Quantification of arsenic adsorption and oxidation on manganese oxides

Oxidation of arsenite (As(III)) by manganese (Mn) oxide minerals commonly found as coatings on aquifer sediments can be an important process controlling arsenic mobility in many aquifers. To date the mechanistic details of As(III) oxidation process have only been described qualitatively in a number of studies, however, these mechanisms vary markedly with respect to the intermediate reactions involved and the products formed. We carried out a detailed geochemical analysis of key literature datasets by translating the known reaction mechanisms into conceptual models of sequentially increasing complexities. These conceptual models were then tested for their feasibility in a numerical modelling framework. The results of this modelling exposed significant limitations in the current conceptual models. Although none of the model simulations produced accurate fit to the data, we were able to ascertain that As(III) oxidation by Mn oxides is most likely a two-step process in which the rate of first oxidation step was dependent only on a small fraction of the Mn oxide initially present in the system. The second rate of oxidation step was slower than first and was responsible for producing Mn(II) ions. The conceptual models can be further improved with data on solid phase characterisation of Mn mineralogy and speciation of adsorbed species during the experiments. This modelling framework provides a good foundation for assessing the influence of other geochemical factors on As(III) oxidation in future research.