Opposite Environmental Responses of Arsenic and Cadmium Mediated by Redox Conditions, Sulfur Cycling, and Microorganisms

Arsenic (As) and cadmium (Cd) often coexist in paddy soils and sediments, yet they respond differently to changes in oxygen availability. Under moderately reducing conditions, the dissolution of iron(III) oxides releases sorbed As, whereas increasing pH and stronger surface sorption commonly decrease Cd mobility. Under strongly reducing conditions, microbial sulfate reduction generates sulfide and promotes the precipitation of sparingly soluble CdS. Arsenic may instead be immobilized in sulfide minerals or remain dissolved as thioarsenic species, depending on iron availability, sulfide concentration, pH, and reaction time. Reoxidation can reverse these trends by rebuilding iron oxides that retain As while oxidizing CdS and potentially remobilizing Cd. Iron-reducing bacteria, sulfate-reducing bacteria, sulfur-oxidizing bacteria, cable bacteria, and As-metabolizing microorganisms regulate these linked transformations. The apparent opposite response is therefore conditional rather than universal. Its direction and magnitude depend on Fe-S balance, organic matter, phosphate competition, and wet-dry frequency. Knowing these controls is important for managing As-Cd co-contaminated soils without decreasing the risk of one contaminant while increasing the other.