Pyrolysis Characteristics Of Corn Straw And Adsorption Performance Of Pb(Ⅱ) On Corn Straw Biochar

As a novel environmental adsorbent,biochar can be used to remove lead ions from aqueous solutions and reduce lead mobility.Corn straw is a large biomass resource in agriculture,but it has not been used effectively at present.In order to make agricultural resources recycle,agricultural waste is used to deal with pollutants.In this study,corn straw was used as raw material to study its pyrolysis characteristics.The adsorption performance of biochar on Pb（Ⅱ）was investigated,and the adsorption rule,adsorption mechanism and modification methods were explored.This is of great significance to increase the added value of agricultural waste,reduce straw burning and promote green development.According to the thermogravimetric analysis,the pyrolysis of process corn straw consists of four steps:drying,transition,main pyrolysis,and carbonization.During pyrolysis,the heating rate increased,and TG and DTG curves shifted to the high temperature side.Both the FWO and the KAS belong to the model-free method,and the average apparent activation energy values obtained by the two methods were were181.7 k J/mol and 181.5 k J/mol respectively.The pyrolysis mechanism equation of corn straw was obtained by the master curve method and the C-R method.The mechanism equation conforms to the Avrami-Erofeev（A-E）equation,and the mechanism is random nucleation and subsequent growth.Whenα=0.1～0.5,n=3,f（α）=1/3（1-α）[-ln（1-α）]^-2and whenα=0.5～0.7,n=2,f（α）=1/2（1-α）[-ln（1-α）]^-1.The study on the preparation technology of corn straw biochar showed that after the pyrolysis of corn straw,the original dense morphology was destroyed and the corn stalk biochar presented fragmentary structure.When the pyrolysis temperature increases,the specific surface area and total pore volume of biochar increase,the yield decreases,and the peak intensity of surface groups such as-OH,C=C,and C-H decreases.Adsorption experiments and biochar characterization showed that the optimal adsorption conditions of Pb（Ⅱ）on corn straw biochar prepared at pyrolysis temperature of 300℃were as follows:solid-liquid ratio of 1.5 g/L,p H=5.5.Under the conditions,changing the initial concentration of Pb（Ⅱ）,the adsorption capacity of corn straw biochar reached an equilibrium value at 150 mg/L,and the equilibrium adsorption capacity was 80 mg/g.Kinetic analysis shows that the adsorption rate of Pb（Ⅱ）on corn straw biochar is controlled by the chemical adsorption mechanism,and the adsorption process includes external liquid film diffusion,surface adsorption and intra-particle diffusion processes.According to the Langmuir isotherm model,the adsorption belongs to monolayer adsorption,and the q_mis 89.21 mg/g.Thermodynamic analysis shows that the adsorption is a spontaneous,endothermic and entropy increasing process.The adsorption mechanism is ion exchange and chemical complexation.The main ions involved in ion exchange are K⁺,and the functional groups involved in adsorption are-OH,C=C,O-C=O.After UV modification,the specific surface area of corn straw biochar increased from 11.3 m²/g to 12.5 m²/g,and the content of functional groups such as-OH and C=C increased.Under the wavelength of 365 nm,the optimal irradiation time of modification by ultraviolet radiation is 24 h.At 25℃and the adsorption time is 24 h,changing the initial concentration of Pb（Ⅱ）,the maximum equilibrium adsorption capacity of corn straw biochar increased from unmodified 80 mg/g to modified 104mg/g,the removal rate of Pb（Ⅱ）is significantly higher than that of the unmodified biochar.Kinetic analysis shows that UV modification can increase the adsorption rate of Pb（Ⅱ）on corn straw biochar,and the process is controlled by chemical adsorption.According to the Langmuir isotherm model,at 45℃,the q_mof GSCB is 112.9 mg/g,which is 1.26 times that of CSB.Thermodynamic analysis shows that the adsorption is an endothermic and spontaneous process.