TitleArsenic release metabolically limited to permanently water-saturated soil in Mekong Delta
Publication TypeJournal Article
AuthorsStuckey J., Schaefer M., Kocar BD, Benner S., Fendorf S.
JournalNature Geoscience
Keywordsaquifer, Bangladesh, Carbon, dissolved organic-matter, Groundwater, iron degradation, reactivity, reduction, sediments

Microbial reduction of arsenic-bearing iron oxides in the deltas of South and Southeast Asia produces widespread arsenic-contaminated groundwater. Organic carbon is abundant both at the surface and within aquifers, but the source of organic carbon used by microbes in the reduction and release of arsenic has been debated, as has the wetland type and sedimentary depth where release occurs. Here we present data from fresh-sediment incubations, in situ model sediment incubations and a controlled field experiment with manipulated wetland hydrology and organic carbon inputs. We find that in the minimally disturbed Mekong Delta, arsenic release is limited to near-surface sediments of permanently saturated wetlands where both organic carbon and arsenic-bearing solids are sufficiently reactive for microbial oxidation of organic carbon and reduction of arsenic-bearing iron oxides. In contrast, within the deeper aquifer or seasonally saturated sediments, reductive dissolution of iron oxides is observed only when either more reactive exogenous forms of iron oxides or organic carbon are added, revealing a potential thermodynamic restriction to microbial metabolism. We conclude that microbial arsenic release is limited by the reactivity of arsenic-bearing iron oxides with respect to native organic carbon, but equally limited by organic carbon reactivity with respect to the native arsenic-bearing iron oxides.

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    Kocar Lab
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    <p><span>Professor Benjamin Kocar</span><br><span>Ralph M. Parsons Laboratory for Environmental Science and Engineering</span><br><span>Massachusetts Institute of Technology</span><br><span>15 Vassar Street</span><br><span>Cambridge, MA 02139</span></p>
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    Laboratory for Process-Based Biogeochemistry
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    Our group measures and deciphers biogeochemical processes responsible for controlling the fate and cycling of nutrients, contaminants, and trace gases in soils, sediments, and natural waters. These biotic and biotic reactions are often chemically intertwined and transpire within complex systems containing aqueous solutions, mineral assemblages, gases, and microorganisms.

    Further, they are often linked with other physical and (geo)chemical processes, including hydraulic transport and photochemical reaction pathways. We are particularly interested in these “coupled” processes, since they often dominate pathways controlling the cycling of elements in natural and engineered systems. We perform laboratory experiments to understand the importance of specific (coupled) processes, and use variety of analytical tools, including conventional and synchrotron-based techniques, to understand mechanisms at the molecular scale. A goal of our group is to understand how these reactions “scale-up”, and this is accomplished by linking molecular-scale reactions and processes with observed field measurements using computational tools such as reactive transport modeling. <

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