Kinetics And Pathways For Aqueous ROS Photooxidation Of Sulfonamide And Tetracycline Antibiotics In Different Dissociation Forms

Antibiotics are frequently detected as emerging pollutants in environmental waters. They are of acute concern as their pseudo-persistence and many of them can induce environmental bacterial resistance. The molecular structures of most antibiotics contain both ionizable groups(e.g.,-COOH,-OH, and-NHn) and heteroatoms(e.g., O, N and S). Theoretically, antibiotics can undergo acid-base dissociation. Thus, they can exist in different dissociation forms in waters. Photochemical degradation has been proved to be a central factor in determining the environmental fate of most antibiotics, and the phototransformation may have an impact on their ecotoxicological effects. Therefore, the photochemical transformation of antibiotics need to be investigated so as to assess their the persistence and risks. Therefore, in this study, sulfonamide antibiotics(SAs) and tetracycline(TC) were adopted as a case to study the photolytic kinetics, products and pathways of antibiotics in different dissociation forms by simulated sunlight experiments and matrix calculations. The purpose of this thesis is to clarify the photochemical behavior and mechanisms of antibiotics in waters. The main research contents and results are as follows:(1) The molecular structure of SAs has two ionizable functionalities, and SAs may exhibit three dominant dissociation forms in water. Photooxidation by the reactive oxygen species is an important transformation pathway for many organic pollutants in natural waters, but little is known for the photooxidation activity, products and reaction pathways of antibiotics in different dissociation forms. In this study, competition kinetics method and matrix calculations were performed to determine the second order reaction rate constants of the three dissociation species with ?OH and 1O2(k?OH,S, k1O2,S). The results indicated that the three SAs dissociation forms showed different ROS photooxidation activities, and their bimolecular rate constants for the reaction between SAs and ROS depended on matrix pH. The reaction activities in different dissociation forms with 1O2 varied by orders of magnitude for some SAs. In the normal pH range of natural waters(6 ~ 9), the corresponding environmental half-lives of SAs were calculated to be 14 ~ 39 h for ?OH oxidation and 0.06 ~ 2.22 h for 1O2 oxidation, respectively. Furthermore, the main photooxidation products were enriched by solid phase extraction and identified by Agilent 6410 B triple quadrupole liquid chromatography-mass spectrometry. The different intermediates and pathways were involved for ?OH oxidation, 1O2 oxidation, and direct photolysis. One single product was formed from 1O2 oxidation, while diverse hydroxylated products were generated from ?OH oxidation.(2) The photoreaction kinetics of the different TC dissociation forms were further investigated to weigh the contributions of direct photolysis and ROS photooxidation to the photochemical fate. Firstly, the absorption spectra and direct photolytic kinetics for the three dissociation forms of TC(TCH20, TCH- and TC2-) were examined. The results indicated that the three dissociation species had dissimilar light absorption properties, quantum yields(Φ) and direct photolytic rate constants(k). With the form of dissociation changed from TCH20 to TCH- and TC2-, absorbance spectrum showed redshift. Φ and k value increased gradually. All the three dissociation species were highly reactive towards ?OH. k?OH for TCH- was the largest, then TC2-, and TCH20 was the smallest. Three TC dissociation forms showed different 1O2 photooxidation activities. k1O2 for TC2- was the largest, then TCH-, and TCH20 was the smallest. The reaction activities in different dissociation forms with 1O2 varied by orders of magnitude. The total degradation rate constants and half-lives of direct photodegradation and reactions with 1O2 and ?OH were estimated for the three dissociation species in surface waters at 39° N latitude. Direct photodegradation was the dominant degradation pathway for all the three dissociation species of TC, and the relative contributions of 1O2 and ?OH reactions were low. In the normal pH range of natural waters(6 ~ 9), the total half-lives of TC range from 0.05 h(summer, pH = 9) to 2.88 h(winter, pH = 6).These findings are of importance toward the goal of understanding the multivariate photochemical behavior of different antibiotic dissociation forms in surface waters. Thus, to assess the environmental fate and ecological risk of organic pollutants like SAs and TC, the effects of acid-base dissociation must be considered.