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) solubility product ( K so ) was determined in a pure CdCO 3 solution, suspensions of soils amended with Cd, and two limed, wetted, and dried agricultural soils amended with Cd to determine if dissolved Cd was controlled by CdCO 3 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 so for octavite was 1.64 × 10 −13 (standard error of the mean was 0.08 × 10 −13 ) at 25°C and zero ionic strength. Suspensions of soil amended with 2 to 1000 mg Cd kg −1 in a 10 −3 M CaCl 2 solution showed more than 10 times supersaturation of the solution relative to the determined CdCO 3 solubility product. Soils amended with 2 to 500 mg Cd kg −1 that were periodically watered and allowed to dry out showed more than five times supersaturation. Long‐term aging (15 mo) had little effect on supersaturation in both experiments. It is likely that precipitation inhibitors such as dissolved organic C were the reasons for the apparent lack of CdCO 3 precipitation. The experiments showed that under environmentally relevant conditions carbonates are not likely to govern aqueous Cd concentrations in calcareous agricultural soils.