Comparing photodegradation model systems: measuring bimolecular rate constants between photochemically produced reactive intermediates and organic contaminants†

Predicting the environmental fate of anthropogenic chemicals remains a top priority for scientists and regulators; however, these efforts are hindered by the complexity of environmental systems. For example, in aquatic photodegradation, multiple photochemically produced reactive intermediates (PPRI) are present simultaneously, such as hydroxyl radicals (˙OH), singlet oxygen (¹O₂), and triplet excited states of chromophoric dissolved organic matter (³CDOM*). This makes it difficult to isolate contributions of individual PPRI to overall photodegradation as well as to measure bimolecular reaction rate constants with target contaminants (k_PPRI), which could subsequently be used to predict degradation rates under variable environmental conditions and in engineered water treatment systems. As an alternative approach, simplified model systems can be used to isolate reactions with each PPRI. Yet, a systematic comparison of the results obtained in different model systems has not been conducted. In this study, at least two model systems were used to quantify k_PPRI between each PPRI (i.e., ˙OH, ¹O₂, and three ³CDOM* proxies) and each of the 28 pesticides evaluated. Results were consistent for most pesticides across the set of model systems used to evaluate a given PPRI. However, significant discrepancies were observed in some cases. For some pesticides, reactions with ˙OH appeared faster than the diffusion-controlled limit, suggesting additional reactions with unidentified PPRI were occurring. In ¹O₂ model systems, unexpected reactions occurred between some pesticides and the triplet excited states of the model sensitizer. Lastly, there was not a consistent trend between the calculated k_PPRI and the photochemical properties of the three ³CDOM* proxies evaluated, as suggested in previous studies. Overall, the results from this study showed that model systems are a powerful tool for investigating indirect photodegradation reactions and should be adopted in formal evaluations of the environmental fate of anthropogenic chemicals. Key considerations and recommendations to ensure accurate and reliable use of model systems are provided and areas benefiting from further investigation are identified.