Study On Oxidative Decomposition Behavior Of Chlortetracycline And Secondary Risk Of Disinfection Byproducts During Chlorine Disinfection

Water disinfection is an important part of preventing the spread of waterborne diseases,especially Severe Acute Respiratory Syndrome Coronavirus 2,Intensive disinfection practices during SARS-COV-2 led to the release of large amounts of chlorine-based disinfectants in surface water^[1],which may lead to the formation of DBPs in the presence of Chlortetracycline.DBPs are produced by interactions between organic/inorganic substances and disinfectants added during water treatment and their presence has been widely documented to adversely affect human health.Although there are relatively many studies on DBPs formation induced by chlorine disinfectants in the presence of Dissolved Organic Nitrogen（DON）in water environment,the oxidative decomposition behavior of tetracycline antibiotic CTC during chlorine disinfection and its secondary risk of DBPs are very limited.In this paper,the kinetics of oxidation and decomposition reaction of CTC in chlorine disinfection process was firstly studied.The main products of CTC chlorination reaction were detected by ultra-high liquid chromatography-time-of-flight mass spectrometry,and the main degradation pathways of CTC were analyzed.Then,the secondary risk of DBPs was further explored.ECOSAR 2.0 software was used to simulate the detected ecotoxicity of DBPs,and the mixed toxicity of some DBPs was assessed by concentration plus model and mixed toxicity index method.In addition,the effect on antibiotic resistance genes（ARGs）level transfer and related mechanism of another major secondary risk of DBPs were preliminarily explored.The morphological characteristics of Escherichia coli and the extracellular polymeric substances（EPS）in the plasmid binding horizontal transfer system of Escherichia coli were detected by field emission electric field scanning electron microscopy,three-dimensional fluorescence spectrometer and Qubit fluorescence quantitative analyzer EPS and extracellular DNA content changes,so as to provide some basis for the ways and reasons of DBPs affecting ARGs level metastasis,and to find the appropriate amount of disinfectant to reduce the secondary risk of DBPs.The main conclusions are as follows:（1）The linear correlation constant range of first-order kinetic fit for the chlorination reaction between CTC and sodium hypochlorite was 0.32-0.99.Compared with the linear correlation constant range of second-order kinetic fit and the rapid removal of CTC in the chlorination process,it can be seen that the degradation of CTC in the chlorine disinfection process is more consistent with the quasi-first-order kinetic model.The oxidation and degradation of antibiotic wastewater by sodium hypochlorite is a first-order reaction.The degradation reaction was significantly affected by p H.The degradation rate of CTC was the fastest when p H=8,and the degradation rate of the alkaline condition was generally faster than that of the acidic condition.However,after10 minutes,the degradation rate of chlortetracycline was suddenly accelerated and the degradation was more complete when p H=6,and the total degradation rate was the highest.The degradation of chlortetracycline can be more complete by adding low dose of calcium and magnesium ions,but when the concentration of calcium and magnesium ions increases,it can inhibit the degradation of chlortetracycline.（2）The main products of CTC chlorination are m/z 270,282,326,512,457,610and 832,and the structures of the main products and other possible products are shown in appendix.Based on the existing literature and experimental results,three possible oxidation pathways were speculated.In the first oxidation pathway,the bond between C-11 and C-11A atoms on the C ring of CTC structure is broken,and the hydroxyl catalyzed intramolecular cyclization produces the ester product M/Z 512 and other products.The second oxidation pathway is that hypochlorite or chlorine radical preferentially reacts with three CTC sites,namely two double bonds,dimethylamino and benzene ring,and then further reacts with other pathways.The third possible oxidation pathway is that the dimethylamino group on CTC is first replaced by ketone group,and then the product M449 is formed by substitution reaction.（3）Toxicity simulation of DBPs using ECOSAR 2.0 showed that chlortetracycline and some disinfection by-products produced by sodium hypochlorite had higher acute and chronic toxicity compared with aureomycin alone;The toxicity of these two compounds was simulated by toxicity addition model and concentration index method and it was found that the toxicity of these two compounds had partial additive effect on fish and algae.（4）Compared with no DBPs（horizontal transfer efficiency of 0.00109%）,DBPs produced by the reaction of 4 mg/L CTC and 1.5mg/L sodium hypochlorite had the highest horizontal transfer efficiency of 0.0112%,which was nearly ten times higher than THAT of ARGs.The level transfer efficiency of DBPs produced by the reaction of adding 12 mg/L CTC and 0.9 mg/L sodium hypochlorite was 0.00219%,and the level transfer efficiency of ARGs was nearly 2 times higher than that of blank group.DBPs can affect the horizontal transfer efficiency of ARGs by affecting the bacterial abundance and morphology of Escherichia coli HB101 and Escherichia coli DH5α,EPS and bacterial extracellular DNA.The change of the horizontal transfer efficiency of ARGs is the result of the combined action of these factors.