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 (O-2(center dot-)) in irradiated dissolved organic matter (DOM) solutions were directly determined by the chemiluminescent method. Under irradiation, uncatalyzed and catalyzed O-2(center dot-) dismutation account for similar to 25% of the total O-2(center dot-) degradation in air-saturated DOM solutions. Light-induced O-2(center dot-) loss, which does not produce H2O2, was observed. Both the O(2)(center dot-)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 O-2(center dot-). In air-saturated conditions, the O-2(center dot-) quantum yields of 12 DOM solutions varied in a narrow range, from 1.8 to 3.3%c, and the average was (2.4 +/- 0.5)%c. The quantum yield of O(2)(center dot- )nonlinearly increased with increasing dissolved oxygen concentration. Therefore, the quantum yield of one-electron reducing intermediates, the precursor of O-2(center dot-), was calculated as (5.0 +/- 0.4)%c. High-energy triplets ((DOM)-D-3*, E-T > 200 kJ mol(-1)) and O-1(2) quenching experiments indicate that 3 DOM* and O-1(2) play minor roles in O-2(center dot-) production. These results are useful for predicting the photochemical formation and decay of O-2(center dot-) in sunlit surface waters.