Study On Photochemical Behavior And Photocatalytic Degradation Of Sulfamethazine In Aqueous Environment

The wide occurrence of clinical and veterinary antibiotics in natural aquatic environment received extensive scientific interest as well as public awareness recently. The environmental fate of these antibiotics, including transportation and transformation, is still largely unknown up to date. Photodegradation is one of the main abiotic degradation approachs of antibiotics in the aqueous environment.In order to assess the ecological impact of antibiotics on aquatic environments,it is very important to understand their photochemical behavior.On the other hand, in order to control and eliminate the contamination of antibiotics, developing an economic,high efficient and environment-friendly advanced oxidation processes (AOPs) became another research hospot. Graphitic carbon nitride (g-C3N4) has recently been found to be a good visible-light-driven photocatalyst with a series of advantages,for example, it is stable in acidic, neutral and basic solution and easy to be modified.In the present study, sulfamethazine (SMT) has been chosen as the model compound of sulfonamide antibiotics,the photochemical and photocatalytic degradation of SMT in aqueous solutions have been systematically investigated with the purpose of evaluating the photofate as a potential loss process and photocatalysis as a promising treatment technology for degrading it. The main conclusions are as follows:(1) Photochemical degradation mechanism and pathway of SMT were investigated under simulated sunlight irradiation.The quantum yield of sulfamethazine direct photolysis in pH 6.9 buffer solution under simulated sunlight irradiation was determined as 2.92×10-2.The results indicated that the photodegradation pathway of SMT proceed via pseudo first-order kinetics well. Quenching experiments demonstrated that the photodegradation of SMT under simulated sunlight was the sum of the degradation by direct photodegradation and self-sensitization in the aquatic environment,and direct photodegradation which contributed to the main degradation of SMT was faster than self-sensitization.The increase of dissolved oxygen concentration was benefit for self-sensitization while against direct photolysis. The contribution rate of self-sensitization photolysis increased with the increasing of the initial concentration of SMT, resulting in the overall photolysis rate constant decreased with the increase of initial concentration. under different light resource, the photolysis of sulfamethazine was in accord with the first order equation well. The photodegradation rate was found to increase with increasing illumination intensity. The effect of pH on the photodegradation of SMT was confirmed to change the speciation of SMT,that was when SMT existed as SMT+ and SMT-,the rate constant was greater than when it existed as SMT0.Toxicity test indicated that there was no more poisonous photolytic products than SMT generated.Two photodegradation intermediates were detected by HPLC-MS,which were PPl:N-(4,6-dimethylpyrimidine-2-yl)benzene-1,4- diamine(C12H14N4,[M+H]+m/z215),and PP2:2-Amino-4,6-dimethylpyrimidine(C6H9N3, [M+H]+m/z124).Under the energy of photon,the product of SO2 extrusion generated,that was PPl,then reactive oxgen species such as hydroxyl radical attack at the carbon-nitrogen bond of the benzene ring,resulting in the product PP2.(2)Effects of various dissolved matters in aquatic environment on photodegradation of SMT under simulated sunlight were studied.Results were as follows; The inhibiting effect because of photo-competitive effect by NO3- and NO2- was weaker than the indirect reaction which initiated by·OH coming from NO3- and NO2-,so NO3- could promote the photodegradation through promoting the generation of·OH while NO2- could inhibit through reducing the amount of·OH in the solution,the influence of ammonium was weak. Carbonate bicarbonate decreased the photodegradation of SMT under the comprehensive function of pH effect and trapping hydroxyl radical. The scondary reaction rate constant between sulfamethazine and carbonate radical is 2.27×109L/(mol·s) which is lower than that with hydroxyl radical 7.77x109mol/(L·s).Fe3+ and Fe2+ inhibited the photodegradation via attenuating light.The influence of the three investigated surfactants on the photodegradation of SMT under simulated sunlight irradiationwere weak, that was CTAB could weakly promote the photolysis of SMT,while Tween-80 and SDBS could weakly inhibit the photolysis of SMT. Acetone promoted the photodegradation of SMT due to formed 3SMT* and H2O2 promoted the photodegradation of SMT when H2O2 concentration was lower than 0.15mmol/L, while higher than 0.15mmol/L,the degradation rate of SMT was found to be decreased with increasing H2O2 concentration. Sorbic acid inhibited the photodegradation which indicated the photodegradation underwent triplet state. SRFA inhibited the photodegradation via both attenuating light and competing radicals.(3)Moreover, the compound effect of dissolved matter coexist in aquatic environment on photodegradationof SMT under simulated sunlight was investigated. On the condition of various pE values of the aquatic environment,when NO3- and NO2-, NH4+ and NO2- coexisted, there was an obvious antagonistic effect on the photodegradation of SMT.Surfactant CTAB could accelerate the catalysis photodegradation of H2O2 on SMT.When H2O2 coexisted with CTAB,the photodegradation of SMT followed the mechanism of ·OH degradation. N4-acetylsulfamethazine could promote the photodegradation of SMT, while itself’s photodegradation declined.On the condition that the total iron was lmmol/L in the aquatic environment there had an obvious antagonistic action between Fe3+ and Fe2+ on photodegradation of SMT under simulated sunlight.Photocatalytic degradation efficiency of SMT was highly dependent on the properties of the water matrix, such as pH, the presence of organic and inorganic species. River water matrix was found to play a promoting effect on SMT photocatalytic degradation with a shorter irradiation time.(4)Finally, bulk, supported and porous g-C3N4 with various surface morphologies and surface area had been synthesized successfully and were characterized by XRD, TGD, BET and UV-Vis, and SMT was used as a representative pollutant to evaluate the photocatalytic activity of the catalysts under simulated sunlight irradiation.It was seen that g-C3N4 was an active photocatalyst for the reaction, and the photolysis of sulfamethazine was in accord with the first order equation well,and the degradation rate of SMT all reached 70%.Radical scavenging experiments indicated that photogenerated hole (h+) was the predominant reactive species responsible for an appreciable degradation of SMT. Stability tests indicated that the catalyst could be recycled, for at least five times with no loss in catalysis activity, making it an interesting material for commercial applications.Acetone promoted the photocatalytic degradation ofSMT due to formed 3SMT*while H2O2 played a detrimental effect on photocatalysis process.The main photocatalysis products of SMT were identified by using high liquid chromatography-mass spectrometry (HPLC-MS) techniques and possible photocatalytic degradation pathways were proposed.