Iron and Arsenic Speciation During As(III) Oxidation by Manganese Oxides in the Presence of Fe(II): Molecular-Level Characterization Using XAFS, Mossbauer, and TEM Analysis

The redox state and speciation of the metalloid arsenic (As) determine its toxicity and mobility. Knowledge of biogeochemical processes influencing the As redox state is therefore important to understand and predict its environmental behavior. Many previous studies examined As(III) oxidation by various Mn-oxides, but little is known concerning environmental influences (e.g., coexisting ions) on the process. In this study, we investigated the mechanisms of As(III) oxidation by a poorly crystalline hexagonal birnessite (delta-MnO2) in the presence of dissolved Fe(II) using X-ray absorption spectroscopy, Mossbauer spectroscopy, and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The As K-edge X-ray absorption near edge spectroscopy (XANES) analysis revealed that, at low Fe(II) concentration (100 mu M), As(V) was the predominant As species on the solid phase, whereas at higher Fe(II) concentrations (200-1000 mu M), both As(III) and As(V) were sorbed on the solid phase. As K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) analysis showed an increasing As-Mn/Fe distance over time, indicating As prefers to bind with the newly formed Fe(III)-(hydr)oxides. Both As(III) and (V) adsorbed on Fe(III)-(hydr)oxides as a bidentate binuclear corner-sharing complex. Both Mossbauer and TEM-EDS investigations demonstrated that oxidized Fe(III) products formed during Fe(II) oxidation by d-MnO2 were predominantly ferrihydrite-, goethite-, and ferric arsenate-like compounds. However, Fe EXAFS analysis also suggested the formation of a small amount of lepidocrocite. The Mn K-edge XANES data indicated that As(III) oxidation occurs as a two electron transfer with delta-MnO2 and that the observed Mn(III) is due to conproportionation of surface-sorbed Mn(II) with Mn(IV) in the delta-MnO2 structure. This study reveals that the mechanisms of As(III) oxidation by delta-MnO2 in the presence of Fe(II) are very complex, involving many simultaneous reactions, and the formation of Fe(III)-(hydr)oxides plays a very important role in reducing As mobility.