Anionic Intercalated CaMgFe-Layered Double Hydroxides for Synchronous Passivation of Heavy Metals Contaminated Farmland Soils with Soil Fertility Enhancement and Microbial Community Reconstruction

Heavy metals such as arsenic (As), cadmium (Cd), and lead (Pb) can accumulate through the soil-plant-food chain, posing severe threats to food safety and human health. Traditional remediation materials often fail to immobilize As, Cd, and Pb synchronously in mixed-contaminated soils and enhance soil fertility simultaneously. From a geochemical perspective, calcium (Ca), magnesium (Mg), and iron (Fe) could synchronously anchor and passivate heavy metal ions (As, Cd and Pb) through isomorphic substitution, surface precipitation, and complexation. Intercalated silicate (SiO ₃ ²⁻ ) and phosphate (H ₂ PO ₄ ⁻ ) simultaneously enhanced heavy metal passivation and soil fertility. This modified layered double hydroxide (LDH) was synthesized via chemical co-precipitation, named CaMgFe/SiO ₃ /H ₂ PO ₄ -LDH. Batch experiments showed that As (V), Cd (II), and Pb (II) adsorption followed Langmuir isotherms and pseudo-second-order kinetics, with higher adsorption performance in mixed systems. Soil incubation experiments demonstrated that 3% passivator addition reduced available As, Cd, and Pb to 0.26–0.45, 0.32–0.43, and 9.52–22.37 mg/kg, respectively, and transformed them into stable residual fractions. Soil pH increased to 6.19–6.26, and cation exchange capacity to 11.94–14.81 cmol/kg at 30 d compared with CK. Available silicon and phosphorus increased significantly to 193.28-314.57 mg/kg and 26.12–56.24 mg/kg, respectively. The passivation further altered microbial diversity and structure. The relative abundance of Gemmatimonas , Pseudomonas , and Comamonas increased, which contributed to heavy metals immobilization and nutrient cycling. Bacteroidetes, Proteobacteria, and Firmicutes were keystone microbes for remediation. CaMgFe/SiO ₃ /H ₂ PO ₄ -LDH had engineering potential for heavy metals immobilization and simultaneous fertility enhancement in contaminated farmland soils.