Sorption
and Redox Reactions of As(III) and As(V) within Secondary Mineral
Coatings on Aquifer Sediment Grains
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Abstract
Important
reactive phenomena that affect the transport and fate of many elements
occur at the mineral–water interface (MWI), including sorption
and redox reactions. Fundamental knowledge of these phenomena are
often based on observations of ideal mineral–water systems,
for example, studies of molecular scale reactions on single crystal
faces or the surfaces of pure mineral powders. Much less is understood
about MWI in natural environments, which typically have nanometer
to micrometer scale secondary mineral coatings on the surfaces of
primary mineral grains. We examined sediment grain coatings from a
well-characterized field site to determine the causes of rate limitations
for arsenic (As) sorption and redox processes within the coatings.
Sediments were obtained from the USGS field research site on Cape
Cod, MA, and exposed to synthetic contaminated groundwater solutions.
Uptake of As(III) and As(V) into the coatings was studied with a combination
of electron microscopy and synchrotron techniques to assess concentration
gradients and reactive processes, including electron transfer reactions.
Transmission electron microscopy (TEM) and X-ray microprobe (XMP)
analyses indicated that As was primarily associated with micrometer-
to submicrometer aggregates of Mn-bearing nanoparticulate goethite.
As(III) oxidation by this phase was observed but limited by the extent
of exposed surface area of the goethite grains to the exterior of
the mineral coatings. Secondary mineral coatings are potentially both
sinks and sources of contaminants depending on the history of a contaminated
site, and may need to be included explicitly in reactive transport
models