Two-step sequential washing with N-acetyl L-glutamic acid and oxalic acid for remediation of multi-PTE contaminated soils

Conventional soil washing for remediating potentially toxic elements (PTEs) is often limited by poor selectivity, secondary pollution, and the presence of recalcitrant oxide-bound fractions. Here, we propose a sequential two-stage washing strategy using biodegradable agents. Results suggest that N-acetyl-L-glutamic acid (NAG, 0.15 M) chelates exchangeable and carbonate-bound metals, while oxalic acid (OA, 0.20 M) subsequently reduces and dissolves iron oxides, releasing encapsulated PTEs. When applied to two contaminated soils from Sichuan Province, China, the sequential NAG-OA wash outperformed single reagents, achieving effective removal of cadmium (Cd, 71.4-91.8%) and zinc (Zn, 86.4-94.5%), though lead (Pb) removal remained relatively limited (24.8-57.7%). Preliminary column-scale evaluations confirmed the engineering viability of the two-step washing, achieving 82.36% Cd and 83.14% Zn removal in Soil 1. Mechanistic investigations using XPS and XRD indicated a 9.4–9.7% decrease in metal-oxide bonding and a shift in the Fe2 + /Fe3 + ratio, consistent with the dissolution of mineral phases. While the treatment preserved soil organic carbon content (11.10–26.90 g/kg) and enhanced catalase activity, it also caused notable decreases in available phosphorus, available potassium, and microbial alpha diversity, indicating trade-offs between PTE removal efficiency and post-treatment soil quality. This laboratory study demonstrates a promising sequential washing approach using biodegradable agents for multi-PTE contaminated soils, though post-treatment soil rehabilitation and further validation across diverse conditions are needed.