| Available and sustainable biomass is an alternative for the conventional fossil energy.Biomass pyrolysis can not only slow down the consumption of fossil energy,but also produce biochar that can reduce greenhouse gas emissionsand increase soil fertility.Recently,biochar has received increasing attention in the fields of sustainable agriculture,environmental control,industrial catalysis,and energy storage.However,the application of biochar was limited by the disadvantages of low surface area,less active sites and insufficient surface functional groups,which need to be modified before used.The composition,structure and surface property of biochar was affected by the pyrolysis process and conditions,such as pyrolysis temperature,pyrolysis atmosphere,heating rate and holding time.Besides,the intrinsic nature of the biomass feedstock also can change the properties of the produced biochar.It is highly desired to analyze the property of biochar,regulate the component and structure of biochar and design novel biochar material for emerging application.In this thesis,a series of novel biochar materials were synthesized by adjusting the component of biomass and pyrolysis parameters.The prepared biochars show excellent performances in various environment application.This paper provides valuable information to build the structure-application relationships for biochar application.The detailed contents and results of this paper are as follow:1.To improve the surface area and recycle of biochar in pollutant removal,a magnetic biochar was prepared by fast pyrolysis of FeCl3 pre-loaded biomass.The reduced components(such as H2,CO and amorphous carbon)released during biomass pyrolysis convert iron ions to zero-valent iron,and the zero-valent iron embedded in the carbon skeleton improve the structure of biochar.At 600℃,FeCl3 can catalyze the deposition of carbon nanofibers on the surface of biochar and enhance the surface area of biochar to 420 m2·g-1.20 mg/L bisphenol A can be completely removed in 5 minutes under optimal conditions.Results show that both radical pathway and nonradical pathway were involved in BPA degradation.In radical pathway,zero-valent iron in biochar reacted with PMS to produce SO5·-and HO·.Those radicals can attack and break the C-C bond of BPA,and transform BPA to CO2 and H2O after a series of ring-opening reaction.The persistent free radical in biochar can also react with PMS to form SO5·-and HO· though electron transport.In the nonradical pathway,the surface nanofibers could affect the electron configuration of PMS,activate and break O-O bond in PMS.The activated PMS molecule(electron acceptor)can directly oxidize BPA(electron donor)via two-electron conduction with the assistance of the nanofibers(electron shuttle).In addition,Fe-BC-700 has relative high cycle stability because the iron nanoparticles were encapsulated in carbon nanofibers.Reactive oxygen induce by the environment persistent free radicals(EPFRs)in biochar is harmful to ecosystem.To avoid the negative effect of biochar on environment,sludge was used as a typical solid waste to investigate the formation mechanism of EPFRs during the waste pyrolysis and adjust the content of EPFRs in biochar by changing the pyrolysis parameters.Results showed that EPFRs are mainly formed in the pyrolytic temperature range of 300～600℃.The content of EPFRs in sludge biochar reached 1.6×1019 spins g-1,which hardly reduced after a month.The content of EPFRs in biochar obtained from slow pyrolysis was higher than that from fast pyrolysis.EPFRs in the biochar obtained from slow pyrolysis are mainly carbon-centered radicals whereas those from fast pyrolysis are carbon and oxygen-centered radicals.Aromatic compounds,including carboxyl,hydroxyl,and aldehyde groups,may be the main precursor of EPFRs based on the correlation of EPFR concentrations and intermediates determined via TG-FTIR-MS.Those compounds were physically or chemically adsorbed on transition metal oxides.O-centered EPFRs were formed from the electron transfer from the oxygen atom to the metal oxide.C-centered EPFRs were formed by adsorption between transition metal oxide and aromatics and aldehyde compounds by the cation-π interaction at active sites.Notably,both N species and ash can react with EPFR to reduce the formation of EPFRs during sludge pyrolysis.2.To enhance the active sites of magnetic biochar,precious metal Pd nanoparticles were fixed on the magnetic biochar by a wet chemical reduction method.The oxygen-containing functional groups on the surface of the biochar can fix and disperse the Pd nanoparticles and increase the activity of the catalyst.To investigate the catalytic performance of prepared catalyst,the 4-nitrophenol reduction was used as a model reaction.The catalytic performance of Pd@Fe3O4/biochar with a Pd content of only 1.58%was twice of the commercial 5%palladium carbon.Results show that a strong metal-support interaction was occured between the Pd nanoparticles and Fe3O4 in magnetic biochar.Such a strong interaction can induce a redistribution of charge of Pd nanoparticles,which endow the Pd nanoparticles with reduction state and reduce its adsorbate binding energies.During the reaction,the modified Pd nanoparticles can accelerate the formation of active hydrogen and rapid desorption of intermediates.Similarly,other noble metal,such as Pt and Ag can also be fixed on the biochar and enhance the catalytic performance.3.To improve the electron transport capacity of biochar in electrochemical,a Fe,N co-doped biochar materials was synthesized by pyrolysis of N-containing biomass(Typha angustifolia)and Fe salt.Electrochemical nitrate reduction was conducted to test the performance of modified biochar.It was found that the nitrogen content of biochar after pyrolysis could reach 2.2%.The pyridine-N and pyrrole-N in biochar will convert to graphite-N with the temperature increase.The electrochemical denitrification performance indicated that the synthesized Fe,N co-doped biochar could reduce 80%of the 50 mg/L nitrate solution within 5 hours,and the nitrogen selectivity was 73%.The nitrate was first adsorbed on modified biochar and reduced to nitrite.Pyridine nitrogen and pyrrole nitrogen in biochar can serve as electron donors and reduce the nitrite to nitrogen.Graphite nitrogen can reduce energy barriers of dissociated water molecules and promote the production of atomic hydrogen,which facilitate the reduction of nitrite to ammonium. |
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