Arsenite sorption and oxidation by synthetic birnessite
Arsenite (As(III)) is a more toxic form of inorganic arsenic and its removal from drinking water is less effective compared with arsenate (As(V)). In the present study, synthetic birnessite (δ-MnO2) was used to investigate the oxidative transformation of arsenite into arsenate using batch experiments under different reaction conditions. Results showed that arsenite transformation occurred and was accompanied by the sorption and fixation of both As(III) and As(V) on δ-MnO2. Approximately 90% of the arsenite (1.0 mg/L) was transformed by δ-MnO2 at 25 °C and pH 6.0, 49% of which was sorbed and 10% fixed by δ-MnO2. Increased δ-MnO2 dosages promoted the transformation rate of As(III) and sorption of As species. As pH increased from a very low value (pH 2.0), the amount of As(III) oxidized by the tested δ-MnO2 initially decreased, then negatively peaked at pH 4.0, and eventually increased remarkably. At the same time, the oxidation of As(III) by δ-MnO2 had a buffering effect on the pH variation in the solution; the oxidation rate decreased and sorbed As(III) and As(V) increased with the increase in initial arsenite concentration. These findings are of great significance in improving our understanding of As geochemical cycling and in controlling As contamination.
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