| Biochar is the carbonaceous product from biomass pyrolysis under oxygen-limited conditions.It has been widely applied in carbon sink enhancement,soil fertility improvement and soil pollution remediation.Biochar containing multiple oxygenated groups and highly aromatic structures exhibits redox activity.It might not only affect the biological reduction of various pollutants,but also influence biological transformation of soil minerals.Additionally,as the core issue of microbial remediation technology,the transport,retention and distribution of microorganisms in soil aquifers directly influence the practical remediation effects,thereby have drawn significant attention.However,there has been limited research on the application of biochar in microbial reduction of pollutants and the indirect effects of biochar-paticipated soil mineral bioreduction on pollutants transformation and fate.The attachment of microbial cell to biochar particle and their transport behaviors through porous media under water flow have also been rarely reported.Based on these considerations,this study used self-prepared biochar particle and typical electroactive strain Shewanella oneidensis MR-1 as research objects,and investigated the effects of biochar on nitrobenzene reduction by MR-1 and Cr(?)removal by carbon/iron composite formed via biochar-mediated dissimilatory iron reduction.The aggregation of coexisting colloidal biochar particles and MR-1 cells,their cotransport behaviors thorough porous media,and MR-1 transport behaviors in the redox-active porous media containing insoluble electron acceptors like bulk biochar particles were investigated.The main research results are as follows:The effects of biochar on the nitrobenzene reduction by MR-1 were firstly investigated.The results showed that biochar contains conductive carbon skeleton structure and oxygencontaining groups(e.g.,quinoid moieties)and exhibits redox activity.In aqueous systems,biochar particles remain in close contact with MR-1 cells,mediate the extracellular electron transfer(EET)of MR-1,and promote biological nitrobenzene reduction.The promotion effects of biochar produced from wheat and cotton straws were similar.Increasing pyrolysis temperature(300-800?)enhanced biochar conductivity,causing stronger stimulation effects on bioreduction.In addition,the enhancement of nitrobenzene reduction was increased with biochar concentration,but was decreased when biochar concentration reached 0.67 g/L.Iron mineral is widespread in natural environments and plays important roles in biogeochemical processes.This study found that biochar improved Fe(?)production during dissimilative reduction of ferrihydrite and goethite,and formed reductive C/Fe composites with secondary iron minerals.The iron mineral-biochar composite removed Cr(?)through adsorption-reduction and generated Fe-Cr coprecipitate to immobilize Cr.The removal effects were decreased with pH(5.5-8.0)and increased with temperature(15-30?),but were basically not affected by ionic strength changes(0-100 mM).The direct contact between MR-1 cell and biochar particle benefits EET.The heteroaggregation of colloidal biochar particles and MR-1 cells were investigated.With the increase of electrolyte concentration and valence(1-100 mM Na+ and 0.1-5 mM Ca2+),the electrostatic repulsion between cell and biochar was weakened,and thus their aggregation rate was enhanced.More importantly,increasing lactate concentration promoted while lacking key genes of EET inhibited the heteroaggregation.The results of microbial taxis experiments further indicated that MR-1 utilizes EET to sense insoluble electron acceptors like biochar,and tactically moved towards media surface via flagella motility,promoting their heteroaggregation.The transport and retention of cell and colloidal particles are important environmental behaviors and can greatly impact the bioremediation and biodegradation processes.Base on the column experiments using saturated porous media of quartz sand,the transport behaviors of biochar colloids and MR-1 cell under hydrodynamic conditions were investigated.Their cotransport behavior was significantly reduced by heteroaggregation and followed the ripening mechanism.The cotransport of MR-1 cell and biochar without electron donor was mainly influenced by DLVO and hydrophobic interactions.In the presence of electron donor,MR-1 utilized EET not only to reduce biochar and enhance its hydrophobicity,but also to induce microbial energy taxis towards biochar particle.The improved heteroaggregation caused physical straining and inhibited their cotransport.Besides flowing biochar colloids,larger biochar particles would remain in soil media.This study further amended quartz sand media with biochar particles and other insoluble electron acceptors(e.g.,ferrihydrite and MnO2)of MR-1.It was found that increasing electron acceptor loadings,adding electron transfer mediators,and enhancing electron donor concentration all significantly inhibited MR-1 transport,while lacking EET capability and flagella motility promoted MR-1 transport by more than 40%.The inversion results of transport model further suggested that MR-1 utilizes EET to induce energy taxis towards the modified media surface via flagella motility,and this tactic movement promoted the dispersion,attachment and deposition of cells in porous media.In the absence or presence of soluble mediator,the transport ability of MR-1 in porous media was mainly controlled by thermodynamic or kinetic properties,specifically the reduction potential or electron-accepting rate of the redox-active media.This study suggested that redox-active biochar could not only mediate microbial EET and promote bioremediation,but also induce microbial taxis,promote cell-biochar heteroaggregation and therefore inhibit transport of electroactive cell.These results improved the understanding of the environmental behaviors of microorganisms and redox-active carbon materials,and could facilitate the effective bioremediation of contaminated sites using biochar. |
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