KINETICS OF CADMIUM AND COBALT DESORPTION FROM IRON AND MANGANESE OXIDES

Oxides of Fe and Mn in soils are capable of sorbing large amounts of t race metal ions and can therefore be important in controlling trace me tal concentrations in soil solution, and hence trace metal bioavailabi lity in soils. There is, however, relatively little information on the rates of desorption of trace metals from oxide materials or on the fa ctors affecting desorption rates. The objective of this study was to e xamine the kinetics of desorption of Cd and Co from two Fe oxides, goe thite and ferrihydrite, and from two Mn oxides, hausmannite and crypto melane. The concentrations of Cd and Co specifically sorbed by the oxi des at pH 6.0 were greater for the Mn oxides than for the Fe oxides. T he metals were also much less readily desorbed from the Mn than the Fe oxides and, in general, Cd was more readily desorbed than Co from all four oxides. Increasing the initial sorption period from 1 to 15 wk s ubstantially decreased the proportion of sorbed Cd or Co subsequently desorbed from goethite, with a similar but much smaller effect also ob served with the Mn oxides. Desorption kinetics for both Cd and Co were found to be described wed by assuming either the occurrence of two si multaneous first-order desorption reactions, or by a continuous distri bution of reaction sites, distributed lognormally with respect to deso rption first-order rate constant. with increasing initial sorption per iod, the parameters obtained from fitting either type of kinetic equat ion to the experimental data could be interpreted as indicating a move ment of metal ions to sites with slower desorption reactions.