Effects Of Different Oxidants On Tetracycline Degradation Characteristics And Solution Biotoxicity

Tetracycline（TC）is a typical tetracycline antibiotic,which is widely used because of its broad spectrum,low price,high antibacterial activity and few side effects.Studies have shown that animals have a low utilization rate of tetracycline,and after 8-24 hours of use,more than 75%of TC is excreted in the form of maternal molecules or active metabolites,posing potential risks to the natural environment.At present,TC has been widely detected in urban sewage treatment plant effluent,groundwater,surface water,and even drinking water sources,with concentrations ranging from several ng/L to several mg/L,which poses a serious threat to human health and ecological environment.Therefore,how to efficiently remove TC in water has become a subject of great concern.Oxidation is one of the main techniques for TC removal.However,there are few reports on the effects of different oxidants on TC degradation pathway and biotoxicity of treated solutions.In this study,TC was taken as the research object,and four oxidants commonly used in water treatment were selected:potassium permanganate,chlorine,chloramine and chlorine dioxide.The effects of oxidizing agent type,solution pH value on tetracycline oxidation and degradation kinetics,degradation product type,degradation pathway and solution biotoxicity were systematically studied,and the corresponding mechanism was discussed.The main research achievements are as follows:（1）Solution pH,type and concentration of oxidant have great influence on solution REDOX potential（OPR）.When the pH of the solution remained unchanged,the ORP of the solution all showed a trend of rapid increase first（oxidizer concentration:0-0.02 mmol/L）and then slow increase（oxidant concentration:0.02-0.1 mmol/L）with the increase of oxidant dosage.Under the same oxidant dosage condition,the ORP value of solution decreased significantly with the increase of pH.In addition,the type of oxidizer also significantly affects the ORP value of the solution,which follows the order of ORP（Cl O₂）>ORP（Cl₂）>ORP（KMn O₄）>ORP（NH₂Cl）.It should be noted that when low concentrations of oxidants are added to water（the concentration range commonly used in water treatment,such as 0 to0.1mmol/L）,the ORP of the solution is significantly lower than its standard ORP value.（2）Solution pH value and oxidant type would affect the degradation kinetics and degradation pathway of TC.The process of TC degradation by KMn O₄ and NH₂Cl conformed to the quasi-first order reaction kinetics,while the process of TC degradation by Cl₂ and ClO₂ conformed to the second-order reaction kinetics,and both have strong pH dependence.For KMn O₄,Cl₂ and NH₂Cl,the degradation rate of TC decreased gradually with the increase of pH value.When Cl O₂ was used as an oxidant,TC degradation showed an opposite trend and increased with the increase of pH value.Compared with NH₂Cl and KMn O₄,the oxidative degradation rate of Cl₂ and Cl O₂ is 2 orders of magnitude higher.The degradation products of TC under different reaction conditions were qualitatively analyzed by UPLC-MS/MS.It was found that when KMn O₄,Cl₂,NH₂Cl and Cl O₂ were used as oxidants,5,7,8 kinds of 8 main intermediate degradation products were generated respectively.The degradation mechanisms included demethylation,dehydroxylation,nucleophilic substitution,deamido and dimethylamine.In addition,the pH value of solution also affects the types of degradation products,and the differences in the occurrence types of TC(TC⁺,TC⁰,TC^-and TC^2-)and/or oxidant types are the main reasons.（3）Solution pH value and oxidant type would affect the biological toxicity of treatment solution.The biotoxicity of Vibrio fischeri,activated sludge bacteria and Scenedesmus obliquus were evaluated under different reaction conditions.It was found that when Cl₂ or NH₂Cl was used as oxidant,the inhibition of the treatment solution to Vibrio fischeri,activated sludge bacteria and Scenedesmus obliquus all showed an increasing trend,indicating that the oxidation product had higher toxicity than its parent.However,when KMn O₄ and Cl O₂ were used as oxidants,the biotoxicity of the solution increased first and then decreased.The biotoxicity of the main oxidative degradation products was evaluated by ECOSAR software.The simulated biotoxicity had a good correlation with the measured biotoxicity.Based on this,the changes of biotoxicity of the solution were discussed in depth.The results showed that oxidant type and solution pH value not only affected the degradation kinetics of TC,but also affected the degradation path and solution biotoxicity.It provides a theoretical basis for the oxidation removal and health risk assessment of TC in water treatment.