Oxidation-controlled doping of molybdenum/iron oxides assisted by reflux for enhancing the adsorption capacity of biochar toward tetracyclines and fluoroquinolones

Metal-doping has emerged as a powerful tool for upgrading the performance of biochar through the synergistic interaction between biochar and metal species. In this work, a controllable incorporation of iron and molybdenum oxides into biochar was achieved via reflux-assisted oxidation of rice straw with phosphomolybdic acid and ferric nitrate followed by pyrolysis to enhance its adsorption capability for tetracycline and fluoroquinolone antibiotics. Through reflux-assisted oxidation, oxygen-containing functional groups were introduced, enabling effective capture and immobilization of molybdenum/iron species. These species were then transformed into uniformly dispersed iron and molybdenum oxides during pyrolysis. Moreover, abundant oxygen-functional groups were retained, while well-developed porous and graphitic structures were simultaneously generated. The biochar composite exhibited 1781.81 and 202.68 mg/g maximum adsorption capabilities for tetracycline and ciprofloxacin, respectively, surpassing the capacities of individually molybdenum-doped and iron-doped biochars. The synergistic effects of molybdenum/iron oxides, oxygen-functional groups, and graphitic domains significantly enhanced adsorption through combined mechanisms including hydrogen bonding, metal complexation, cation exchange, electrostatic interactions, and electron donor–acceptor interactions. Furthermore, the composite displayed high adsorption capacities for other tetracycline and fluoroquinolone antibiotics as well. The proposed oxidation-mediated method enables simultaneous introduction of oxygen functional groups and homogeneous metal oxide dispersion in biochar, yielding superior adsorption performance.