Ion exchange resin and MINTEQA2 speciation of Zn and Cu in alkaline sodic and acidic soil extracts

Abstract

The bioavailability of trace metals, their accumulation by organisms, and their toxicity to terrestrial and aquatic organisms can be understood better in terms of trace metal species. Although there are numerous chemical techniques used for the determination of ionic species, most are tedious, time-consuming, and not practical for routine analyses. Alternatives to these chemical techniques are computer speciation models that often have an incomplete database on metal–organic chemistry, yet scientists continue to use these techniques for estimation of trace metal activities. In this study we compare the cation exchange resin (Amberlite) and the MINTEQA2 geochemical model for the speciation of zinc (Zn²⁺) and copper (Cu²⁺) in the aqueous phase of 11 soils varying 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 µg/L, respectively. Preliminary equilibration studies revealed that 24-h contact between resin and sample solution was sucient to exchange all free ions onto the resin bead. By using this equilibration time, the effects of temperature, solution composition, and pH on the resin distribution coeficients (K_d) for Zn and Cu were investigated. These studies revealed that temperature did not significantly influence the Zn and Cu resin distribution coecient. Solution composition such as the presence of Ca, Mg, 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 increased at pH >7·1. These results suggest that soil solution composition must be considered during metal ion speciation using the resin beads. Changes in Zn and Cu concentrations, however, did not influence the K_d values of these metals. The cation exchange resin method was modified for speciation of solutions containing very low concentration (<50 µg/L) of Zn. Comparison of the 2 speciation techniques demonstrated excellent agreement for free hydrated Zn²⁺ and Cu²⁺ in the soils studied.