Non-radical pathway dominated catalytic oxidation of As(III) with stoichiometric H2O2 over nanoceria

Abstract

Heterogeneous catalysis is a promising approach to achieve efficient As(III) oxidation by H2O2 at circumneutral pH. However, radical-attack pathways dominated catalytic As(III) oxidation over most metal oxides is undesirably associated with low utilization of H2O2 induced by rapid self-quenching of radicals. In this study, we developed a non-radical catalytic route to improve H2O2 utilization efficiency for catalytic As(III) oxidation based on nanoceria. The performance and mechanism of As(III) oxidation by H2O2 was investigated by employing four types of nanoceria with different crystallite sizes (9.1–80.6 nm). At the H2O2/As(III) molar ratio of 2.0, nanoceria exhibited crystallite size-dependent catalytic activity for oxidation of As(III) over a wide pH range (5–9). Based on comprehensive study including scavenging, enzymatic, and pretreatment experiments and miscellaneous spectroscopy techniques, the catalytic As(III) oxidation over nanoceria was proved to be mainly mediated by the non-radical Ce-hydroperoxo surface complexes, whilst the surface hydroperoxyl radical served as a minor oxidant. In contrast, the roles of dissolved oxygen, homogeneous H2O2, and OH radical were all negligible. No obvious interconversion of Ce(III)/Ce(IV) was observed by XPS for the catalytic process. Such non-radical pathway enabled a stoichiometric reaction with high H2O2 utilization efficiency (90.5%–122%). Moreover, nanoceria could be recycled for five consecutive runs without desorption/regeneration treatment. This study sheds new light on the development of nanoceria-based catalytic processes for efficient oxidation of As(III). Keywords: Arsenite oxidation, Hydrogen peroxide, Cerium oxide, Hydroperoxo surface complex, H2O2 utilization efficiency, Nano-size effect