EDTA-enhanced phytoremediation of heavy metal contaminated soil with Indian mustard and associated potential leaching risk

A glasshouse pot experiment and a laboratory leaching column experiment were conducted to study the EDTA enhancement of the mobility and phytoextraction of heavy metals and the potential for leaching of metals during the phytoextraction process. Addition of EDTA (disodium salt, 3 mmol kg−1) to pots of a paddy soil (an Fe-accumulic Gleyi-Stagnic Antrosol) historically polluted with Cu and experimentally spiked with Zn, Pb and Cd significantly enhanced the mobilities of soil Cu and Pb but not of Zn and Cd. EDTA increased shoot Cu and Pb concentrations in Indian mustard (Brassica juncea) plants growing in the soil but the resulting offtakes were low and a sequence of at least 200 crops would be required to remediate the soil. Addition of oxalic, citric or malic acid to soil at the same rate (3 mmol kg−1) had virtually no effect on uptake of the metals by Indian mustard. EDTA addition led to elevated soil solution concentrations of TOC, Cu, Zn, Pb and Cd for about 1 month. Rainfall after EDTA application, as simulated by the column leaching experiment, increased the concentrations of Cu, Zn, Pb and Cd linearly in leachate with increasing EDTA dosage (0–12 mmol kg−1). EDTA addition also led to losses of soil macronutrients including Fe. About 68% of the added EDTA tended to chelate soil Cu, Zn, Pb and Cd and the remaining 32% was chelated with and leached other ions. Total Cu, Zn, Pb and Cd losses were significantly correlated with EDTA dosage. The low shoot offtakes of Pb and Cu and the risk of groundwater pollution as EDTA remains active for several weeks make chelate-enhanced phytoremediation with Indian mustard unsuitable for this soil, especially during periods of high rainfall.