Kinetics of metal fixation in soils: measurement and modeling by isotopic dilution

The bioavailability of metal contaminants in soils varies widely, depending on soil characteristics and the source of the contaminant. As a consequence, site-specific risk assessment requires accurate prediction of the bioavailable (or labile) fraction of soil metal. Moreover, metals in soil are subject to time-dependent processes, which affect their bioavailability and thereby complicate the prediction of future risk. The aim of the present study was to describe the development of simple, readily applicable models for the time-dependent changes in lability of Cd and Zn in soils. We present data showing the time-dependent behavior of radiolabile and soil solution concentrations of Cd and Zn during an incubation study over a period of 813 d in 23 diverse soils. The data are used to parameterize candidate models of metal fixation in soils designed to be readily applicable and therefore relevant to risk assessment. We conclude that the final extent of metal fixation increases with pH and generally is greater for Zn than for Cd; however, the rate of fixation is independent of pH and equivalent to a half-time to equilibrium of 29 and 89 d for Cd and Zn, respectively. It is possible that longer-term processes occur, especially for Zn, but these could not be detected in the present study.