Effects of Ca(NO3)2 pretreatment on structure of coal-based porous carbon and its adsorption properties for toluene

Volatile organic compounds (VOCs) are important precursors of secondary pollutants such as fine particulate matter (PM2.5) and ozone (O3),which are harmful to the atmospheric environment and human health.Adsorption technology by porous carbon is widely used in VOCs treatment.However,traditional coal-based porous carbon exhibits underdeveloped pore structures,which may limit the mass transfer process.The in-situ structure control during synthesis process of porous carbon and the diffusion/adsorption mechanism of VOCs were studied in this paper.Porous carbon was prepared from lignite by acid washing together with Ca(NO3)2 impregnation pretreatment,and toluene was chosen as adsorbate.Results showed that the stacking height (Lc) of aromatic structure decreased after Ca(NO3)2 modification,while the interlayer spacing (d002) increased.This indicated the deep damage of carbon microcrystalline,which was favorable to the formation of pores.The molecular diameter of toluene (0.57 nm) was used as the dividing scale of pore size range.PC-Ca0.2 exhibited the largest pore volume in the range of 1-3 times of toluene molecular diameter (0.57-1.71 nm),which also had the highest toluene adsorption capacity of 746.2 mg/g.Weber-Morris intraparticle diffusion model was used to study the rate determining step of toluene adsorption.It was found that the adsorption process was controlled by both internal and external diffusion.PC-Ca0.2 had the maximum adsorption rate constant and the minimum diffusion boundary layer constant.Therefore,when the porous carbon had a larger pore volume in the range of 1-3 times of molecular diameter,it was more conductive to the diffusion and mass transfer in the adsorption process.