Unlocking biochar's carbonyl group: Fe-Mn oxide synergy enables high-efficiency As(III) immobilization via non-radical oxidation.

Arsenite (As(III)) contamination in water poses a significant global health threat. The Fe-Mn modified biochar has been reported to be excellent adsorbent for aqueous As(III), yet the synergy between Fe-Mn oxides and biochar still needs further exploration. In this study, Fe₃O₄/MnO₂ composite and MnFe₂O₄ were successfully loaded on biochar, termed as Fe-MnOBC and FeMnOBC, respectively. The adsorption of As(III) was identified as a chemisorption-dominated mechanism on heterogeneous surfaces, with kinetics for Fe-MnOBC and FeMnOBC best described by the pseudo-second-order and Elovich models, respectively, and isotherms well fitted by the Langmuir-Freundlich model, revealing maximum capacities of 36.6 ± 1.2 mg/g and 241.6 ± 4.7 mg/g for Fe-MnOBC and FeMnOBC, respectively. The distribution of As(III) and As(V) and the limited impact of oxygen and reactive oxygen species scavengers on As removal along with electron paramagnetic resonance and X-ray photoelectron spectroscopy spectra demonstrate that As(III) was first adsorbed to Fe-MnOBC and FeMnOBC via inner-sphere complexation with Fe-O and Mn-O, subsequently oxidized to As(V) by Mn(IV)/Mn(III) rather than by reactive oxygen species, and finally immobilized through complexation with C=O. Our study suggested that the loaded Fe-Mn oxides not only provide additional active sites for As(III) adsorption and oxidation, but also enable the original functional group (C=O) to immobilize As, which is a breakthrough overlooked in prior studies. This work provides novel insights into the enhanced performance by modification of Fe-Mn oxides and advances future biochar design for targeted immobilization of potentially toxic elements in environmental systems.