Arsenite Depletion by Manganese Oxides: A Case Study on the Limitations of Observed First Order Rate Constants

Abstract Arsenic (As) contamination of drinking water is a threat to global health. Manganese(III/IV) (Mn) oxides control As in groundwater by oxidizing more mobile As III to less mobile As V. Both As species sorb to the Mn oxide. The rates and mechanisms of this process are the subject of extensive research; however, as a group, study results are inconclusive and often contradictory. Here, the existing body of literature describing As III oxidation by Mn oxides is examined, and several potential reasons for inconsistent kinetic data are discussed. The oxidation of As III by Mn(III/IV) oxides is generally biphasic, with reported first order rate constants ranging seven orders of magnitude. Reanalysis of existing datasets from batch reactions of As III with δ-MnO 2 reveal that the first order rate constants reported for As depletion are time-dependent, and are not well described by pure kinetic rate models. This finding emphasizes the importance of mechanistic modeling that accounts for differences in reactivity between Mn III and Mn IV, and the sorption and desorption of As III, As V, and Mn II. A thorough understanding of the reaction is crucial to predicting As fate in groundwater and removing As via water treatment with Mn oxides, thus ensuring worldwide access to safe drinking water. View Full-Tex