Cadmium solubility in alkaline paddy soils with contrasting Cd sources and biogeochemical properties under redox...

Paddy soil in carbonate regions is susceptible to cadmium (Cd) contamination from geogenic or anthropogenic sources. However, Cd solubility in carbonate-derived alkaline paddy soils with contrasting Cd sources and properties under redox alternations is poorly understood. In this study, paddy soils enriched with geogenic or mine drainage-derived Cd and presenting contrasting manganese and sulfur abundances were incubated under alternating redox conditions. The changes in aqueous chemistry, Cd bioavailability and speciation were investigated. The results revealed that mine drainage significantly elevated the content of soluble sulfate and the abundance of sulfur-related bacteria (e.g., Sulfurifustis and Thiobacillus) in paddy soils. For contaminated soils, the reductive dissolution of Fe/Mn oxides and cation exchange induce Cd mobilization from soils during the initial reduction period. More than 90 % of the soluble Cd was subsequently re-sequestered, concomitant with the increase in residual and organically bound Cd fractions. Nevertheless, the soluble Cd gradually decreased in geogenic soil during the reduction period. During the reoxidation period, the Cd solubility increased in all the soils in the initial 3 days. Afterward, soluble Cd was partially immobilized by different soil constituents. As a result of Cd redistribution, Cd bioavailability decreased with reduction time and increased with ongoing oxidization of the soil. These results indicate that the solubility and fate of Cd vary in soils and strongly depend on soil biogeochemical properties in the karst region.