Enhanced reductive extraction of arsenic from contaminated soils by a combination of dithionite and oxalate

Iron oxide minerals are the most important sinks of arsenic in arsenic contaminated soils. Therefore, the effective extraction of arsenic bound to the iron oxides is essential to increase the efficiency of arsenic removal from soils. In this study, the reductive extraction of arsenic from contaminated soils was studied with a combination of dithionite and oxalate in order to remediate arsenic-contaminated soils via the reductive dissolution of arsenic associated iron oxides. The addition of oxalate greatly enhanced the reductive arsenic extraction by forming strong complexes with iron, which could prevent the precipitation of a new iron oxide phase and also enhance the iron oxide dissolution via a non-reductive dissolution pathway. Iron in soils can either be extracted as a soluble Fe(C₂O₄)₂^2- or remain in the soil as a solid Fe(C₂O₄) precipitation depending on the oxalate to iron molar ratio. Arsenic extraction was hindered by an excess of dithionite, and the optimum dithionite concentration was affected by the arsenic concentrations and the speciations present in the soils. Relatively high arsenic extraction could be obtained by a combination of dithionite and oxalate at a wide range of pH conditions.