The Process And Mechanism Study Of Graphene Coupled With Geobacter For Effective Removal Of Nitrobenzene From Aqueous Environment

In recent years,the pollution of nitrobenzene（NB）is getting worse,threatening the safety of human beings and aquatic organisms.The removal of nitrobenzene with environmental-friendly and efficient methods has become a research hotspot in the field of environmental remediation.With the discovery of extracellular respiration of microorganisms,extracellular respiration bacteria,especially those with electrochemical activity,are of great significance to the anaerobic reduction process of microorganisms.As a typical electrochemically active bacterium（EAB）,Geobacter can participate in the extracellular electron transfer（EET）process,and transfer electrons to extracellular electron acceptors.However,the function of Geobacter alone is relatively slow,and the reduction of extracellular electron acceptors by Geobacter can be accelerated by appropriate bio-enhancement methods.Based on above,in this study,we chose G.sulfurreducens PCA as the model strain.Graphene oxide（GO）and reduced graphene oxide（r GO）acted as electron shuttle（ES）mediating the EET process,and NB acted as a target pollutant and a terminal electron acceptor.The main contents of this study included:（1）The GO treatment groups with different concentration gradients(0.2 mg·m L^-1,0.4 mg·m L^-1,0.6 mg·m L^-1 and 0.8 mg·m L^-1)were set up in order to explore the different removal effects of 100μM NB.Meanwhile,the control group of pure G.sulfurreducens PCA and pure 0.6 mg·m L^-1 GO and blank NB treatment were set up,and the study confirmed that the 0.6 mg·m L^-1 GO coupled with G.sulfurreducens PCA was the optimal experimental system for the removal of 100μM NB.The removal efficiency of NB in 12 days was 87.1%.（2）The intrinsic mechanism of NB removal by by GO coupled with G.sulfurreducens PCA was explored through characterization of Scanning electron microscope（SEM）,Fourier transform infrared（FTIR）,X-ray photoelectron spectroscopy（XPS）,Raman spectra and electrochemical analyses.The results showed that the removal mechanism of NB could be summarized into two aspects:Ⅰ)G.sulfurreducens PCA transferred electrons directly to the extracellular acceptor NB,so that NB was reduced and removed;Ⅱ)GO and a part of r GO acted as ES,and promoted the reduction of NB by accelerating the EET process.（3）The green pollution-free r GO material was prepared by reduction of GO through G.sulfurreducens PCA,and the successful preparation of r GO was verified by SEM,X-ray diffraction（XRD）,FTIR,XPS and Electrochemical impedance spectroscopy（EIS）analyses.（4）By comparing the removal effect of the 0.6 mg·m L^-1 r GO coupled with G.sulfurreducens PCA（0.6r GO-PCA-NB）system,pure G.sulfurreducens PCA and pure r GO control system and blank NB system for the removal of 100μM NB treatment,it was found that 0.6r GO-PCA-NB was the best system,which could achieve 100%removal of 100μM NB within 1 day,and the corresponding degradation efficiency was73.3%.（5）By setting different NB concentrations（50μM,100μM,200μM）,the optimal system of 0.6 mg·m L^-1 r GO coupled with G.sulfurreducens PCA for the removal of NB at different concentrations was explored.The results showed that all the three systems could all achieve 100%NB removal within one day.Meanwhile,the degradation efficiencies of NB in 50μM,100μM and 200μM systems were 61.7%,74.5%and 83.9%,respectively and the r GO-PCA-200NB was the best system with the highest degradation efficiency.（6）The internal mechanism of r GO coupled with G.sulfurreducens PCA for the removal of NB was explored through characterization of SEM,XPS,Raman spectra and electrochemical analyses and the mechanism mainly included:I)Direct EET process,which transferred electrons from G.sulfurreducens PCA to electron acceptor NB,and this process was mainly due to function of cytochrome c;Ⅱ)r GO transferred electrons to NB as exogenous ES,and this process was the main way of NB reduction.This study is beneficial to further understand the role of graphene as exogenous ES in the EET process.Meanwhile,exploring the NB removal process and internal mechanism by GO and r GO coupled with G.sulfurreducens PCA provide a theoretical support for the removal of nitroaromatic compounds in the environment.This study is meaningful in the field of environmental ecology.