Soil contamination by heavy metals poses serious risks to human health and the environment. This study investigates the removal of Pb, Cu, Zn, Cd, and Cr from heavily contaminated slightly acidic to neutral soil (pH 6.5) using organic washing agents (humus, malic acid, and gluconic acid) at concentrations of 1% and 3% and a solid-to-liquid ratio (S/L) of 1:8. The results reveal that metal mobilization depends strongly on the type and concentration of the extraction agent, the target metal, and soil properties. Cd was highly mobilized, reaching more than 90% with 3% gluconic acid, whereas Cu and Pb remained largely immobile ( Cr > Zn > Cu ≈ Pb) and demonstrates that metal speciation, soil chemistry, pH, and S/L ratio critically govern extraction efficiency. These findings provide mechanistic insights into metal–ligand interactions and practical guidance for optimizing soil remediation strategies using organic acids.

Soil contamination by heavy metals is a significant sustainability and ecological issue, impacting on the health of ecosystems and groundwater. This study assessed the efficacy of malic acid as a biodegradable and environmentally benign agent for the remediation of soils contaminated with cadmium, chromium, copper, and zinc. Two soils with contrasting textures were treated with a 10% malic acid solution at solid/liquid ratios of 1:5 and 1:10 for contact times of 2, 4, 6, and 8 h. The extraction efficiency varied depending on metal type, soil texture, and washing conditions. Cadmium removal ranged from 26% to 55%, zinc removal ranged from 10% to 25%, while copper showed variable extraction (5–45%) depending on initial soil concentration. Chromium exhibited the highest removal efficiency (30–90%), quantified as total chromium; however, the absence of speciation analysis (Cr(III)/Cr(VI)) represents a key limitation and may affect the interpretation of the removal performance. FTIR and UV–Vis analyses confirmed the formation of metal–carboxylate complexes and changes in soil functional groups during the washing process. In addition, significant mobilization of nitrogen and potassium was observed, whereas phosphorus remained relatively stable. The results highlight the influence of soil texture and multi-metal interactions on malic acid washing efficiency and provide a laboratory-scale environmental assessment of malic acid as a sustainable remediation alternative for soil remediation, while emphasizing the need for further evaluation regarding chromium speciation and post-treatment soil quality and sustainability impacts.