ION-EXCHANGE RESIN AND MINTEQA2 SPECIATION OF ZN AND CU IN ALKALINE SODIC AND ACIDIC SOIL EXTRACTS

The bioavailability of trace metals, their accumulation by organisms, and their toxicity to terrestrial and aquatic organisms can be underst ood better in terms of trace metal species. Although there are numerou s chemical techniques used for the determination of ionic species, mos t are tedious, time-consuming, and not practical for routine analyses. Alternatives to these chemical techniques are computer speciation mod els that often have an incomplete database on metal-organic chemistry, yet scientists continue to use these techniques for estimation of tra ce metal activities. In this study we compare the cation exchange resi n (Amberlite) and the MINTEQA2 geochemical model for the speciation of zinc (Zn2+) and copper (Cu2+) in the aqueous phase of 11 soils varyin g widely in chemical composition. The pH(w(1:5)) of these soils ranged from 5.3 to 9.1, while the total soil aqueous phase concentrations of Zn and Cu were in the ranges 1-71 and 5-74 mu g/L, respectively. Prel iminary equilibration studies revealed that 24-h contact between resin and sample solution was sufficient to exchange all free ions onto the resin bead. By using this equilibration time. the effects of temperat ure, solution composition, and pH on the resin distribution coefficien ts (K-d) for Zn and Cu were investigated. These studies revealed that temperature did not significantly influence the Zn and Cu resin distri bution coefficient. Solution composition such as the presence of Ca, M g, and Al, however, had a marked effect on the K-d value. Although the K-d value was not sensitive to changes in solution pH (4.5-7.1), it i ncreased at pH >7.1. These results suggest that soil solution composit ion must be considered during metal ion speciation using the resin bea ds. Changes in Zn and Cu concentrations, however, did not influence th e K-d values of these metals. The cation exchange resin method was mod ified for speciation of solutions containing very low concentration (< 50 mu g/L) of Zn. Comparison of the 2 speciation techniques demonstrat ed excellent agreement for free hydrated Zn2+ and Cu2+ in the soils st udied.