CADMIUM SOLUBILITY IN AEROBIC SOILS

Cadmium solubility in calcareous soils is assumed to be controlled by precipitation of Cd carbonate minerals. The Cd carbonate (octavite) so lubility product (K-50) was determined in a pure CdCO3 solution, suspe nsions of soils amended with Cd, and two limed, wetted, and dried agri cultural soils amended with Cd to determine if dissolved Cd was contro lled by CdCO3 precipitation in the soils. Calculation of solution ion activities was done accounting for the ionic strength and formation of Cd complexes in solution. The K-50 for octavite was 1.64 x 10(-13) (s tandard error of the mean was 0.08 x 10(-13)) at 25 degrees C and zero ionic strength. Suspensions of soil amended with 2 to 1000 mg Cd kg(- 1) in a 10(-3) M CaCl2 solution showed more than 10 times supersaturat ion of the solution relative to the determined CdCO3 solubility produc t. Soils amended with 2 to 500 mg Cd kg(-1) that were periodically wat ered and allowed to dry out showed more than five times supersaturatio n. Long-term aging (15 mo) had little effect on supersaturation in bot h experiments. It is likely that precipitation inhibitors such as diss olved organic C were the reasons for the apparent lack of CdCO3 precip itation. The experiments showed that under environmentally relevant co nditions carbonates are not likely to govern aqueous Cd concentrations in calcareous agricultural soils.