The oxidation of anoxic As(III)-rich groundwater, which often contains co-occurring Fe(II) and Mn(II), is a simple groundwater treatment method. In this method, an oxidant is introduced to convert aqueous Fe(II) to insoluble Fe(III) (oxyhydr)oxides that bind As and can be removed by filtration. While aeration is inexpensive, dosing O2 to As(III)-rich groundwater often cannot achieve sufficiently low As levels because O2 alone cannot rapidly oxidize As(III) to the readily-sorbed As(V) oxyanion. Therefore, water providers have proposed switching to strong oxidants to meet stringent As limits. In this presentation, the co-oxidation of As(III), Fe(II) and Mn(II) is compared using three common groundwater oxidants: O2, NaOCl and KMnO4. Oxidation experiments were performed in bicarbonate-buffered solutions at pH 7.5 using environmentally-relevant initial concentrations of 90 µM Fe(II), 9 µM Mn(II) and 0.7 µM As(III) (50 µg/L As). The solutions were either aerated (i.e. O2 = 9 mg/L) or were mixed with stoichiometric amounts of NaOCl or KMnO4. Measurements of As, Fe and Mn removal using ICP-MS were combined with solid phase characterization by synchrotron-based As, Fe and Mn K-edge X-ray absorption spectroscopy. The results indicated O2 was able to oxidize Fe(II), yielding Mn(III)-incorporated lepidocrocite, but was unable to decrease As to &lt;10 µg/L from the initial 50 µg/L, which was attributed to inefficient As(III) oxidation. Dosing NaOCl consistently decreased As to &lt;10 µg/L (and often &lt;5 µg/L), and removed As(III) via oxidation and As(V) sorption to co-precipitated Mn(III)-rich hydrous ferric oxide (HFO). KMnO4 addition removed As(III) most effectively, yielding lower residual As concentrations than the NaOCl experiments. The enhanced As removal with KMnO4 coincided with a systematically shorter As-metal bond length in the solids (RAs-Fe/Mn=3.24±0.02 Å) compared to NaOCl addition (RAs-Fe/Mn=3.29±0.02 Å), consistent with As(V) sorption to both HFO and MnO2 produced from KMnO4 reduction. The implications of these results will be discussed in view of optimizing groundwater treatment to meet stringent As limits while considering key particle properties.

Co-oxidation of Fe(II), As(III) and Mn(II) by O2, NaOCl and KMnO4: Contrasting reaction pathways of weak and powerful oxidants

ACS Fall 2021

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