Influence of Fe(II) on Arsenic(III) Oxidation by Birnessite in Diffusion-Limited Systems

Manganese­(III/IV) oxides are naturally occurring oxidants of arsenic (As) and can transform the more mobile and toxic arsenite [As­(III)] to the less mobile and less toxic arsenate [As­(V)]. However, physical heterogeneity of soils contribute to the formation of redox transition zones which can host the interaction of Fe­(II) with Mn­(III/IV) oxides, leading to altered Mn­(III/IV) oxide reactivity. In the current study, we use a diffusion-controlled reactor to simulate such a redox interface to determine how As­(III) oxidation by the Mn­(III/IV) oxide birnessite is affected by Fe­(II) within transport-limited environments. Our results show that Fe­(II) oxidation by birnessite in diffusion-limited systems forms Fe­(III) (oxyhydr)­oxides with a range of crystallinities, with ferrihydrite being the dominant phase even in the presence of high Fe­(II) concentrations. Fe­(III) (oxyhydr)­oxide formation is concomitant with birnessite transformation and release of Mn­(II), which leads to a decrease in Mn AOS without significant alteration to Mn mineralogy. Using X-ray photoelectron spectroscopy depth-profiling analysis and scanning electron microscopy imaging, we found that even as Fe­(II) is gradually introduced to birnessite, Fe­(III) (oxyhydr)­oxide precipitates did not coat the birnessite surface uniformly upon oxidation but instead formed discrete and unevenly dispersed surface-associated phases, leaving birnessite surfaces exposed to the surrounding solution. Average oxidation state of Mn in birnessite decreased rapidly after exposure to Fe­(II) coincident with a fraction of solids transforming from hexagonal to triclinic birnessite. When As­(III) was added to the diffusion-limited Fe–Mn oxide system, our results showed that pre-exposure of birnessite to high Fe­(II) concentrations leads to a delay in the appearance of As­(V) in solution as compared to oxidation by birnessite exposed to lower Fe­(II). Additionally, the maximum steady state concentration of As­(V)aq was suppressed in the high Fe system. Taken together, these findings show that though pre-exposure of birnessite to high concentrations of Fe­(II) inhibited As­(III) oxidation by Mn oxides within these systems, the precipitation of higher total mass of Fe­(III) (oxyhydr)­oxides in the high Fe system leads to greater retention of As