Electrocatalytic Arsenite Oxidation Using Iron Oxyhydroxide Polymorphs (Alpha-, Beta-, And Gamma-Feooh) In Aqueous Bicarbonate Solution

alpha-, beta-, and gamma-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. alpha-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 poly -morphs. Despite the highest current density, beta-FeOOH shows the lowest FE values for both As(III) and water oxidation. In contrast, gamma-FeOOH exhibits a significantly higher catalytic activity for As(III) oxidation, up to twofold and five-fold greater FE values compared to alpha and beta-FeOOH, respectively. The superior activity of gamma-FeOOH is attributed to the combined effects of large As(III) adsorption, fast charge transfer, and facile generation of hydroxyl radicals.