Occurrence and Fate of the Pharmaceutical Drug Diclofenac in Surface Waters: Rapid Photodegradation in a Lake

The pharmaceutical drug diclofenac (2-[(2,6-dichlorophenyl)amino]benzeneacetic acid) was detected in rivers and lakes in Switzerland. The data strongly suggest inputs of diclofenac from human medical use via wastewater treatment plants. Interestingly, the concentrations in a major tributary to a lake (Greifensee) were significantly higher (up to 370 ng/L) than those in the outflow of this lake (up to 12 ng/L). It is estimated that more than 90% of the diclofenac entering the lake is eliminated in the lake, most likely by photolytic degradation. Diclofenac was not detected in the sediments of the lake, and in a laboratory experiment, it showed negligible adsorption onto sediment particles. Incubation of lake water, fortified with diclofenac, showed no degradation in the dark, suggesting minimal chemical and biological degradation. However, when the fortified water was exposed to sunlight, rapid photodegradation was observed with a (pseudo) first-order kinetic and a half-life of less than 1 h (October and 47° N latitude). Modeling these experimental data for the situation of Greifensee, the data indicated that photodegradation can account for the rapid elimination of diclofenac in the lake. Several photoproducts were characterized in the laboratory experiments but were so far not detected under the natural conditions in the lake. Whereas photodegradation is often one among several degradation pathways for environmental contaminants, the photolysis experiments and the computer simulation suggested this process to be the predominant one for diclofenac in the lake.