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 ϳ1.5 ϫ 10 Ϫ3 , 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 ϫ 10 Ϫ5 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 microporous adsorbents in the D-R-L model and in the corresponding D-R equation.

Breakthrough Curves for Toluene Adsorption on Different Types of Activated Carbon Fibers: Application in Respiratory Protection

“…Several studies that tested ACF materials used toluene as a representative for the VOC group (Brasquet and Le Cloirec, 1997;Cheng et al, 2004;Das et al, 2004;Fournel et al, 2005;Lorimier et al, 2005). Additionally, toluene is important to characterize because, at exposures ranging from 200 to 1100 p.p.m., the adverse effects of toluene on humans range from eye, skin, and mucous membrane irritation, to drowsiness, nausea, and memory loss, to damaged liver and kidney, unconsciousness, or even death (Agency for Toxic Substances and Disease Registry, 2000 (Hung et al, 2007;Ramos et al, 2010;Li et al, 2012).…”

Effect of the adsorbate kinetic diameter on the accuracy of the Dubinin–Radushkevich equation for modeling adsorption of organic vapors on activated carbon

“…In this case, a portion of the narrow micropores are not accessible to the adsorbate. Based on the results, the limiting pore volume of the adsorbent might be predicted using the pore size distribution of the adsorbent and the kinetic diameter of the adsorbate [5]. Table 2 shows the calculated affinity coefficients (using Eqs.…”

“…The Dubinin-Radushkevich (D-R) equation, based on Polanyi's potential theory of physical adsorption, could be used for estimating the adsorption capacity for volatile organic compounds (VOCs) on microporous adsorbents such as activated carbon [5,9]. The D-R equation can include the effect of temperature as well as the properties of both adsorbent and adsorbate [10].…”

“…= P e P e ref (4) and P e = (n 2 − 1)M (n 2 + 2) (5) where n is the refractive index of the adsorbate in liquid phase, M is the molecular weight of the adsorbate, (g/mol) and is the density of the adsorbate in liquid phase (g/cm 3 ). could also be estimated based on the ratio of molecular parachors, ˝ [9,11]: =r ef (6) and…”

“…The adsorption capacity of activated carbon is an important parameter for the design of a reliable adsorption system. Experimentally determining the adsorption capacity of AC needs considerable effort, time, and cost especially at low concentrations or for the case of hazardous compounds [5][6][7]. Hence, there is interest in developing reliable models to predict the adsorption capacity of adsorbents [5,6,8].…”