Chelant extraction of heavy metals from contaminated soils using new selective EDTA derivatives
Introduction
Heavy metal contamination of soils, resulting from rapid industrialization, increased urbanization, modern agricultural practices, and inappropriate waste disposal methods, has become a serious problem worldwide [1], [2], [3]. The available remediation technologies for heavy metal-contaminated soils are mainly divided into two groups: namely immobilization, such as in situ chemical fixation, and separation, such as soil washing. Chelant-enhanced soil washing is a technology that is potentially useful for the economically feasible remediation of contaminated soils [4], [5], [6], [7], [8].
The chelating agent ethylenediaminetetraacetic acid (EDTA) and its salts have been extensively studied for their potential use in soil washing [9], [10], [11], [12]. They have been reported to appreciably increase the dissolution and mobilization of cationic heavy metals [13], [14]. EDTA has low biodegradability in soil and a high efficiency of metal extraction through the formation of thermodynamically stable and soluble metal–EDTA complexes [15], [16]. In addition, recent advances in recovery and recycling techniques of used EDTA have enhanced its appeal [17].
A majority of the literature focused on demonstrating the remediation capabilities of EDTA has found that extraction of heavy metals was faster and more complete with increased quantities of added EDTA. Competition between the major cations of the soil (e.g., Ca2+, Mg2+, and Fe3+) and minor cations for chelation by EDTA may be one of the factors affecting the efficiency of trace metal extraction [18], [19], [20], [21], [22], [23]. As illustrated in previous studies [24], [25], when Ca solubility in calcareous soils is raised, the effectiveness of EDTA extraction is diminished significantly, increasing the cost of remediation. For non-calcareous soils, Fe and Al dissolution may be more crucial, in view of their high tendency for complexation (i.e., large stability constants). Excessive addition of chelating agent can cause extensive dissolution of soil minerals and organic matter, leading to alteration of soil physical and chemical properties and even disintegration of soil structure, which could render the soil unfit for future use for vegetation or construction. Therefore, there has been a growing need to develop highly selective chelating agents for the extraction of heavy metal ions from polluted soils.
In previous studies, EDTA has been modified to improve its selectivity in chelating target metal ions [26], [27], [28]. Highly selective EDTA derivatives have a wide range of application in the fields of analytical chemistry, biology and medicine, as well as in many industrial processes. In this study, two new EDTA derivatives were designed and synthesized with the goal of enhancing its selectivity as a chelating agent. These EDTA derivatives contain a phenyl or benzyl group directly bonded to the nitrogens of the ethylenediamino group, and thus potentially are more sterically constrained than the parent compound. The objective of this study was to characterize these EDTA derivatives in aqueous solution and assess their potential as selective washing agents. Batch experiments were conducted to determine their efficacy in the simultaneous extraction of trace metal ions and major cations from contaminated soils and to investigate the extraction mechanisms.
Section snippets
Soil Characteristics
Soil samples were collected from 0.7 to 1.7 m below the ground surface at a demolished industrial site in South China, air-dried at room temperature (20–30 °C), and passed through a 2 mm sieve. The soil properties in Table 1 were the average of three replicates.
Various soil physical and geochemical characterization tests were carried out. The physical and chemical characteristics of the soil are shown in Table 1. The metal concentrations in soil were determined by acid digestion with HF–HClO4–HNO3
Acid–base properties of EDTA derivatives
The protonation constants of PDTA and BDTA in water were determined with a view to assessing their acid–base properties, since these properties control what species exist in solution at various pH values. Both PDTA and BDTA have six potential sites that can bind with a proton, including the two nitrogens of the ethylenediamino groups and the four carboxyl groups. However, only three deprotonation events were observed by potentiometry (Table 2). For comparison, Table 2 also includes the
Conclusions
Two new EDTA derivatives, BTDA and PDTA, were synthesized and their metal–ligand complexation equilibrium constants and selective capabilities in aqueous media were investigated, along with those of EDTA and CDTA. Titration results showed that PDTA had the highest stability constants for Cu and Ni and the highest overall selectivity for trace metals over major cations. Equilibrium batch experiments were conducted to evaluate the efficacy of the EDTA derivatives at extracting Cu, Zn, Ni, Pb, Ca,
Acknowledgements
The project was supported by National Natural Science Foundation (No. 41171374), National Funds for Distinguished Young Scientists of China (No. 41225004), Guangdong Province Higher Vocational Colleges & Schools Pearl River Scholar Funded Scheme, the Ministry of Environmental Protection of China (No. 201109020) and the Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (No. 2011K0007).
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2022, ChemosphereCitation Excerpt :However, EDTA has a low biodegradability and a long retention time in the soil. The long-term accumulation of chelating agents can lead to the extensive dissolution of minerals and organic matter in the soil, further leading to changes in soil physicochemical properties and, more seriously, to the disintegration of the soil structure so that the soil will be unsuitable for vegetation or construction (Zhang et al., 2013). EDDS and HIDS are biodegradable chelating agents that can extract heavy metals under neutral or weakly alkaline conditions, but the extraction rate is low (Hasegawa et al., 2019; Begum et al., 2013).