Mechanistic insights into Sb(iii) adsorption and oxidation on MnO2 facets

Antimony (Sb) contamination from mining and industrial activities poses a major environmental concern. Manganese dioxide (MnO₂) exhibits strong potential for Sb removal from aqueous solutions through adsorption and redox processes. The surface reactivity of MnO₂ facets critically governs interfacial Sb(III) reactions; however, the structural and mechanistic controls underlying facet-dependent reactivity remain unclear. This study investigates Sb(III) adsorption and oxidation on three MnO₂ nanomaterials with distinct exposed facets, δ-001, α-310, and α-110, to elucidate surface structure-dependent mechanisms. Sb(III) adsorption followed the Langmuir isotherm, with maximum capacities of 153.9, 107.3, and 70.1 mg g⁻¹ for δ-001, α-310, and α-110, respectively. The oxidation of Sb(III) to Sb(V) followed the order α-310 (13.2%) > α-110 (10.0%) > δ-001 (2.5%). ATR-FTIR analysis showed that Sb(III) forms inner-sphere surface complexes whose structures evolve during adsorption and transformation. XPS results revealed that Sb(III) oxidation is coupled with Mn reduction, controlled by facet-dependent Mn(III)/Mn(IV) and O_ads/O_lat ratios. XRD confirmed mineral transformations from MnO₂ to MnO(OH) and Mn₃O₄. EPR measurements demonstrated that Mn(II)/Mn(IV) species play key roles in Sb(III) adsorption and oxidation. Overall, the three MnO₂ facets exhibited distinct Sb(III) adsorption and oxidation behaviors. These findings provide new mechanistic insights into facet-dependent redox processes and enhance predictive understanding of Sb transformation at mineral surfaces.