Structural transformation of manganese oxides induced by the oxidation of As(III) and Mn(II)

Manganese (Mn) oxides are ubiquitous in natural systems, occurring as reactive nanoparticles with high surface area that play key roles in controlling the fate of trace elements and contaminants. These minerals may effectively scavenge arsenite [As(III)] from solution via oxidation and adsorption reactions, generating solid-phase Mn(II/III) as well as dissolved Mn(II). At redox interfaces, dissolved Mn(II) similarly generates solid-phase Mn(II/III) via adsorption and comproportionation reactions with Mn oxides, inducing structural transformations. It is unclear how the co-reaction of As(III) and Mn(II) with Mn oxides at redox interfaces affects both arsenic oxidation and mineral structure. This study investigates the reaction of As(III) and δ-MnO2, a phyllomanganate mineral that has a turbostratic structure similar to natural biogenic Mn oxides, at pH 4, 7, and 8.5. Dissolved Mn(II) is introduced in some systems to investigate the synergistic and competitive effects of its co-reaction with As(III) on the structure of Mn oxides. Results show that partial adsorption of dissolved As after 2 d, with all remaining dissolved As being oxidized to As(V). The adsorption of As increased with decreasing pH. As(III) was fully oxidized to As(V) in the solid phase, and thus also the system, regardless of pH and dissolved Mn(II) concentration. Adsorbed As(V) occurred as a bidentate, binuclear surface complex with a coordination geometry unaffected by chemical conditions. Reactions of As(III) and Mn(II) with δ-MnO2 each decreased the average manganese oxidation state of the mineral. When these co-occurred, their effects were largely additive. Both Mn(II) and Mn(III) were generated in the solid phase, with a greater relative proportion of Mn(III) at higher pH. Mn(II) induced a partial transformation of δ-MnO2 to nsutite at pH 4, but this was inhibited by As(III) despite being similar to the greater generation of solid-phase Mn(II/III). At pH 7, the reaction with Mn(II) increased layer stacking, with a lesser increase from reaction with As(III) and weak to no change in stacking when dissolved As(III) and Mn(II) initially co-occurred. In contrast, at pH 8.5, reaction with As(III) induced greater layer stacking than dissolved Mn(II), which had little effect, and As(III) and Mn(II) co-existence produced even stronger stacking and initiated a partial conversion of the sheet symmetry from hexagonal to orthogonal. Co-addition of dissolved As(V) and Mn(II) had similar effects as a mixture of dissolved As(III) and Mn(II) at pH 4 and 7 but caused a complete inhibition of structural changes at pH 8.5. This study indicates that the effects of the co-existence of Mn(II) and As(III) on the transformation of Mn oxides are pH-dependent, showing an inhibitory effect at acidic and neutral pH while a synergistic effect under alkaline conditions. This study also suggests that As(III) could act as an important reducing agent, similar to other inorganic redox-active species as well as organic acids, that triggers the reductive transformation of Mn oxides in the natural environment, and may further affect geochemical interactions between Mn oxides and trace metals.