Effects Of Dissolved Matter On Photodegradation Behavior Of Sulfonamide Antibiotics In Estuarine Waters

Antibiotics have been frequently detected in the aquatic environment as emerging pollutants. They are pseudo-persistent and have been proved to induce bacterial resistance, making them of acute concern. Photochemical transformation has been proved to be an important degradation pathway in determining the environmental fate of antibiotics in surface waters. These photochemical processes can be impacted by many dissolved matter in natural waters. So far, studies largely focused on the effects of univariate aqueous dissolved matter on the photochemical behavior of antibiotics, but photochemical processes of antibiotics may be intricate in natural waters where water constituents are concomitant. In estuarine waters, the concentrations of dissolved matter are strongly modified from freshwater downstream into the seawater. However, the effects of water constituent gradients on the photochemical transformation of antibiotics are largely unknown. Therefore, in this study, sulfonamide antibiotics (SAs) were adopted as target compounds to investigate the photolytic kinetics of SAs in the Yellow River Estuary waters. Moreover, mechanisms for the effects of water constituents on the photolytic kinetics of SAs were probed. The primary research contents and results are as follows:(1) Based on dissolved organic matter (DOM), NO3-, and Cl-as target factors, we employed response surface methodology (RSM) to develop the prediction model for the photolytic kinetics of five kinds of SAs (i.e., sulfapyridine, sulfamethazine, sulfachloropyridazine, sulfameter, and acetyl sulfapyridine) in estuarine waters. Results show that DOM is the only statistically significant (p< 0.05) impacting factor on the k values of the SAs under this study; and k follows a parabolic function of DOM for the five SAs. For sulfapyridine, sulfamethazine, sulfachloropyridazine, and sulfameter, the k values increase with increasing the concentrations of DOM when the concentrations of DOM lower than that of vertex. Radical quenching experiments show that 3DOM* is largely responsible for the enhancing effects of DOM on the photodegradation of SAs. For the four SAs, effects of DOM on their k values present an inverse relationship at high DOM concentrations when the concentrations of DOM higher than that of vertex. Laser flash photolysis (LFP) found that the quenching of 3D0M* by DOM leads to the inhibiting effects of DOM on the photodegradation of sulfapyridine, sulfamethazine, sulfachloropyridazine, and sulfameter. For acetyl sulfapyridine, DOM at low concentrations inhibits the photodegradation, while enhances the photodegradation at high DOM concentrations. Steady state photochemical experiments found that acetyl sulfapyridine does not react with 3D0M*, and light-screening effects of DOM are responsible for the inhibiting effects of DOM on the photodegradation of acetyl sulfapyrdine. Furthermore, the quenching of 3DOM* by DOM also results in the photobleaching of DOM, which weakens the light-screening effects of DOM and leads to the enhancing effects of DOM on the photodegradation of acetyl sulfapyridine.(2) Based on the LFP and quantum chemical calculation, mechanism for the reactions of SAs with 3DOM* was investigated. Results show that amino-N and sulfonyl-N atoms of SAs are the reaction sites between 3DOM* and SAs. Density functional theory (DFT) calculation found the electron transfer from the two reaction sites to the carbonyl oxygen atom of 3DOM*, followed by proton transfer, leading to the formation of SAs radicals. Identification of photoproducts for SAs indicates that the radicals can form desulfonated, hydroxylated, and dimerized products. LFP experiment was performed to confirm the formation of SAs radicals, which verified the DFT prediction, i.e., electron coupled proton transfer process between 3DOM* with SAs.(3) Photochemical transformation involved halide ions is one of important pathways for the formation of halogenated organic pollutants in natural environment. However, photoinduced halogenation of SAs has not been reported in previous studies. The effects of halide ions on the photodegradation of sulfapyridine, sulfamethazine, and sulfamethoxazole were investigated by steady state photolysis experiment and DFT calculation under simulated sunlight. Result show that halide ions does not have significant effects on the photodegradation of sulfapyridine and sulfamethoxazole compared to pure water; but significantly promotes the photodegradation of sulfamethazine. DFT calculation shows that triplet-excited sulfamethazine can oxidize Cl-and Br-to generate halide radicals that react with sulfamethazine, leading to the photoinduced halogenation of sulfamethazine. Halogenated intermediates of sulfamethazine were identified by time-of-flight mass spectrum. Furthermore, sulfamethazine can also proceed in photoinduced halogenation in seawater, leading to the formation of chlorided and bromined intermediates of sulfamethazine.In summary, we developed the prediction model as a function of main water constituents to predict the photolytic kinetics of the SAs in the Yellow River estuarine waters; and unveiled the mechanism for the effect of main water constituents on their photodegradation. These results are helpful in deeply understanding the photochemical fate of SAs in estuarine waters.