| Hexavalent chromium(Cr(?))is a common toxic heavy metal in wastewater.Zero-valent iron(ZVI)can reduce Cr(?)to less toxic trivalent chromium(Cr(?)),which can be removed via precipitation and co-precipitation mechanisms.However,aggregation and passivation of ZVI nanoparticles could compromise electron transfer capacity of ZVI and thus reduce Cr(?)removal.Thereafter,harboring ZVI by conductive and porous carbon matrix can improve dispersion of ZVI particles and enhance electron transfer from ZVI to Cr(VI),possibly attributing to increased Cr(?)removal efficiency.It has been documented that graphitic structure of biochar prepared via pyrolysis at high temperature exhibited well electron transfer capacity.However,the quantitative relationship between ZVI reductive capacity and electrical conductivity of carbon matrix has yet been established,which made it difficult to manipulate the carbon structure for optimized Cr(?)removal.Besides,biomass composition could affect graphitic carbon structures and electron tranfer of resulting biochar,but there was still lack of systemic investigation of their contribution to ZVI electron transfer.In this thesis,individual component(cellulose,hemicellulose and lignin)of residual biomass from agriculture and forestry were co-pyrolysis with hematite under N2 atmosphere to synthesize biochar-supported ZVI composites.The as-obtained composites were characterized by electrochemical analysis,Raman spectroscopy and other techniques to investigate the effects of indivial and combined biomass components,the mass ratio of hematite to biomass components on electrical conductivity,carbon graphitization and the electron transfer ability and Cr(VI)reduction of resulting composites.In addition,biochar-supporting ZVI composites(ZVI/BC)were synthesized by co-pyrolysis of hematite-treated pinewood(composition of cellulose,hemicellulose and lignin was 47%,25%and 26%respectively)at 600-1000?.The capacity and mechanisms associated with enhanced electron transfer capacity of ZVI/BC were investigated by sorption experiments and characterization.Finally,graphene and p-benzoquinone was used to mimic carbon matrix structure and functional groups of biochar,respectively,in order to understand the contribution of these two major conductive moieties for ZVI electron transfer,and quantify the relationship between biochar conductivity and electron transfer capacity by using electrochemical analysis.The main results are as follows(1)Thermogravimetric analysis of biomass pyrolyzed between 20-800? under nitrogen atmosphere showed that decomposition of cellulose occurred mainly between 310-370? with most rapid weight loss rate at 346?(2.19wt.%/?),considerable weight loss was observed between 210-32? for hemicellulose,with greatest decomposition rates of 0.55wt.%/? and 0.63wt.%/? at 245? and 295?,respectively.Lignin decomposed quickly at 220-460? and 665-705? with greastest decomposition rates(0.08wt.%/? and 0.11wt.%/?).The biochar yields of cellulose,hemicellulose and lignin were 10%,17%and 42.5%,respectively.The surface area of BCCE,BCHE and BCLG was 432.95m2/g,279.84m2/g and 1.45m2/g,respectively.Raman spectra indicated that the ID/IG ratio of BCCE,BCHE,and BCLG was 0.928,0.925 and 0.848,respectively,which implied a greater graphitization degree of BCCE and BCHE over BCLG.Adsorption capacity of BCCE,BCHE and BCLG was 6.67mg/g,4.49mg/g and 3.42mg/g,which showed opposite trend of Tafel corrosive potential(BCCE<BCHE<BCLG),confirming BCCE tranferred electron faster than other two biochars.