Electrocatalytic arsenite oxidation using iron oxyhydroxide polymorphs (α-, β-, and γ-FeOOH) in aqueous bicarbonate solution

α-, β-, and γ-FeOOH polymorphs have been synthesized and used for electrocatalytic As(III) oxidation into less toxic As(V) in aqueous bicarbonate solution. As-synthesized FeOOH electrodes exhibit unique crystalline structures and morphologies. All three FeOOH polymorphs have specific complexation with bicarbonate under alkaline conditions, whereas the complexation disappears at pH 6.7. α-FeOOH is active for both As(III) oxidation and competitive water oxidation, and the Faradaic efficiency (FE) of the latter is the greatest among the polymorphs. Despite the highest current density, β-FeOOH shows the lowest FE values for both As(III) and water oxidation. In contrast, γ-FeOOH exhibits a significantly higher catalytic activity for As(III) oxidation, up to two-fold and five-fold greater FE values compared to α- and β-FeOOH, respectively. The superior activity of γ-FeOOH is attributed to the combined effects of large As(III) adsorption, fast charge transfer, and facile generation of hydroxyl radicals.