Predicting photodegradation rates in environmental waters: quantifying the role of individual degradation pathways

Predicting aquatic photodegradation remains challenging due to the simultaneous occurrence of multiple degradation pathways. While direct photolysis rates can be predicted from molar absorptivity and quantum yield, predicting indirect photodegradation requires quantifying both the bimolecular reaction rate constants with various photochemically produced reactive intermediates (PPRI) - including hydroxyl radicals (˙OH), singlet oxygen (¹O₂), and triplet excited states of chromophoric dissolved organic matter (³CDOM*) - and the steady-state concentrations of the PPRI. Yet, using laboratory measurements of these properties to predict photodegradation in environmental waters and quantify the relative contributions of individual pathways have not been evaluated across diverse chemical structures. In this study, photodegradation rates of 30 pesticides were measured in two CDOM solutions and compared to predicted values. The dominant degradation pathway was predicted to be direct photolysis for five pesticides, ˙OH reactions for five pesticides, and ³CDOM* reactions for 20 pesticides. Nevertheless, predicted rates often overestimated measured rates seemingly because of (1) higher reactivity of the selected triplet excited state model sensitizer (3-methoxyacetophenone, ³3-MAP*) relative to ³CDOM*, (2) the effects of antioxidants, and (3) overestimating reactive [³CDOM*]_ss due to using a probe compound that is more reactive with ³CDOM* than many organic pesticides. Adjusting for these factors, when possible, and accounting for quenching of ˙OH by the probe compound resulted in predicted rates 0.24-13.1 times the measured rates. Reactions with ˙OH became the dominant pathway for most of the pesticides previously predicted to primarily react with ³CDOM*. Based on these results, environmental half-lives under near surface conditions were predicted to range from 0.04 to 202 days across pesticides depending on the dominant pathway and environmental conditions. Notably, pesticides sharing the same dominant degradation pathway had similar t_1/2 ranges, indicating that environmental conditions will have a large influence on potential photodegradation rates. Consequently, identifying the relevant photodegradation mechanisms for a given chemical can be used to more accurately model environment-specific persistence, and this mechanistic approach should be integrated into regulatory frameworks.