Groundwater-native Fe(II) oxidation prior to aeration with H₂O₂ to enhance As(III) removal

Abstract

Groundwater contaminated with arsenic (As) must be treated prior to drinking, as human exposure to As at toxic levels can cause various diseases including cancer. Conventional aeration-filtration applied to anaerobic arsenite (As(III)) contaminated groundwater can remove As(III) by co-oxidizing native iron (Fe(II)) and As(III) with oxygen (O₂). However, the As(III) removal efficiency of conventional aeration can be low, in part, because of incomplete As(III) oxidation to readily-sorbed arsenate (As(V)). In this work, we investigated a new approach to enhance As(III) co-removal with native Fe(II) by the anaerobic addition of hydrogen peroxide (H₂O₂) prior to aeration. Experiments were performed to co-oxidize Fe(II) and As(III) with H₂O₂ (anaerobically), O₂ (aerobically), and by sequentially adding of H₂O₂ and O₂. Aqueous As(III) and As(V) measurements after the reaction were coupled with solid-phase speciation by Fe and As K-edge X-ray absorption spectroscopy (XAS). We found that complete anaerobic oxidation of 100 µM Fe(II) with 100 µM H₂O₂ resulted in co-removal of 95% of 7 µM As(III) compared to 44% with 8.0-9.0 mg/L dissolved O₂. Furthermore, we found that with 100 µM Fe(II), the initial Fe(II):H₂O₂ ratio was a critical parameter to remove 7 µM As(III) to below the 10 µg/L (0.13 µM) WHO guideline, where ratios of 1:4 (mol:mol) Fe(II):H₂O₂ led to As(III) removal matching that of 7 µM As(V). The improved As(III) removal with H₂O₂ was found to occur partly because of the well-established enhanced efficiency of As(III) oxidation in Fe(II)+H₂O₂ systems relatively to Fe(II)+O₂ systems. However, the XAS results unambiguously demonstrated that a large factor in the improved As(III) removal was also due to a systematic decrease in crystallinity, and thus increase in specific surface area, of the generated Fe(III) (oxyhydr)oxides from lepidocrocite in the Fe(II)+O₂ system to poorly-ordered Fe(III) precipitates in the Fe(II)+H₂O₂ system. The combined roles of H₂O₂ (enhanced As(III) oxidation and structural modification) can be easily overlooked when only aqueous species are measured, but this dual impact must be considered for accurate predictions of As removal in groundwater treatment.