CHARACTERIZATION OF SECONDARY MINERAL COATINGS ON CAPE COD SAND GRAINS AND THEIR ROLE IN OXIDATION OF ARSENIC(III)

Many important geochemical reactions occur at the mineral-water interface, including sorption and redox reactions of contaminants. In well-stirred suspensions of Cape Cod aquifer sand grains, it was found that arsenic(V) and (III) sorption and redox reactions took many days to achieve steady-state concentrations and the progress of As(III) oxidation was slow. The reactions primarily took place within secondary mineral coatings on quartz grains that were 20 microns thick and composed of clay minerals cemented with nano-goethite. Microfocused-XRF imaging was used to determine the spatial variations of Fe, Mn, and As concentrations within the coatings and µ-XANES was used to monitor progress in As(III) oxidation as a function of reaction time. The primary oxidant for As(III) appeared to be Mn(III) present in Mn-substituted, nano-goethite in the coatings. As(III) oxidation was greatest at the exterior of coatings, suggesting that it was necessary to release Mn(II) to solution in order to complete the redox reaction.