Redistribution of forms of zinc, cadmium and nickel in soils treated with EDTA

Organic amendments are often added to soils in the form of waste materials which increase the soluble organic ligands in soil. To study the interaction of these soluble organics with metals in soil, Ethylenediaminetetraacetic acid (EDTA) can be used as a model compound for natural organics. The addition of EDTA to soil may alter the physicochemical forms of metals in soil because of its strong chelating capacity. In this work, samples of eight agricultural soils collected in Georgia were spiked with soluble salts of Zn, Cd, and Ni in proportions of 75.9, 1.62, and 4.30 mg kg(-1), respectively. Both natural and metal-amended soils were treated with Na-2-EDTA (disodium ethylenediamine tetraacetate) solution at amounts of 0, 1.0 and 2.0 g kg(-1). After five months incubation in a greenhouse, the soil samples were fractionated into exchangeable (EXC), organic matter (OM), Mn oxide (MNO), amorphous Fe oxide (AFEO), crystalline Fe oxide (CFEO), and residual (RES) fractions. The added Zn was distributed among all fractions in the soils, and added Cd and Ni were found predominantly in the EXC and OM fractions with some of the added Ni occurring in the RES fraction. Increasing the amounts of EDTA significantly raised Zn concentrations in the EXC fraction in both natural and metal-amended soils, and the EDTA also increased Ni and Cd concentrations in the EXC fraction of the metal-amended soils. The results suggested that added EDTA enhanced the solubility of these metals in these soils. The OM-fraction metals decreased with increasing amounts of EDTA, except for Ni in the natural soils. The increase in Zn and Cd associated with the EXC fraction may have been related to solubilization of metals from the OM fraction by EDTA. In the metal-amended soils, EDTA not only removed Ni from the OM fraction, but also from the MNO, AFEO and CFEO fractions, and redistributed it into the EXC fraction. Based on relative percent, Cd in the EXC fraction was higher than Zn, while Ni in this fraction was lowest. This suggests that the order of solubility was: Cd > Zn > Ni for metals added to soils, but when EDTA was present, added Ni may be more vulnerable to solubilization than added Zn and Cd.