Mercapto-modified layered double hydroxide: high-efficiency emerging amendment for cadmium immobilization in cadmium-contaminated acidic and alkaline soil

Soil cadmium (Cd) contamination has emerged as a critical environmental challenge in Chinese agricultural systems. The remediation of Cd-contaminated farmland necessitates high-efficient immobilization amendments. The present study successfully synthesized mercapto-modified MgAl- and ZnAl-layered double hydroxides (MgAl/ZnAl-MP-LDHs) as structurally stable and high-performance amendments via a coprecipitation-condensation method. Batch adsorption experiments revealed superior Cd 2+ sorption capacities for MP-LDHs, with the maximum Cd 2+ capacities increased by 1.6–1.8 times compared to the unmodified materials. The XPS analysis further elucidated the dual sorption mechanisms involve in Cd 2+ absorption by MP-LDHs: complexation via mercapto groups and precipitation mediated by hydroxyl groups. Moreover, soil application of MP-LDHs at 0.025% dosage (w/w) demonstrated rapid immobilization efficacy, reducing available Cd content by 38.41%–93.20% within 24   h in alkaline and acidic soil. Subsequent analysis revealed that MP-LDHs exhibited pH-resilient performance in soil Cd immobilization. Potting experiment further validated practical applicability of MP-LDHs, reducing pakchoi shoot Cd concentration by 53.52%–95.24% at 0.025%–0.1% dosage. The findings established MP-LDHs as multifunctional amendments with ultralow application dosage requirements, excellent immobilization efficiency, and universal performance across varying soil pH levels, hence positioning MP-LDHs as a scalable solution for safe utilization of Cd-contaminated soil.