Adsorption of Chloroaromatic Models for Dioxins on Porous Carbons: The Influence of Adsorbate Structure and Surface Functional Groups on Surface Interactions and Adsorption Kinetics

Abstract

Polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran species are classes of extremely toxic compounds generated in very low concentrations in postcombustion gases and these may be removed by adsorption on porous carbons. Their extreme toxicity and very low volatility prevent detailed study of their adsorption characteristics, and therefore, models for dioxins have been used in this study. Chlorobenzene, 2-chlorotoluene, 1,3-dichlorobenzene, and 2-chloroanisole were used as models to investigate factors influencing the adsorption characteristics of dioxins on porous carbons. Adsorption studies were carried out under conditions of very low concentration and temperatures up to 453 K, which simulate those found in dioxin abatement systems. Adsorption of 2-chloroanisole on three carbons with various micro/mesoporous structures showed that microporous structure was a critical adsorbent characteristic under these conditions. A microporous activated carbon was selected for detailed thermodynamic and kinetic studies of adsorption of chloroaromatic species in relation to adsorbate structure and adsorbent surface functional groups. Virial equation analysis of adsorption isotherms was used to determine the Henry’s Law constants and isosteric enthalpies of adsorption at zero surface coverage to compare adsorbate−adsorbent interactions. The van’t Hoff equation was used to determine the enthalpy of adsorption as a function of surface coverage. The role of surface functional groups on adsorption thermodynamics was investigated by oxidizing and reducing the carbon in nitric acid and hydrogen, respectively. The important factor influencing adsorption at very low concentrations is the adsorbate adsorbent interaction. Oxidation of the carbon adsorbent only has a small effect on the isosteric enthalpy of adsorption. The adsorption kinetics for each isotherm pressure increment were described by the stretched exponential equation. The activation energies and enthalpies of activation were calculated as a function of surface coverage for adsorption kinetics of chloroaromatic species. The planar molecules studied had lower activation energies and enthalpies of activation than isosteric enthalpy of adsorption indicating that a site-to-site surface hopping mechanism is the main factor in determining the adsorption kinetics. In comparison, 2-chloroanisole is nonplanar with a methoxy group giving rise to a larger minimum cross-section size and higher barrier to diffusion than isosteric enthalpy of adsorption at low surface coverage leading to the adsorption kinetics being mainly determined by diffusion through constrictions in the porous structure under these conditions. The isosteric enthalpies of adsorption initially increase with increasing surface coverage and this is attributed to π−π interactions of planar aromatic molecules confined in microporosity. The trends in the kinetic barriers and isosteric enthalpies of adsorption with surface coverage for 2-chlorotoluene are similar irrespective of adsorbent oxidation/reduction, indicating that surface functional groups only have a relatively small effect on adsorption characteristics.