Effects of elevated sulfate concentration on the mobility of arsenic in the sediment-water interface

The adsorption/desorption of arsenic (As) at the sediment-water interface in lakes is the key to understanding whether As can enter the ecosystem and participate in material circulation. In this study, the concentrations of As(III), total arsenic [As(T)], sulfide, iron (Fe), and dissolved organic carbon (DOC) in overlying water were observed after the initial sulfate (SO42-) concentrations were increased by four gradients in the presence and absence of microbial systems. The results indicate that increased SO42- concentrations in overlying water triggered As desorption from sediments. Approximately 10% of the desorbed As was desorbed directly as arsenite or arsenate by competitive adsorption sites on the iron salt surface; 21% was due to the reduction of iron (hydr)oxides; and 69% was due to microbial activity, as compared with a system with no microbial activity. The intensity of microbial activity was controlled by the SO42- and DOC concentrations in the overlying water. In anaerobic systems, which had SO42- and DOC concentrations higher than 47 and 7 mg/L, respectively, microbial activity was promoted by SO42- and DOC; As(III) was desorbed under these indoor simulation conditions. When either the SO42- or DOC concentration was lower than its respective threshold of 47 or 7 mg/L, or when either of these indices was below its concentration limit, it was difficult for microorganisms to use SO42- and DOC to enhance their own activities. Therefore, conditions were insufficient for As desorption. The migration of As in lake sediments was dominated by microbial activity, which was co-limited by SO42- and DOC. The concentrations of SO42- and DOC in the overlying water are thus important for the prevention and control of As pollution in lakes. We recommend controlling SO42- and DOC concentrations as a method for controlling As inner-source pollution in lake water.