Investigation On Selective Oxidation Of Phenol And Modification Of Microbial Fuel Cell Anode With Its Product

Phenol is a typical environmental endocrine disrupting chemical,which is widely detected in the discharge of many industrial wastewaters.Electrochemical oxidative process?EOP?for the degradation of phenol can result in the formation of hydroquinone?HQ?/benzoquinone?BQ?,which can function as electron mediators by taking advantage of the reversible redox reaction between them.However,the side reaction of phenol electrochemical oxidation leads to the generation of an insulating layer on the electrode,which can passivate the electrode and hinder the further degradation of phenol.A microbial fuel cell?MFC?can directly convert chemical energy of organic pollutants in the wastewater into electric energy.However,it suffers problems from practical application in the wastewater treatment,owing to the main disadvantages of low level of energy output and high cost of electrode materials.To show the possibility of using the EOP for resource recovery of phenol from the wastewater,in this study,we attempt to achieve selectively electrocatalytic oxidation of phenol to HQ/BQ using a nitrogen-doped reduced graphene oxide?N-rGO?electrode,and further investigate the feasibility of using the HQ-modified anode to improve the power performance of MFCNitrogen-doped carbons have been previously shown as a successful electrocatalyst for many redox reactions.The first effort was made to prepare a N-r GO/SSM electrode?N-doped reduced graphene oxide coated on a stainless steel mesh substrate?and apply it to the selectively electrocatalytic oxidation of phenol.It was found from the cyclic voltammetry?CV?tests that the oxidation potential of phenol was 0.84 V,at which the peak current density for the first cycle is 2.3 times larger than that associated with the non-doped control electrode?rGO/SSM?.The CV results also suggested the N-rGO/SSM electrode enabled a remarkably higher anodic peak current and the generation of two pairs of redox peaks(E_1/2=0.28 and0.51 V)compared to the rGO/SSM electrode.The reactive redox peaks were confirmed to be the voltammetric feature of hydroquinone,a product of selective oxidation of phenol.The electrochemical impedance studies indicated that there was an insignificant increase in the ohmic and charge transfer resistances upon multisweep cycling on N-rGO/SSM,in contrast to the pronounced increases associated with rGO/SSM.Electrochemical tests and surface composition analysis indicated that HQ/BQ adhered to the surfaces of N-rGO/SSM after the selectively electrocatalytic oxidation of phenol.Introducing a pseudocapacitance to the carbon-based electrode is widely adopted as a method to increase the power ouput of the MFC.The second attempt was made to electrochemically coat polymerized HQ?polyhydroquinone,PHQ?onto the surface of graphite felt?GF?substrate to prepare the anode with pseudocapacitance characteristics,and the application of this redox-active product to shuttle electron transfer in the anode system of an MFC.The microscopic and spectroscopic results showed that the treatment of the graphite felt?GF?substrate with acids was effective in improving the amounts of surface-bound oxygen-containing groups,enabling better adhesion of PHQ onto the GF surfaces.The electrochemical measurements indicated that the resulting PHQ-AGF?acid treated GF?possessed high pseudocapacitance due to the fast and reversible redox cycling between hydroquinone and benzoquinone.The MFC equipped with the PHQ-AGF anode achieved a maximum power density of 633.6 mW m-2,which was much higher than 368.2,228.8,and119.7 mW m-2 corresponding to the MFC with the reference PHQ-GF,AGF,and GF anodes,respectively.The increase in the power performance was attributed to the incorporation of the redox-active PHQ abundant in C-OH and C=O groups that were beneficial to the increased extracellular electron transfer and enhanced bacterial adhesion on the anode.On the whole,we have successfully demonstrated that the N-rGO electrode was effective in selectively catalyzing phenol conversion to HQ/BQ,significantly alleviating the passivation effect.The solid-phase PHQ-modified AGF anode synthesized by electropolymerization from the aqueous HQ was shown to appreciably improve the anode performance,and thus boost the power output of the MFC.These findings might provide insights into the resource recovery from phenol in the wastewater using the EOP.