Iron-modified calcium-based biochar for multi-metal stabilization in mine soil: Quantitative mechanistic insights,...

Soils in Chinese mining areas, severely degraded and disturbed, commonly contain high, mixed heavy-metal loads, necessitating the development of highly efficient and durable remediation materials. This study developed an iron-calcium synergistic strategy to synthesize iron-modified calcium-based biochar (Fe-Ca-BC) from waste rice straw and eggshells. Iron oxides provide high specific surface area and metal-binding sites, while calcium components enhance stability through precipitation and ion exchange. In a heavily contaminated soil (Cu: 297.38 mg kg^-1, Pb: 502.13 mg kg^-1, Cd: 21.05 mg kg^-1), a 3 % Fe-Ca-BC application reduced the available fractions of Cu, Pb, and Cd by 75.47 %, 63.93 %, and 43.56 %, respectively, after 60 days. This far outperformed single-component biochar (12.15 %-25.01 % reductions). Mechanistic apportionmen indicated heavy metal stabilization resulted from physical adsorption/ion exchange (10.80 %), co-precipitation (4.77 %), metal complexation (31.28 %), and microbial contributions (10.17 %). Density functional theory calculations confirmed adsorption energies of -0.93 eV (Cu), -1.01 eV (Pb), and -0.89 eV (Cd), indicating spontaneous and stable chemisorption. Furthermore, Fe-Ca-BC significantly improved soil pH, cation-exchange capacity, and soil organic matter, and enriched heavy-metal-tolerant microbial communities (Cupriavidus, Stenotrophomonas, Sphingobacterium), establishing a biochar-driven microbe-material synergy. Fe-Ca-BC emerges as a green, efficient soil remediation material for mining areas, providing new insights into a synergistic stabilization mechanism centered on material-microbe-metal interactions.