Adsorption of Low-Concentration VOCs on Modified Activated Carbons in a Humid Atmosphere

A large amount of water vapor present in low-concentration organic exhaust gases severely inhibited the adsorption of volatile organic compounds (VOCs). In this paper, three coal-based activated carbon samples were prepared by heat treatment, sodium hydroxide impregnation, and ammonia solution impregnation and were analyzed by N2 adsorption isotherm, X-ray photoelectron spectroscopy (XPS), Boehm titration, and water and toluene adsorption isotherms. Besides, the low-concentration toluene adsorption performance of samples was investigated under different humidities, and the desorption activation energies (DAEs) of toluene and water on samples were estimated through temperature programmed desorption (TPD) experiments. Furthermore, the effect of molecule polarity was investigated under 0 and 60 RH%. The results indicated that samples treated by heating (CAC-1) significantly improved the uptake of low-concentration toluene under humid conditions compared to samples modified by alkali impregnation, which is related to surface chemistry. The analysis confirmed that CAC-1 possessed more graphitic (C + N), enhancing the π–π interaction, and possessed lower acidic groups, reducing the bonding sites with water. The Yoon and Nelson (Y–N) model indicated that CAC-1 presented a higher diffusion rate of toluene than CAC under high humidity. TPD experiments further demonstrated that the CAC-1 had smaller DAE of water and larger DAE of toluene, suggesting a stronger hydrophobic and stronger interaction with toluene. In addition, the experiments demonstrated that CAC-1 had superior adsorption performance both for polar and weakly polar VOCs at high humidity, especially for weakly polar VOCs. These conclusions would help to emphasize that activated carbons after heat treatment have better selectivity in removing low-concentration VOCs from high-humid conditions.