Reactivity of binary manganese oxide mixtures towards arsenite removal: Evidence of synergistic effects

Highlights: • Hausmannite possess a higher arsenic removal capacity than manganite. • Hausmannite-manganite mixtures show a higher arsenic removal than hausmannite alone. • Arsenic removal processes by hausmannite and manganite show a significant pH effect. • Edge sites of hausmannite and manganite are preferred and initially reacted during As(III) oxidation. • The presence of manganite helps limit hausmannite aggregation and enhances As(III) oxidation. The effects of manganese (Mn) mineral mixtures on arsenite (As(III)) removal (i.e., the sum of As(III) oxidation to As(V) and As species adsorption) were systematically quantified for the first time using varying ratios of hausmannite and manganite, common Mn(III)-containing oxides that often exist as mixtures in natural environments. Due to smaller particle sizes and a higher surface area, hausmannite alone exhibited a total As(III) removal of 8.86 μM m⁻² at pH 5, almost double that of manganite, 4.63 μM m⁻², with initially fast but then subsequently slower As(III) oxidation and Mn(II) production rates. Both minerals showed a substantial decrease in As(III) removal as pH increased. High resolution transmission electron microscopy (HRTEM) analysis showed mineral edge sites initially and preferably consumed for As(III) oxidation. Mixtures of hausmannite and manganite resulted in enhanced As(III) removal (9.62–11.2 μM m⁻²) relative to the single minerals at pH 5, increasing with increasing manganite quantities. The mineral mixtures also displayed two reaction phases, where As(III) oxidation and Mn(II) production were initially fast but then slowed after the first hour. Further, the mineral mixtures produced a Mn(II):As(V) ratio higher than the theoretical two, indicating enhanced mineral dissolution than that of a single Mn oxide. Enhanced reactivity was attributed to the aggregation structure of mixtures, as the presence of manganite effectively limited the aggregation of hausmannite particles, as observed in HRTEM, promoting the exposure of highly active edge sites for the surface-mediated reactions. Thus, mineral mixtures may serve as a better surrogate than a single mineral to examine the extent and magnitude of As(III) removal in natural environments, by more closely reflecting the heterogeneity and complexity in surface interactions and aggregation structures between the minerals.