A distributed reactivity model for sorption by soils and sediments .7. Enthalpy and polarity effects on desorption under supercritical fluid conditions

Mechanisms of phenanthrene desorption from five subsurface materials in supercritical carbon dioxide (SC CO2) were investigated by measuring isotherms in the presence of fixed quantities of a polar cosolvent (methanol) and by calculating desorption enthalpies from the temperature dependence of the isotherms. The addition of 7.4 mol % methanol to the SC CO2 phase resulted in 2-11-fold reductions in Freundlich capacity factors at 120 atm and 50 degrees C. The capacity reduction greatly exceeded that expected from the 21% increase in phenanthrene solubility accompanying cosolvent addition. Isotherms became more linear at 120 atm and 50 degrees C upon methanol addition for all sorbents except a shale sample. Solubility and organic carbon-normalized phenanthrene sorption capacities declined with increasing solvent polarity, in the order dry SC CO2 > methanol-amended SC CO2 > aqueous solution, and declined with sorbent organic carbon content Sorption enthalpies from an ideal gas reference state ranged from -106 to -70 kJ/mol, similar to phenanthrene's heat of condensation of -70.7 kJ/mol. The results indicate that the primary mechanism for polar cosolvent desorption enhancement is competitive displacement of phenanthrene from polar sites within soil organic matter or on mineral surfaces.