Study On The Physicochemical Structure Regulation And Lead Adsorption Mechanism Of Corn Stalk Hydrothermal Charcoal

Lead is a common toxic heavy metal,which is highly toxic,difficult to degrade and easy to accumulate.Lead pollution in water bodies will cause long-term and lasting harm to human health and ecological safety.The adsorption method has the advantages of simple process and environmental protection.The preparation process of traditional adsorbents（such as activated carbon,alumina and silica,etc.）is complicated and costly,which severely restricts the wide application of this technology.Corn is widely planted throughout my country,and the annual output of corn stalks is about 250 million tons,of which 30～50%of the corn stalks are burned or discarded,which not only pollutes the environment but also causes a waste of resources.Hydrothermal carbonization is a new type of thermochemical conversion technology,which can directly convert corn stalks with high moisture content into carbon-based functional materials and use them as heavy metal adsorbents.Therefore,studying the evolution of the physical and chemical structure of corn stalk hydrothermal charcoal,clarifying the relationship between the physical and chemical structure and its adsorption characteristics,revealing the adsorption mechanism of different corn stalk hydrothermal charcoal to lead ions in water,it can be a low-cost and high-efficiency adsorbent The preparation provides theoretical guidance and has important engineering application value.First,this paper uses corn stalks as raw materials to prepare a series of corn stalk hydrothermal carbon and thermal pyrolysis carbon by hydrothermal carbonization method（180,220,280℃）and thermal cracking method（400,600,800℃）,respectively.Comparing the physical and chemical structures of two types of corn stalk charcoal,it is found that hydrothermal charcoal has lower solid yield,volatile and ash content,and higher fixed carbon content.The specific surface area of hydrothermal carbon is relatively low,showing a dense and smooth surface morphology,forming a disordered crystal structure and abundant surface oxygen-containing functional groups;the specific surface area of pyrolytic carbon is relatively high,and its surface oxygen-containing functional groups are relatively high.Less,and formed an ordered crystal structure.The results of the lead adsorption experiment show that with the increase of the initial solution p H（2.0～7.0）and the initial lead ion concentration（25～150 mg/L）,the lead adsorption capacity of hydrothermal carbon and pyrolytic carbon both increase first and then Decreasing trend,when p H=6 and 100 mg/L,the two kinds of corn stalk charcoal reached the maximum lead adsorption capacity.With the prolongation of the reaction time（～12 h）,both types of corn stover chars experienced two stages of rapid adsorption and slow equilibrium adsorption,but the hydrothermal chars reached adsorption equilibrium faster（6 h）,and saturated lead adsorption capacity higher.Under the optimal adsorption conditions,it is found that the hydrothermal carbon（H220）prepared at 220℃has a higher adsorption capacity（29.33 mg/g）and a carbon yield（56.34%）.Then,using H220 as raw material,low-concentration Na OH solution（0～1 mol/L）was used to impregnate it to obtain a series of alkali-modified hydrothermal carbon.Through the lead adsorption experiment,the influence of Na OH concentration,initial p H,initial concentration,dosage and reaction time on the adsorption performance was explored.The results show that under the optimal adsorption conditions,0.5 mol/L Na OH modified hydrothermal carbon（H220-0.5）has a higher adsorption capacity（91.79 mg/g）and yield（81.56%）.The results of adsorption kinetics and isotherm fitting show that under different temperature conditions,the quasi-second-order kinetics and Freundlich isotherm model are more suitable to describe the H220-0.5 pair of lead adsorption process,indicating that a physical-chemical composite adsorption reaction has occurred.The apparent activation energy of H220-0.5 is 42.38 k J·mol^-1,which proves that chemical adsorption is more important in the adsorption process.The results of thermodynamic studies show that the adsorption process is a spontaneous endothermic chemical adsorption,and increasing the temperature increases the adsorption capacity of H220-0.5 for lead ions.By analyzing the surface morphology,pore structure,functional group structure,and element composition of H220-0.5 before and after adsorption,it is revealed that the lead adsorption mechanism of alkali-modified corn stalk hydrothermal carbon includes ion exchange,surface complexation and cation-πinteraction.Finally,2 mol/L HNO3 was used to impregnate H220 to obtain hydrothermal nitric acid modified carbon（H220-2N）.In addition,2 mol/L HNO3 was used to impregnate the corn stalks first,and then 220 Hydrothermal carbonization treatment at℃to obtain hydrothermal nitric acid modified carbon（2N-H220）,combined with the physical and chemical structure characterization and adsorption kinetics,adsorption isotherm model fitting results,explored the nitric acid modified corn stalk hydrothermal carbon The mechanism of lead adsorption.The results show that the content of oxygen-containing functional groups in 2N-H220 and H220-2N are similar,but the former has a well-developed mesoporous structure,while the latter has a significant microporous structure.Under the optimal adsorption conditions,the saturated lead adsorption capacity of 2N-H220（164.68 mg/g）and H220-2N（117.58 mg/g）were significantly higher than that of the original hydrothermal carbon（54.03 mg/g）.The liquid film diffusion plays an important role in the adsorption process of H220-2N,which is manifested as a monolayer chemical adsorption.The adsorption of lead ions by 2N-H220 is controlled by intra-particle diffusion,which is manifested as multi-molecular layer adsorption.Through the analysis of the physical and chemical structure of 2N-H220 and H220-2N before and after adsorption,it is found that the adsorption mechanism of HNO3 modified hydrothermal carbon lead mainly includes surface complexation,electrostatic attraction,cation-πbond interaction and physical adsorption.However,the well-developed mesopores of 2N-H220 can promote the mass transfer rate,ensure that the functional groups in the pores/channels are fully combined with lead ions,and significantly improve the lead adsorption performance.