Preparation Of Lignin-based Activated Carbon And Study On Its Adsorption Performance For Toluene

Activated Carbon,as a traditional adsorption material,has been widely used in the treatment of VOCs because of its large specific surface area,high adsorption rate and strong regeneration ability.The Lignosulfonates is a major by-product of the pulping process,formed by phenylpropane units into a three-dimensional network of macromolecule,it is an ideal precursor for the preparation of activated carbon with high specific surface area due to its advantages of large annual output,low cost,wide source and rich carbon element.However,the natural macromolecular structure of Lignin undergoes hydrolysis in the pulping process to produce Lignosulfonates,and when phosphoric acid is used to activate Lignosulfonates to prepare activated carbon,the preparation conditions and activation mechanism of the activated carbon are quite different from those using natural plants as carbon source.In addition,activated carbon prepared from Lignosulfonates is a loose powder that is easily lost during use.This paper carries out a series of research on these problems,the specific contents and conclusions are as follows:Firstly,high specific surface area activated carbon was prepared by simultaneous carbonization and activation with Na-Ls as carbon source and phosphoric acid as activator.The effects of phosphoric acid concentration,activation temperature and activation atmosphere on the iodine adsorption value and yield of activated carbon were investigated,and the preparation conditions were optimized.The pore size distribution and micro morphology of activated carbon were analyzed by gas adsorption instrument and scanning electron microscope.The results showed that the optimum conditions for the preparation of activated carbon were as follows:the mass ratio of 34 wt%phosphoric acid solution to sodium lignosulfonate was 0.5:1,the activation temperature was 800℃,and the activation time was 90min.Under these conditions,the iodine adsorption value of activated carbon is 1259.3 mg/g,and the yield is 8.3%.The pore structure of activated carbon is fissured,mainly composed of 0.5 nm micropores and2.6 nm mesopores,with a total pore volume of 1.7 cm³/g and a specific surface area of2066.2 m²/g.In order to elucidate the activation mechanism of phosphoric acid,the changes of materials during phosphoric acid impregnation and pyrolysis and the pore formation of activated carbon were analyzed by means of scanning electron microscopy,thermogravimetric analysis,X-ray diffraction and infrared spectroscopy.The results show that the thermal decomposition of Na-Ls impregnated with phosphoric acid involves hydrolysis,crosslinking,aromatization and oxidation.The addition of phosphoric acid changes the pyrolysis process of Na-Ls.Phosphoric acid concentration and activation temperature are the key factors affecting the pore formation and adsorption properties of activated carbon.Secondly,in order to solve the problem of easy loss of powdered activated carbon in the use process,based on the study of the preparation of activated carbon,using attapulgite as binder,phosphoric acid as activator and Na-Ls as raw material,the molding activated carbon was prepared by one-step molding and activation method,and hydrogen was introduced in the activation process to reduce carbon ablation.Taking the iodine adsorption value and compressive strength of formed activated carbon as indexes,the effects of soil material ratio,activation atmosphere,forming pressure and forming time on the properties of formed activated carbon were discussed.By means of scanning electron microscope,X-ray diffraction and infrared spectrum analysis,the changes of micro morphology and pore size distribution of the formed activated carbon and the reasons for the compressive strength of the formed activated carbon were analyzed.The results show that the optimum conditions for the preparation of formed activated carbon are as follows:the ratio of soil to material is 1.5:8.5,the activation atmosphere is 5%H₂ and 100%N₂,the molding pressure is 100 MPa,and the molding time is 3 min.Under these conditions,the compressive strength of the formed activated carbon can reach 13.8 MPa,and the iodine adsorption value is 386.6mg/g.The pore structure is mainly 0.5 nm micropore and 3.7 nm mesopore.The total pore volume is 0.5 cm³/g and the specific surface area is 331.7 m²/g.In addition,the compressive strength of the formed activated carbon is attributed to the formation of Si-O-C bond cross-linking between attapulgite and activated carbon and the formation of Ca₂Al₂SiO₆（OH）₂ crystal.At the same time,it is found that the introduction of hydrogen source has no obvious effect on the phase structure of the formed activated carbon.Finally,the static and dynamic adsorption of toluene on activated carbon and formed activated carbon under the optimal preparation conditions were carried out,and the effects of toluene initial concentration and carrier gas flow rate on the dynamic saturated adsorption capacity of activated carbon were investigated.In addition,the desorption conditions were optimized with toluene desorption rate as index,desorption temperature and desorption time as main factors.After determining the optimal desorption conditions,the regeneration adsorption capacity of activated carbon for static and dynamic adsorption was studied.The results showed that the static saturated adsorption capacity of activated carbon and formed activated carbon for toluene was354.9 mg/g and 197.2 mg/g.after 10 cycles of regeneration adsorption,the static saturated adsorption capacity of activated carbon and formed activated carbon for toluene reached more than 88%of the first adsorption capacity.The dynamic saturated adsorption capacity of activated carbon varies with the initial concentration of toluene and the flow rate of carrier gas.The initial concentration of toluene is 0.0121-0.0335mg/m L,and the corresponding saturated adsorption capacity is 308.9 mg/g-348.9 mg/g.The saturated adsorption capacity is 321.9 mg/g-344.3 mg/g when the carrier gas flow rate is 80-120 m L/min.The dynamic adsorption of activated carbon after regeneration for three times still reached 86%of the first adsorption capacity.In addition,the optimal desorption conditions of activated carbon and formed activated carbon were 180℃and4 h,and the desorption rates were 98.88%and 98.73%respectively.