Synergistic adsorption and oxidation of arsenite by Fe–Mn binary oxide-modified bamboo biochar in the presence of air

The adsorption and oxidation processes of arsenite (As(III)) are investigated in this work utilizing bamboo biochar (BC) modified with Fe, Mn, and Fe-Mn binary oxides (FBC, MBC, FMBC). The modification resulted in substantial enhancements to the bamboo biochar's surface area, pore structure, and functional groups. Batch adsorption experiments showed that FMBC achieved the highest As(III) removal capacity, reaching 5.86 mg g^{- 1}, outperforming both FBC and MBC. The kinetics data were fitted with a pseudo second-order model and analyzed using the Langmuir-Freundlich isotherm model, resulting in a good correlation coefficient. Characterization techniques, including XRD, FTIR, XPS, and BET analysis, confirmed successful loading of metal oxides and demonstrated As(III) oxidation to arsenate (As(V)) during adsorption, particularly under oxic conditions. Manganese oxides played a pivotal role in catalyzing the oxidation of As(III), whereas iron oxides predominantly facilitated the adsorption of arsenic (As) species. pH and coexisting anions were found to influence As removal, with FMBC maintaining high performance across a broad pH range. Additionally, the presence of dissolved oxygen (DO) enhanced As(III) oxidation, improving overall removal efficiency. The synergistic action of the Fe-Mn oxides significantly enhanced both the rate and extent of As(III) oxidation, a critical step for subsequent adsorption. These combined features position FMBC as an efficient, cost-effective, and highly promising functionalized biochar for As removal.