Removal of trace and major metals by soil washing with Na₂EDTA and oxalate

Abstract

Background, aim, and scope

Various metals such as cationic metals (Cu, Pb, Zn) and anionic metals (As, Cr) often coexist in real soils, and normal soil washing techniques for the removal of cationic metals with a single-washing reagent make it rather difficult to simultaneously remove all of them. Oxalate could effectively remove anionic As and EDTA could effectively remove the cationic metals, so it was possible to remove all coexisting cationic and anionic metals by washing with the combination of Na₂EDTA and oxalate. The objective of this study was to (1) discuss the possibility of removing five metals, As, Cd, Cu, Pb, and Zn, effectively from the soil by washing with Na₂EDTA-combined oxalate; (2) optimized through the consecutive washing.

Materials and methods

The soil sample was collected, air-dried, and sieved. The typical extraction tests were conducted in 50-mL-capacity polyethylene tubes. Tubes containing 1 g samples and a measured volume of extractant (L/S = 20 mL g⁻¹) were stirred for a given time. The soil was first treated with 0.01 M Na₂EDTA and 0.1 M oxalate individually to compare the removal efficiencies. Then, consecutive extractions were performed so that 1 g of soil was treated by four cycles of washing with 2 h for each washing. To optimize the combination of Na₂EDTA and oxalate, three methods of consecutive washing were designed: washing with Na₂EDTA followed by oxalate, for two rounds (A); washing with oxalate followed by Na₂EDTA, for two rounds (B); and washing with the admixture of oxalate and Na₂EDTA for four cycles (C).

Results

After 24 h washing, Na₂EDTA removed only 2.3% of As from the soil, while oxalate removed 59.9%. In contrast, the removal of Pb for oxalate was 1.5% while it was 27.4% for Na₂EDTA. A large amount of Ca (379.72 mg l⁻¹) was released when washing with Na₂EDTA; while the released Ca was far lower (15.86 mg l⁻¹) with oxalate. In contrast, washing with oxalate resulted in a large amount of Al (123.34 mg l⁻¹) and Fe (305.9 mg l⁻¹) in the solution, far higher than washing with Na₂EDTA (14.03 mg l⁻¹ for Al and 38.40 mg l⁻¹ for Fe). The consecutive experiments indicated that after four cycles of washing, three methods could effectively remove all of both cationic and anionic metals, and the accumulative removals of five metals were 54.7–65.6% for As, 28.6–33.8% for Cd, 80.3–86.6% for Cu, 15.8–42.9% for Pb, and 43.2–45.2% for Zn, respectively. In the first cycle of method A, the total molar concentration of major elements was 11.53 mmol l⁻¹, higher than the molar concentration of Na₂EDTA. Arsenic released was linearly correlated with the concentrations of Fe and Al in the eluate to a significant degree and the correlation coefficients (r) were 0.942 and 0.920, respectively.

Discussion

The reason for the low As removal for Na₂EDTA was that the anionic form of As does not allow for the formation of a stable complex with Na₂EDTA. The reason for the low removal of Pb by oxalate was due to the formation of low-solubility lead oxalate precipitates. Extract analyses indicated that major elements were extracted by Na₂EDTA and oxalate as well. Oxalate is able to promote greater Al/Fe/Mn dissolution than an equivalent level of EDTA through ligand-promoted oxide dissolution and reductive dissolution mechanisms.

Method

A removed more Cd and Pb, but a little less Cu, Zn, and As than methods B and C. Since it could remove all of the five metals effectively, but the other two methods could remove only 15.8–20.7% of Pb, it was considered as the best method to decontaminate this studied soil. In the first cycle of method A, the total molar concentration of major elements was higher than the molar concentrations of Na₂EDTA. It was reasonable to assume that some of the major substances in the washing solutions were therefore not complexed to chelant. In the first cycle, a large amount of Ca (388.7 mg l⁻¹) was released when washing with Na₂EDTA (method A), while the concentration of Ca was far lower for method B (11.38 mg l⁻¹) and C (6.51 mg l⁻¹). It seemed that oxalate could inhibit the release of Ca through the formation of insoluble CaOx, which is not EDTA-exactable. No matter with which method, the release of Fe and Al was large when washing with oxalate, while it was small when washing with Na₂EDTA. Arsenic released was significantly linearly correlated with the concentrations of Fe and Al in the eluate and the correlation coefficients (r) were 0.942 and 0.920, respectively. It further confirmed the conclusion that the releasing of As can be mainly attributed to the release of Fe and Al oxides.

Conclusions

After 24 h washing, Na₂EDTA led to high removal for cations such as Cu, Pb, Zn, and Cd, but less As removal. On the contrary, oxalate removed more As, Cd, Cu, and Zn than Na₂EDTA, while the removal of Pb was very low (1.5%). Four cycles of washing with three methods combining Na₂EDTA and oxalate washing could effectively remove both cationic and anionic metals. About 96.54–99.20% of chelant was found binding with major elements in the eluate. Washing with Na₂EDTA followed by oxalate (method A) was considered to be the best method to decontaminate the studied soil. Arsenic released by washing was significantly correlated with the concentrations of Fe and Al in the eluate, indicating that the release of As was mainly ascribed to the release of Fe and Al oxide.

Recommendations and perspectives

The removal of metals in this study is efficient, but the high cost of chelating reagents such as EDTA has precluded their field application. Since the adsorption mechanism between the metal and the specific soil fraction differs, the lack of understanding the chemistry of soil metal speciation, interparticle extraction dynamics, and spent extractant recycling techniques have limited this promising technology to small-scale applications. The behavior of the individual fractions during the extraction process, the selectivity between target heavy metals and chelators, and the reuse and recoverability of chelators will be the major tasks for further studies.