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 ^•- ) in irradiated dissolved organic matter (DOM) solutions were directly determined by the chemiluminescent method. Under irradiation, uncatalyzed and catalyzed O ₂ ^•- dismutation account for ∼25% of the total O ₂ ^•- degradation in air-saturated DOM solutions. Light-induced O ₂ ^•- loss, which does not produce H ₂ O ₂ , was observed. Both the O ₂ ^•- 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 ^•- . In air-saturated conditions, the O ₂ ^•- 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 O ₂ ^•- nonlinearly increased with increasing dissolved oxygen concentration. Therefore, the quantum yield of one-electron reducing intermediates, the precursor of O ₂ ^•- , was calculated as (5.0 ± 0.4)‰. High-energy triplets ( ³ DOM*, E _T > 200 kJ mol ^-1 ) and ¹ O ₂ quenching experiments indicate that ³ DOM* and ¹ O ₂ play minor roles in O ₂ ^•- production. These results are useful for predicting the photochemical formation and decay of O ₂ ^•- in sunlit surface waters.