Remediation of Sb-Contaminated Soil by Low Molecular Weight Organic Acids Washing: Efficiencies and Mechanisms

Low molecular weight organic acids (LMWOAs) are promising agents in the remediation of heavy metal contaminated soil with strong complexing ability and less environmental impact. However, the application of LMWOAs for washing the Sb-contaminated soil still faces great challenges, such as the selection of suitable washing agents, optimal washing parameters, and the unclear Sb removal mechanism. In this study, five suitable LMWOAs were screened from ten common washing agents and their optimum washing parameters were determined. The results showed that oxalic acid (OA) and HEDP were the top two outstanding agents, and the removal efficiencies of Sb were 68.79% and 49.73%, respectively, under optimal parameters (OA at 0.5 mol/L, HEDP at 0.2 mol/L, washing for 480 min, and the liquid-to-solid ratio of 15). The soil was analyzed for chemical speciation, morphology, functional groups, and mineralogy before and after washing. The results indicated that Fe/Al minerals in the soil are the main reason for the adsorption of Sb, and the possible mechanisms of Sb removal by LMWOAs included the dissolution of minerals, complexation reaction, and ligand exchange. Our findings highlight the potential application of LMWOAs as efficient washing agents to remove Sb from contaminated soils.