Selective recovery of ferrous oxalate and removal of arsenic and other metals from soil-washing wastewater using a reduction reaction

Abstract

Abstract Oxalic acid can effectively extract arsenic bound to amorphous iron oxides via dissolution of iron oxides containing arsenic. Therefore, soil washing with oxalic acid is a promising method to remediate arsenic-contaminated soil, since arsenic is often associated with amorphous iron oxides in soil. However, high cost of oxalic acid compared to the other inorganic acids commonly used for remediation of metal-contaminated soils is the main disadvantage of using oxalic acid for soil washing. In this study, in order to increase the economic feasibility of soil washing using oxalic acid, selective recovery of iron and oxalate as resources was suggested via two step reduction reaction using dithionite as a reductant during soil-washing wastewater treatment. In the first step, high levels of oxalate and iron present in wastewater was recovered as a ferrous oxalate phase. Under mild reducing conditions, ferric iron in the wastewater was reduced to ferrous iron, which could form a complex with oxalate and precipitate as the ferrous oxalate phase with low solubility. The ferrous oxalate phase did not affect dissolved arsenic retention in this step. The recovered ferrous oxalate phase can be applied in the industry as useful resources, which can contribute to the sustainable development and cleaner production. In the second step, arsenic in wastewater was removed by forming a realgar-like phase from the reaction with dithionite. Sulfide produced by the decomposition of dithionite reacted with arsenic and other metals in wastewater, and formed sulfide phases such as realgar (As4S4)/orpiment (As2S3) and lead sulfide (PbS).