Adsorption Of Humid Volatile Organic Compounds On Hydrophobic Modified Activated Carbon

Volatile organic pollutants（VOCs）exposed to the environment can cause damage to health andpollution of atmosphere,soil and even groundwater.Activated carbon adsorption is one of the common methods for the treatment of VOCs.Whether in industrial VOCs exhaust emission or VOCs from thermal desorption of organic polluted soil,water vapor is inevitable,leading to deterioration of activated carbon adsorption performance.Therefore,target-oriented modification of activated carbon to improve its adsorption capacity of VOCs at high humidity has become a hot spot of research in recent years.To improve the adsorption selectivity of VOCs at high humidity,trimethyl chlorosilane（TMCS）was chosen as modifier based on literature research,which was widely used in the surface treatment of silicon material.Firstly,tetraethyl orthosilicate（TEOS）was used to introduce more hydroxyl into activated carbon by hydrolysis and polycondensation.Then these hydroxyl groups were replaced with hydrophobic groups-Si（CH₃）₃ to realize hydrophobic modification of activated carbon,by means of impregnation of TMCS.BET specific surface area analysis,scanning electron microscope,X-ray energy spectrum（XPS）and water contact Angle measurement were used to characterize the activated carbon before and after TMCS modification.According to the results of the thermal desorption exhaust measurement,toluene（benzene）and trichloroethylene（halogenated hydrocarbon）were selected as representative VOCs.Dynamic adsorption experiments were conducted to quantitatively study the influence factors of activated carbon fixed bed and validate the adsorption ability of TMCS impregnating modified activated carbon at high humidity,and the Yoon-Nelson adsorption model was used to analysis dynamic adsorption process.The conclusions of this paper were as follows:Polycondensation of TEOS hydrolysate mainly formed a spatial structure,leading to the decrease in mesopore capacity.While TMCS substitution reaction mainly occurred on the surface,leading to a decrease in specific surface area.With the increase of TMCS immersion concentration,the reaction equilibrium reached faster and the hydrophobicity was enhanced,but with the increase of TMCS concentration,this enhancement was weakened.Through a series of controlled experiments,it is concluded that the best modification condition is to impregnate the activated carbon water with 30%TMCS for 24 h,after hydrolysis and polycondensation of TEOS,which can increase the water contact Angle of activated carbon to 143.5°.With the increase of VOCs concentration,the breakthrough time shorten,and there is a linear relationship between the breakthrough time and the logarithm of VOCs concentration.Within a certain range of VOCs（trichloroethylene）concentration,the breakthrough adsorption capacity and saturation adsorption capacity remain at the same level.Although the breakthrough time prolongs at extremely low VOCs concentration,but the adsorption rate of VOCs is slow which causes the reduction of breakthrough adsorption capacity and saturation adsorption capacity.At low linear velocity,with the increase of gas flow rate,the breakthrough time shortened,and the breakthrough adsorption capacity and saturation adsorption capacity increased.However,this increment rapidly weakened with the continuous increase of gas flow rate.Relative humidity has a significant influence on the ability of activated carbon to adsorb VOCs,and the adsorption of trichloroethylene was more sensitive to the moisture than the adsorption of toluene.Especially when the relative humidity exceeds 45%,the breakthrough time shortened sharply,and the breakthrough adsorption capacity and saturation adsorption capacity significantly reduced.Due to the weakening of pore structure,TMCS modified activated carbon showed no advantage under the dry airflow condition,and the saturation adsorption capacity decreases by about 22%compared with the bare activated carbon.However,TMCS modified activated carbon showed insensitivity to the change of humidity.With the relative humidity increased from 0%to90%,the toluene saturated adsorption capacity and the trichloroethylene saturated adsorption capacity of bare activated carbon dropped by 67.1%and 75.9%,respectively.While the toluene saturated adsorption capacity and the trichloroethylene saturated adsorption capacity of AC/TEOS-30%TMCS only dropped by 10%and 21.1%,respectively.