Dual-templating Activation And Functional Modification Of Soybean Cake Biochar And Properities

Environmental and energy problems are the two main topics of concern in today’s world.In the pulp and paper making industry,organohalogens（AOX）contained in pulp bleaching wastewater andazo dyes used in paper and pulp dyeing can be seriously toxic to aquatic organisms and humans,with strong carcinogenic and teratogenic effects.Adsorption is the most commonly used technique to remove small and medium-sized molecular pollutants from water due to its advantages of simple treatment,low cost and high efficiency.Biochar is a good adsorption material,and its adsorption performance is related to the raw materials,preparation process,specific surface area,pore size distribution and surface functional groups,etc.In this study,biochar was obtained by direct carbonization using soybean cake as raw material,but the adsorption capacity is not good enough due to the low specific surface area of raw biochar.The specific surface area of biochar can be greatly improved after chemical activation,but there is the disadvantage of concentrated pore size distribution.The characteristics of raw materials can also affect the conversion process and product properties of biochar.In this study,hierarchically porous self-doped nitrogen soybean cake biochar was prepared by using K₂C₂O₄·H₂O and Ca CO₃as dual-templating activators,which optimized the pore size distribution,increased the porosity and specific surface area of soybean cake biochar;to further overcome the problem that it is difficult to recover the activated biochar（powder）,K₃[Fe（C₂O₄）₃],which can be regarded as a complex of K₂C₂O₄and Fe₂（C₂O₄）₃,was used to magnetize,activate,and the nitrogen-doped soybean cake biochar in a one-pot method,and its adsorption properties were investigated;the effect of N/Fe doping in biochar on the adsorption of AOX and dyes was illustrated with DFT calculations.This study provides a theoretical basis for the preparation of high-performance biochar and the influence of specific elements in carbon materials on adsorption.Finally,since the prepared hierarchically porous soybean cake biochar has rich micro-and mesoporous structures,it can be loaded with Fe₃O₄nanoparticles in its pores and used as anode material for lithium-ion batteries for energy applications to achieve high value utilization of soybean cake biochar.The mainly research contents and conclusions are as follows:1.Preparation of self-doped nitrogen soybean cake biochar and its adsorption performance on dyes and AOX model agents.Self nitrogen-doped soybean cake biochar（SCB）was prepared by direct carbonization of soybean cake.Its specific surface area is 18 m²g^–1,elemental analysis results showed that the N content of SCB reached 6.8 wt.%;XPS results confirmed that the surface of soybean cake biochar was rich in pyridine-N,pyrrole-N,graphite-N and oxidized-N.Adsorption studies showed that the theoretical capacity of SCB is 17.25 mg g^-1and 54.91 mg g^-1for the cationic dyes rhodamine B（Rh B）and 2,4,6-trichlorophenol（TCP）,respectively.2.Dual-templating activators modulated the pore size distribution of soybean cake biochar to prepare hierarchically porous self nitrogen-doped soybean cake biochar（NSB）and its adsorption properities on dyes and AOX.The soybean cake biochar prepared by using K₂C₂O₄·H₂O and Ca CO₃as dual-templating agents had abundant micro-and mesoporous structures,its average pore size and specific surface area is 3.83 nm and 2413 m²g^–1,respectively.XPS showed that the NSB surface was rich in pyridine-N,pyrrole-N and graphite-N.Compared with SCB for Rh B(17.25 mg g^–1)and TCP(54.91 mg g^–1),the adsorption capacities of NSB for Rh B,CR and TCP reached 840 mg g^–1,882 mg g^–1and 1550 mg g^–1,respectively.The adsorption capacities of NSB for Rh B and TCP were increased by 51 and 28 times,respectively.Such high adsorption capacity of NSB for TCP were attributed to the hierarchically porous structure of biochar on the one hand,and to the small molecular weight and hydrophobic nature of TCP on the other hand.The thermodynamic parameters reveal that NSB is easy to adsorb CR dyes according toΔG°values;positiveΔH°values indicate that the adsorption of dyes and AOX on NSB is a heat-absorbing process,and more heat is required for the adsorption of CR by NSB.And positiveΔS°values indicate that adsorption of adsorbate on NSB leads to increased disorder at the biochar/adsorbate solution interface.DFT calculations indicate that nitrogen-doped biochar has higher adsorption energies for Rh B,CR and TCP,more favorable adsorption,and more stable equilibrium structure after adsorption.3.Magnetic self nitrogen-doped biochar（MNB）was prepared by the one-pot method of K₃[Fe（C₂O₄）₃].Using（K₃[Fe（C₂O₄）₃]）as an activator and iron provider,the soybean cake biochar was simultaneously activated and functionally modified to prepare MNB;the specific surface area of MNB is 1034 m²g^–1and there is an abundant micro-and mesoporous structure.The maximum adsorption of MNB for CR and Rh B was 1360 and 320 mg g^–1,respectively.Since the content of loaded iron on MNB reached 38.7 wt.%,it could be recovered using magnets easily,and the adsorption capacity remained above 70%of the initial one after 5 cycles.DFT calculations showedπ-πstacking,electrostatic attraction,ionic dipole and hydrogen bonding interactions between the two dyes and MNB;the adsorption energies of the MNB-Fe²⁺/CR and MNB-Fe³⁺/CR complexes were-85.5 and-60.3 kcal mol^–1,respectively,which are significantly higher than the adsorption energies of MNB-Fe²⁺/Rh B and MNB-Fe³⁺/Rh B complexes(-22.2 and-16.7 kcal mol^–1);thus indicating that MNB has a stronger adsorption effect on CR than Rh B;the adsorption energy of MNB-Fe²⁺on dye is also higher than that of MNB-Fe³⁺,and the Fe²⁺loaded on biochar is more effective.The adsorption energy of MNB-Fe²⁺was also higher than that of MNB-Fe³⁺,and the Fe²⁺loaded on biochar played a better role than Fe³⁺in dye adsorption.4.NSB loaded with Fe₃O₄nanoparticles to prepare Fe₃O₄/NSB lithium-ion battery anode material for high value utilization of soybean cake biochar.Fe₃O₄/NSB was prepared from soybean cake with 16.3 wt.%Fe content and 1495 m²g^–1specific surface area after the activation by dual-templating activators and loading Fe₃O₄nanoparticles by co-precipitation method.NSB and Fe₃O₄/NSB were tested as lithium-ion battery anode materials.It was found that the discharge specific capacity of the lithium-ion battery assembled with NSB as anode material was almost the same as the secondary discharge specific capacity after 1000 charges and discharges at600 m A g^–1current density without decay;while the secondary discharge specific capacity of the battery assembled with Fe₃O₄/NSB lithium-ion battery anode material increased by up to 60%compared with that of the NSB anode material;the discharge specific capacity of the battery assembled with Fe₃O₄/NSB anode material increased by up to 60%compared with that of the NSB anode material at 600 m A g^–1current density.The secondary discharge capacity was maintained at 89.7%after 1000 charge/discharge cycles.In this study,we made full use of the chemical composition and biological structure of soybean cake,and the biochar with hierarchically porous structure prepared by a mild double-template method has excellent adsorption performance for dyes and AOX,which is important for the clean production of pulp and paper,and is expected to be used as anode material for lithium-ion batteries in the energy field.