Arsenite Oxidation by a Poorly Crystalline Manganese-Oxide. 2. Results from X-ray Absorption Spectroscopy and X-ray Diffraction

Arsenite (As(III)) oxidation by manganese oxides (Mn-oxides) serves to detoxify and, under many conditions, immobilize arsenic (As) by forming arsenate (As(V)). As(III) oxidation by Mn(IV)-oxides can be quite complex, involving many simultaneous forward reactions and subsequent back reactions. During As(III) oxidation by Mn-oxides, a reduction in oxidation rate is often observed, which is attributed to Mn-oxide surface passivation. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) data show that Mn(II) sorption on a poorly crystalline hexagonal birnessite (delta-MnO(2)) is important in passivation early during reaction with As(III). Also, it appears that Mn(III) in the delta-MnO(2) structure is formed by conproportionation of sorbed Mn(II) and Mn(IV) in the mineral structure. The content of Mn(III) within the delta-MnO(2) structure appears to increase as the reaction proceeds. Binding of As(V) to delta-MnO(2) also changes as Mn(III) becomes more prominent in the delta-MnO(2) structure. The data presented indicate that As(III) oxidation and As(V) sorption by poorly crystalline delta-MnO(2) is greatly affected by Mn oxidation state in the delta-MnO(2) structure.