| Abstract
| - Knowledge of trace metal speciation in soil pore watersis important in addressing metal bioavailability and riskassessment of contaminated soils. Numerous analyticalmethods have been utilized for determining trace metalspeciation in aqueous environmental matrixes; however, mostof these methods suffer from significant interferences.The Donnan dialysis membrane technique minimizes theseinterferences and has been used in this study to determinefree Zn2+, Cd2+, Cu2+, and Pb2+ activities in pore watersfrom 15 agricultural and 12 long-term contaminated soils. Thesoils vary widely in their origin, pH, organic carboncontent, and total metal concentrations. Pore water pM2+activities also covered a wide range and were controlledby soil pH and total metal concentrations. For the agriculturalsoils, most of the free metal activities were belowdetection limit, apart from Zn2+ for which the fraction offree Zn2+ in soluble Zn ranged from 2.3 to 87% (mean 43%).Five of the agricultural soils had detectable free Cd2+with fractions of free metal ranging from 59 to 102% (mean75%). For the contaminated soils with detectable freemetal concentrations, the fraction of free metal asa percentage of soluble metal varied from 9.9 to 97%(mean 50%) for Zn2+, from 22 to 86% (mean 49%) for Cd2+,from 0.4 to 32.1% (mean 5%) for Cu2+, and from 2.9 to48.8% (mean 20.1%) for Pb2+. For the contaminated soils,the equilibrium speciation programs GEOCHEM and WHAMModel VI provided reasonable estimates of free Zn2+fractions in comparison to the measured fractions (R2 ∼0.7),while estimates of free Cd2+ fractions were less agreeable(R2 ∼0.5). The models generally predicted strongerbinding of Cu2+ to DOC and hence lower fractions of freeCu2+ as compared with the observed fractions. Thebinding of Cu2+ and Pb2+ to DOC predicted by WHAMModel VI was much stronger than that predicted by GEOCHEM.
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