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 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 (OERI), the precursor of O2•−, was calculated as (5.0 ± 0.4)‰. High-energy t...