Modeling photodegradation kinetics of organic micropollutants in water bodies: A case of the Yellow River estuary

Predicting photodegradation rate constants (k) of pollutants in water bodies is important for assessing their persistence and fate. This prediction used to be based on the k values determined under laboratory conditions that seldom consider underwater downward sunlight attenuation in the field. We studied a procedure to predict k taking the Yellow River estuary and two model chemicals (sulfamethoxazole and acyclovir) as a case. Models were developed for predicting underwater sunlight intensities from optically-active substances. Based on the predicted underwater sunlight intensities, hourly variation of k for the model compounds was predicted as a function of water depth, for a fresh water, an estuarine water and a seawater body in the estuary. Results show that photodegradation half-lives (t_1/2) of the two compounds will be underestimated by dozens of times if underwater downward sunlight attenuation and intensity variation are not considered. Outdoor validation experiments show the maximum deviation between the predicted and measured k values is a factor of 2. The developed models can be employed to predict k of environmental chemicals in coastal water bodies once they are locally calibrated.