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| - Examination of HydrophobicContaminant Adsorption in MineralMicropores with Grand CanonicalMonte Carlo Simulations
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| - A molecular level understanding of the interactionsbetween hydrophobic organic contaminants (HOCs) andsediments is needed in order to assess contaminant fatein the environment. Grand canonical Monte Carlo simulationswere performed to investigate water and trichloroethylene(TCE) adsorption in slit micropores confined by chargedand uncharged silica surfaces. Gas-phase single-sorbatesimulations with water or TCE were performed as well asmixture simulations of bulk water containing TCE at 1%of its saturation concentration. Gas-phase isosteric heatsfor water adsorption in the uncharged pores rangedfrom −40 to −52 kJ/mol, and the densities of the adsorbedwater phases were always less than that for bulk water. Gas-phase isosteric heats for water adsorption in the chargedpores ranged from −79 to −170 kJ/mol, and the densitiesof the adsorbed water phases were close to that forbulk water. The isosteric heats and water densities indicatedthat the uncharged pores were mildly hydrophobic, andthe charged pores were very hydrophilic. In mixturesimulations of adsorption from solution, the presence ofwater promoted TCE adsorption in uncharged pores withwidths between 14 and 20 Å. The isosteric heats for TCEadsorption from solution ranged from −14 to −27 kJ/molin the uncharged pores and from −9.3 to −50 kJ/mol in thecharged pores. Strong attractions to the pore surfaceswere significantly diminished after adsorption of the firsttwo monolayers of either adsorbate. Aqueous-phase TCE ata concentration equal to 1% of its saturation concentrationwas able to completely displace adsorbed water inuncharged pores. Even in highly hydrophilic pores, TCE atthis concentration was able to displace up to 50% ofthe adsorbed water. Apparent differential enthalpies ofadsorption determined from the temperature dependenceof TCE adsorption isotherms underestimated the magnitudeof the true isosteric heats of adsorption by up to 30 kJ/mol. This shows that HOC adsorption enthalpies determinedfrom the temperature dependence of their adsorptionisotherms underestimate the true strength of HOC−adsorbentinteractions.
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