Iron modified biochar derived from diverse feedstock: Enhancing denitrification and mechanistic insights into the detoxification and removal of Cu and Pb.

Simultaneous removal of nitrate and heavy metals (HMs) from wastewater is a critical challenge due to their distinct chemical behaviors and impacts on microbial processes. Biochar-based materials have emerged as promising multifunctional platforms for integrated pollutant remediation. This study investigates the dual functionality of iron-modified biochar (prepared from rice husk, waste shell, and crayfish shell) in enhancing denitrification by Aquabacterium sp. XL4 and removing toxic Cu and Pb. Under low C/N ratios, iron-modified biochar, particularly rice husk-derived (NRHB), significantly improved nitrate removal efficiency (NRE) by 24.6 % via electron transfer and iron redox cycling. NRHB exhibited superior adsorption capacities (29.78 mg/g for Cu and 29.95 mg/g for Pb) through chemisorption and monolayer mechanisms. Low Cu concentrations enhanced denitrification by accelerating Fe oxidation, while Pb exhibited higher toxicity, reducing microbial activity. Co-existing Cu and Pb mitigated Pb inhibition via synergistic adsorption and bio-precipitation. Biochar aging released humic/fulvic acids, alleviating metal toxicity and promoting microbial metabolism. Characterization confirmed FeO loading, surface roughness, and CaCO participation in metal immobilization. The findings highlight iron-modified biochar as a sustainable solution for simultaneous nitrate and HMs remediation in wastewater.