In groundwaters, indigenous iron- and manganese-oxidizing bacteria are able to catalyze the oxidation of dissolved manganese [Mn(II)], which can subsequently be removed by filtration of the insoluble products (hydrous manganese oxides). This process results in the creation of a natural coating on the surface of the filter medium, and if arsenic is simultaneously present in the groundwater, it can subsequently be removed by sorption. The results indicate that this method can serve as an alternative treatment option for the removal of low-level arsenic concentrations [35 and 42 mug/L for As(III) and As(V), respectively]. Rapid oxidation of As(III) was observed prior to removal by sorption onto the biogenic manganese oxide surfaces. The rates of As(III) oxidation were estimated, yielding an apparent first-order constant of 0.23 min(-1) (half-life of 3 min). A comparison of these values with the respective data obtained from the literature indicated that the rate of oxidation of As(III) was significantly higher than the rates reported for abiotic As(III) oxidation by manganese oxides, supporting the indication that bacteria play an important role in both the oxidation of As(III) and the generation of reactive manganese oxide surfaces for the removal of As(III) and As(V) from solution. The following sequence of reactions have been found to occur in the treatment system: (a) oxidation of Mn(II) to Mn(IV) and Fe(II) to Fe(III), (b) oxidation of As(III) to As(V), (c) precipitation of manganese(IV) oxides, (d) abiotic oxidation of As(III) by manganese(IV) oxides, and (e) As(V) sorption by manganese(IV) oxides, where steps a and b are biotic and steps c-e are abiotic. The presence of phosphates at concentrations of around 600 mug/L had an adverse effect on As(III) removal (competitive adsorption), reducing the overall removal efficiency by 50% (from 80 to 30%), although it did not affect the oxidation of As(III).