Studies On Enhanced Anaerobic Biodecolorization Of Azo Dyes By Quinone And Graphene Modified Polyurethane Foam

At present,water pollution problems resulted from the release of refractory organic pollutants are quite serious.Quinones as redox mediators can significantly enhance the anaerobic biotransformation of these pollutants,but dissolved quinones can flow out in the effluent in practical applications,which will bring secondary pollution and high-cost.To realize the quinones application,various immobilization techniques have been developed,however,the long-term catalytic stability of immobilized quinones remains to be investigated.In addition,the interaction effect between the immobilized quinone and microorganisms,as well as other electron mediators have not been elucidated yet.Based on this,in this dissertation,azo dyes were used as target pollutants,the catalytic stability of anthraquinone-2-sulfonate modified polyurethane foam(AQS-PUF)and its impact on the microbial community were studied during bio-decolorization;AQS and reduced graphene oxide(rGO)co-modified PUF(AQS-rGO-PUF)was prepared,followed by investigating the extracellular electron transfer mediated by the immobilized AQS and rGO;the mechanism of synergistic effect between AQS and rGO and the interactive relationship between microorganisms and immobilized quinones were clarified,which provided theoretical basis for the practical application of immobilized quinones in the biological treatment of refractory organic pollutants.AQS-PUF was amended in an up-flow anaerobic bioreactor,and its catalytic performance and stability in bio-decolorization of reactive red K-2G(RR K-2G)under high salt conditions were investigated.The results showed that AQS-PUF-amended bioreactor(R1)exhibited a better decolorization performance of 93.8%,which was approximately 50%higher compared to that of the PUF-amended one(R2),when the influent concentration of RR K-2G was 50 mg·L-1.Moreover,the decolorization efficiency of R1 maintained above 85%during the 75-d continuous operation.R1 could maintain a high decolorization efficiency at a reactive red K-2G concentration range of 50-400 mg·L-1,indicating a high stability and shock load resistance of R1.Liquid chromatography analysis showed that RR K-2G was reduced to aromatic amines by the cleavage of an azo bond.Microbial community structure analysis showed that the addition of AQS-PUF could enrich microorganisms with quinone-reducing ability.The catalytic performance of AQS-PUF only reduced less than 9%after 75 d usage of AQS-PUF,indicating that AQS-PUF was potential of application.A facultative anaerobic strain with quinone-reducing ability was isolated from R1 and used in subsequent studies.This strain was identified by 16S rDNA and named as Shewanella sp.RQs-106.AQS-rGO-PUF was first prepared by co-immobilizing AQS and rGO on PUF,and then AQS-rGO-PUF mediated bio-decolorization of acid red 18(AR 18)by strain RQs-106 was studied.When the concentrations of the immobilized AQS and rGO were 50?M and 1.63 mg·L-1,respectively,the first-order rate constant(k)of AR 18 bio-decolorization mediated by AQS-rGO-PUF was 0.53 h-1.which was higher than the sum of k values mediated by AQS-PUF(0.35 h-1)and rGO-PUF(0.10 h-1),indicating a significant synergistic effect between AQS and rGO.Such a synergistic effect was produced mainly due to the presence of additional electron transfer pathway by adding rGO,which contributed approximately 33.5%of AR 18 decolorization.Moreover,the immobilized AQS could induce strain RQs-106 to secrete more flavins to enhance the above-mentioned synergistic effect.Liquid chromatography-mass spectrometry analysis showed that strain RQs-106 can reduce AR 18 by cleaving an azo bond,and the corresponding aromatic amines were generated under anaerobic conditions.Considering the limited immobilization of graphene oxide(GO)on PUF(5.53 mg·g-1),the bio-reduction of GO(0-40 mg·L-1)and the synergistic mechanism between rGO and AQS were further investigated in the absence of PUF.The results showed that the optimal conditions for GO reduction were pH 7.5 and temperature 30?,and AQS at a concentration of 10 ?M could significantly accelerate GO reduction.With a ratio of GO(10 mg·L-1)to cells(50 mg·L-1)of 0.2,after a 24-h reduction of GO,the reduced GO-mediated bio-decolorization rate of AR 18 increased 10.5-fold compared with that in the absence of the reduced GO.When strain RQs-106 was combined with rGO,the ability of strain RQs-106 to reduce AQS and AR 18 was enhanced by improving its direct electron transfer capacity(conductivity,by 70.9%)and the dehydrogenase activity(by 61.6%),resulting in the formation of a synergistic effect between AQS and rGO during bio-decolorization of AR 18.The effect of flavins secreted by strain RQs-106 on AQS-PUF-mediated AR 18 decolorization and GO reduction was studied.The results showed that there were positive correlations between the content of extracellular flavins and the concentration of the immobilized AQS(0-100 ?M),as well as the AR 18 decolorization efficiency mediated by AQS-PUF.The amounts of the extracellular and total flavins increased by approximately 10.4 nM(13.3%)and 22.8 nM(27.0%),respectively,in the presence of 100 ?M immobilized AQS.The deletion of the flavin adenine dinucleotide exporter gene(bfe)of strain RQs-106 resulted in approximately 60%decrease of the extracellular flavins secreted by the mutant ?bfe.The knock-out of gene bfe had little effect on the decolorization of AR 18 by strain RQs-106,but resulted in around 41.7%decrease in the AQS-reducing ability of strain RQs-106,further causing a 19.6%decrease in the decolorization rate of AR 18 mediated by AQS-PUF.During the non-direct contact bio-reduction of GO,the extracellular flavins were proven to participate in the reduction of GO,and the decrease of flavins secretion lead to a decrease in GO reduction rate.