Process And Mechanism Of Oxytetracycline Degradation And The Effect Of Enterbacter Hormaechei Resistance In Electro-fenton System With Modified Graphite-felt Cathode

In recent years,there is rapid developement and gradual expandment in China’s livestock industry whose wastewater has become an important factor in environmental pollution.The traditional biological technology has some problems such as long reaction time,high energy consumption,ineffective in low chemical oxygen demand(COD_Cr)concentration and biorefractory substance（such as antibiotics）.Cathode electro-Fenton technology as an electrochemical advanced oxidation processes（EAOPs）,is environment-friendly technology for degrading recalcitrant pollutants,which can effectively improve the biodegradability of livestock wastewater.The electrochemical treatment efficiency of wastewater depended on electrode materials and modification of electrodes.Therefore,it is a hot research topic to explore new electrode materials and modification methods with good performace.In this paper,the problems of residual antibiotics（oxytetracycline,OTC）,antibiotic-resistant bacteria（ARB,Enterbacter hormaechei）and antibiotic-resistance gene（ARG）in livestock wastewater were investigated.The sodium hydroxide activated graphite felt（Na OH-GF）cathodes was used to degrade OTC in electro-Fenton（EF）systems.Moreover,the performance of Na OH-GF impacting on the activity of Enterbacter hormaechei and the abundance of ARGs were evaluated.After the characterization of materials,the main operating parameters affecting the degradation kinetics of OTC,such as the applied current densities and the initial concentration were investigated.Besides,the degradation pathway and mechanisms of the OTC were illuminated.At the same time,the activity of Enterbacter hormaechei and the abundance of ARGs（tet A,tet O,sul II,EBC,FOX）were investigated in EF and non-EF,respectively.The changes of OD₆₀₀,conductivity,p H,bacterial morphology and ATP which may influence the mechanism of migration of Enterbacter hormaechei and ARGs was discussed.The main findings were as follows:The scanning electron microscopy（SEM）images of the GF showed that the surface of Na OH-GF was much rougher than that of Raw-GF.The increase of oxygen containing ligands on the surface of Na OH-GF was observed in the X-ray photoelectron spectroscopy（XPS）.Video optical contact angle measuring instrument demonstrated that the Na OH activation could remarkably enhance the hydrophilicity of GFs,and the Na OH-GF-Repeatedly still performed completely hydrophilic properties.Fourier transform infrared（FT-IR）spectrometer showed that Na OH-GF-Repeatedly had similar absorption peaks under the same wavenumber.Linear sweeping voltammetry（LSV）indicated that the oxygen reduction activity of Na OH-GF was much stronger than that of Raw-GF,and Na OH-GF still showed good electrochemical properties after the cycle experiments.The result of hydrogen peroxide（H₂O₂）accumulation showed that the concentration of H₂O₂ was higher than that of Raw-GF in the same current density.The electron paramagnetic resonance（EPR）results demonstrated that the concentration of hydroxyl radical（·OH）of Na OH-GF in the EF system was significantly higher compared to Raw-GF at 5.17 m A·cm^-2.Therefore,Na OH-GF had better electrocatalytic performance than Raw-GF.The experimental wastewater was selected from first-stage aerobic wastewater in a pig farm wastewater treatment process in Yunfu City,Guangdong Province.Ultra High-Resolution Quadrupole Combined Electrostatic Field Track Trap Liquid Mass Spectrometer（LC-MS-MS）was used to analyze the substances in wastewater including