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Competitive microbially and Mn oxide mediated redox processes controlling arsenic speciation and partitioning
Samantha C Ying1, Benjamin D Kocar, Sarah D Griffis
1Department of Environmental & Earth System Science, Stanford University, Stanford, California 94305, United States.
Environmental Science & Technology
|June 9, 2011
Summary
Arsenic cycling in soils is hindered when manganese oxide surfaces become passivated by microbial byproducts. This study shows microbial reduction and mineral precipitation limit arsenic redox reactions.
Area of Science:
- Environmental Science
- Geochemistry
- Microbiology
Background:
- Redox processes control arsenic (As) speciation and partitioning in soils and sediments.
- Redox gradients drive competitive As reduction-oxidation reactions, influencing its environmental fate.
Purpose of the Study:
- To determine the fate and redox cycling of arsenic at the interface of birnessite (a manganese oxide) and dissimilatory As(V)-reducing bacteria.
- To investigate As redox dynamics in a controlled, diffusively limited system.
Main Methods:
- Utilized a Donnan reactor to isolate birnessite and Shewanella sp. ANA-3 with a semipermeable membrane.
- Tracked As migration, oxidation, and reduction between chambers.
- Employed modeling and experimental data to analyze surface passivation.
Main Results:
- Arsenic(III) was rapidly oxidized to As(V) by birnessite, and As(V) was reduced to As(III) by bacteria, establishing continuous As cycling.
- A rapid decline in As(III) oxidation rate was observed due to birnessite surface passivation.
- High Mn(II) and carbonate from microbial respiration led to rhodochrosite precipitation, passivating the birnessite surface.
Conclusions:
- Microbial As(V) reduction and the formation of passivating metabolites limit the capacity of birnessite to oxidize As(III).
- Synergistic effects create biogeochemical conditions deviating from thermodynamic predictions.
- Surface passivation mechanisms are critical for understanding arsenic redox cycling in subsurface environments.
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