Valorization of rice husk biochar into Fe- and Mn-modified adsorbents: Contrasting mechanisms of metal oxides in tetracycline remediation

Current studies involve excessively complex synthesis of metal-modified biochar for the adsorption of tetracycline hydrochloride (TC), focusing on the effects of a single strategy and neglecting systematic comparisons. For environmental sustainability, Fe₃O₄/MnOₓ-modified rice husk biochar (BC) was simply synthesized by impregnation-precipitation. Batch adsorption experiments were conducted on BC, Fe-modified (BC-Fe), and KMnO 4 -modified (BC-Mn) biochars to assess their TC removal efficiency. The mechanisms were investigated by FTIR, XRD, XPS, and UV–vis. Results demonstrated BC-Fe and BC-Mn exhibited significantly enhanced TC adsorption capacities compared with BC (170.06/266.94/132.86 mg/g, respectively). This improvement is attributed to increased specific surface area, refined microporous architecture, enriched O-containing functional groups, and the incorporation of Fe 3 O 4 /MnO x nanoparticles. Divergent adsorption behaviors were observed: BC-Fe operates through an endothermic process dominated by diffusion, pore filling, electrostatic adsorption, coordination interactions, and hydrogen bonding; this process is pH-sensitive and further promoted by the presence of SO 4 2− , PO 4 3− and CO 3 2− ions. In contrast, BC-Mn exhibits a stable adsorption mechanism minimally affected by pH or ionic composition, combining diffusion, pore filling, and hydrogen bonding with catalytic oxidation (Mn (III), Mn (IV)) that disrupts and degrades the conjugated structure of TC. Fe/Mn-modified adsorption guides TC targeted remediation and rice husk sustainable utilization. By converting agricultural byproducts into high-performance environmental remediation materials, this study advances the ecological circular economy, realizing the dual value of waste recycling and pollution control.