Gibbs free energies for dual Langmuir-like adsorption onto hard carbon materials in sediment and soils.

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.