Oxidation and sorption of arsenite by manganese dioxide as influenced by surface coatings of iron and aluminum oxides and calcium carbonate

Abstract

Manganese dioxide (birnessite) was coated with two levels of Fe and Al oxides and CaCO3, and the influence of these coatings on the surface features and the reactivity of MnO2 with respect to the oxidation and sorption of As(III) (arsenite) was examined. For all untreated and coated MnO2 samples, the depletion (oxidation plus sorption) of As(III) by the samples follows first-order kinetics. The rate constants are smaller for the samples with the high levels of coating of Fe and Al oxides and CaCO3 on MnO2 than they are for the untreated MnO2 and the MnO2 with the low levels of coating. The extent of masking of the electron-accepting sites on the MnO2 for converting the toxic As(III) to the less toxic As(V) significantly varies with the kinds and levels of coatings. Coatings of Fe and A1 oxides and CaCO3, on MnO2 distinctively affect the sorption of As. Manganese oxide evidently catalyzes the sorption of As by Al oxide through oxidation of As(III) to As(V). The relative affinities of the oxides of Mn, Fe, and Al and CaCO3, toward As(III) and As(V) account for the coating effects.