Impact of Direct and Indirect Photolysis of Selected Environmentally Relevant Pesticides on Their Fate in River Water and Seawater

Pesticides pose significant hazards to aquatic ecosystems and public health; therefore, understanding their fate in aquatic systems is critically important. Photochemical processes driven by direct and indirect photolysis, particularly hydroxyl radical (•OH) reactions, play a pivotal role in the transformation of these contaminants in natural waters. This study employed an efficient and selective •OH production technique using a high-power UV light-emitting diode (UV-LED) combined with nitrite photolysis to determine the second-order reaction rate constants between •OH and selected pesticides (kX,OH). This approach enabled reliable estimation of the indirect photodegradation rate constants (kIP) for the selected pesticides in aquatic systems. In addition, direct photodegradation rate constants (kDP) of the selected pesticides were determined under simulated sunlight conditions using a solar simulator equipped with a 500 W xenon lamp. The photochemical half-lives of selected pesticides in river water and seawater were calculated from kDP and kIP under assumed steady-state HO• concentrations. The results demonstrated that direct photolysis rate constants of the selected pesticides ranged from 1.62 × 10−7 to 5.52 × 10−4 s−1. The second-order reaction rate constants between the investigated pesticides and •OH ranged from 0.045 × 109 to 14.8 × 109 M−1 s−1. Estimated half-lives under direct photolysis in seawater ranged from hours to days, whereas half-lives attributed to indirect photolysis in seawater extended to several years. In contrast, half-lives of pesticides in river water ranged from hours to days for indirect photolysis. Under the assumed steady-state •OH concentrations, direct photolysis generally produced shorter calculated half-lives than the •OH pathway in seawater, whereas the higher assumed •OH concentration substantially reduced the calculated indirect-photolysis half-lives in river water. These findings should be interpreted as condition-specific kinetic comparisons rather than direct measurements of environmental persistence.