Characteristics And Mechanism For The Degradation Of Norfloxacin In Water By MPUV/PAA Process

At present,the problem of antibiotics residue in the source water poses a threat to the safety of the water supply in our country.Conventional treatments are difficult to effectively remove antibiotics from the water.Advanced treatment processes represented by membrane technology and ozone/activated carbon also have the limitations that they cannot completely degrade antibiotics.In order to solve this problem,medium-pressure ultraviolet/peracetic acid（MPUV/PAA）advanced oxidation process was used in this work.Norfloxacin（NOX）was selected as the typical representative of fluoroquinolones in this study.This study evaluated the degradation effect of MPUV/PAA on NOX and explored the mechanism of NOX degradation by MPUV/PAA,which provided a theoretical basis for the application of this technology in practical engineering.The results are as follows:（1）MPUV/PAA had the highest degradation rate of NOX.The order of the degradation rate of NOX in different treatment processes was MPUV/PAA>MPUV/H2O2>MPUV>PAA.PAA is difficult to effectively degrade NOX.When the initial concentration of NOX was 2 mg/L,the concentration of PAA was 10 mg/L and the reaction time was 35 min,the degradation rate of NOX by PAA alone was only 12.50%.However,under the same conditions,the degradation rates of NOX by MPUV alone,MPUV/H₂O₂ and MPUV/PAA were 73.51%,93.63%and 95.91%,respectively.The degradation rates based on UV dose were 0.0026 cm²/m J,0.0057 cm²/m J and 0.0067 cm²/m J,respectively.（2）The degradation efficiency of the MPUV/PAA were influenced by the dosage of oxidant,initial concentration of NOX,p H,and inorganic ions.The increase in the concentration of PAA,Cl^-and HCO₃^-can all increase the NOX degradation rate,and the order of its promotion efficiency was:PAA>chloride ion>bicarbonate.However,the degradation rate of NOX in MPUV/PAA system decreases with the increase of NOX concentration.In addition,the p H value in the MPUV/PAA reaction system has a significant impact on the NOX degradation efficiency.MPUV/PAA has the worst degradation effect on NOX under acidic conditions,and the best degradation effect under alkaline conditions.（3）Two methods,TBA capture method and competitive kinetics,were used to study the contribution rate of direct photolysis,·OH and carbon center free radicals to NOX degradation in the MPUV/PAA system under different p H or different inorganic ion concentrations.The results show that under acidic conditions,the contribution of·OH to NOX degradation is 82.93%-94.44%.With the increase of p H,the contribution rate of·OH keeps decreasing in the system,while the contribution rate of direct photolysis and carbon center radical keeps increasing.When p H=9,the contribution rate of·OH dropped to 7.35%-17.58%,while the contribution rate of direct photolysis rose to43.24%-52.94%,and the contribution rate of carbon center free radicals rose to 39.17%.In addition,under the condition of p H=7,with the increase of Cl^-or HCO₃^-,the contribution rate of·OH and direct photolysis keeps decreasing,while the contribution rate of carbon center free radicals and other free radicals gradually increases.（4）This paper explored the degradation path of NOX under different p H conditions in the MPUV/PAA.The degradation of Norfloxacin in the MPUV/PAA system is mainly through decarboxylation,ring-opening cleavage of piperazine ring,hydroxylation substitution of fluoride ion on benzene ring,cleavage rearrangement of quinolone ring,defluorination and cleavage of piperazine ring,etc.In addition,the degradation products,and pathways of NOX in MPUV/PAA advanced oxidation technology at different p H were different.At p H=7,the degradation products of NOX were the most abundant.