Arsenite Oxidation by a Poorly Crystalline Manganese-Oxide 1. Stirred-Flow Experiments

Manganese-oxides (Mn-oxides) are quite reactive, with respect to arsenite (As(III) oxidation. However, studies regarding the pathways of As(III) oxidation, over a range of time scales, by poorly crystalline Mn-oxides, are lacking. In stirred-flow experiments, As(III) oxidation by (delta-MnO(2) (a poorly crystalline form of hexagonal birnessite) is initially rapid but slows appreciably after several hours of reaction. Mn(II) is the only reduced product of delta-MnO(2) formed by As(III) oxidation during the initial, most rapid phase of the reaction. There seems to be evidence that the formation of Mn(III) observed in previous studies is a result of conproportionation of Mn(II) sorbed onto Mn(IV) reaction sites rather than from direct reduction of Mn(IV) by As(III). The only evidence of arsenic (As) sorption during As(III) oxidation by delta-MnO(2) is during the first 10 h of reaction, and As sorption is greater when As(V) and Mn(II) occur simultaneously in solution. Our findings indicate that As(III) oxidation by poorly crystalline delta-MnO(2) involves several simultaneous reactions and reinforces the importance of studying reaction mechanisms over time.