Photodegradation Of Typical Veterinary Drug In Aqueous Solutions

Veterinary drugs play an active role in protecting animal health and promoting animal growth.In recent years,with the rapid development of massive concentrated animal feeding operation,the variety,dosage and application range of veterinary drugs is increasing year by year.Fluoroquinolone and phenylarsonic compounds are heavyly used veterinary drugs.As both of them are less metabolized in the human or animal bodies,but excreted unchanged,both of them are widely detected in aquatic environment.The chemical structures of most fluoroquinolones contain ionizable groups,3-carboxyl and N-4amino,can undergo acid-base dissociation and form different chemical speciation.Since photolysis is the main pathway of eliminating fluoroquinolones in surface waters,the photodegradation kinetics and photodegradation pathways of fluoroquinolones in different chemical speciation are studied to clarify the mechanism of photochemical conversion in aqueous environments.It is important to investigate the effects of water environmental factors on the photodegradation of fluoroquinolones,so as to describe the general rules of this kind of compunds photodegradate in natural water environment.It has been indicated that direct photolysis are mainly photodegradation pathway for fluoroquinolones while self-sensitized photolysis also plays an important role in photodegradation.Therefore,phenylarsonics which degraded mainly by self-sensitized photolysis was selected as the model compounds for further study this type of photodegradation pathway.The former researches have indicated that main pathway for phenylarsonics is ¹O₂mediated self-sensitized photo-oxidation.In ¹O₂ mediated self-sensitized photodegradation,degradation rate of substrate/sensitizer is determined simultaneously by the reaction between ¹O₂ and substrate/sensitizer in spatially correlated pairs,as well as in in homogeneous solution.It has been know that pseudo-first-order kinetic models and pseudo-second-order kinetic models can not accurately describe the degradation rates of target compouds in self-sensitized photodegradation.To develop a kinetic model which can accurately describe ¹O₂mediated self-sensitized photo-oxidation can effectively support research on the elimination of such pollutants in the water environment,and is extremely important for accurately predicting the fate of various pollutants degraded by ¹O₂ mediated self-sensitized photo-oxidation under different natural conditions.Lomefloxacin,norfloxacin,and ofloxacin were selected as model fluoroquinolones,while p-aminobenzoic acid was choosen as model phenylarsonics,the main research contents and conclusions in this dissertation are as follows:（1）Photodegradation mechanism of lomefloxacin,norfloxacin,and ofloxacin under simulated sunlight irradiation was intensively and systematically investigated.Photodegradation kinetics and quantum yield of 3 model fluoroquinolones was determined.It was indicated that the photodegradation rate of this kind of veterinary drugs was dependent on the electron-withdrawing substituents in C-8.The effect of dissolved oxygen was not consistent for the three model compounds:photodegradation rate of lomefloxacin and norfloxacin was inhibited by dissolved oxygen,while the opposite was observed for photolysis of ofloxacin.The reactive oxygen species quenching experiment of fluoroquinolones in different initial concentration indicated that direct photodegradation is the most important photo-transformation pathway for fluoroquinolones in pure water,though·OH plays a key role in self-sensitized photolysis of these compunds.Photodegradation products for lomefloxacin,norfloxacin,and ofloxacin in different dissociation states were identified,and 7 new intermediate products were first found and 4 mian photo-transformation pathways were proposed（including:defluorination,direct cleavage of piperazinyl ring side ring,photo-oxidation of piperazinyl ring side ring,and decarboxylation）,and the photodegradation pathways for different kind of fluoroquinolones in different dissociation states are structure dependent.Both C-8defluorination and C-6 defluorination are the most important pathway for lomefloxacin and norfloxacin in different dissociation states,respectively（except anionic norfloxacin）;Both C-8 defluorination and C-6 defluorination in zwitterionic form are more prevalent than in the other two forms.At the same time,decarboxylation is one of the most important pathway for zwitterionic and anionic ofloxacin;Besides,direct cleavage of piperazinyl ring side ring is the most important degradation path for three of the target antibiotics in anionic form,while photo-oxidation of piperazinyl ring side ring occurs more easily in the other two forms（except cationic lomefloxacin）.Vibrio fischeri bioluminescence inhibition tests indicated that the photo-transformation products of zwitterionic lomefloxacin was more toxic than the parent compound,while the toxicity of zwitterionic norfloxacin and ofloxacin decreased after photodegradation.（2）Environmental factors effect on photolysis of lomefloxacin,norfloxacin,and ofloxacin was systematically studied based on the actual concentration of dissolved matters in natural water samples and the natural sunlight intensity.The pH dependence of the initial solution of 3 fluoroquinolones under simulated sunlight irradiation was investigated.It can be deduced that photodegradation rate of lomefloxacin,norfloxacin,and ofloxacin under different pHs dependent on the chemical speciation,which from high to low is:zwitterionic form（neutral pH）>anionic form（basic p H）>cationic form（acidity pH）.In the natural water of the urban area of Guangzhou,China,photodegradation rate of fluoroquinolones was rarely effect by the concentration of Na⁺,K⁺,Ca²⁺,Mg²⁺,Cl^-and the temperature of water,but dependent on the pH and light intensity,and to a certain extent promoted and suppressed by NO₃^-and dissolved organic matter,respectively.Direct photolysis is the most important photodegradation pathway for fluoroquinolones in natural water,thouth·OH plays a key role in indirect photolysis of fluoroquinolones.（3）A mechanistically based kinetic model which can adequately describe ¹O₂mediated self-sensitized photo-oxidation was developed.All major steps in ¹O₂mediated self-sensitized photodegradation were incorporated,including reaction between the ¹O₂ and target compunds in both geminate pairs and homogeneous solution,quenching of the ¹O₂ in solution by scavengers and solvent,¹O₂ mediated self-sensitized photo-oxidation,and dissociation of the geminate pairs.Therefore,this kinetic model could accurately describe the photochemical behavior of p-arsanilic acid,and the effect of D₂O matrices and chemical scavengers under simulated sunlight irradiation.Besides,the validation of this kinetic model with literature data indicated that,it can adequately predicting the self-sensitized photolysis of3,4-dihydropapaverine,l-benzyl-3,4-dihydroisoquinoline and p-aminobenzoic acid.In conclusion,photodegradation of 3 model fluoroquinolones（including lomefloxacin,norfloxacin,and ofloxacin）in sunlit surface water through direct,indirect,and self-sensitized photolysis,which mainly by direct photo-transformation.·OH plays a major role in self-sensitized and indirect photolysis of fluoroquinolones.Photodegradation of fluoroquinolones in sunlit surface water dependent on the pH and light intensity,and to a certain extent influence by dissolved matters.The photodegradation pathways for different kind of fluoroquinolones in different dissociation states are structure dependent.A mechanistically based kinetic model which can adequately describe ¹O₂ mediated self-sensitized photo-oxidation was developed.It can illustrate the concentration dependence of the photodegradation rate,and role of ¹O₂ during the self-sensitized photolysis.This kinetic model can also predict the influence of solution chemistry on self-sensitized photodegradation of organic pollutant.Researches above provide strong data support for predicting the environmental fate and ecological risk assessment of veterinary drugs photodecomposed by ¹O₂ mediated self-sensitized photo-oxidation as p-aminobenzoic acid.