INFRARED STUDIES OF NON-POLAR CHLORINATED SOLVENT ADSORPTION ON MONTMORILLONITE

The adsorption of liquid-phase carbon tetrachloride (CCl₄), tetrachloroethylene (PCE) and trichloroethylene (TCE) onto homoionic montmorillonites was studied using diffuse reflectance infrared spectroscopy. Non-polar solvents are known to change the physical properties of clay minerals, however, the mechanism of the interaction is poorly understood. Identifying how clay and non-polar solvents interact, at the molecular scale, provides a better understanding of the mechanisms that affect the fate and transport of these solvents in the environment. Lithium, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺ and La³⁺ homoionic clays were prepared to observe the effect of different exchangeable cations on the adsorption. Absorption band positions of solvent and clay spectra were compared to the spectra of clay-solvent admixtures. The shifts observed in spectral band positions due to the adsorption process were typically less than 1 percent. The magnitude of observed band shifts were found to be relative to the polarizing power of the cations, suggesting solvent molecules coordinate directly with exchangeable cations. Lithium produced the largest solvent band shifts, whereas K⁺ produced the smallest. The solvent-cation interaction is an ion-dipole attraction between the electropositive cation and the partial negative charge on the chlorine atom of polarized C-Cl bonds. The spectra of clay-solvent admixtures were searched for absorption bands indicative of adsorption reaction products. The only new absorption feature identified is thought to be a solvent overtone band activated by surface adsorption. The energy of the clay-solvent adsorption is less than the solvents energy of vaporization. Evaporation experiments show that solvents adsorbed to clays will completely desorb at ambient temperature and pressure. The small magnitude of spectral bands shifts, lack of reaction product adsorption bands, and the low adsorption energy indicate the clay-solvent interaction is a physical adsorption processes.