Effects of the antioxidant moieties of dissolved organic matter on triplet-sensitized phototransformation processes: Implications for the photochemical modeling of sulfadiazine.

Previous studies have shown that the photodegradation of some pollutants, induced by the excited triplet states of chromophoric dissolved organic matter (CDOM*), can be inhibited by back-reduction processes carried out by phenolic antioxidants occurring in dissolved organic matter (DOM). Here, for the first time to our knowledge, we included such an inhibition effect into a photochemical model and applied the model predictions to sulfadiazine (SDZ), a sulfonamide antibiotic that occurs in surface waters in two forms, neutral HSDZ and anionic SDZ (pK = 6.5). The input parameters of the photochemical model were obtained by means of dedicated experiments, which showed that the inhibition effect was more marked for SDZ than for HSDZ. Compared to the behavior of 2,4,6-trimethylphenol, which does not undergo antioxidant inhibition when irradiated in natural water samples, the back-reduction effect on the degradation of SDZ was proportional to the electron-donating capacity of the DOM. According to the model results, direct photolysis and OH reaction would account for the majority of both HSDZ and SDZ photodegradation in waters having low dissolved organic carbon (DOC < 1 mgC L). With higher DOC values (>3-4 mgC L) and despite the back-reduction processes, the CDOM* reactions are expected to account for the majority of HSDZ phototransformation. In the case of SDZ at high DOC, most of the photodegradation would be accounted for by direct photolysis. The relative importance of the triplet-sensitized phototransformation of both SDZ and (most importantly) HSDZ is expected to increase with increasing DOC, even in the presence of back reduction. An increase in water pH, favoring the occurrence of SDZ with respect to HSDZ, would enhance direct photolysis at the expense of triplet sensitization. SDZ should be fairly photolabile under summertime sunlight, with predicted half-lives ranging from a few days to a couple of months depending on water conditions.