Electron Transfer Mediated Microbial Synthesis Of Reduced Graphene Oxide And Its Application In Cd(Ⅱ) Removal From Aqueous Solution

Graphene,as an emerging two-dimensional carbon material with excellent electrical,mechanical,optical properties,exhibits inimitable chemical properties and physical structure.For breaking the deadlock caused by surface electrical structure and spatial effect in common adsorbents,the application of graphene and its related functional materials are gradually increasing in environmental remediation,such as adsorption,catalytic reduction,and photocatalytic degradation.Reduced graphene oxide（rGO）not only has remarkable properties of graphene,but also has a high capacity for chemical adsorption due to the presence of oxygen-containing functional groups which are not only partially removed during the reduction process.Microbial reduction of GO is eco-friendly,free of secondary pollution,low production cost,and can be applied for mass industrial production.Therefore,microorganisms possess a tremendous development potential for the production of rGO.However,the possible formation mechanism for biosynthetic rGO by Gram-positive bacteria is still unclear.Therefore,microbial reduced graphene oxide（M-rGO）prepared by Gram-positive strain Lysinibacillus sphaericus HJ-3 was investigated in this study.The interfacial electron transfer and a possible mechanism for the reduction between GO and strain HJ-3 were discussed.In addition,the process of reduction was regulated by additional electron transfer mediators（ETMs）.Finally,potential applications of functionalized M-rGO for Cd（Ⅱ）removal were suggested.（1）A Gram-positive bacterium with reducing power was isolated from the crude ore of kaolin.The combination of physiological,biochemical methods and 16S rRNA gene sequence designated the strain as L.sphaericus HJ-3.The color changes,characterization results of Raman and X-ray diffraction（XRD）for M-rGO indicated that the oxygen-containing functional groups of GO were effectively removed by strain HJ-3.（2）After adding respiration inhibitors and extracellular filtrate,the possible reduction mechanism was proposed by three-dimensional fluorescence spectroscopy and electrochemical analysis.The autocrine soluble redox mediators,such as flavin and humic acid,might play an essential role in the extracellular electron transfer（EET）by L.sphaericus HJ-3.The reduction of GO using strain HJ-3 was caused by electron transfer between intracellular electrons and extracellular electron receptors.Gram-positive bacteria exhibit weak electrochemical activity,so the electron transfer efficiency could be enhanced by adding additional redox compounds as ETMs.The results showed that different ETMs have diverse contributions to EET（Riboflavin,Carmine,Anthraquinone-2-sulfonic acid,Anthraquinone-2-carboxylic acid,2-hydroxy-1,4-naphthoquinone,and Humic acid）,and the presence of flavins and anthraquinones could accelerate the synthesis of M-rGO by L.sphaericus HJ-3.（3）The potential application of functionalized M-rGO for Cd（Ⅱ）removal was investigated.Functionalized M-rGO has successfully removed 10.0 mg L^-1 Cd（Ⅱ）with an efficiency of 99.2%,while that only 34.2%and 50.9%for graphene oxide and tea reduced graphene oxide（T-rGO）.Kinetic analysis and adsorption isotherms revealed that the adsorbing Cd（Ⅱ）by M-rGO fitted with pseudo-second-order adsorption kinetics and Langmuir isotherm.Raman spectroscopy,Fourier transform infrared spectroscopy（FTIR）,Scanning Electron Microscope and Energy-Dispersive X-ray Spectrometer（SEM-EDS）,and X-ray photoelectron spectroscopy（XPS）confirmed that the potential interaction mechanism of Cd（Ⅱ）by M-rGO includes van der Waals forces,electrostatic attraction,and complexation.The abundant functional groups in M-rGO could facilitate the adsorption to Cd（Ⅱ）.The removal efficiency was greatly influenced by pH and the strength of coexisting ions.M-rGO exhibited outstanding adsorption performance,the removal efficiencies of Cd（Ⅱ）in the municipal and simulated wastewaters were 92.3%and 65.9%,respectively.Then M-rGO can be regenerated through 5 times consecutive adsorption/desorption cycles with a slight loss in the removal performance.