Preparation,structure And Adsorptive Properties Of Carbon Material From Cotton Stalk

Xinjiang is the largest cotton-producing area of China.Million tonnes of cotton stalk waste are generated from the agricultural production of cotton.Local environmental problems are associated with this stalk waste since much of cotton stalk is disposed of by either burning or dumping.Therefore,in the context of environmental protection and resource utilization,carbon materials including biochar and activated carbons were prepared from cotton stalk in this research.First,the surface structural evolutions of biochars using muffle furnace heating and activated carbons prepared using two heating methods(muffle furnace heating and microwave heating)were investigated.Then,the relationship between surface structural properties of the different carbon materials and the adsorption capacities of methylene blue and idiom was discussed respectively.Finally,the biochar and activated carbons prepared by two heating methods were used in organic pollutants removal to reveal the effects of heating method and property of adsorbate on adsorption process and adsorption mechanism.The results of this research were listed as following:(1)The biochar yield was mainly determined by pyrolytic temperature,and its change was not obvious under different holding times.With the increase of the pyrolytic temperature,acidic functional groups and polarity of biochar decreased,whereas the the carbon content,basic functional groups and aromaticity dramatically increased.We observed the mesopores increased with increasing charring temperatures.However,increasing temperature to 700? promoted the formation of micropores.The sorption of iodine and methylene blue with actived carbon were found to positively correlate to the BET surface area and micropore surface area of actived carbon produced from cotton stalk(p-value<0.01 for correlation test).(2)The biochar prepared from cotton stalk could regard as an effective adsorbents using p-nitrotoluene removal.The pseudo second-order kinetic model was more appropriate to describe the adsorption behavior of p-nitrotoluene onto biochars(C300,C500 and C700).In terms of equilibrium sorption quantity,the three kinds of biochars followed an order of C700>C500>C300.The adsorption free energy for p-nitrotoluene in present study onto different biochars(C300,C500 and C700)were 0.25kJ/mol,0.31kJ/mol and 2.25kJ/mol,respectively.Physisorption and chemisorption coexisted during p-nitrotoluene adsorptions.Batch adsorption experiments found that the adsorption data of p-nitrotoluene onto biochars(C300 and C500)were better fitted by Langmuir model,suggesting that the adsorption process for p-nitrotoluene was monolayer in nature.While the adsorption isotherms of p-nitrotoluene onto C700 were well fitted by Langmuir equation and Freundlich equation,suggesting that the adsorption process for p-nitrotoluene were monolayer and multilayer adsorption,and the maximum sorption capacity was 100mg/g.Moreover,all the calculated RL values were below 1,indicating a favorable nature of adsorption.(3)Actived carbon were prepared from cotton stalk using H3PO4 as activating reagent with conventional heating method.Phosphoric acid concentration had the greatest effect on on the carbon yield,and the pore structure,elements and surface functional groups of actived carbon were mainly determined by activation temperature.The maximum BET surface area,total pore volume and micropore surface area for activated carbon derived from cotton stalk were achieved under the optimized conditons of 40%phosphoric acid concentration,impregnation ratio of 2,activation temperature of 500? and a holding time of 1.5h.Iodine number and amount of methylene blue adsorption of activated carbon prepared under optimum conditions were 1109.3 mg/g and 269.6 mg/g,respectively.The sorption of methylene blue and iodine with actived carbon were found to positively correlate to the BET surface area,total pore volume and micropore surface area of actived carbon produced from cotton stalk(p-value<0.01 for correlation test).(4)Activated carbons were prepared with a microwave-induced activation process using H3PO4 as the activating reagent.The carbon yield,pore structure,elements and surface functional groups of actived carbon were mainly determined by microwave power.A high BET surface area and total pore volume for activated carbon derived from cotton stalk were achieved under the optimized conditons of 50%phosphoric acid concentration,impregnation ratio of 2.5,radiation power of 560 W and radiation time of 8 min.Iodine number and amount of methylene blue adsorption of activated carbon prepared under optimum conditions were 907.5mg/g and 288.7mg/g,respectively.The sorption of iodine and methylene blue with actived carbon were found to positively correlate to the micropore surface area,BET surface area and external surface area of actived carbon produced from cotton stalk(p-value<0.01 for correlation test).(5)Two activated carbons both belonged to mesoporous carbon material and had similar specific surface areas and surface groups,but the surface morphology,amount of surface groups and pHpzc of the activated carbons were different.MAC and EAC could be employed as efficient alternative for the removal of the MO,but the adsorption capacities were significantly influenced by the adsorbent dosage,ionic strength,initial dye concentration and solution pH.Batch adsorption experiments found that the adsorption data of MO were well fitted by Langmuir model and pseudo second-order kinetic model,respectively.It could be inferred that the adsorption process for MO was monolayer and chemisorption might be the rate-limiting step.Moreover,pH condition could affect the adsorption mechanism for both adsorbents,and chemisorption is dominant mechanism in the both adsorption process under the neutral condition,but ion-exchange mechanism is dominating in the adsorption process for both adsorbents under the acid or basic condition.