The Electron Mediating Role Of Biochar-based Catalysts To Promote The Advanced Oxidation Process On Pollutant Removal From Aqueous Solution

With the rapid development of society,the environmental pollution and shortage of water resources are the key issues affecting the sustainable development of mankind.There is a wide application prospect of heterogeneous advanced oxidation progresses（AOPs）in removing pollutants and realizing the reuse of water resources,due to their simple operation,mild reaction and high mineralization efficiency.Developing an economical,efficient,and green catalyst to promote AOPs has become the hot spot in the field of water treatment.Biochar prepared from organic solid waste had been a popular environmental remediation mat erial with sustainable benefits,owing to its excellent surface properties,pore structure and biocompatibility.The heteroatom functional groups,defect sites,persistent free radicals and active mineral components produced in the preparation process endow biochar with catalytic performance.However,the disordered structure of biochar cannot achieve the effective directional transfer of electrons,which limits the practical application of biochar in catalytic application.In this study,agricultural wastes were used as raw materials to prepare biochar-based catalysts with different functional trigger ing sites and electron conduction capacity through a facile and green synthesis route.On the basis of maintaining the excellent adsorption,low toxicity and stability of biochar,the catalytic ability of biochar was improved by promoting the electron transfer.Prepared biochar-based catalyst was coupled with AOPs to achieve the removal of pollutants from aqueous solution.The structure and property characteristics of biochar-based materials and their functions in promoting AOPs were analyzed.The degradation mechanism and removal pathway of pollutants in biochar catalytic system were also revealed.The specific research work involves in the following four sections:In the 1st section,the mechanism of catalytic degradation of organic pollutants by graphitized porous biochar nanocomposites was studied.Graphitized porous biochar（PGBC）was obtained by pyrolysis using iron-based compounds as graphitization activator.PGBC with high specific surface area can be a good loading platform for semiconductors.g-MoS₂ nanosheets were self-assembled with PGBC as carrier by hydrothermal reaction.The absorption intensity and response range of visible light of composite catalysts were enhanced by the incorporation of biochar with improved graphitization degree and porosit y.With the acceleration of photogenerated electron transfer,the instantaneous photocurrent intensity of g-MoS₂/PGBC is more than twice as high as that of monomer MoS ₂.Under the condition of visible light radiation,the removal efficiency of tetracycline（TC）by g-MoS₂/PGBC through the synergy of adsorption and catalysis is 37.04%and 32.15%higher than that of monomer MoS₂ and MoS₂/BC,respectively.The better catalytic performance comes from the formation of Mo-S-C bond at the interface of the composite,which contributes to the cross-interface transfer of photogenerated electrons.It helps to reduce the recombination of carriers,and produce more oxidizing active species.h⁺and·OH work together on the degradation of TC,and the mineralization rate is more than 40%with 60 min radiation.In the 2nd section,the mechanism of organic pollutants degradation by nitrogen-doped graphitized biochar activated persulfate was studied.In order to avoid the release of toxic metal ions in the reaction process,and reduce the dependence on visible light radiation,the 2nd section prepares a metal-free nitrogen-doped graphitized biochar（N-PGBC）by the co-pyrolysis of biochar with urea and potassium ferrate.The experiments of electrochemical characteristics show that N-PGBC has the lowest electrochemical impedance and the maximum current density.By coupling persulfate（PS）system,the degradation rate of TC by N-PGBC with stronger electron transfer was 7 times higher than that of BC/P S heterogeneous system.Graphitization and nitrogen doping functionalization of biochar-based materials help to accelerate electron transport and increase the contribution of persulfate non-radical pathway in TC degradation.The vacancy and defect edge for med on the sp²-hybrid carbon skeleton,as well as nitrogen doping sites and ketone functional groups were considered to be the main active sites of N-PGBC for PS activation.The degradation of pollutants by N-PGBC/PS system does not depend on free radicals thereby has high anti-interference ability to organic matter or salt ions in solution.In the 3rd section,the mechanism of organic pollutants degradation by nitrogen-doped biochar fibers activated persulfate was studied.In order to reduce the use of corrosive reagents in the modification of biochar and avoid its subsequent treatment process,the 3rd section employs green reagents and synthesis path for biochar optimization:using deep eutectic solvents（DESs）modification and slow pyrolysis to realize the separation of biochar fiber structure and nitrogen doping.The characterization results show that the nitrogen-doped biochar fiber（NRBF）has more regular morphology,higher carbon content,more aromat ic cluster structure,with the strongest electron conduction properties.N atom doping increased the electron density of the aromatic ring,created more defect sites and improved the electron mobility.The non-radical pathway of electron shuttle mediated b y NRBF bridge is the main pathway for pollutant degradation by NRBF activated PS.The degradation rate of TC in NRBF/PS system is 0.0221 min^-1,which is 4.5 times higher than that of pristine biochar/PS system.In addition,NRBF/PS system can achieve bacte rial inactivation through the oxidation of cell membrane substances.In the 4th section,the mechanism of metals removal from microplastics by magnetic porous biochar activated persulfate was studied.In order to further improve the universality of biochar-based catalysts for the removal of pollutants and realize the catalyst recovery,the 4th section realizes the simultaneous carbonization,graphitization,pore-creation and magnetization of biomass by one-step pyrolysis.The graphitized magnetic biochar（PGMB）has typical graphite structure and crystallinity.The slight change of saturation magnetization and the minor leaching of Fe ion into solution after reaction show that the close combination of biochar with Fe⁰ and Fe₃O₄.Coating endows the biochar-based catalyst with magnetism and improves the catalytic ability for PS activation through valence change.The experimental results show that the organic matter layer of microplastics is the key to pollutant absorption.Based on3D fluorescence measurement,it is proved that PGMB activated PS can decompose the organic matter on the microplastic surface,which induces the Pb desorption from microplastic surface.The removal rate of Pb on the surface of aged microplastics by PGMB/PS system can reach more than 60%,and the adsorption capacity of Pb on the recovered PGMB in this system is 31.29 mg/g,which is much higher than that on PGMB alone（7.07 mg/g）.It indicates that PGMB/PS system can realize the transfer of Pb from microplastic surface to PGMB,and the PGMB adsorbed with heavy metals could be removed from aqueous solution by magnetism.This paper systematically studies the performance and mechanism of biochar-mediated electron transfer to promote the removal of pollutants by AOPs.It provides a lot of valuable information for the resource utilization of organic solid wastes and the design and development of biochar-based catalysts for polluted-water remediation.It also provides important experimental basis and theoretical guidance for the application of biochar technology in practical site engineering.