Prediction of the Adsorption Capacity for Volatile Organic Compounds onto Activated Carbons by the Dubinin–Radushkevich–Langmuir Model

Abstract

Prediction of the adsorption capacity for volatile organic compounds (VOCs) onto activated carbons is elucidated in this study. The Dubinin–Radushkevich (D–R) equation was first used to predict the adsorption capacity of nine aromatic and chlorinated VOCs onto two different activated carbons. The two key parameters of the D–R equation were estimated simply from the properties of the VOCs using quantitative structure-activity relationship and from the pore size distribution of the adsorbent. The approach based on the D–R equation predicted well the adsorption capacity at high relative pressures. However, at the relative pressures lower than ~1.5 × 10^-3, the D–R approach may significantly overestimate adsorption capacity. To extrapolate the approach to lower relative pressures, the integration of the D–R equation and the Langmuir isotherm, called the D–R–L model, was proposed to predict adsorption capacity over a wide range of relative pressures of VOCs. In this model, the Langmuir isotherm parameters were extracted from the predicted D–R isotherm at high relative pressures. Therefore, no experimental effort was needed to obtain the parameters of the D–R–L model. The model successfully predicted the adsorption capacity of aromatic and chlorinated hydrocarbons tested onto BPL and Sorbonorit B carbons over relative pressures ranging from 7.4 × 10^-5 to 0.03, suggesting that the model is applicable at the low relative pressures of VOCs often observed in many environmental systems. In addition, the molecular size of organic compounds may be an important factor affecting the adsorption capacity of activated carbons. For BPL carbon, an ultramicroporous adsorbent, the limiting pore volume W _o of the D–R equation decreased when the kinetic diameter of the adsorbate was larger than 6 Å. However, for Sorbonorit B carbon, no reduction of W _o was found, suggesting that the W _o may be related to the pore size distribution of the adsorbents, as well as to their molecular size. This size exclusion effect may play an important role in predicting the adsorption capacity of VOCs onto micro-porous adsorbents in the D–R–L model and in the corresponding D–R equation.