Temperature and Hydroxyl Radical Abundance Limit the Photochemical Degradation Kinetics and Photoproducts of Fluridone in High-Latitude Aquatic Systems

Abstract

Temperature is often overlooked as an environmental driver of aquatic pollutant photodegradation kinetics; however, it may strongly impact contaminant persistence in polar climates characterized by low summertime temperatures and near-continuous sunlight. The photochemical degradation of fluridone (FLU), an herbicide applied worldwide to waterways for the eradication of invasive freshwater species, was investigated under simulated subarctic conditions typical of high-latitude surface waters. Temperature had a strong effect on the photochemical degradation of FLU, with half-lives for direct photochemical degradation ranging from approximately 32 h at 22 °C to 71 h at 9 °C under constant irradiation. Assessment of indirect processes involving reactive oxygen species indicated that FLU will primarily react with hydroxyl radicals (·OH) and not singlet oxygen (¹O₂) produced by chromophoric dissolved organic matter (CDOM) in the environment. These results were corroborated by Fenton experiments, resulting in a calculated second order rate constant for the reaction with ·OH of 8.37 × 10⁹ M^–1 s^–1. Photoproduct identification revealed four main pathways for direct and indirect FLU photodegradation. Taken together, this work shows that direct photochemical degradation, which is dominant, is temperature dependent. Also, the interplay between light screening and ·OH production of environmental CDOM, which is site dependent, will strongly influence FLU persistence.