Effective removal of nitrate nitrogen from water and soil using biochar-loaded nano zero-valent iron: performance and mechanisms

Abstract Nitrogen leaching from agricultural soils serves as a main contributor to water pollution and nitrogen losses, posing significant challenges to sustainable agricultural practices. However, existing mitigation strategies often exhibit limited efficiency in reducing nitrate nitrogen (NO 3 − –N) losses and enhancing ammonium nitrogen (NH 4 + –N) retention under varying environmental conditions. To address this, our research investigated the effectiveness of biochar-supported nanoscale zero-valent iron (nZVI@BC) composites. The combination of nZVI with biochar is driven by the need to enhance biochar's adsorption capacity and provide an additional mechanism for nitrate reduction via the strong reductive properties of nZVI. Our findings indicated that nZVI@BC significantly enhanced nitrate reduction and ammonium retention compared to biochar alone, with the nZVI@BC0.6 achieving the best overall performance. The NO 3 − –N concentration in leachate was reduced by up to 71.31%, while NH 4 + –N retention increased by 53.12%, with notable improvements in nitrogen retention even in deeper soil layers. The adsorption kinetics revealed that nZVI@BC composites exhibited both rapid initial adsorption and sustained chemical reduction of NO 3 − –N, with the pseudo-second-order model verifying the dominance of chemical adsorption for nZVI-enriched treatments. The nitrate nitrogen reduction potential of nZVI@BC at various pH levels revealed optimum performance at low pH. Structural analyses (XRD, FTIR, and XPS) showed that nZVI incorporation altered the chemical environment of biochar, enhancing its surface reactivity and functional group availability, which improved nitrogen immobilization efficiency. This study highlights the potential of nZVI@BC composites, particularly nZVI@BC0.6, as an effective and sustainable strategy for mitigating nitrogen leaching, improving nitrogen use efficiency, and addressing agricultural non-point source pollution. Graphical Abstract