Arsenite removal in groundwater treatment plants by sequential Permanganate―Ferric treatment

The Dutch drinking water sector is actively investigating methods to reduce arsenic (As) to <1 μg/L in drinking water supply. We investigated (1) the effectiveness of sequential permanganate (MnO_4^¯)–ferric (Fe(III)) dosing during aeration–rapid sand filtration to achieve <1 μg/L As (2) the influence of MnO_4^¯–Fe(III) dosing on pre-established removal patterns of As(III), Fe(II), Mn(II) and NH₄⁺ in rapid sand filters and (3) the influence of MnO_4^¯–Fe(III) dosing on the settling and molecular-scale structural properties of the filter backwash solids. We report that MnO_4^¯–Fe(III) dosing is an effective technique to improve arsenite [As(III)] removal at groundwater treatment plants. At a typical aeration—rapid sand filtration facility in the Netherlands effluent As concentrations of <1 μg/L were achieved with 1.2 mg/L MnO₄^¯–and 1.8 mg/L Fe(III). The optimized combination of MnO_4^¯ –and Fe(III) doses did not affect the removal efficiency of Fe(II), Mn(II) and NH₄⁺ in rapid sand filters, however, the removal patterns of Fe(II) and Mn(II) in rapid sand filter were altered, as well as the settling behaviour of backwash solids. The characterization of backwash solids by Fe K-edge X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) showed that the changed settling velocity of backwash solids with MnO_4^¯–Fe(III) in place was not due to changes in the molecular-scale structure of Fe-precipitates that constitute the major portion of the backwash solids.