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.