Green Preparation Of Biomass Activated Carbon Based On Hydrothermal Method And Its Adsorption Properties Study

As a general adsorbent,activated carbon is widely used to adsorb pollutants in water and air,and as energy storage,catalysis and electrode materials,because of its good adsorption performance and porous structure.Therefore,activated carbon still has huge market demand and broad development prospects.At present,the commercial activated carbon market is mainly coal-based activated carbon.Due to the non-renewable limitation of coal resources,the supply and price of activated carbon will be affected.Therefore,in order to reduce the excessive dependence on coal resources and reduce the cost of activated carbon,people began to focus on biomass activated carbon.Compared with fossil precursor activated carbon,biomass activated carbon has the advantages of wide source of raw materials,renewable,cheap and easy to obtain,environment-friendly and so on,and it has great market potential and application prospect.Although there are many researches on biomass activated carbon now,most of them focus on the selection of raw materials,the preparation process is still the traditional physical and chemical activation method.And the preparation process of biomass activated carbon with high specific surface area still needs chemical activators such as KOH,H₃PO₄and ZCl₂,or physical activation processes such as CO₂and H₂O with additional equipment.The production process emits toxic and harmful gases and waste liquids,and causes environmental pollution problems.At the same time,acid and alkali can also reduce the life of the equipment and increase the production cost.Based on the analysis of the application prospect and the existing preparation process of biomass activated carbon,this thesis aimed at achieving green preparation of biomass activated carbon with high specific surface area,and explored new processes and methods for the preparation of biomass activated carbon,so as to realize its preparation process without adding chemical activators,and reducing environmental pollution and reducing production costs.In this study,pine sawdust was used as the precursor of activated carbon,based on the theory of biomass composition and hydrothermal reaction principle,steam explosion method,low temperature hydrothermal reaction and high temperature hydrothermal carbonization reaction were used as the basic methods.Pine sawdust was pretreated on the self-designed hydrothermal steam explosion test platform,and then pyrolyzed at 900℃to obtain high specific surface area activated carbon.By means of physical adsorption,SEM,XRD,FTIR and XPS,the effects of different pressure,temperature and time factors on BET specific surface area,pore size distribution,microcrystalline structure and surface oxygen-containing functional groups of final pine sawdust activated carbon were explored and compared.The adsorption properties of the prepared pine sawdust activated carbon for perfluorooctanoic acid（PFOA）and methylene blue（MB）in aqueous solution were investigated.The pseudo first-order,pseudo second-order,intra-particle diffusion and pore diffusion kinetic models,and Langmuir and Freundlich adsorption isotherm models were used to fit the experimental data,so as to study the adsorption mechanism and difference of PFOA and MB on the prepared pine sawdust activated carbon.The study on the effects of steam explosion pretreatment on the properties of pine sawdust activated carbon showed that increasing the steam explosion pressure will reduce the BET specific surface area,micropore volume and total pore volume of the final activated carbon product;Increasing the steam explosion temperature can increase the specific surface area and micropore volume of activated carbon;Prolonging the residence time of steam explosion can increase the specific surface area of activated carbon,but when the residence time was extended from 5 min to 10 min,the increment of specific surface area was limited.At the same time,prolonging the residence time will help to increase the micropore volume,but reduce the mesopore volume;However,the changes of pressure,temperature and residence time have limited effects on the type and relative content of oxygen-containing functional groups on the surface of final activated carbon.The specific surface area and micropore volume of pine sawdust activated carbon decrease with the increase of steam explosion pressure,while the macropore volume increases,which showed that increasing the initial pressure enhances the steam bursting strength,caused the tearing and damage of pine cell structure,and improved the macropore volume of the final activated carbon product.The change of steam explosion initial pressure had a certain impact on the type and number of oxygen-containing functional groups on the surface of final activated carbon,which can be attributed to the change of steam burst pressure under low-temperature hydrothermal conditions affecting the hydrolysis degree of cellulose and hemicellulose in pine cell wall.The pine sawdust pretreated by steam explosion can be pyrolyzed at 900℃to obtain high specific surface area activated carbon.The maximum BET specific surface area was 988 m²/g and the maximum pore volume was0.526 cm³/g.In the study of the effect of hydrothermal-pressure preconditioning on the characteristics of activated carbon and the adsorption performance of PFOA,the effect of pressure and explosion on the characteristics of activated carbon was mainly investigated.Through comparative study on the effects of three pretreatment methods of hydrothermal-pressure steam explosion（HPE）,hydrothermal pressure non explosion（HP）and hydrothermal non pressure non explosion（HY）on the final pine sawdust activated carbon products.It was found that the pressure could significantly improve the micropore volume and BET specific surface area of activated carbon;The steam explosion factor reduced the micropore volume and BET specific surface area of activated carbon,and improved the mesopore volume of activated carbon.Hydrothermal pressure non burst（HP）provided the maximum micropore volume（0.344 cm³/g）and BET surface area（990 m²/g）.The adsorption capacity of PFOA on HP was 232 mg/g and the adsorption capacity of MB was 350 mg/g.Hydrothermal pressure steam explosion（HPE）activated carbon showed more 100-1000?pore traces than hydrothermal pressure non explosion（HP）samples.These differences may mean that HPE steam explosion method actually had enough strength to convert more micropores into mesopores.The preparation of high specific surface area activated carbon from pine sawdust by high temperature hydrothermal carbonization was studied.The results showed that the BET specific surface area and microporous volume of pine sawdust activated carbon increased with the increase of initial pressure of hydrothermal carbonization;Increasing the hydrothermal carbonization temperature of pine sawdust can increase the BET specific surface area,micropore and total pore volume of the final activated carbon;However,the mesoporous volume of activated carbon first decreased and then increased with the increase of hydrothermal carbonization temperature;Prolonging the hydrothermal carbonization time can improve the BET specific surface area,micropore and mesopore volume of activated carbon.Compared with the factors of pressure and temperature,the hydrothermal carbonization time had a greater impact on the pore structure of the final activated carbon.The results also showed that,biomass activated carbon with high specific surface area and pore volume can be obtained by pyrolysis of pine sawdust at 900℃after high-temperature hydrothermal carbonization pretreatment.The maximum BET specific surface area was 1507 m²/g,the maximum total pore volume and micropore volume were 0.780 cm³/g and 0.493 cm³/g respectively,and the maximum MB equilibrium adsorption capacity was 502 mg/g.One-way analysis of variance（ANOVA）and least significant difference（LSD）comparison based on MB adsorption capacity showed that,there were significant differences in pine sawdust activated carbon prepared at different levels of temperature and time factors,but there was no significant difference between the samples under 0 psi and 400 psi pressure.