Quantifying geochemical controls on site-specific reactivity and species-specific passivation during arsenite oxidation by manganese oxides.

Arsenite [As(III)] oxidation by manganese oxides (MnOx) is a key pathway for As attenuation in natural and engineered water systems, proceeding through surface-mediated, multi-step reactions. Despite extensive experimental and conceptual advances, the interdependent geochemical controls governing this pathway remain insufficiently quantified and mechanistically integrated. Here, we address this gap by developing a process-based modeling framework that distinguishes highly reactive Mn(IV) sites from less reactive Mn(III) sites and integrates site-specific redox kinetics with competitive surface complexation. The model was constrained and validated by a near-complete set of published batch experiments spanning diverse pH, MnOx specific surface area (SSA), reagent ratios, and competing cation conditions. Results reveal a biphasic kinetic regime characterized by hierarchical site control, where Mn(III) sites determine baseline oxidation rates, while Mn(IV) sites govern sensitivity to pH and surface passivation. Apparent pH dependence arises primarily from pH-driven redistribution of reactive surface species and competitive site occupation, with a minor but non-zero contribution from proton-dependent kinetics. Elevated pH promotes enhanced Mn(II) adsorption, accelerating passivation of Mn(IV) sites and thus suppressing early-phase oxidation. Reactive site abundance exerts dual, nonlinear control. Increasing SSA enhances oxidation capacity by expanding the pool of reactive sites, whereas As(III)-to-MnOx molar ratio regulates Mn(IV) site saturation and As(III) partitioning onto Mn(III) sites. Coexisting non-redox-active metal cations, e.g., Zn(II), further inhibit oxidation via site competition and enhanced passivation, with effects amplified at higher pH. This framework provides a unified, quantitative description of As(III)-MnOx redox interactions and a transferable basis for predicting transformation dynamics of redox sensitive metal(loid)s across heterogeneous environments.