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

Abstract

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*, <i>E</i>_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.