Mechanisms On Direct Photolysis Of Antibiotic Enrofloxacin In Aqueous Solutions Using A Microwave Discharge Electrodeless Lamp

Recently,more attentions have been paid on photodegradation of organic pollutants using microwave discharge electrodeless lamps(MDELs)because of the high efficiency.Many researchs generally focused on combination of oxidant and the catalyst with MDELs for remove the pollutants in water,while the research on the direct photolysis by MDELs was scared.Nowadays,the abuse of antibiotics has caused a series of serious environmental problems and endangering human health,there is an urgent need for developing efficient process to remove antibiotics from water.In this paper,enrofloxacin(ENRO)was degraded by direct photodegradation using a MDEL which can emit vacuum ultraviolet(VUV)light(185 nm)and high intensity UVC light(254 nm).Effect of dissolved oxygen(DO)and initial pH on ENRO degradation performance and their reaction mechanisms in different MW/UV systems were studied.The intermediates were detected and its possible degradation pathways were pruposed.At the same time,the chelating phenomenon of ENRO with Fe(Ⅱ/Ⅲ)and its effect on degradation of ENRO in MW/UV process were also studied.The results are as follows:(1)The MW/UV system could efficiently degrade high concentration(50 mg L-1)ENRO in the aqueous phase under acidic conditions(pH=3.0).Degradation of ENRO by MW/UV process under different aeration conditions all followed with pseudo first order reaction kinetics,and the rate constants were 0.095 min-1(O2),0.075 min-1(Air)and 0.083 min-1(N2);Effect of DO on MW/UV mineralization was very significant under three aeration conditions and the TOC removal was 70.3%,61.7%and 46.6%for O2,Air and N2 aeration conditions,respectively,indicating that DO might participate in the mineralization of ENRO.Four carboxylic acids(oxalic acid,oxalic acid,fumaric acid and formic acid)and three inorganic products(F-,NH4+,NO3-)were measured by HPLC and IC,respectively.Fourteen aromatic intermediates were detected by the UPLC-QTOF-MS/MS system.On this basis,the three possible degradation pathways of ENRO in the MW/UV system were proposed,in which ENRO was degraded maily by the way of(ⅰ)fluorine solvolysis,(ⅱ)hydroxylated and(ⅲ)piperazinyl aldehyde.(2)The MW/UV system could also efficiently degrade high concentration(50 mg L-1)ENRO solutions under neutral condition(pH = 7.0).DO and initial pH might affect ENRO degradation in MW/UV process.In the range of pH = 2.0-10.0,the degradation rates of ENRO decreased with the following:O2>Air>N2 in the pH = 2.0-5.0 range,while in the range of pH = 6.0-10.0,the degradation rates of ENRO decreased with the following:N2>Air>O2,which was because of the zwitterionic state of ENRO.The mineralization capacity was following O2>Air>N2 and 46.5%TOC was removed under O2 aeration condition.Also four carboxylic acids and three inorganic ions were analysed by HPLC and IC methods.The degradation of low concentration(5 mg L-1)ENRO could be effectively degraded,and over 90%ENRO was removed within 5 min.The anions(NO3-,SO42-,Cl-,CO32-and HCO3-)had different effects on ENRO in MW/UV process.(3)DO and initial pH had great influence on the reaction mechanism of MW/UV process.By analyzing the H2O2 generated by each system,it was found that the concentration of H2O2 was increased with the increasing DO,while more H2O2 was generated in acid media.The maximum concentration of H2O2 was 41.3 μM in acidic medias and 38.3 μM in neutral condition under O2 aeration condition,respectively;The reaction mechanism of MW/UV process was also verified by means of radical scavenger,the results show that in the different DO conditions,when four kinds of OH scavengers were added into the MW/UV process,lower concentration of H2O2 was generated in the N2 atmosphere conditions,while H2O2 content was significantly higher in O2 and Air atmosphere.Photodegradation of ENRO has slowed down by MW/UV process in the presence of various trapping agents,its rate constant decreased from 0.095 min-1 to 0.057 min-1(Tert-butanol)0.042 min-1(2-propanol),0.031 min-1(Methanol)and 0.019 min-1(N-butanol),suggesting that ·OH plays a significant role on ENRO degradation in MW/UV process.(4)The chelation between ENRO and Fe(Ⅲ)might inhibit to ENRO degradation by MW/UV process.The concentration ENRO and Fe(Ⅲ)as well as initial pH might affect the chelation with a complexation ratio of[ENRO]:[Fe(Ⅲ)]= 1:1,which was determined by Job’s method.Under the same concentration of Fe(Ⅱ/Ⅲ)(0.1 mM),ENRO degradation was decreased to 0.056 min-1(R2=0.996)and 0.048 min-1(R2=0.990)respectively.Under different Fe(Ⅱ/Ⅲ)concentrations,ENRO degradation rate in MW/UV systems decreased with different DO and initial pH,which suggested the presence of Fe(Ⅱ/Ⅲ)might inhibite ENRO degradation.Without Fe(Ⅱ/Ⅲ),the 70.1%TOC was removed after 120 min in MW/UV(O2,pH = 3.0),while TOC removal decreased to 45.4%(0.10 mM Fe(Ⅱ))and to 45.8%(0.10 mM Fe(Ⅲ)),respectively.