The influence of environmental conditions on kinetics of arsenite oxidation by manganese-oxides

BACKGROUND: Manganese-oxides are one of the most important minerals in soil due to their widespread distribution and high reactivity. Despite their invaluable role in cycling many redox sensitive elements, numerous unknowns remain about the reactivity of different manganese-oxide minerals under varying conditions in natural systems. By altering temperature, pH, and concentration of arsenite we were able to determine how manganese-oxide reactivity changes with simulated environmental conditions. The interaction between manganese-oxides and arsenic is particularly important because manganese can oxidize mobile and toxic arsenite into more easily sorbed and less toxic arsenate. This redox reaction is essential in understanding how to address the global issue of arsenic contamination in drinking water. RESULTS: The reactivity of manganese-oxides in ascending order is random stacked birnessite, hexagonal birnessite, biogenic manganese-oxide, acid birnessite, and δ-MnO2. Increasing temperature raised the rate of oxidation. pH had a variable effect on the production of arsenate and mainly impacted the sorption of arsenate on δ-MnO2, which decreased with increasing pH. Acid birnessite oxidized the most arsenic at alkaline and acidic pHs, with decreased reactivity towards neutral pH. The δ-MnO2 showed a decline in reactivity with increasing arsenite concentration, while the acid birnessite had greater oxidation capacity under higher concentrations of arsenite. The batch reactions used in this study quantify the impact of environmental variances on different manganese-oxides' reactivity and provide insight to their roles in governing chemical cycles in the Critical Zone. CONCLUSIONS: The reactivity of manganese-oxides investigated was closely linked to each mineral's crystallinity, surface area, and presence of vacancy sites. δ-MnO2 and acid birnessite are thought to be synthetic representatives of naturally occurring biogenic manganese-oxides; however, the biogenic manganese-oxide exhibited a lag time in oxidation compared to these two minerals. Reactivity was clearly linked to temperature, which provides important information on how these minerals react in the subsurface environment. The pH affected oxidation rate, which is essential in understanding how manganese-oxides react differently in the environment and their potential role in remediating contaminated areas. Moreover, the contrasting oxidative capacity of seemingly similar manganese-oxides under varying arsenite concentrations reinforces the importance of each manganese-oxide mineral's unique properties.