Microbial Role in Immobilization and Subsequent Mobilization of Cadmium in Soil Suspensions

Abstract

AbstractThe effect of microorganisms on the fate of Cd in soil suspensions was investigated. Samples of four soil types were suspended in growth media amended with Cd(NO3)2 (20 mg/L) and incubated aerobically or anaerobically at 28 ± 1°C. At various time intervals, aliquots were withdrawn, and Cd concentrations (total and free Cd2+) and pH were determined in the supernatant. The results of the short‐ and long‐term studies indicated that in all soils tested the extent of Cd disappearance (total and free Cd2+) in nonsterile soil supernatants was significantly greater than in the corresponding sterile controls. However, during the subsequent long‐term 8‐week aerobic incubation, a significant amount of Cd (including free Cd2+) was released into solution of nonsterile suspensions, while no increase in Cd concentration was observed in the sterile suspensions. Under anaerobic conditions, a continued disappearance of Cd from the non‐sterile soil supernatants was observed during an incubation period of up to 24 weeks. Under aerobic conditions, the pH of the nonsterile soil suspensions decreased to approximately pH 5.5, whereas the pH remained approximately 8.0 during anaerobic incubation. The rate and degree of Cd immobilization or mobilization was also dependent on the soil types used. Of the four soils tested, Morrison (Ultic Hapludalfs) sandy loam from a coniferous forest site exhibited the highest degree of Cd mobilization, accounting for up to 100% of total Cd immobilized. Our results suggested that soil microbial activity is responsible for both binding of Cd to and its subsequent release from soil.