Synergistic cadmium immobilization and maize growth promotion by chitosan modified biochar loaded with Bacillus subtilis composites: mechanistic insights and life-cycle assessment.

Cadmium (Cd) contamination endangers soil health and food safety, with conventional biochar-microbe composites limited by low bacterial loading and poor microbial persistence. This study developed a novel chitosan-modified biochar loaded with Bacillus subtilis (CSBC@BS.sp) for synergistic Cd immobilization and maize growth promotion in contaminated soil. Chitosan modification introduced abundant Amino (-NH2) and hydroxyl group (-OH), enhancing biochar bacterial loading capacity and Cd2+ complexation. The composite significantly reduced DTPA-extractable Cd by 57.5%, acid-soluble Cd by 62.1%, and uptake in maize, while increasing plant biomass (92.5%). Mechanistic investigations revealed that CSBC@BS.sp facilitated microbial colonization, enriched beneficial bacteria, promoted extracellular polymeric substance (EPS) secretion, and upregulated metabolic pathways related to metal chelation and nutrient cycling. Life-cycle assessment (LCA) confirmed the CSBC@BS.sp low environmental impact and cost-effectiveness ($26.5/kg). The work demonstrates a sustainable soil amendment strategy leveraging biochar-microbe synergy for effective heavy metal remediation and agricultural enhancement.