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

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.