Stabilization Efficiency and Mechanisms of Iron-Manganese Phosphate Modified Biochar for Cadmium, Lead, and Zinc Co-Contaminated Soil
The co-contamination of cadmium (Cd), lead (Pb), and zinc (Zn) in agricultural soils near mining areas poses significant risks to ecosystems and human health. Conventional stabilization materials often exhibit insufficient performance for Zn, particularly in multi-metal systems. This study synthesized a novel composite biochar (PFMBC) by loading phosphate and iron-manganese oxides onto biochar via phosphoric acid impregnation followed by secondary pyrolysis at 600 °C. The stabilization efficiency of PFMBC was evaluated against pristine biochar (BC) and iron-manganese modified biochar (FMBC) in a soil collected from a lead-zinc mining area (total Cd: 43.77 mg·kg−1, Pb: 3355.94 mg·kg−1, Zn: 1296.57 mg·kg−1). After 60 days of incubation with 5% PFMBC, the DTPA-extractable (bioavailable) fractions of Cd, Pb, and Zn decreased by 73.44%, 90.10%, and 69.33%, respectively, significantly outperforming BC and FMBC. Sequential extraction indicated that PFMBC promoted the transformation of Cd, Pb, and Zn from acid-soluble and reducible fractions to more stable residual fractions. Characterization via FTIR, SEM, and XRD revealed enhanced surface functional groups and the formation of stable mineral phases. The synergistic effects of phosphate precipitation, iron-manganese oxide adsorption, and surface complexation contributed to the superior stabilization, particularly overcoming the challenge of Zn immobilization. These findings demonstrate that PFMBC is a promising amendment for the remediation of Cd-Pb-Zn co-contaminated soils, offering high efficiency and long-term stability.