Molecular modeling of sorption phenomena in environmental engineering

Abstract

This research investigated the adsorption mechanisms of hydrophobic chlorinated contaminants in mineral micropores and on iron metal surfaces. Activated adsorption and desorption of trichloroethylene (TCE) in mineral micropores was studied using experimental and molecular modeling techniques. Adsorption of TCE on a silica gel adsorbent was measured using a frontal analysis chromatography technique at atmospheric and elevated fluid pressures. The results showed that the increase in pressure was able to rapidly induce the formation of a desorption resistant fraction. Grand Canonical Monte Carlo (GCMC) modeling was used to elucidate the nature of water and TCE behavior within silica micropores. TCE adsorption was energetically most favorable in pores that were minimally large enough to accommodate one TCE molecule. A molecular level study of the interactions between hydrophobic chlorinated contaminants and sediments was performed. GCMC simulations were preformed to investigate water and TCE adsorption in slit micropores confined by charged and uncharged silica surfaces. Gas-phase single-sorbate simulations with water or TCE were performed as well as mixture simulations of bulk water containing TCE at 1% of its saturation concentration. Aqueous-phase TCE at a concentration equal to 1% of its saturation concentration was able to completely displace adsorbed water in uncharged pores. In highly hydrophilic pores, TCE at this concentration was able to displace up to 50% of the adsorbed water. Metallic iron filings are becoming increasingly utilized as reactive agents for reductive dechlorination of solvents in contaminated groundwaters. This research also used molecular modeling to study chemical adsorption of TCE and PCE to iron surfaces. Quantum mechanical calculations were performed to determine the thermodynamic favorability and resulting structures for chemical adsorption of TCE and PCE to iron surfaces. Molecular mechanics modeling was used to study the effects of atomic hydrogen on the thermodynamic favorability for chemically adsorbed TCE and PCE. Because TCE and PCE react with iron surfaces, their adsorption to iron cannot be investigated experimentally. This makes molecular modeling approaches a useful complement to experimental investigations of chemical reaction phenomena.