The Mechanism Of Enhanced Reductive Degradation Of Nitroaromatic Compounds In Bioelectrochemical System

Because of its biorefractory property and biological toxicity,nitroaromatic compounds(NACs)have been ranked with the priority controlled contamination in many countries and need to be treated properly.In this study,anode/cathode materials with high catalytic efficiency were designed and manufactured,two kinds of membrane-free bioelectrochemical system(MFBES)were developed to enhance the reductive degradation of NACs.Firstly,polyaniline/graphene oxide(PANi/GO)composite was fabricated via a convenient and low-cost electropolymerization method.Scanning electron microscope,Raman spectrum and Fourier transform infrared spectrum demonstrated that PANi/GO composite was successfully coated onto the graphite felt(GF)surface.The electrochemical activity,pH dependence and stability of the PANi/GO modified GF electrode were significantly improved after modification,as proved by cyclic voltammetry and chronoamperometric tests.The application of PANi/GO modified GF electrode in a bioelectrochemical system(BES)confirmed its superior electrogenesis performance,that higher maximum power density was achieved.One-step electrosynthesis of polypyrrole/N-doped graphene(PPY/NG)composite on the GF electrode was achieved by electropolymerization of pyrrole while using NG as the anionic dopant.The resulting PPY/NG composite was uniformly coated onto the GF surface,with increased surface area and electrochemical activity as compared to PPY alone-modified GF electrode.Detailed electrochemical studies indicated that the PPY/NG modified GF electrode possessed high sensitive response to p-nitrophenol(PNP)and presented exceptional electrocatalytic performance toward PNP reduction.The application of PPY/NG modified GF electrode in a BES confirmed its superior electrocatalytic reduction of PNP.An integrated upflow anaerobic sludge blanket(UASB-BES)system was developed and operated continuously for 240 d to systematically investigate the feasibility of the enhanced reduction of 2,4-dinitrochlorobenzene(DNCB).The results indicated that high voltage supplied had a positive effect on DNCB reduction but a negative impact for the overhigh voltage(>1.6 V).The ability to resist shock loading was strengthened in the UASB-BES system in comparison with the control UASB system.High-throughput sequencing analysis suggested that the enhanced reduction of DNCB in UASB-BES could be attributed to higher diversity and the enrichment of reduction-related species,potential electroactive species and fermentative species.Both DNCB removal and dechlorination gradually increased with the increase of operation time,indicating the improved performance of the coupled UASB-BES system.Finally,a MFBES has been developed for the enhanced reductive transformation of PNP.The results showed that the electric field played a key role in both PNP reduction and p-aminophenol(PAP)formation.At cathode potential of 1000 mV vs Ag/AgCl and hydraulic retention time(HRT)of 8.9 h,PNP removal rate as high as 18.95±0.10 mol m-3 d-1 could be achieved in the MFBES with acetate as the electron donor.High PNP removal and PAP formation could be achieved at low acetate dosage,high initial PNP concentration and short HRT,indicating the strong ability of the MFBES to resist shock loading.The microbial communities were evaluated and compared for treating different structures of nitrophenols(NPs),i.e.,o-nitrophenol(ONP),m-nitrophenol(MNP)and PNP,in the MFBES.The results demonstrated that NPs reduction in the MFBES decreased in efficiency in the following order:ONP>MNP>PNP.Illumina MiSeq sequencing results showed that richness and diversity of bacterial species in the anodic and cathodic communities decreased when fed different NPs.Though remarkable differences in community composition were found between anodic and cathodic biofilms in the MFBES,three core genera—Treponema,Desulfovibrio and Geobacter-were dominant in the anodic or cathodic biofilm,regardless of various NPs.Other functional genera in the anodic or cathodic biofilm were selectively enriched in the MFBES treating the three NPs with different structures.The observed efficient and stable performance highlights the potential for long-term operation and full-scale application of the MFBES system,particularly for highly recalcitrant NACs removal.