Study On The Bioenhanced Degradation Effect And Mechanism Of Levofloxacin In Microbial Electrochemical Reactor

Fluoroquinolones(FQs)are a class of highly effective broad-spectrum antibiotics,which are widely used in clinical application and livestock industry.Currently,FQs exist in many pharmaceutical wastewater with high concentrations.Meanwhile,due to the difficulty in degradation of FQs,the removal of FQs from pharmaceutical wastewater by conventional wastewater treatment processes is limited.Therefore,most of FQs enter the environment in the form of raw drugs,resulting in the accumulation of FQs in natural water and soil environment,which poses a serious threat to human and animal health.Microbial electrochemical technology is an effective technology for degradation of refractory organic pollutants,which has a good potential to the enhanced removal of antibiotics.Compared with traditional physical,chemical,and biological methods,it is more efficient,environmentally friendly,and low cost.Therefore,this study choosed a typical FQs antibiotic —levofloxacin(LEV)as the target pollutant,and investigated the performance of FQs removal from pharmaceutical wastewater by the microbial electrolysis cell(MEC).Firstly,the start-up and condition optimization of sequential batch MEC reactor were carried out.Then,the contribution of microbial and electrochemical effects,cathode and anode effects to the degradation of LEV was investigated.Subsequently,the degradation intermediates of LEV were analyzed by LC-TOF-MS,and the degradation pathway was inferred.After that,16 S r RNA high-throughput sequencing and function prediction were used to study the microbial community and functional structure of the reactor.Finally,the reactor was scaled up to a laboratory scale continuous MEC reactor for researches on startup,condition optimization,continuous operation,identification of degradation intermediates,microbial community and functional structure,and to explore the long-term operation characteristics of continuous MEC reactor for enhancing LEV degradation wastewater.The main conclusions are as follows:(1)The microbial electrochemical reactor has a good removal effect on LEV.Under the conditions of applied voltage of 1.4 V,initial glucose concentration of 0.25 g/L,and external addition of 0.69 m M sulfate,the sequential batch MEC reactor achieved a 7-day removal of LEV over 90%,with LEV initial concentration ranging in 5 mg/L to 40 mg/L.Under the conditions of applied voltage of 1.2 V,influent glucose concentration of 0.1 g/L,exogenous addition of 0.69 m M sulfate,and HRT of 10 days,the continuous MEC reactor can also achieve a LEV removal of over 90%,with influent LEV concentration of 5 mg/L.At the same time,the continuous MEC reactor has operated stably during the whole operation process for nearly half a year,indicating that it is feasibility of the enhanced degradation of LEV in wastewater.(2)The degradation of levofloxacin in the MEC is the result of collabration of microorganisms and electrochemistry.The high applied voltage is the key to the LEV degradation contribution of the electrochemistry part.Meanwhile,applied voltage enhanced the degradation of LEV by microorganisms.Under the high applied voltage condition,the synergistic effect of microorganisms and electrochemistry jointly achieved the efficient degradation of LEV.The redox performance of MEC’s bioelectrodes were improved compared with that of abiotic electrodes,which is also conducive to the enhancement of LEV degradation.(3)The degradation of LEV was mainly included in three degradation pathways:defluorination,piperazine ring opening,and quinolone ring opening.There was no significant difference in the degradation pathways of LEV degraded by the anode compared to that by and the cathode,with defluorination and piperazine ring opening as the main reactions.Under the joint action of the anode and cathode,LEV degradation achieved more reactions in quinolone ring opening,and more complex degradation intermediates were converted into smaller intermediates,resulting in the more deeper degradation.Meanwhile,in the absence of glucose,the degradation of LEV probably prioritized in fluoride removal,while in the presence of glucose,more reactions in quinolone ring opening were achieved,which may be related to the degradation of LEV by co-metabolism with glucose.(4)The dominant microorganisms in sequencing batch MEC and continuous MEC reactors were very similar.The difference in microbial community between the sequencing batch MEC and the continuous MEC might be mainly due to the differences in different glucose and LEV concentrations.Firmicutes,Desulfobacterota,Proteobacteria,Bacteroidota,and Euryarchaeota were the dominant phyla in the MEC reactors.Enterococcus,norank＿f＿＿PHOS-HE36,Methanobrevibacter,Geobacter,Desulfovibrio,Azospirillum,and Ochrobactrum were the main genera existed in both sequencing batch MEC and continuous MEC.Thereinto,Geobater is more prone to be enriched at the anode,which might improve the LEV degradation capacity of the anode by enhancing electron transfer ability.Desulfovibrio existed at both the anode and cathode,which might promoting the degradation of LEV by co-metabolism of LEV and glucose with sulfate.(5)The KEGG function prediction indicated the similar functional structures of microbial communities of the sequencing batch MEC and continuous MEC reactors.High relative abundances of xenobiotics biodegradation and metabolism function genes in the MEC reactors,including benzoate degradation,chloroalkane and chloroalkene degradation,aminobenzoate degradation,nitrotoluene degradation,polycyclic aromatic hydrocarbon degradation,bisphenol degradation,ethylbenzene degradation,atrazine degradation,naphthalene degradation,toluene degradation,cytochrome P450,and drug metabolism-other enzymes function genes,were indicated,which might be benefial to LEV degradation.