Preparation Of Porous Biochar From Kenaf And Its Adsorption Properties For Methylene Blue And Pb²⁺

With the rapid development of economy in China,the demand for textiles is increasing year by year,textile printing and dyeing industry is also booming,and the resulting water pollution has seriously threatened the health of the water ecological environment.Dye wastewater discharged has high chromaticity,odor,hinders the normal photosynthesis of aquatic organisms.Heavy metals contained in printing and dyeing wastewater themselves are toxic.Wastewater can not only pollute the air,but also affect the growth of crops when it flows into the soil,reaches the human body through the food chain,and thus does harm to human health.Therefore,the effective removal of dyes and heavy metals ions in printing and dyeing wastewater is of great significance to promote the sustainable development of industry and agriculture,and ensure the residential water safety.At present,the treatment methods of printing and dyeing wastewater mainly include physical method,chemical method,biological method,etc.,of which the adsorption method in the physical method is favored because of its simple operation and low cost.Therefore,finding an adsorbent with good adsorption performance and environment friendliness is the key to the research of adsorption method.In this study,kenaf,which was rich in resource but poor in utilization ratio,was used as raw material,and was pyrolyzed in a tubular furnace under N₂ atmosphere to obtain biochar.The physical and chemical properties were tested and analyzed,and the effects of different structures,pyrolysis temperatures,pretreatment methods on the biochar formation of kenaf were explored.Taking methylene blue（MB）and heavy metal ion Pb²⁺as typical models of dyes and heavy metal pollutants,the adsorption capacity and removal rate were used as indicators to characterize the dye and heavy metal adsorption of kenaf biochar:combined with MB kinetic experiments,the effects of the initial MB concentration,adsorbent addition and temperature on adsorption were analyzed;the effects of p H,the initial Pb²⁺concentration,adsorbent addition and temperature on adsorption were analyzed.The adsorption conditions were optimized,and the effects of different pyrolysis temperatures and pretreatment methods on adsorption were explored.Finally,the adsorption-desorption regeneration experiment was carried out on the biochar with the best adsorption performance to explore its recyclability.Firstly,different parts of kenaf（bark,stem and fiber）were pyrolyzed at high temperature.Through the comparison of specific surface area and MB pre-adsorption experiment,it was found that the adsorption performance of kenaf fiber was superior under the same pyrolysis condition.The biochar（KBC400,KBC500,KBC600,KBC700,KBC800）was prepared by pyrolysis at 400～800℃with kenaf fiber as raw material.Kenaf fibers were delignification and hemicellulose,respectively,and two biochar（no lignin kenaf fiber biochar,NL-KBC700,and no hemicellulose kenaf fiber biochar NH-KBC700）were prepared by pyrolysis at 700℃.In the temperature range from 400℃to 700℃,the specific surface area and pore volume of biochar showed an increasing trend with the increase of pyrolysis temperature,and the specific surface area began to decrease when the temperature reached 800℃.It can also be observed that KBC700 has more abundant pore structure by SEM.In terms of NH₃-TPD,KBC600 and KBC700,NL-KBC700 and NH-KBC700 all had acidic sites,and the peak area of NL-KBC700 was large,which may play a catalytic role in adsorption.In the kenaf fiber biochar prepared at 400～800℃,KBC700 had the best adsorption property on MB and Pb²⁺,and the adsorption capacity of KBC700 was greater than that of KBC600.The initial concentration and the weight of adsorbent added had a relatively large impact on the adsorption capacity,and the experimental condition for the maximum adsorption capacity were as follows:the initial concentration of MB was 200 mg·L^-1,the adsorbent added was 100 mg(0.5 g·L^-1),temperature was 45℃,adsorption time was 120 min.However,considering the experimental adsorption efficiency and operating costs,when the initial concentration of MB was 150 mg·L^-1,the temperature was 25℃,KBC700 had a better adsorption capacity of 169.21 mg·g^-1.Through the kinetic model analysis,it was found that the adsorption of MB by all biochar conformed to the pseudo-first-order kinetic model,which means physical adsorption dominated the process.For adsorption of Pb²⁺,the optimal conditions were an initial concentration of 100 mg·L^-1,p H value was 4.0,adsorbent added was 50 mg(1.0 g·L^-1),the temperature is 25℃,the adsorption capacity of Pb²⁺was 284.57 mg·g^-1,and the adsorption capacity rose slightly.Adsorption of MB and Pb²⁺may be endothermic reactions.Pretreatment had a certain impact on the adsorption performance of kenaf biochar.After treatment of getting rid of lignin or hemicellulose in kenaf,the adsorption capacity of the biochar to MB and Pb²⁺was different from that of biochar without pretreatment:the adsorption capacity NL-KBC700 was higher than NH-KBC700,and both were higher than KBC700.The optimal conditions for adsorption of NL-KBC700 were:the initial concentration was 150 mg·L^-1,the adsorbent added was 100 mg,the temperature was 25℃,adsorption time was 120 min,and by adsorption kinetic fitting,it was found that the adsorption of MB of NL-KBC700 and NH-KBC700 was more in line with the pseudo-secondary-order kinetic model.For the adsorption of Pb²⁺,the optimal conditions were:p H value was 4.0,the initial concentration was 150 mg·L^-1,the adsorbent added was 50 mg(1.0 g·L^-1),the temperature was 25℃.Overall,the pretreatment of delignification slightly improved the adsorption capacity of biochar.Through adsorption-desorption regeneration experiments of NL-KBC700,which was of the best adsorption performance,it was found that the recyclability of it was not high,and the binding ability with pollutants was strong.