Adsorption and oxidation of arsenic by two kinds of β-MnO2.

MnO2 is one of the most widespread and cheapest materials in nature that can both adsorb arsenic and oxidize arsenite [As(III)] to arsenate [As(V)]. In this study, column β-MnO2 [CM] with the main facet of {110} and pincer β-MnO2 [PM] with the facets of {110} and {101} are synthesized and used to remove arsenic in water under different conditions. For the adsorption process, the experimental data are fitted well with the pseudo second-order kinetic model; the Langmuir model is better than the Freundlich model to describe the adsorption equilibrium isotherms. Furthermore, the As(III) oxidation rate can be denoted by the pseudo zero-order kinetic model and is related to the O2 concentration, the pH value, the light source and the initial concentration of As(III). Finally, the oxidation mechanism is investigated, and the oxidant should be related to O2. It is interesting to find that these two kinds of β-MnO2 exhibit different pH effects for both adsorption and oxidation. For As(III), the adsorption and oxidation abilities of CM follow the order pH 9 > pH 7 > pH 4, whereas the adsorption and oxidation orders of PM are pH 4 > pH 7 > pH 9.