Use of isotopic dilution techniques to assess the mobilization of nonlabile Cd by chelating agents in phytoremediation

Metals are unavailable to plants when fixed within relatively inert mineral matrices in soil. The success of phytoremediation may be limited by restri cted bioavailability of metals in such nonlabile forms. We conducted a pot trial with EDTA added at rates of 0, 0.1, 1.0, 5.0, and 10 mmol kg(-1), to soil historically contaminated with sewage sludge to increase the availabil ity of metals to a test crop, Indian Mustard (Brassica juncea). The soil wa s found to have large total concentrations (mg kg(-1)) of Cd (59.3), Cu (66 4), Ph (605), Ni (489), and Zn (1780); the EDTA-extractable metal contents (mg kg(-1)) were Cd (36.2), Cu (483), Pb (302), Ni (372), and Zn (1280). Is otopic dilution of Cd-109 was used to measure labile Cd both (i) chemically (the E value: 28.3 mg Cd kg(-1)) and (ii) biologically (the L value). Comp arison of E and L values was then used to determine whether nonlabile Cd wa s mobilized by the chelate treatment. Addition of EDTA increased plant upta ke of Cd, Zn, Pb, Ni, and Cu from sludge-amended soil but did not facilitat e access to the nonlabile pool of Cd in the soil; E values and L values wer e similar across the range of EDTA applications. Thus it was apparent that the small EDTA concentrations applied in phytoremediation to increase metal solubility are probably insufficient to mobilize fixed forms of Cd.