Mechanisms Of Efficient Removal Of Enrofloxacin In Water By The Enhanced Photochemical Oxidation Processes

Due to the abuse of antibiotics,a large number of antibiotics and their metabolites enter the sewage system.However,conventional sewage treatments cannot degrade these pollutants effectively.They may enter the aquatic environment through various ways,which may cause many problems endangering human health,such as bacterial resistance.Therefore,efficient removal of these pollutants is an urgent and important topic of environmental engineering.In this paper,direct photolysis of vacuum ultraviolet(VUV)was developed as an effective process.Five kinds of ultraviolet(UV)light sources with different wavelengths were selected,while Enrofloxacin(ENR)was chosen as a target pollutant.The effects of UV wavelength,VUV intensity and dissolved oxygen(DO)on ENR degradation under neutral(simulated drinking water)and acidic(simulated wastewater)conditions were studied.By adding Fee+/Fe3+,the oxidation ability of the VUV system was greatly improved,and the mechanism of ENR degradation was also investigated.The results are as follows:(1)VUV intensity can affect the degradation and mineralization of ENR in the photodegradation process.Under neutral conditions,ENR could be completely degraded within 90 minutes under the irradiation of five light sources.The results showed that the TOC removal increase with the increasing of VUV intensity,while the mineralization followed the order of VUV(100%)>VUV(50%)>VUV(10%)>UVC.The highest and lowest TOC removal rates were 73.1%and 12.5%,respectively.All the five UV light sources could remove fluorine atoms from ENR molecule completely.Nitrogen in ENR molecule could be converted into NH4'-N,and N03V-N was formed under VUV(50%)and VUV(100%)irradiation.The formed H2O2 concentration also increased with the increasing of VUV intensity.The H2O2 concentration was 9.39 ?M under the irradiation of VUV(100%).9 degradation products were detected under VUV(100%)irradiation.(2)In neutral condition,the photodegradation and mineralization of ENR were affected by DO largely.The degradation of ENR in VUV(100%)system reached 100%,while the mineralization was following O2>80%O2>60%O2>40%O2>Air>N2.The efficiency of fluorine removal can also reach 100%.Nitrogen in ENR molecule could be converted into NH4+-N and NO3--N with the exception of N2 condition.Under the condition of O2,nitrogen conversion rate was the highest,and the total inorganic nitrogen concentration reached up to 2.54 mg L-1.Almost no carboxylic acid was detected in N2 condition,while oxalic acid,oxalic acid,formic acid and fumaric acid were detected in other gas conditions.(3)In acidic medium,DO can also affect the degradation efficiency of ENR.ENR could be degraded completely under VUV(100%)irradiation in 6 gas conditions.The TOC removal increased with the rising of DO.The TOC removal in O2 condition was 83.3%.Oxalic acid,oxalic acid,formic acid and fumaric acid were detected.Yellow flocs were formed with turbidity of 0.83 and 1.04 NTU under N2 conditions in both acidic and neutral mediums,respectively.No similar phenomenon was observed in other gas conditions.(4)The addition of Fe3+/Fe2+ can enhance the ENR removal in UVC photolysis.After adding 0.10 mM Fe3+ or Fee+,the ENR removal rate could be increased from 64.4%to 67.1%or 73.0%,respectively.The rate constants increased from 6.50×1O-3 min-1 to 7.42×10-3 min-1 and 8.32×10-3 min-1,respectively.The TOC removal rate increased from 12.5%to 25.8%and 23.1%.In Fe3+/Fe2+-UVC system,the ENR removal followed Air>O2>N2.(5)The addition of Fe3+/Fe2+ can also enhance the ENR removal greatly in VUV photolysis.100%ENR degradation was reached within 90 min photolysis,while the mineralization efficiency increased from 83.3%to 95.1%(Fe3+)and 92.8%(Fe2+),respectively.Both the degradation and mineralization were following in the order of O2>Air>N2.The formation of oxalic acid,oxalic acid,formic acid and fumaric acid was detected,while 5 degradation products were also detected in the presence of Fe3+/Fe2+.The ENR degradation pathways mainly included:1)photosubstitution;2)N-C bond breaking between quinolone and piperazine ring;3)side chain oxidation of piperazine ring;4)N4-alkyl removal of piperazine ring.