Enhanced Reductive Transformation Of Chloronitrobenzene By Anaerobic Process Coupled With Bioelectrode Based On Cathode Potential Control

Chloronitrobenzenes（CINBs）as important basic materials,are widely used in many industries such as chemicals,medicines,and dyes.These compounds are teratogenic,carcinogenic and mutagenic with stable chemical property and poor biodegradability,threatening the ecological security and human health when discharged into the environment.Currently,the conventional CINBs containing wastewater treatment is based on anaerobic process,but there are several disadvantages including long started-up period,shortage of electron donor and low reaction rate.2,4-dichloronitrobenzene（2,4-DClNB）is used as the target pollutant in this thesis and microbial electrolysis cell（MEC）-upflow anaerobic sludge blanket（UASB）coupled system is developed to investigate the enhanced performance from the point of cathode potential,electrodes position and low organic loading.The mechanism based on the control of cathodic potential is preliminarily revealed by analyzing the degradation efficiency of target pollutants and community structure of electrode biofilm.The main results are as follows:1.A dual-chambered biological electrochemical reactors was designed.The cathodic chamber was without electron donor while the anodic chamber contained 500mg/LCOD.The concentration of the target pollutant was 50mg/L and a voltage of 0.6V was supplied.2,4-DClNB in the cathodic chamber was completely transformed within 24h.While in the control with open circuit,transformation of 2,4-DClNB couldn’t be observed.The results indicated that the cathodic microorganisms could transform 2.4-DClNB with electrode serving as sole electron donor.Synergistic effect of bioelectrode and anaerobic sludge was studied in single-chambered biological electrochemical reactors.When the initial conditions of the COD,2,4-DClNB,anaerobic sludge were set at 500mg/L.50mg/L,and 3g/L,respectively,the degradation rate of 4-chloroaniline（4-ClAn）in the anaerobic sludge reactor was 2.24 mg·L^-1·d^-1.However,the corresponding values in the coupled reactors with a cathodic potential of-450 mV,-660 mV,-870 mV were 3.51mg·L^-1·d^-1,4.61 mg·L^-1·d^-1 and 4.84 mg·L^-1·d^-1,respectively.The results indicated that the reductive transformation of 2,4-DClNB was improved in the coupled bioelectrode-anaerobic sludge system and lower cathodic potential was advantageous to the fracture of C-Cl bond,promoting dechlorination reaction.2.The coupled MEC-UASB reactors were developed to investigate the influence of cathodic potential on the performances of 2,4-DClNB degradation.When the influent COD loading,2,4-DClNB loading,cathodic potential and hydraulic retention time（HRT）were set at 500g·m^-3·d^-1,50 g·m^-3·d^-1,-660mV and 24 h,respectively,the rates of 4-ClAn removal and aniline（An）production were 19.56±0.84 mg·L^-1·d^-1 and 4.12±1.33 mg·L^-1·d^-1,respectively,which were higher than that when the cathodic potential was-450mV.The analysis suggested that cathodic potential was the intrinsic electrochemical parameter that steers the biochemical reaction of the cathode and lower cathodic potential can provide electrons with higher energy and H2 to facilitate dechlorination reaction.Electrode position can also affect dechlorination efficiency.When the electrode position was lifted 10 cm from the bottom of the reactor to the middle of the reactor,the rate of 4-ClAn removal increased to 20.92士0.82 mg·L^-1·d^-1.It is presumed that lifting electrode enhanced the dechlorination of the intermediate products in the upper region of the upflow reactor.Besides,when the influent organic loading decreased from 500 gCOD·m^-3·d^-1 to 200 gCOD·m^-3·d^-1 and 2,4-DClNB loading was maintained at 50g·m^-3·d^-1,the dechlorination rate of the coupled reactor was 56.41±3.51%which was higher than that of the traditional UASB（43.25±2.88%）,indicating that the coupled process was able to use the limited electrons more efficiently for 2,4-DClNB reduction conversion.3.Microscopic observation of the biocathode found that microorganisms such as coccus,bacillus and filamentous bacteria with flagella structure were attached to the surface of the biocathode.Meanwhile,biocathodic biofilm fluorescence staining analysis showed that living cells mainly existed in the surface and middle layer of the cathode biofilm while the dead ones were near the inner layer,suggesting that there may be long-range electron transport mechanism similar to nanowires.The results of Illumina high-throughput sequencing indicated that applied voltage would significantly influence the succession of microbial community.On the genus level,the relative abundances of the genera Methanoregula,Methanolinea,Pseudomonas,Desulfovibrio,Longilinea,Dehalococcoides,Dehalobacter and Anaeromyxobacter increased significantly,indicating that methanogens,anaerobic fermentation bacterium and dechlorination microorganisms were enriched in the coupled system.Further analysis of microbial community on cathodic biofilm found that biocathode served as the active interface for catalytic reduction were enriched with Dehalobacter,Dehalococcoides and Anaeromyxobacter which were related to dechlorination.Furthermore,above-mentioned functional bacteria accounted for higher proportion at the biocathode with a potential of-660mV.It was speculated that these functional bacteria in lower cathodic potential could obtain more energy for reductive dechlorination and microbial growth.