Soil solution metal concentrations and hence metal bioavailability are controlled by sorption-desorption reactions at the surfaces of soil colloids. Compared with the numerous studies on sorption of metals by soils, there are relatively few that examine desorption, and even fewer that have measured desorption kinetics. The objective of this study was, using techniques and models developed previously for studies with synthetic oxides, to examine the effect of sorption period on the subsequent desorption kinetics of Cd and Co from two natural soil clay fractions. Cobalt was less readily desorbed from the clay fractions than Cd, but with both metals and clay fractions, increases in sorption period were accompanied by substantial decreases in the proportions of sorbed metal desorbed. As with the oxides studied previously, desorption kinetics for Cd and Co sorbed onto clay fractions were found to be described well by both a two-site first-order model and a model involving a lognormal distribution of first-order rate constants. Reaction half-lives derived from the models suggested a movement of Cd and Co to slower desorption reactions with increasing sorption period. However, some caution should be used in interpreting such parameters.[Show full abstract]
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