The influence of environmental conditions on kinetics of arsenite oxidation by manganese‑oxides
Date
2015-09-16
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Publisher
BIOMED CENTRAL LTD
Abstract
BACGROUND: 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.
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Citation
Fischel, Matthew HH, et al. "The influence of environmental conditions on kinetics of arsenite oxidation by manganese-oxides." Geochemical transactions 16.1 (2015): 1-10.