Experimental Study On Adsorption Of Gaseous Dioxins And Toluene By Urea Modified Activated Carbon

With the growth of population and the advancement of industrialization,soil has been seriously damaged.Thermal desorption is the most suitable soil remediation technology in China.However,the issue of dioxins and VOCs emissions from thermal desorption exhaust has drawn public attention and restricted the promotion of thermal desorption project.Therefore,it is very important for the removal and purification of dioxins and VOCs in exhaust gas.However,the emission of dioxins and VOCs from thermal desorption exhaust has limited the promotion of thermal desorption project,so it is very important for the removal and purification of dioxins from exhaust gas.Thermal desorption tail gas purification is an important part of thermal desorption repair technology.Activated carbon fixed bed adsorption and jet adsorption are the most widely used method to remove dioxins and VOCs from thermal desorption exhaust.Thermal desorption tail gas has the characteristics of complex composition and various types,and usually contains a large amount of water vapor.At present,commercial activated carbon is microporous activated carbon with developed specific surface area,while activated carbon that adsorbs dioxins from thermal desorption tail gas needs to be rich in small scale mesoporous and can adapt to lower concentration of adsorption.The development of small scale mesoporous and nitrogenous activated carbon and the adsorption of dioxins and toluene were studied in this paper.Using commercial coal-based activated carbon as raw material,the modified coal-based activated carbon was treated with urea impregnation method coupled with nitrogen at high temperature.It was found that the performance of urea could effectively improve the pore structure of activated carbon and at the same time add some nitrogen elements.The effects of mass ratio of urea to activated carbon,modification temperature and modification time on the pore structure and nitrogen content of activated carbon were studied.It was found that the modified activated carbon had the best overall effect when the modified temperature was 600°C,the mass ratio of urea to activated carbon was 1:1,and the high temperature treatment was 2h.After modification,the pore volume of activated carbon reached 0.38cm³/g,accounting for 57.58% of the total pore volume.At 600℃,a certain amount of nitrogen was introduced into the carbon ring of activated carbon to form pyridine and pyrrole groups,can enhance the adsorption of dioxins with low concentration on activated carbon.CO₂,H₂O produced by urea decomposition reacts with activated carbon,which is the main reason for adding urea to expand mesoporous,while NH₃ produced by decomposition provides nitrogen source.The dioxin generation and adsorption system experiment platform was used to select the activated carbon modified by urea and unmodified commercial coal-based activated carbon for dioxin adsorption comparison,so as to understand the adsorption efficiency of activated carbon rich in mesoporous and nitrogen elements to dioxins,and its adaptability to different adsorption time,adsorption temperatures and dioxins concentrations.Adsorption data showed that temperature of 600°C,urea and activated carbon mass ratio of 1:1,high temperature treatment 2h of modified activated carbon adsorption efficiency increased from 83.64%to 97.65%of dioxins,and nitrogen-doped activated carbon was more suitable for the adsorption of dioxins with low concentration.Although the pore parameters of activated carbon at 800℃ are not good,the adsorption efficiency of dioxins at low concentration is up to 85.24% due to more pyrrole groups.Using the laboratory VOCs online adsorption system,modified activated carbon and original activated carbon were selected for the toluene adsorption experiment.The results show that the penetration time and saturation adsorption capacity of the two modified activated carbons are greater than that of the original activated carbons.At the initial concentration of 400 ppm,the saturated adsorption capacity of AC600 and AC800 was 486.7 mg/g and 447.7 mg/g respectively,and the original activated carbon was 390.2 mg/g.By studying the adsorption characteristics of activated carbons at different concentrations,it was found that the penetration time of the three activated carbons was shortened with the increase of the initial concentration,and the saturation adsorption capacity increased with the increase of the initial concentration.