Electron Transfer In Biochar Mediated Reduction Of Chromium

Biochar refers to the pyrolyzed char of biomass in relatively low temperature（less than 700^oC）with anaerobic condition,which contains high level of carbon,with large specific surface area and high aromatization.On the surface of biochar,there is large amount of redox-active groups,including phenolic,quinone and thick rings,which can reversibly gain or lose electrons and be involved in the REDOX reaction via proton coupled electron transport process.Recently,the researches on biochar mainly focus on its use of carbon sequestration,adsorption,soil amendment and biological energy source.However,the information of its application as electron donor and the electron shuttle in the REDOX reaction is scarce.In this study,electron transfer process between biochar and Cr（Ⅵ）was investigated in both nonbiological and biological reactions to understand its electron transfer mechanism.The main contents and conclusions of this paper are as follows:（1）Interaction in abiotic reduction of Cr（Ⅵ）by 350^oC peanut shell-derived biochar and lactate was studied via dynamic experiments,electrochemical property determination and sample characterization（FTIR,XPS,XRD and EPR）.The results showed that BC350 could effectively reduce Cr（Ⅵ）to Cr（III）,and lower pH was favorable for the reduction of Cr（Ⅵ）.The O-containing functional groups,e.g.–C-O and–C=O,were probably the electron donor moieties of biochar,which could be oxidized to–COOH during the redox reactions.BC350 could also act as electron shuttle between Cr（Ⅵ）and lactate,enhancing the reaction rates and extents of Cr（Ⅵ）reduction by lactate,especially at higher pH.The reduction rates of Cr（Ⅵ）by lactate at pH=4 in the presence of BC350 is much higher than those by either BC350 or lactate alone.The MEO analysis showed that BC350 addition could increase the electron donating capacity of lactate by up to 23 times at pH=4.The EPR analysis showed that O-centered radicals,e.g.semiquinone-type radicals,were related with BC350 as electron shuttle.The effect of electron donor was more favourable in lower pH,and the effect of electron shuttle was more favourable in higher pH.（2）The mediating effect of different peanut shell-derived biochar（350^oC and 700^oC）to Escherichia coli BL21 in its reduction of Cr（Ⅵ）was studied by determination of electron exchange capacity（EEC）with MER and MEO reactions.The results showed that the EEC of BC350 was 1.431mmol e^-/g and 3.34 times that of BC700.Dynamic experiments indicated that peanut shell-derived biochar could act as electron shuttle to mediate Escherichia coli BL21 induced reduction of Cr（Ⅵ）,and the mediating ability of BC350 was remarkably higher than that of BC700.SEM scan showed the attaching and growth of Escherichia coli BL21 on biochar,thus facilitating the direct electron transfer between biochar and Escherichia coli BL21.（3）The mediating effect of biochar with modified functional groups to Escherichia coli BL21 for its reduction of Cr（Ⅵ）in realistic Cr（Ⅵ）contaminated soil solution was studied by electrochemical performance testing to modified and unmodified biochar.Results showed that the EEC of biochar varies greatly in biochar with different modification and pyrolysis temperature.Physical adsorption of AQS,H₂O₂ oxidation and blocking of carboxyl and hydroxyl led to the enhancement of biochar EAC and diminution of biochar EDC.Experiments on surface group modification of biochar and reduction kinetics showed close relation between both mediating ability and EEC of biochar and semiquinone free radicals,thus revealing the effect of REDOX active groups in biochar mediated electron transfer between Escherichia coli BL21 and Cr（Ⅵ）.In addition,Eh value of Cr（Ⅵ）contaminated soil solution decreased after adding biochar and occurring of bioremediation,indicating that Cr（III）could be stable in this system.Overall,our paper gives a brand-new insight on biochar application to remediation of Cr（Ⅵ）contaminated sites via its electron shuttle function.