface areas, make them suitable for adsorption of metal ions [5, 6]. LDH used in this paper are the antitypes of clay minerals. Layered double hydroxides (LDHs) are lamellar ionic compounds containing a positively charged layer and exchangeable anions in the interlayer. They consist of brucite-like layers and are represented by

lent and trivalent metals and occupy the octahedral holes in the brucite-like layer. *An*<sup>−</sup> is the interlayer exchangeable anions, which is located in the hydrate layered galleries, and *x* is the layer charge density x <sup>=</sup> [*MII*]/([*MIII*] <sup>+</sup> [*MIII*]) [7–9]. The layered

The ethylenediaminetetraacetic acid (EDTA) is a chelating agent widely used in industry and agriculture. It forms strong complexes with the ratio 1:1 between heavy metal ions and ligand. The EDDS (N, N′-1, 2-Ethanediylbis-1-Aspartic Acid) is also a chelating agent, which may offer a biodegradable alternative to EDTA and is currently used on a large scale in numerous applications [10, 11]. The structure of

Considering the structure of LDHs, it is suggested that these compounds can be intercalated with different polydentate ligands. Recently, the study using LDHs modified with chelating agents as the potential adsorbents of heavy metals from aqueous solution has been reported [12, 13]. The aim of this work is at first to synthesize and to characterize LDHs intercalated with EDTA or EDDS and to study the uptake of heavy metals (Cu2+, Pb2+, Cd2+) by these hybrid compounds. The following five kinds of compounds synthesized in this work are ZnAl-NO3 (L1), ZnAl-EDTA (L2), MgAl-NO3 (L3), MgAl-EDTA (L4), and MgAl-EDDS (L5). To confirm the effect of intercalation with EDTA, the adsorption of metallic ions onto L1 and L2 is also compared. This study investigated the adsorption ability of LDHS as adsorbent for Pb, Cu, and Cd from aqueous solution. Finally, the further

)*x*/*<sup>n</sup>* <sup>∙</sup> *mH*2*<sup>O</sup>*, where cationic *MII* and *MIII* are diva-

biodegradability for plants and animals at higher concentrations [3, 4].

**166**

**Figure 1.**

*The layered structure of LDHs.*

developments of LDHs as useful adsorbent with the future of application in the environmental chemistry are mentioned.
