Arsenite Oxidation Initiated by the UV Photolysis of Nitrite and Nitrate

Abstract

This study demonstrates that the production of reactive oxidizing species (e.g., hydroxyl radical (●OH)) during the photolysis of nitrite (NO₂^-) or nitrate (NO₃^- leads to the oxidative conversion of arsenite (As(III)) to arsenate (As(V)). While the direct UV photolytic oxidation of As(III) was absent, nitrite (20 or 200 μM) addition markedly accelerated the oxidation of As(III) under UV irradiation (λ > 295 nm), which implies a role of NO₂^- as a photosensitizer for As(III) oxidation. Nitrate-mediated photooxidation of As(III) revealed an initial lag phase during which NO₃^- is converted into NO₂^- UV-Photosensitized oxidation of As(III) was kinetically enhanced under acidic pH condition where nitrous acid (HNO₂) with a high quantum yield for ●OH production is a predominant form of nitrite. On the other hand, alkaline pH that favors the photoinduced transformation of NO₃^- to NO₂^- significantly facilitated the catalytic reduction/oxidation cycling, which enabled the complete oxidation of As(III) at the condition of [As(III)]/NO₂^-] ≫ 1 and markedly accelerated NO₃^--sensitized oxidation of As(III). The presence of O2 and N2O as electron scavengers enhanced the photochemical dissociation of NO₂^- via intermolecular electron transfer, initiating the oxidative As(III) conversion route probably involving NO₂^-● and superoxide radical anion (O₂●^-) as alternative oxidants. The outdoor experiment demonstrated the capability of NO₂^- for the photosensitized production of oxidizing species and the subsequent oxidation of As(III) into As(V) under solar irradiation.