The Mechanism Of Enhanced Anaerobic Degradation Of Halogenated Aromatic Hydrocarbon And Nitrogen Heterocyclic Compounds By Electrical Stimulation

Because of the biological toxicity and biorefractory property of halogenated aromatic hydrocarbon and nitrogen heterocyclic compounds in chemical wastewater,conventional anaerobic processes is confined by its low removal efficiency,long startup time,poor stability for toxic contaminants and high demand of electron donor to promote the necessary reductive condition.In this study,an up-flow electricity-stimulated anaerobic system(ESAS)was employed to achieve enhanced degradation of 4-bromophenol(4-BP).The key role of electrical stimulation on the microbial communities was investigated at the same time.Besides,a modified anode with high catalytic efficiency was designed and manufactured to enhance the dagradation of phenol.What's more,an integrated bioelectro-photocatalytic system(IBPS)equipped with a N-doped graphene/?-Fe₂O₃ modified graphite felt(GF)photoanode,a GF biocathode was constructed to realize simultaneous removal of pyridine and nitrogen.In this study,an up-flow ESAS was developed by installing cathode underneath and anode above to realize simultaneous anaerobic debromination and mineralization of 4-BP.When cathode potential was-600 m V,high TOC removal efficiency(98.78±0.96%),complete removal of 4-BP and phenol could be achieved at 4-BP loading rate of 0.58 mol m^-3 d^-1,suggesting debrominated product of 4-BP from cathode(i.e.,phenol)would be utilized as the fuel by the bioanode of ESAS.Under high 4-BP loading rate(2.32 mol m^-3 d^-1)and low electron donor dosage(4.88 m M),4-BP could be completely removed at acetate usage ratio as low as4.21±1.42 mol acetate mol^-1 4-BP removal in ESAS,whereas only 13.45±1.38%of 4-BP could be removed at acetate usage ratio as high as 31.28±3.38 mol acetate mol^-1 4-BP removal in control reactor.Besides,electrical stimulation distinctly facilitated the growth of various autotrophic dehalogenation species,phenol degradation related species,fermentative species,homoacetogens and electrochemically active species in ESAS.Moreover,based on the identified intermediates and the bacterial taxonomic analysis,possible metabolism mechanism involved in enhanced anaerobic debromination and mineralization of 4-BP in ESAS was proposed.In order to develop a highly efficient anode material for recalcitrant phenol degradation in ESAS,fabrication of polypyrrole(PPy)/(?-,?-and?-)MnO₂ composite onto graphite felt(GF)electrode through facile one-step electrodeposition was investigated in this study.The successful coating of PPy/MnO₂ onto GF surface was verified by scanning electron microscopy,Raman spectrum and XPS.The best electrochemical and catalytic property of GF/PPy/?-MnO₂among the modified electrodes was confirmed by cyclic voltammetry analysis,chronoamperometric and electrochemical impedance spectra.The application of GF/PPy/?-MnO₂ in ESAS notarized the superior degradation performance towards phenol.Shorter startup time,higher mineralization efficiency and improved bacteria adhesion was achieved in ESAS using GF/PPy/?-MnO₂ anode.Coulombic efficiencies of 17.3±0.5%in ESAS using GF/PPy/?-MnO₂ modified GF was much higher than those in ESAS using GF/PPy and blank GF as anode,which were as low as 12.1±2.4%and 6.6±1.3%,respectively.The key role of MnO₂ and possible degradation pathway involved in phenol degradation was further proposed.The milder fabrication condition,improved electrochemical activity and increased phenol degradation efficiency suggest that GF/PPy/?-MnO₂ has a promising future in BES application for recalcitrant phenol catalytic degradation.In order to realize simultaneous removal of pyridine and nitrogen,an integrated bioelectro-photocatalytic system(IBPS)equipped with a N-doped graphene/?-Fe₂O₃ modified graphite felt(GF)photoanode,a GF biocathode was constructed and operated in ON-OFF illumination mode.The N-doped graphene/?-Fe₂O₃ modified photoanode showed superior stability,photo-electrochemical and catalytic activity owing to the excellent electron transporting properties of N-doped graphene and unique structure of N-doped graphene/?-Fe₂O₃ composite.In IBPS,driven by the concentration gradient and electric field between two chambers,ammonia produced from pyridine photocatalytic degradation was supposed to diffuse from anode to cathode through the cation-exchange membrane,so that simultaneous nitrification and denitrification was realized in the cathode chamber under limited dissolved oxygen.At the same time,the harvested photo-electrons derived from photoanode was delivered to the biocathode via an external circuit for enhanced cathodic denitrification.Complete removal of pyridine and a high TOC removal efficiency of 85.90±5.48%in the anode and complete denitrification in the cathode after four ON-OFF cycles(96 h)reaction was achieved in IBPS utilizing the modified photoanode.Furthermore,the underlying mechanism for the enhanced removal of pyridine and nitrogen was proposed preliminarily.This integrated reactor synergically utilized photoenergy,electrical energy and bioenergy for efficient removal of pyridine and nitrogen,should have a promising future in designing new systems for water environmental remediation from solar energy.