Long-distance Electron Transfer By Biochar To Promote The Degradation Of Organic Pollutants In Bioelectrochemical Systems

Extracellular electron transfer of electroactive biofilms(EABs)has attracted increasing attentions and shows great application potentials in diverse areas,including pollutant degradation,wastewater treatment and clean energy production.However,the electron transfer of EABs is mainly limited at the electrode/microbial interface,which reduces the remediation efficiency of EABs.Inspired by the long-range electron transfer(Long-distance electron transfer,LDET)of natural microorganisms,biochar as a conductive substance may mediate the biogeochemical process between EABs and organic pollutants,and affect the transformation and fate of organic pollutants.This paper explores the feasibility of biochar promoting LDET and its application in the degradation of organic pollutants in the soil bioelectrochemical systems(SBESs),and provides theoretical and technical support for strengthening the bioelectrochemical remediation of organic pollutants.In this study,based on the main line of bidirectional LDET mediated by biochar in soil bioelectrochemical systems(SBESs),a new method to measure the distance of biochar-mediated electron transfer was designed.The bidirectional LDET process and distance effects between biochar-mediated EABs and electrodes were evaluated fo the microbial reductive dechlorination of pentachlorophenol(PCP)and mineralization of phenanthrene(PHE),and the underling electron transfer mechanisms were analyzed.The main conclusions were as follows:(1)The electrochemical characterization of LDET in biochar-mediated soil was established to realize the effective measurement of LDET distance for the first time.The conductivity of soil and soil amended with biochar were measured,and it was found that biochar could effectively improve soil coductivity;however,it could not reflect the effect of biochar on the electron transfer of flooded soil in natural environment.Therefore,electrochemical characterization was used to test the distance effect of biochar mediated electron transfer in flooded soil.Our results showed that the electron transfer efficiency and distance of electron transfer in soil were related to the soil conductivity.In the non-biological systems,the LDET mediated by 6% BC900 was at least 6 cm.This study provides an effective electrochemical method to measure the electron transfer distance promoted by biochar in soil.(2)Biochar mediated LDET of petroleum contaminated soil to promote SBESs for phenanthrene(PHE)degradation.Results showed that low content of BC900 significantly promoted the performance of SBESs,and the effective distance of SBES was at least 28 cm.With the increase of BC900 addition,the degradation rate increased.Based on the bioelectrical and electrochemical analysis,we found that biochar promoted PHE mineralization through electron shuttle and conductive mechanisms.Additionally,biochar can increase the abundance of some microbes,such as Azotobacter,Achromobacter,Caproiciproducens,Dysgonomonas and Lutispora.Among them,Azotobacter and Achromobacter were enriched and spatially distributed in all the biochar-amended SBESs.What's more,the abundance of PHA-RHD? genes encoding polycyclic aromatic hydrocarbon-ring hydroxylating dioxygenase were also decreased with the increasing distance away from the bioanode.Accordingly,biochar coupled with SBESs is an effective bioremediation and potential approach for bioremediation of PAHs-contaminated soils.(3)Biochar mediated LDET of paddy soil to promote SBESs for degradation of pentachlorophenol(PCP).Results showed that PCP transformation in the control SBES mostly occurred within 4 cm around the electrode.However,the effective distance of ~16 cm toward the electrode could be achieved for PCP biodegradation in the SBES amended with a highly conductive biochar pyrolyzed at 900 °C(BC900),which was significantly greater than those of low conductive biochars(BC300 and BC600).Based on the high-throughput sequencing analysis,Desulfitobacterium and Geobacter were enriched and spatially distributed in the biochar-amended SBES,indicating the formation of conductive networks between bacteria and biochar,which in turn promoted PCP degradation.(4)DNA-based stable isotope probing(DNA-SIP)and BESs were applied to explore functional microorganisms in the dichlorination and mineralization of PCP.The results showed that PCP was completely degraded within 10 days.In BESs,the thickness of the anode biofilm was significantly higher than that of the cathode biofilm.,High abundance of Comamonas,Stenotrophomonas and Geobacter were observed in the anode biofilm,indicating that these microorganisms were responsible for the PCP reductive dechlorination and mineralization.In the cathode biofilm,Comamonas,Pseudomonas,Methylobacillus and Dechlorosoma were enriched in the heavy DNA fractions of the 13C-PCP samples with high buoyancy density values of more than 1.73 g/m L,hinting that these microbes were involved in the mineralization of PCP dechlorination products in BESs.This study provides new understandings for the diversity of functional of PCP degrading microorganisms.