Synergistic adsorption of ammonia and cadmium on oxychar for sustainable remediation of water and soil

Abstract Cadmium (Cd) contamination in agricultural soils poses a significant threat to environmental sustainability and human health. This study proposes a sustainable remediation approach using oxychar derived from rice straw via a low-temperature partial-oxidation process which is an energy-efficient, waste-free method yielding over 55%. We demonstrate the synergistic co-adsorption of Cd(II) and ammonia (NH₃) on oxychar, with adsorption capacity increasing from 31.7 mg g⁻ 1 (raw straw) to 38.9 mg g⁻ 1 (oxychar), and reaching 53.8 mg g⁻ 1 with oxychar-NH₃ in aqueous solution. In soil incubation, oxychar-NH₃ exhibited the most effective and stable performance, reducing diethylenetriaminepentaacetic acid (DTPA)-extractable Cd by up to 25.2% and achieving an immobilization rate of 69.3% within 20 days. Enhanced microbial respiration, indicated by 49.9% higher CO₂ emissions than the control on day 1, suggests reduced Cd toxicity. Mechanistic analysis identified phenolic groups as key Cd(II) binding sites on oxychar, while on oxychar-NH 3 , the introduced nitrogen-containing functional groups can covalent-like complexes with Cd(II), improving adsorption strength and stability. Economic evaluation revealed that oxychar could reduce total cost by 120 to 310 USD·t⁻ 1 compared to traditional biochar without decreasing the remediation efficiency of Cd. These results highlight oxychar-NH₃ as a scalable, efficient, and cost-effective amendment for remediating Cd-contaminated agricultural soils. Graphical Abstract