| The current urban wastewater treatment process is energy-intensive,which results in a lot of energy/carbon source consumption and carbon emissions.Microbial fuel cell（MFC）is a new wastewater treatment technology that utilizes exoelectrogens to degrade organics while simultaneously generating electricity.The closed circuit connection between cathode and anode in MFC enriches exoelectrogens and forms electrochemical biofilms effectively,which can significantly improve the degradation efficiency of pollutants.Therefore,it is a promising method of utilizing the energy rich in wastewater.At present,one of the bottlenecks restricting the large-scale application of MFCs is the inefficient electron transfer between exoelectrogens and electrodes.Adding external electron mediators（EMs）to the MFC can improve the electron transfer efficiency effectively.However,most of the EMs currently used are synthetic chemical substances,which have the disadvantages of poor environmental friendliness and high cost.Some even have microbiological toxicity that limits their wide applications.In this study,the plants rich in anthraquinones widely spread in nature were used as sustained-release EM carriers to investigate the effects of plant-derived EMs addition on bioelectricity generation and conventional/refractory pollutant removal in MFCs.Under the condition of EMs addition,the mechanism of the effects on MFCs were revealed by analyzing the formation of electrochemical biofilms,the variation of community structure and abundance of functional microorganisms,and the expression of functional genes.On this basis,studies have been carried out on azo dyes decolorization in MFC by adding plant-derived EMs,and the use of anthraquinone-rich wastewater as an electricity-generating substrate to enhance the performance of MFC.The effects of plant-derived EMs addition on the cytotoxicity and microbial toxicity of refractory pollutants were evaluated.The main results are as follows:（1）The redox characteristics of the electrochemically active substances in the six natural plants rich in anthraquinones were evaluated by electrochemical methods and the plants suitable for the microbial electrochemical system were screened out.In order to accelerate the release of electrochemical active substances in plants,acid immersion and ultrasonic heat treatment were used to pretreat plants.Cyclic voltammetry and antioxidant capacity test were used to analyze the electrochemical activity of the alternative plant materials’water leach liquor,supplemented by FT-IR and SEM analysis to reveal the changes in functional groups and morphological structure of plants before and after pretreatment,so as to screen out more proper method.The results indicated that the plant extracts of B.rheum,T.fallopia,Catsia tora Linn and L.Rubia.have reversible redox properties and had the potential as EMs.Acid treatment has a stronger promotion on the release of electrochemically active substances in plants than ultrasonic treatment,and 2%acid treatment performed better.The results of electrochemical analysis showed that the electron transfer mediated by anthraquinones in plants was a single-electron process.（2）Four plant powders treated by 2%acid were added to MFCs in different proportions,and the influence of plant-derived EMs on the electricity generation and pollutant removal of MFC was explored.The results showed that B.rheum powder addition obtained higher coulombic efficiency（29.03%）and maximum power density（18.67 W/m3）compared with other groups,which had greater potential to act as EMs.Correspondingly,the addition of B.rheum had the most significant enhancement to the pollutant removal.Adding 0.01 g/L of B.rheum improved the electricity generation more,which was beneficial to COD removal and NH4+-N migration.（3）The plant-derived EMs regulated and affected the microbial community structure and relative abundance in MFCs.Compared with the control group,the level of microbial diversity in the B.rheum powder addition group was improved.Electrochemically active microorganisms（e.g.,Geobacter and Bacteroides）and pollutant degrading microorganisms（e.g.,Aeromonas,Paludibacter,and Azospirillum）were enriched efficiently.In addition,the laser confocal analysis showed that plant-derived EMs had a significant impact on the morphology and composition of suspended sludge and biofilm.Compared with the control group without addition,adding B.rheum increased the protein content of electrode biofilm more（6.89%）,which was beneficial to improve the tolerance of microorganisms to the biological toxicity caused by the EMs and increased extracellular electrons transfer.This was consistent with the performance of electricity generation.（4）The effect of B.rheum extract addition on MFCs’performance was explored.The results indicated that adding 5%（V/V）B.rheum extract solution can effectively improve the electricity generation of MFC by reducing the internal resistance,increasing the electrochemical activity of the electrode biofilm,and reducing the anodic polarization,which was proved to be a more suitable dosage proportion.To further reveal the effect of plant-derived EMs addition on the removal of refractory pollutants,5%（V/V）B.rheum extract was added into the MFC using reactive red 2（RR2）synthetic wastewater as substrate.The results showed that the decolorization efficiency（86.35%）,decolorization rate（2.93 mg/h）,maximum power density（2.75 W/m3）,and coulombic efficiency（6.45%）were significantly improved compared with the control group without any addition.Meanwhile,the electron transfer resistance was reduced.Otherwise,the addition of B.rheum extract improved microbial activity,enriched obligate dye-degrading microorganisms and electrochemically active microorganisms.For the first time,this study proved that plant-derived EMs represented by B.rheum extract not only can be used as an"accelerator"for improving microbial activity and regulating the composition of microbial communities,but also as an"antidote"for alleviating the biological toxicity of RR2.（5）The feasibility of using wastewater rich in anthraquinones as both anode substrate and EMs library of MFC was studied.The wastewater rich in anthraquinone was used as a co-substrate for dye decolorization and the performance was investigated.The results showed that the wastewater rich in anthraquinones can be effectively used by MFC for electricity generation.Microbial community structure analysis and functional gene prediction results indicated that anthraquinones in wastewater played key roles in enriching electroactive bacteria,promoting biofilm formation,and enhancing the tolerance of biofilms to adverse external environments.Using wastewater rich in anthraquinones as co-substrate of RR2 decolorization can enhance the decolorization efficiency（92.45%）of RR2 and enrich the obligate electricity-producing bacteria and dye-degrading bacteria effectively.UV-Vis analysis showed that the presence of anthraquinones in the simulated wastewater made the dye degradation more thorough.The absorption peak intensity of aniline in the effluent was significantly reduced compared with the control group.Therefore,using wastewater rich in anthraquinones as a co-substrate and EMs library for the treatment of refractory wastewater is a promising strategy. |
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