Kinetic Consideration of Photochemical Formation and Decay of Superoxide Radical in Dissolved Organic Matter Solutions

The photochemical formation and decay rates of superoxide radical ions (O2 •–) in irradiated dissolved organic matter (DOM) solutions were directly determined by the chemiluminescent method. Under irradiation, uncatalyzed and catalyzed O2 •– dismutation account for ∼25% of the total O2 •– degradation in air-saturated DOM solutions. Light-induced O2 •– loss, which does not produce H2O2, was observed. Both the O2 •– photochemical formation and light-induced loss rates are positively correlated with the electron-donating capacities of the DOM, suggesting that phenolic moieties play a dual role in the photochemical behavior of O2 •–. In air-saturated conditions, the O2 •– quantum yields of 12 DOM solutions varied in a narrow range, from 1.8 to 3.3‰, and the average was (2.4 ± 0.5)‰. The quantum yield of O2 •– nonlinearly increased with increasing dissolved oxygen concentration. Therefore, the quantum yield of one-electron reducing intermediates, the precursor of O2 •–, was calculated as (5.0 ± 0.4)‰. High-energy triplets (3DOM*, E T > 200 kJ mol–1) and 1O2 quenching experiments indicate that 3DOM* and 1O2 play minor roles in O2 •– production. These results are useful for predicting the photochemical formation and decay of O2 •– in sunlit surface waters.