Synergistic removal of As, Cd, and Pb from soil using composite eluent of oxalic acid and tetrasodium salt of N,N-bis(carboxymethyl)-L-glutamic acid

The simultaneous removal of anionic metalloid arsenic (As) and cationic heavy metals cadmium (Cd) and lead (Pb) from co-contaminated soil via soil washing remains challenging due to their distinct geochemical behaviors. In this study, we developed a novel composite eluent (OG) based on oxalic acid (OA) and tetrasodium N,N-bis(carboxymethyl)-L-glutamic acid (GLDA) to synergistically enhance the removal of As, Cd, and Pb. Under optimized conditions (0.1 mol L⁻¹ OA and 0.2 mol L⁻¹ GLDA, pH 4, 2 h), OG increased As removal (33%) by approximately two-fold compared to OA alone and ten-fold compared to GLDA alone, demonstrating a significant synergistic effect (p< 0.05). Cd and Pb removal efficiencies (54% and 31%) were also enhanced by approximately three-fold relative to OA alone. Mechanistically, OG deprotonation released H⁺, promoting the dissolution of surface-adsorbed metals. As removal was primarily driven by OA ligands via non-reductive ligand exchange and reductive dissolution, while Cd and Pb removal was dominated by GLDA ligands complexation. Through coupled dissolution-complexation dynamics, OG preferentially targeted Fe/Al-bound and residual As, exchangeable Cd, and reducible Pb without compromising soil functionality. This work establishes OG as an environmentally benign and efficient strategy for remediating heterogeneous multi-metal-contaminated soils.