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 pHw(1:5) of these soils ranged from 5¢3to9¢1, while the total soil aqueous phase concentrations of Zn and Cu were in the ranges 1{71 and 5{74 ¹g/L, respectively. Preliminary equilibration studies revealed that 24-h contact between resin and sample solution was su±cient to exchange all free ions onto the resin bead. By using this equilibration time, the e®ects of temperature, solution composition, and pH on the resin distribution coe±cients (Kd) for Zn and Cu were investigated. These studies revealed that temperature did not signi¯cantly in°uence the Zn and Cu resin distribution coe±cient. Solution composition such as the presence of Ca, Mg, and Al, however, had a marked e®ect on the Kd value. Although the Kd 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 in°uence the Kd values of these metals. The cation exchange resin method was modi¯ed for speciation of solutions containing very low concentration (<50 ¹g/L) of Zn. Comparison of the 2 speciation techniques demonstrated excellent agreement for free hydrated Zn2+ and Cu2+ in the soils studied.

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