Oxidation of arsenate(III) with manganese oxides in water treatment

Arsenate(III) is the more toxic form of inorganic arsenic and its removal from drinking water is less effective as compared to arsenate(V). Arsenate(III) persists in aerated water, even at high pH, but is easily oxidized by managanese dioxides. The oxidation of As(III) follows a second order rate law with respect to As(III). The reaction rate is effected by the initial molar ratio of MnO2 to As(III), the respective rate order is 1.5. Calcium reduces the rate constant by the order of −0.2, whereas the pH has no influence (pH 5–10). There is no desorption of reduced manganese in the batch tests at high initial molar ratios. Flow-through-tests in small sand filters, loaded with managanese dioxide, showed a decreasing oxidation of As(III) in the first 60 h. Afterwards the oxidation increased again and was quantitative after 10 days. It is concluded that the enhanced oxidation after this initial period is caused by bacteria, as some ubiquitous bacteria are involved in managanese oxidation. An application of this oxidation technique in drinking water treatment should be possible, because As(III)-oxidation is efficient and the release of soluble manganese is low.