Gibbs free energies for dual langmuir‐like adsorption onto hard carbon materials in sediment and soils

Abstract

Adsorption of organic compounds onto hard carbon constituents of soils and sediments may be described by a dual Langmuir-like equation for adsorption onto high-energy sites and low-energy sites. To describe quantitatively the sorbate-sorbent interactions on these high-energy sites and low-energy sites, Gibbs free energies for adsorption onto several soils and sediments were calculated using suitable experimental sorption data from the literature. A large part of the variation in these Gibbs free energies relative to the pure solid state appeared to be related to differences in sorbate molecular symmetry. Generally, for a broad range of nonpolar organic compounds, from substituted benzenes to five-ring polycyclic aromatic hydrocarbons and hexachlorinated biphenyls, sorbate molecular symmetry-corrected Gibbs free energies for high-energy and low-energy adsorption relative to the pure solid state were within a narrow range or approximately -23 and -11 kJ/mol, respectively. These two average values for the geosorbents were comparable to corresponding values for adsorption onto activated carbon.