A mechanistic kinetic model for singlet oxygen mediated self-sensitized photo-oxidation of organic pollutants in water

Singlet oxygen (1O2) mediated self-sensitized photo-oxidation in aqueous solution is a relatively uncommon photodegradation pathway that has been reported for some organic pollutants. Such reactions have been modeled by the empirical first-order kinetics, but significant deviation occurs at high substrate concentrations and in the presence of 1O2 scavengers and in D2O matrices. A mechanistically based kinetic model was developed for 1O2 mediated self-sensitized photo-oxidation by accounting for the sensitized generation of 1O2 and the heterogeneous reaction between the substrate/sensitizer and 1O2 in the photochemical system. The initially formed 1O2 is treated in the model to be spatially correlated with its sensitizer, which could simultaneously undergo reaction and dissociation, instead of being evenly distributed in the solution instantaneously once formed. The oxidation of the substrate/sensitizer by 1O2 occurs in both the geminate pairs and in solution, while the 1O2 in solution is also quenched by the solvent and scavengers present. The model could well describe the solar photodegradation of p-arsanilic acid (p-ASA) under various initial concentrations, and predict the effect of 1O2 scavenger NaN3, and D2O matrices on degradation rate. The performance of the model was also validated by the kinetics of 1O2 mediated self-sensitized photo-oxidation of p-aminobenzoic acid (PABA), l-benzyl-3,4-dihydroisoquinoline, and 3,4-dihydropapaverine reported in the literature. Overall, this kinetic model could help better understand the fundamental processes involved in 1O2 mediated self-sensitized photo-oxidation and predict the photochemical fate of organic pollutants that undergo such photochemical transformation in sunlit surface water.