Photodegradation Characteristics And Mechanisms Of Typical Quinolone Antibiotics In Ice

As a solid form of water,ice is an important environmental medium in cold regions,which plays an important role in regulating the environmental behavior of pollutants.Photodegradation is one of the important ways to transform pollutants in ice.Many studies have focused on the photodegradation behavior of persistent organic pollutants in cryosphere.Antibiotic is a new pollutant frequently detected in seasonal ice and snow covered water in middle and high latitudes,and its photodegradation characteristics and mechanism in ice are still poorly understood.In this study,four typical quinolone antibiotics with certain environmental persistence were selected as the target antibiotics,namely,ciprofloxacin,norfloxacin,ofloxacin,and pipemidic acid,among which pipemidic acid was the non-fluorine-containing structural analogue.By constructing the photodegradation system of typical quinolone antibiotics in ice under simulated sunlight,the photodegradation characteristics of target antibiotics in pure water ice（ice phase samples frozen from ultra-pure water containing only quinolone antibiotics）were studied.The effects of different p H on the photodegradation of four quinolone antibiotics in pure water ice were studied to analyze the photodegradation characteristics of quinolone antibiotics in water ice with different ionic forms.Evaluated the antibacterial activity of the photodegradation products against Escherichia coli.The effect of fulvic acid on the photodegradation of quinolone antibiotics in ice was investigated by selecting fulvic acid as the representative of dissolved organic matter（DOM）.The photodegradation products,pathways,and mechanisms of four quinolones in ice were investigated by theoretical calculation based on Density Functional theory（DFT）,mass spectrometry,and spectral characterization.The study reached the following conclusions:The increase of light flux was the main reason for the enhanced photodegradation of four quinolones in pure water ice.The increase of light intensity and the decrease of particle size of pure water ice increased the light flux of the four quinolones in pure water ice by 0.7–4.1 times and 2.0–4.1 times,respectively.In addition to direct photodegradation,the four quinolone antibiotics also underwent self-sensitized photodegradation in pure water ice.Direct photodegradation mainly led to the decarboxylation and defluorination while self-sensitized photodegradation mainly led to cleavage and oxidation in the piperazine ring.The defluorination rates of ciprofloxacin and norfloxacin in pure water ice were 2.1 and 1.3 times higher than those in pure water,respectively.Photodegradation rate constants of the four quinolones in pure water ice decreased in the order of zwitterionic morphology,anion morphology,and cation morphology.Quantum yields of quinolones with different ionic forms in pure water ice determined the rate constants and decreased in the order of amphoteric ion,anion,and cation forms.The cleavage/oxidation of piperazine ring was an important photodegradation pathway for the cationic forms of norfloxacin and ofloxacin in pure water ice,while the defluorination at the C-6 position was an important pathway for the zwitterionic and anionic forms of ciprofloxacin and norfloxacin in pure water ice.Decarboxylation was an important pathway for ofloxacin in amphoteric and anionic forms.The products of norfloxacin,ciprofloxacin,and pipemidic acid in pure water ice had little effect on the inhibition rate of Escherichia coli activity,while the products of ofloxacin showed a higher inhibition rate on Escherichia coli activity at the initial period of photodegradation.The photodegradation rate constants of the four quinolones in natural water ice were 2.6–3.7 times higher than that in pure water ice in the role of soluble substances such as NO₃^–、Fe（III）,and fulvic acid.When the molar concentration ratio of fulvic acid to quinolone≤60,the photosensitization of fulvic acid promoted the photodegradation of quinolone antibiotics in ice.Fulvic acid accelerated the cleavage/oxidation of piperazine ring,the formation of oxidation products,and the formation of defluorination products of quinolone antibiotics.The defluorination rates of ciprofloxacin and norfloxacin in ice increased by 33.9%and 11.3%,respectively.When the molar concentration ratio of fulvic acid to quinolone was between 60 and 650,the photodegradation of quinolone antibiotics by fulvic acid changed from promoting effect to inhibiting effect.When the molar concentration ratio of fulvic acid to quinolone ranged from 650 to 2600,the inhibition of the photodegradation of quinolones by fulvic acid was caused by the quenching effect（196.7–51.3%）and light screening effects（0–48.7%）.Fulvic acid inhibited the cleavage/oxidation in piperazine ring by inhibiting the formation of aniline free radical cations.When the molar concentration ratio of fulvic acid to quinolone was?2600,the light screening effect（?50%）was greater than the quenching effect.When the molar concentration ratio of fulvic acid to quinolone was between 650and 2600,fulvic acid combined with quinolone antibiotics in ice by hydrogen bonding,which produced the quenching effect.The binding ability of ciprofloxacin and fulvic acid was the strongest,which made the strongest light quenching effect between ciprofloxacin and fulvic acid.When the molar concentration ratio of fulvic acid to quinolone is≤60,the concentration of singlet oxygen（¹O₂）generated by the excitated fulvic acid was 4.5 times concentrated in the liquid-like layer.¹O2 as an important ROS promoted the photodegradation of four quinolone antibiotics in ice.The ratio of the binding energy of ciprofloxacin,norfloxacin,pipemidic acid,and fulvic acid in the liquid-like layer to that in ice was 0.79,0.76,0.70,and 1.25,respectively.Fulvic acid was more likely to be retained in the liquid-like layer.By comparing the binding energy of quinolones in ice or liquid-like layer and the properties of fluorine substituents,concluded that norfloxacin was easier than pipemidic acid to get into the liquid-like layer.This resulted in the enhanced photodegradation of norfloxacin in ice.This study will help to reveal the environmental behavior and trend of fluorinated antibiotics in cold regions and provide a reference for the research of new environmental pollutants and cryosphere as well as the targeted pollution control.