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

Mrinal Roy, Case M. van Genuchten, Luuk Rietveld, Doris van Halem

Research output: Contribution to journalArticleResearchpeer-review

6 Citations (Scopus)

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.

Original languageEnglish
Article number119007
Number of pages9
JournalWater Research
Volume223
DOIs
Publication statusPublished - 1 Sept 2022

Keywords

  • Arsenic
  • Drinking water
  • Groundwater
  • Hydrogen peroxide
  • Iron

Programme Area

  • Programme Area 2: Water Resources

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