Biochar-nZVI Mediated Immobilization of Soil As and Cd Reduces Their Gastrointestinal Bioaccessibility and Cytotoxic Potential in Caco-2/HL-7702 Cells.

Soil-bound arsenic (As) and cadmium (Cd) reach the human body through accidental ingestion, yet most risk assessments rely on total soil metal content rather than the fraction that is actually released and absorbed. The present study linked these steps directly by coupling simulated gastrointestinal digestion with human intestinal (Caco-2) and hepatic (HL-7702) cell models and evaluated whether biochar supported nano-zero-valent iron (BC-nZVI) could interrupt the pathway from contaminated soil to cellular injury. SEM-EDS and XRD confirmed the successful doping of crystalline Fe0 nanoparticles on biochar surface. Linear regression revealed strong correlation between DTPA-extractable and total soil As (R2 = 0.9963) and Cd (R2 = 0.9984), confirming DTPA-extraction as a reliable proxy for extractable metal pool. Bioaccessibility increased with increase in co-contamination levels reaching 72.0% for As and 62.2% for Cd at the highest co-contamination level (T3). BC-nZVI significantly reduced bioaccessibility to 27.3% (As) and 17.3% (Cd) at 3% application rate. This immobilization translated to biological protection, where HL-7702 viability (MTT) reduced to 15.8% and membrane damage (LDH release) reached 85.2% at highest co-contamination. BC-nZVI enhanced the cell viability to 94.5% and reduced membrane damage to 49.2% with 3% dose. Multivariate modeling identified bioavailable As as a significant predictor of cytotoxicity (each 1% increase in bioavailable As predicted ≈1.1% reduction in cell viability), while Cd toxicity followed a pH-mediated pathway decoupled from its bioavailable fraction. This study traces the full exposure chain from soil chemistry through gut-liver response to remediation outcomes within co-contaminated system. Therefore, it provides a mechanistic framework for contaminated soil and highlights the potential of BC-nZVI for reducing dietary exposure in co-contaminated agricultural lands.