Study On Denitrification Performance Of Birnessite Modified Carbon Material Cathodes In Bioelectrochemical System

Bioelectrochemical Systems?BES?is a kind of technology which combines microbes and electrochemistry together.It uses solid electrodes as electron donors or acceptors for the growth of electroactive bacteria?EABs?,and uses microbes as catalyst to catalyze redox reaction and achieve the degradation of pollutants.In this paper,BES reactors were set up in which Tlhiobacillus denitrificans was inoculated in the cathode chamber as the biocatalyst.The cathodes with an applied constant potential of-500 mV?vs.SHE?were used as the sole electron donor for T.denitriflcans and nitrate was the electron acceptor.Birnessite modified carbon felt and cerium-doped birnessite modified carbon felt were prepared by electrodeposition and respectively used as the cathodes.The effects of modified cathodes on the denitrification performance of BES were studied.The denitrification performances of BES with different types of cathodes and different nitrate concentrations were also studied.The main conclusions were as follows:At the potential of-500 mV,T.denitriflcans could attach to the surface of cathodes and absorb electrons to maintain the autotrophic metabolism.After acclimatization,the electroactive biofilm formed on the cathodes.Compared with the unmodified carbon felt,the birnessite modified carbon felt cathode prepared by electrodeposition had larger specific surface area,and it enhanced the electroactivity of T.denitrificans biofilm,reduced the cathodic biofilm resistance and the interfacial resistance between the cathode and the biofilm,and accelerated the reduction of nitrate.The nitrate removal rate of the modified cathode was 45.35±2.31 mmol NO₃^--N L^-1d^-1m^-2,which was 76.46%higher than that in the control group(25.70±2.07 mmol NO₃^--N L^-1d^-1m^-2).No ammonia was detected,indicating that dissimilatory nitrate reduction to ammonium?DNRA?didn't occur and denitrification was the only pathway for the reduction of nitrate by T.denitrificans biofilm.Cerium-doped birnessite was electrodeposited on carbon felt electrode and the cerium ions entered into the layer of birnessite instead of occurring as individual oxides.The carbon felt modified by cerium-doped brnessite had better electrochemical performance and enhanced the electroactivity of T.denitrificans biofilm and thus promoted the nitrate reduction rate.After modification,the nitrate reduction rate increased from 26.85±2.23 mmol NO₃^--N L^-1d^-1m^-2 to 54.47±2.46 mmol NO₃^--N L^-1d^-1m^-2.It was found that the doping of cerium in birnessite could better optimize the denitrification performance of BES.The experimental results also showed that the BES reactor designed in this experiment had a good treatment effect on groundwater with the NO₃^--N concentration in the range of 20?40 mg L^-1.In addition,when graphite and carbon felt were used as the cathodes,the nitrate reduction rates were 37.96±2.45 mmol NO₃^--N L^-1d^-1m^-2 and 28.93±1.84 mmol NO₃^--N L^-1d^-1m^-2,respectively,indicating that graphite may be a better choice as cathodes in BES for nitrate removal.