Arsenite Oxidation Initiated by the UV Photolysis of Nitrite and Nitrate

摘要

This study demonstrates that the production of reactive oxidizing species (e.g., hydroxyl radical (•OH)) during the photolysis of nitrite (NO_2~-) or nitrate (NO_3~-) 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 (γ ＞ 295 nm), which implies a role of NO_2~- as a photosensitizer for As(Ⅲ) oxidation. Nitrate-mediated photo-oxidation of As(Ⅲ) revealed an initial lag phase during which NO_3~- is converted into NO_2~-. UV-Photosensitized oxidation of As(Ⅲ) was kinetically enhanced under acidic pH condition where nitrous acid (HNO_2) 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 NO_3~- to NO_2~- significantly facilitated the catalytic reduction/ oxidation cycling, which enabled the complete oxidation of As(Ⅲ) at the condition of [As(Ⅲ)3/[NO_2~-] 》 1 and markedly accelerated NO_3~--sensitized oxidation of As(Ⅲ). The presence of O_2 and N_2O as electron scavengers enhanced the photochemical dissociation of NO_2~- via intermolecular electron transfer, initiating the oxidative As(Ⅲ) conversion route probably involving NO_2• and superoxide radical anion (O_2•~-) as alternative oxidants. The outdoor experiment demonstrated the capability of NO_2~- for the photosensitized production of oxidizing species and the subsequent oxidation of As(Ⅲ) into As(Ⅴ) under solar irradiation.