IMPACT OF OXIDIZING AGENT ON GROUNDWATER ARSENIC TREATMENT BY AS(III), FE(II) AND MN(II) CO-OXIDATION (Invited Presentation)

Recent evidence suggests exposure to arsenic (As) in drinking water at concentrations even below the 10 μg/L World Health Organization guideline leads to increased cancer rates and diabetes. In response to concern over low-level As exposure, several states in the US (e.g. New Jersey and New Hampshire) and some European countries (e.g. Denmark and The Netherlands) have decreased the target for As in drinking water to as low as 1 μg/L. Therefore, there is a pressing need for reliable treatment methods that can decrease aqueous As as low as possible without major changes to existing drinking water infrastructure.

In this presentation, we compare the impact of three common oxidants used in groundwater As treatment, O₂, NaOCl and KMnO₄, on the co-oxidation of As(III), Fe(II) and Mn(II). 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 (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.

We found that dosing O₂ was least effective, unable to meet the 10 µg/L As guideline for any experiment, which was partly attributed to incomplete As(III) oxidation. When NaOCl was dosed, the residual As levels were always <10 µg/L and often near 1 µg/L. The addition of NaOCl completely oxidized As(III), leading to As(V) removal by sorption to Mn(III)-rich hydrous ferric oxide (HFO) in the binuclear, bridging (²C) geometry. The most effective oxidant was KMnO₄, which produced even lower residual As levels than NaOCl and completely oxidized As(III), Fe(II) and Mn(II) to As(V), HFO and solid-phase MnO₂. The lower residual As levels using KMnO₄ were concomitant with a systematically shorter As-metal interatomic distance detected by As K-edge EXAFS shell-by-shell fits of KMnO₄ samples, which is consistent with As-Mn backscattering from As(V) adsorbed to MnO₂. These results suggest that KMnO₄ plays a dual role in groundwater As treatment: it serves as an oxidant for As(III), Fe(II) and Mn(II) and as source of solid-phase MnO₂ that can bind As(V), leading to more effective As removal.