(2)Hematite was co-pyrolyzed with cellulose,hemicellulos and lignin at the mass ratio of 1:20 respectively and the obtained composites was named as ZVI/BCCE,ZVI/BCHE and ZVI/BCLG.In Raman spectra,the ID/IG ratios of ZVI/BCCE,ZVI/BCHE and ZVI/BCLG was 0.947,0.941 and 0.888,which were greater than respective pristine BCCE(0.928),BCHE(0.925)and BCLG(0.848),this indicated that hematite facilitated carbon condensation of biomass but to different extents.Relative to ZVI/BCHE and ZVI/BCLG,the ZVI/BCCE was characterized with lower Tafel corrosive potential and electron conductivity,which was favorable for electron tranfer and Cr(VI)reduction.(3)Hematite was co-pyrolyzed with dural or tri-components of cellulose,hemicellulose and lignin at the mass ratio of 1:20 respectively and the obtained composites was named as ZVI/BCCH,ZVI/BCCL,ZVI/BCHL and ZVI/BCCHL.In Raman spectra,the ID/IG ratios of ZVI/BCCH,ZVI/BCCL,ZVI/BCHL and ZVI/BCCHL was 0.972,0.896,0.862 and 0.903,Raman characteristic indicated that ZVI/BCCH exhibited well graphitization.Cr(VI)reduction capacity on ZVI mass basis were ZVI/BCCH(176.71mgCr/gFe)>ZVI/BCCHL(91.00 mgCr/gFe)>ZVI/BCCL(72.99 mgCr/gFe)>ZVI/BCHL(48.30mgCr/gFe),ZVI/BCCH had well reductive capacity.The Cr(VI)reduction capacity on ZVI basis showed opposite trends of Tafel corrosive potential and electron conductivity(ZVI/BCCH<ZVI/BCCHL<ZVI/BCCL<ZVI/BCHL),demonstrating good electrical conductivity could enhance electron transfer and Cr(VI)reduction.(4)Pinewood biomass and hematite were used to synthesize ZVI/BC600 ZVI/BC800 and ZVI/BC1000 composites.At 600?,Fe2O3,Fe3O4 and FeO were dominant Fe minerals with minor ZVI peaks in ZVI/BC600.At 800?,ZVI/BC800 exhibited ZVI peak.At 1000?,hematite was successfully converted to ZVI.As pyrolysis temperatures increased,ID/IG ratios of Raman spectra increased in the order of ZVI/BC600(0.75),ZVI/BC800(0.82)and ZVI/BC1000(0.97),which indicated graphitization of biochar.Cr(?)removal capacity of composites was:ZVI/BC600(0.98mg/g)<ZVI/BC800(29.45mg/g)<ZVI/BC1000(34.29mg/g),ZVI/BC1000 exhibited well Cr(?)removal.At pH=3,the Cr(VI)adsorption kinetics data were better simulated by pseudo-second order kinetics model.In addition,intraparticle diffusion model confirmed that adsorption invoked surface complexation,electrostatic attraction and intraparticle diffusion.The adsorption isotherms of composites were better fitted with Langmuir isotherm model,and the predicted maximum Cr(?)adsorption capacity was 36.12mg/g.Coupled with desorption and XPS spectrum of Cr-spent sorbent,the Cr(VI)was mainly retained by reduction,followed by ion exchange and surface complexation.Electrochemical analysis demonstrated that BC1000 and ZVI/BC1000 had lower electrical resistance than ZVI1000,and ZVI1000 showed lower electron transfer rate.The quantitative electron transfer of ZVI/BC1000 was 1.182 ?mol e-relative to BC1000(5.35 nmol e-),suggesting biochar mainly function as an electron meditor to promote Cr(?)reduction by ZVI.(5)Graphene and p-benzoquinone were used to mimic the carbon matrix structure and functional groups of the biochar to optimize Cr(?)reduction capacity of ZVI/BC1000.As for pristine ZVI,graphene and p-benzoquinone treatment increased Cr(?)reduction by 1.12 and 1.46 times,indicating both functional groups and graphite carbon matrix were important moieties for electron transfer.Besides,Cr(?)reduction increased by 29.84%and 52.53%after graphene and p-benzoquinone treatments,respectively.This indicated that graphitic carbon in ZVI/BC1000 was the major electron transfer moiety. |
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