Selective copper immobilization in contaminated soils using cysteine-modified chitosan: Mechanisms and efficacy

Copper (Cu) contamination in soils, especially sludge-made nutrient soils, poses environmental and agricultural risks due to its high bioavailability and toxicity. Current stabilization methods often lack efficiency and selectivity. Here, we developed a cysteine-modified chitosan composite (Cys-CS) as an eco-friendly stabilizer for targeted Cu remediation. In the single-metal system, Cys-CS's adsorption capacity for Cu gradually increases as solution pH rises from 3.5 to 6.5, reaching the maximum at pH near 6.5, with a Langmuir maximum adsorption capacity of 97.8 mg/g (1.54 mmol/g). In mixed-metal systems, Cu adsorption (0.687–1.718 mmol/g) greatly exceeded that for Pb, Zn, Cd, and Ni (0.003–0.120 mmol/g), giving K Cu / K other ratios 11.09–118 and demonstrating strong selectivity. Soil incubation with only 0.5wt‰ Cys-CS transformed Cu from bioavailable fractions (BCR1/BCR2) into stable forms (BCR3/BCR4), while preserving soil pH, soil organic matter, cation exchange capacity, and microbial community structure, and minimally affecting non-target metals. Field-relevant trials in Cu-polluted sludge-made nutrient soils confirmed its high stabilization efficiency and negligible ecological disturbance. FTIR spectroscopy reveals that functional groups of L-cysteine, including SH, NH , and CONH 2 , form coordination bonds with Cu 2+ . Density functional theory (DFT) calculations further confirm Cu-selective binding: the interaction energy of Cys-CS with Cu 2+ is significantly lower than with coexisting metals (Pb, Zn, Cd, Ni), and CONH 2 coordination with Cu exhibits a lower band gap energy (Egap) than other Cys-CS functional groups, indicating higher binding affinity. This work demonstrates that functional group regulation of biopolymer-based adsorbents enables precise, low-dose, and environmentally safe remediation of Cu-contaminated soils, offering a practical pathway toward selective in situ stabilization in complex multi-metal environments.