OTC,Lincomycin,Epinephrine,Norepinephrine,Aminopyrine,Benzophenone,Diclofenac,Diphenylamine,Tributyl Phosphate,Tri-Isobutyl Phosphate,4-Aminobiphenyl,Gabapentin,Oxadixyl,Tranexamic Acid,Metolcarb and Xylylcarb.Further research on the effect of electrolysis on the degradation of OTC,which was the main antibiotic detected from aquaculture wastewater.The cathode current density played a crucial part in EF technique for degrading OTC.The OTC removal rate increased from 66%to 98%(K_app,OTC:0.061 to 0.200 s^-1)with an increased cathode current density from 0.86 to5.17 m A·cm^-2 after 30 s of electrolysis in the EF–Pt/Na OH-GF.22,44 and 66μM of OTC reacted in the EF–Pt/Na OH-GF system were completely removed after 40,60 and 90 s,respectively.However,OTC could not be completely eliminated in the EF–Pt/Raw-GF system.According to the apparent rate constant obtained in oxidation of 44μM OTC at 5.17 m A·cm^-2,the OTC removal rate in EF–Pt/Na OH-GF was higher than that in Pt/Na OH-GF.The COD_Crremoval rates of OTC（44μM）in EF-Pt/Raw-GF and EF-Pt/Na OH-GF cells at 5.17 m A·cm^-2were 59.2%and 83.7%,respectively.The removal efficiencies of OTC after 50 s in EF–Pt/Na OH-GF were 98.2-98.8%at five iterations under the same condition(5.17 m A·cm^-2).Ultra High-Resolution Quadrupole Combined Electrostatic Field Track Trap Liquid Mass Spectrometer（LC–MS/MS）was used to analyze the degradation products of OTC during electrolysis.The results showed that OTC could be degraded through the following six pathways in EF–Pt/Na OH-GF:hydroxylation（+16 Da）,secondary alcohol oxidation（–2 Da）,demethylation（–14 Da）,decarbonylation（–28 Da）,dehydration（–18 Da）and deamination（–15 Da）.BOD₅/COD_Cr of first-stage aerobic wastewater increased from 0.09 to 0.56 and p H decreased from 6.57 to 3.92 after 10 min at 5.17 m A·cm^-2 in a self-made electro-Fenton system with Na OH-GF as cathode and Pt as anode（EF–Pt/Na OH-GF）,the biodegradability of wastewater was improved significantly.Enterbacter hormaechei bacteria which was the common multi-drug resistant bacteria in aquaculture system,was used as an object to study the effects of electrolysis on the activity of bacteria and the abundance of ARGs.In Pt/Na OH-GF,the OD₆₀₀ of the Enterbacter hormaechei bacteria suspension decreased from 0.45 to 0.26,the p H increased from 6.84 to8.76 and the conductivity increased from 9.12 ms·cm^-1 to 9.78 ms·cm^-1 after 180 min at 5.17m A·cm^-2.In EF–Pt/Na OH-GF,the OD₆₀₀ and p H decreased significantly after 30 minutes.The OD₆₀₀ decreased from 0.46 to 0.17,the p H decreased from 6.64 to 4.76,and the conductivity increased from 9.22 ms·cm^-1 to 10.42 ms·cm^-1 after 180 min at 5.17 m A·cm^-2.The SEM images showed that bacteria was damaged in both Pt/Na OH-GF and EF–Pt/Na OH-GF,and EF–Pt/Na OH-GF caused more serious damage to bacteria.The results of flow cytometry,coating plate,and ATP showed that EF–Pt/Na OH-GF had a greater effect on bacterial activity than that of Pt/Na OH-GF.DNA results showed that the relative abundances of tet A,tet O,EBC and FOX decreased significantly after 180 min in Pt/Na OH-GF,with removal rates of 37%,75%,30%,and 73%,respectively;while the relative abundances of sul II increased slightly,with an increase rate of 15%.After 180 min in EF-Pt/Na OH-GF,the relative abundances of tet O and FOX decreased,with removal rates of 47%and 74%;while the relative abundances of tet A,sul II and EBC increased,with increase rates of 146%,173%,and 51%,respectively.This study provided the basis for improving the biodegradability of first-stage aerobic wastewater,removing non-biodegradable antibiotics,and reducing ARB and ARGs.The related mechanism still need to be further studied and may provided inspiration for practical applications.