| Manganese oxides (Mn-oxides) are some of the most reactive minerals in the environment, and are known to readily oxidize toxic arsenite (AsIII) to less toxic arsenate (AsV). However, AsIII oxidation by Mn-oxides can be quite complex, involving many simultaneous reactions. Also, when AsIII is oxidized by Mn-oxides, a reduction in the oxidation rate is often observed, which has been attributed to Mn-oxide surface passivation. Although AsIII oxidation by Mn-oxides has been studied, fundamental understanding of the mechanisms of AsIII oxidation, and subsequent Mn-oxide passivation by poorly crystalline, layered Mn-oxides (i.e. phyllomanganates), is lacking. In stirred-flow experiments, AsIII oxidation by delta-MnO 2 (a poorly crystalline phyllomanganate) is initially rapid but slows appreciably as the mineral surface became passivated. MnII is the only reduced product of AsIII oxidation by delta-MnO 2 during the initial period of the reaction, indicating that As III oxidation does not proceed through a MnIII intermediate. Also, X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) show that MnII sorption is the primary cause of delta-MnO 2 passivation during the early periods of AsIII oxidation. There is also evidence that formation of MnIII observed in previous studies is a result of conproportionation of MnII sorbed onto MnIV reaction sites of delta-MnO2. It is possible that MnIII formed through MnII/IV conproportionation also plays a role in delta-MnO2 passivation. Only AsV is observed bound to delta-MnO2 during AsIII oxidation, and it is present in several adsorption complexes that change as the MnIII content in delta-MnO2 increases. Although AsV is directly bound to the delta-MnO 2 surface, a significant fraction is quite mobile. These findings show that AsIII oxidation by poorly crystalline delta-MnO 2 involves several simultaneous reactions and emphasizes the importance of Mn oxidation state in the reactivity of Mn-oxides. Also demonstrated is the value of studying reaction mechanisms over a range of time scales. |
