Preparation Of Tobacco Stem-based Porous Carbon By Calcium Carbonate-potassium Oxalate Activation And Its Adsorption Properties

Tobacco stem is the thick vein part of tobacco leaves,which is an important by-product in tobacco processing.Every year in China,hundreds of thousands of tons of tobacco stems are not fully utilized and abandoned,resulting in waste of resources and environmental pollution.Porous carbon,as an adsorbent,has been widely used in water treatment,essential oil and drug encapsulation and slow release because of its high specific surface area,abundant pores,good thermal stability and no toxicity.The preparation of porous carbon adsorption materials from tobacco stems is an effective way to realize its high-value utilization,which has important research significance and application value.However,the porous carbon prepared by traditional single-component activator is mainly micropores,and more importantly,tobacco stems are insoluble in water and compact in structure,which has poor compatibility with activator,resulting in low specific surface area of tobacco stem carbon,which greatly limits its adsorption performance and industrial application.In this paper,calcium carbonate-potassium oxalate was used as binary activator,and tobacco stem powder and activator were effectively compounded by mechanical ball milling pretreatment method,and tobacco stem-based porous carbon materials with high specific surface area and adjustable pore channels were prepared.Furthermore,SEM,BET,XRD,FT-IR,Raman,TGA and other technical means were used to characterize its micro-morphology and physical and chemical properties,and the formation mechanism of multi-stage pores was explored.The adsorption properties of SMZ and essence were studied by static adsorption at constant temperature,which provided basic data for its application in antibiotic wastewater and cigarette flavoring adsorbent.The main research contents and conclusions are as follows:(1)The effects of activator types on the structure of porous carbon based on tobacco stems were studied.With potassium oxalate or calcium carbonate as one-component activator,the specific surface area and mesopore ratio of porous carbon prepared are 2299 m²/g and 15.9%,and 1979 m²/g and 40.4%,respectively.When calcium carbonate-potassium oxalate(mass ratio:1:1)is used as binary activator,compared with single-component activator,the specific surface area of the prepared porous carbon increases(2795 m²/g),and the mesoporous rate increases significantly(71.4%).The effects of preparation parameters on the structural characteristics of porous carbon were further studied when calcium carbonate-potassium oxalate was used as binary activator.With the increase of ball milling time,the specific surface area of porous carbon gradually increases,while the micropore rate decreases and the mesopore rate increases.With the increase of carbonization temperature,the specific surface area and mesopore ratio of porous carbon increased at first and then decreased.The optimal process parameters are as follows:the activator is calcium carbonate-potassium oxalate(mass ratio 1:1),the ball milling time is 2 h,and the carbonization temperature is 800?.The tobacco stem-based porous carbon(PC-Ca-K-800)has the largest specific surface area(2795 m²/g)and high mesoporous rate of 71.4%.In addition,the graded porous carbon material with high specific surface area(3056 m²/g)and high mesoporous rate(70%)was prepared by using calcium carbonate-potassium oxalate binary activator as raw material,which indicated that the binary activator has certain universality for preparing biomass porous carbon with high specific surface area and high mesoporous rate.(2)The adsorption properties of porous carbon prepared by different activators and carbonization temperature on SMZ were studied.The adsorption performance of porous carbon prepared by binary activator is better than that of single-component activator,and its adsorption capacity for SMZ increases at first and then decreases with the increase of carbonization temperature.PC-Ca-K-800 prepared under the above optimum conditions has the largest equilibrium adsorption capacity(852.4 mg/g)for SMZ,and porous carbon has the largest adsorption capacity when the p H of SMZ solution is 3-6.The results of isothermal adsorption experiment and adsorption kinetics study show that the adsorption of SMZ by PC-Ca-K-800accords with Langmuir model,and the adsorption process accords with pseudo-second-order kinetic model.At T=318 K and initial concentration C₀=200 mg/L of SMZ solution,the theoretical saturated adsorption capacity Q_m=1121 mg/g,and the adsorption equilibrium can be quickly reached in about 10 min.Cyclic experiments show that PC-Ca-K-800 has good stability and regeneration performance,and the adsorption capacity can still reach 781 mg/g after four cycles.(3)The adsorption properties of menthol,allyl caproate,blueberry and orange oil essence on porous carbons prepared by different activators and carbonization temperatures were studied.Binary activators are superior to single-component activators,and the adsorption capacity of porous carbon for four kinds of flavors gradually increases with the increase of carbonization temperature.Among them,the porous carbon prepared at 900?with calcium carbonate-potassium oxalate as binary activator has the largest adsorption capacity for four kinds of flavors,which are 1475 mg/g,1562 mg/g,1421 mg/g and 1024 mg/g,far exceeding the molecular sieve(<100 mg/g)and activated carbon(<200 mg).Furthermore,the retention rate of flavor by porous carbon with different pore structure parameters was studied.The results show that the higher the specific surface area of the material,the higher the retention rate of essence;When porous carbon has higher specific surface area,the higher the micropore rate and the higher the retention rate of essence.With the increase of carbonization temperature,the micropore rate decreases gradually.The porous carbon prepared at carbonization temperature of 700?has higher specific surface area(2642 m²/g)and higher micropore rate(18.5%),and has the highest retention rate for the above four flavors under open and closed conditions:closed conditions(35?,The retention rates of menthol,allyl caproate,blueberry and orange oil essence were 84.5%,67.1%,55.1%and 83.4%respectively in 15 days,and 68.5%,61.4%,58.1%and 82.1%respectively in open environment(25?,1 day).To sum up,tobacco stem is an excellent biomass carbon source.The tobacco stem-based porous carbon prepared by ball milling pretreatment and binary activator activation has the characteristics of high specific surface area and high mesoporous rate.It has the advantages of high adsorption capacity and high retention rate of essence when applied to cigarette filter flavoring and antibiotic adsorption,and has potential application value in the adsorption field.