Development of carboxyl-based composite washing agent for the simultaneous removal of heterogeneous metal(loid)s from co-contaminated site soils: performance and mechanisms

Abstract

The simultaneous removal of arsenic (As), cadmium (Cd), and lead (Pb) from co-contaminated site soils remains challenging because these contaminants differ markedly in geochemical behavior, binding phase, and response to washing agents. Conventional soil washing systems based on single-function agents show limited co-removal efficiency and poor transferability across soils.
This thesis developed and optimized a carboxyl-based composite washing agent (OG), composed of tetrasodium N,N-bis(carboxymethyl)-L-glutamate (GLDA) and oxalic acid (OA). Its removal performance, mechanisms, soil-dependent behavior, and post-washing environmental safety were systematically evaluated by tracking the removal of pollutants and matrix elements, contaminant fractionation, soil surface changes, and soil-eluent physicochemical properties. Under optimized conditions, OG removed 18-38% of As, 47-76% of Cd, and 48-65% of Pb from three naturally contaminated soils. Compared with individual agents, OG showed a pronounced synergistic effect on As removal while maintaining high Cd and Pb extraction. Mechanistic analyses showed that OG operated through coupled dissolution-complexation pathways. OA promoted the release of iron (Fe)/aluminum (Al)-bound As through ligand-assisted and reductive dissolution, whereas GLDA extracted exchangeable Cd and reducible Pb through strong complexation. GLDA also complexed dissolved Fe, limiting As re-adsorption, while OA enhanced mineral accessibility. Across soils, removal efficiencies were strongly regulated by initial soil pH, cation exchange capacity, organic matter, Fe content, and competition from calcium (Ca) for complexation sites. Post-washing assessments showed that OG largely preserved soil structure and nutrient status, while reducing contaminant leachability, mobility, and bioaccessibility by 47-96%. Transient ecotoxicity caused by residual eluents was mitigated after natural aging.
This thesis establishes a mechanistically informed composite washing strategy for remediating As, Cd, and Pb co-contaminated soils under variable soil conditions.

Date of Award	15 Nov 2026
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Yong Sun (Supervisor), Chao Cai (Supervisor) & Jun He (Supervisor)