ABSTRACT
The co-removal of Fe(II), As(III) and Mn(II) during oxidation by O2, NaOCl and KMnO4 was investigated by combining ICP-MS measurements of ion uptake and solid phase characterization by X-ray absorption spectroscopy. We studied O2, NaOCl and KMnO4 as oxidants because they have different reaction rates with reduced species (O2 < NaOCl < KMnO4) and all have potential use in groundwater treatment. Our results indicated that the oxidant identity largely controlled the solid phase speciation and efficiency of Fe(II), As(III) and Mn(II) removal. In the O2 system, Fe(II) oxidation was complete (0.5 h reaction time), but As(III) and Mn(II) were only partially oxidized and removed, producing Mn(III)-incorporated lepidocrocite with sorbed As(III,V). Co-oxidation was more effective with NaOCl, but a significant fraction of sorbed Mn(II) was still present in the reaction products, which consisted of As(V)-sorbed hydrous ferric oxide (HFO). KMnO4 was the most effective oxidant, producing solids containing a mixture of As(V)-sorbed HFO and MnO2. However, the solids produced in the KMnO4 system were colloidally stable in solutions free of bivalent cations (i.e. Ca) and difficult to remove from the treated water, which is explained by the highly negative charge of MnO2 at neutral pH preventing particle aggregation. Our results suggest a potential tradeoff between effective Fe(II), As(III) and Mn(II) co-oxidation with KMnO4 in soft waters and reduced particle trapping in rapid sand filters. This diversity in particle behavior must be considered when enhancing As(III) oxidative removal in water treatment, especially with KMnO4.
