As(III) removal and speciation of Fe (Oxyhydr)oxides during simultaneous oxidation of As(III) and Fe(II)

• As(III) significantly slows down the oxidation rate of Fe(II).
• As(III) can be readily oxidized to As(V) during the oxidation of aqueous Fe(II).
• Speciation of Fe (oxyhydr)oxides is dependent on pH and As:Fe ratios.
• As(III) inhibits the development of lepidocrocite and goethite at pH 6.0 and 7.0.
• As(III) significantly inhibits the formation of magnetite at pH 8.0.

Abiotic oxidation of Fe(II) is an important pathway in the formation of Fe (oxyhydr)oxides. However, how can As(III) affect the oxidation rate of Fe(II) and the speciation of Fe (oxyhydr)oxides, and what's the extent of the newly formed Fe (oxyhydr)oxides on the removal of aqueous arsenic are still poorly understood. Oxidation of Fe(II) under neutral pH conditions was therefore investigated under different molar ratios of As:Fe. Our results suggest that co-existence of aqueous As(III) significantly slows down the oxidation rate of Fe(II). Speciation of Fe (oxyhydr)oxides is dependent on pH and As:Fe ratios. At pH 6.0, formation of lepidocrocite and goethite is apparently inhibited at low As:Fe ratios, and ferric arsenate is favored at high As:Fe ratios. At pH 7.0, lepidocrocite gradually degenerates with the increasing As:Fe ratios. At pH 8.0, arsenite significantly inhibits the development of magnetite and favors a formation of lepidocrocite. XPS analysis further reveals that more than half of As(III) is oxidized to As(V) at pH 6.0 and 7.0, whereas at pH 8.0, the rapid oxidation of Fe(II) as well as the rapid formation of Fe (oxyhydr)oxides facilitate a rapid removal of dissolved As(III) before its further oxidation to As(V).

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