Photodegradation of the pharmaceutical drug diclofenac in a lake: Pathway,field measurements, and mathematical modeling

Vertical concentration profiles of diclofenac were measured in Lake Greifen see (Switzerland) under mixed (February/December) and stratified (July) lak e conditions. The concentrations ranged from 1 to 12 ng/L and were lower in summer than in winter, especially near the lake surface, pointing to an ef ficient elimination of diclofenac by photodegradation in the lake. Laborato ry experiments confirmed the rapid photodegradation of diclofenac in water when exposed to sunlight. First-order reaction rates varied seasonally acco rding to actual solar radiation (half-lives, tau = 0.2-1.7 h). The initial photoproduct was 8-chlorocarbazole-1-acetic acid, which photodegraded even faster than the parent compound. Carbazole-1-acetic acid, previously report ed as the main photoproduct, was only formed in the presence of a H-source, such as methanol. In the absence of a H-source and of air, hydroxycarbazol e-1-acetic acid was formed. However, hydroxycarbazole-1-acetic acid was not observed in the presence of air and, thus, under conditions similar to tho se in a lake, likely because of its rapid further photooxidation. Computer simulations using a one-dimensional lake model taking actual solar radiatio n and flushing data of the lake into account confirmed that photolysis is t he predominant elimination pathway for diclofenac in Lake Greifensee. These calculations further showed that the expected concentrations of the photop roduct 8-chlorocarbazole-1-acetic acid are less than the current detection limits of approximately 3 ng/L.