| Antibiotics are frequently detected in environmental waters as emerging pollutants. They are of acute concern as their pseudo-persistence and many of them can induce environmental bacterial resistance. In recent years, the combined pollution of antibiotics and metals became prominent, especially in natural waters adjacent to aquaculture and livestock breeding areas. However. the environmental behavior of antibiotics in metal co-contamination waters is still unclear. 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 both undergo acid-base dissociation, and complex with coexisting metals. Thus, they can exist in different dissociation and metal complexing forms. Photochemical degradation has been proved to be a central factor in determining the environmental fate of most antibiotics. Therefore, in this study, ciprofloxacin (CIP, an antibiotic with representative structure and frequently detected in environmental waters) and heavy metal Cu(II) were adopted as a case to study the photolytic kinetics, products and pathways of antibiotics in different dissociation and metal complexation forms by simulated sunlight experiments and density functional theory (DFT) calculations. The purpose of this thesis is to clarify the photochemical behavior and mechanisms of antibiotics in metal co-contamination waters. The main reasearch contents and results are as follows:(1) The molecular structure of CIP has four ionizable functionalities, and CIP may exhibit five dominant dissociation forms in water. In this study, simulated sunlight experiments and matrix calculations were performed to differentiate the absorption spectra and apparent photolytic kinetics for the five dissociation forms of CIP (H4CIP3+, H3CIP2+, H2CIP+, HCIP0and CIP"). The results proved that the five dissociation species do have dissimilar light absorption properties, apparent photolytic rate constants and quantum yields. Under the light source employed, it is quantum yields for the different dissociation species that dominates their apparent photodegradation. Based on the results of product identification and the dissociation species distribution, the major products and pathways for the five dissociation species were proposed. H4CIP3+mainly undergoes stepwise cleavage at the piperazine ring, while H2CIP+mainly undergoes defluorination. For H3CIP2+, HCIP0and CIP-the major photolytic pathway is oxidation. Different from other fluorobenzenes, photoinduced defluorination is a major photolytic pathway for CIP. Thus, DFT calculations were employed to clarify the defluorination mechanisms. All the five dissociation species can defluorinate by the reaction with hydroxide ions (OH-), and H2CIP+can also undergo C-F bond cleavage.(2) 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 five dissociation species with1O2and·OH (K1O2, kOH). The k102values for H4CIP3+, H3CIP2+, H2CIP+and HCIP0(～105L mol-1s-1) is smaller than that of CIP-(～107L mol-1s-1). All the five dissociation species are highly reactive towards·OH (kOH>1010L mol-1s-1). kOH for H2CIP+is the largest, then HCIP0and CIP-, H4CIP3+is the smallest. All the five dissociation species can react with1O2through oxidation cleavage of the piperazine ring. HCIP0and CIP-also can be oxidized to nitro-products. For·OH reactions, proposed degradation pathways involve substituent groups on quinolone ring substituted by·OH, dehydrogenation of·OH adducts, and oxidation cleavage induced by·OH. The total degradation rate constants and half-lives of apparent photodegradation and reactions with1O2and·OH were estimated for the five dissociation species in surface waters at40°N latitude. Apparent photodegradation is the dominant degradation pathway for all the five dissociation species of CIP, and the relative contributions of1O2and-OH reactions are low. In the normal pH range of natural waters (6～9), the total half-lives of CIP range from0.07h (spring and summer, pH=8) to1.40h (winter, pH=6).(3) The influencing mechanisms of Cu(II) on the photochemical behavior of H2CIP+(predominant dissociation species of CIP in the normal pH range of natural waters) were studied systematically by combining simulated experiments with DFT calculations. The results showed that Cu(II) can bond with ketonic and carboxylate oxygen atoms of H2CIP+to form [Cu(H2CIP)(H2O)4]3+with a conditional stability constant of1.23×106L mol-1, and then the apparent photodegradation of H2CIP+was inhibited. The complexation of Cu(II) can alter the atomic charge distributions, molecular orbital components of the light excitation and orbital structures of H2CIP+. As a result,[Cu(H2CIP)(H2O)4]3+shows distinct light absorption properties, slower direct photolytic rate, lower1O2photogeneration ability and weaker reactivity towards1O2. Furthermore, the reaction pathways of direct photodegradation and1O2oxidation for H2CIP+were also altered due to the Cu(II) complexation.In this study, we first differentiated and confirmed that different dissociation and Cu(II) complexation species of CIP have distinct environmental photochemical behavior. Thus, for accurate environmental fate and ecological risk assessment of organic pollutants (e.g., antibiotics), the effects of acid-base dissociation and metal complexation must be considered in metal combined pollution waters. |
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