Specific and non-specific sorption of cadmium by soil clays as influenced by zinc and calcium

The sorption of low concentrations of cadmium by the calcium-saturated <2 µm fraction of four soils was studied in the presence of variable concentrations of calcium and zinc, as a model of reactions in contaminated soils. The sorbed cadmium and zinc was determined by radioisotopic procedures and partitioned experimentally into specifically and non-specifically sorbed forms. With cadmium the non-specifically sorbed form usually exceeded specifically sorbed. Specific sorption of cadmium was depressed by competition with zinc but not the reverse. The non-specifically sorbed component of sorbed cadmium, as defined by rapid exchange with calcium nitrate solution, was preferred over calcium by the clay exchanger, with separation factors of 4-10 for cadmium saturations of <1% CEC. Some implications for the management of soil as a sink for heavy metal pollutants are discussed.