Oxidation of Chromium (III) and Arsenic (III) By Iron Oxide Coated Birnessite.

Iron and manganese are prevalent in soils and are known to interact with each other under anaerobic conditions.  Fe(II) has been found in the presence of manganese oxides under natural conditions at neutral pH.  Manganese oxides are known to oxidize many metals and metalloids commonly found in soil, including arsenic and chromium, which can be oxidized to As(V) and Cr(VI), respectively, altering their toxicity and mobility through the soil profile.  Past studies have shown that Fe(II) can be oxidized by Mn oxides, coating the manganese oxides with a Fe(III) (hydr)oxides layer. We hypothesize that though the Mn oxide appears to be passivated by an Fe oxide coat, As(III) and Cr(III) oxidation will still occur through electron transfer through the Fe oxide matrix.  To simulate diffusion-limited environment of soil matrices, we used a Donnan reactor to investigate the oxidation of Cr(III) and As(III) by Mn oxides. Two chambers were separated by a polycarbonate filter, where birnessite was placed in one chamber, while Fe(II) was injected into the opposing chamber and allowed to diffuse, adsorb, and oxidize to Fe(III) in the birnessite chamber for 72 hours.  Cr(III) or As(III) was then added to examine subsequent sorption and redox reactions.  The rate of oxidation of As(III) and Cr(III) by Fe oxide-coated manganese oxides over time was determined by measuring the concentration of aqueous species including Mn(II) using ICP-OES, and As speciation and Cr speciation through HG-ICP and DPC colorimetric method, respectively.  Change in adsorbed Cr and As speciation was determined using X-ray absorption near-edge spectroscopy (XANES). This study demonstrates the oxidative impact of Mn oxides in soils in the presence of Fe(II) and the impact these processes have on redox and mobilization of metal contaminants.