A distributed reactivity model for sorption by soils and sediments .6. Mechanistic implications of desorption under supercritical fluid conditions

Phenanthrene desorption isotherms for five dry soil materials were measured in supercritical carbon dioxide (SC CO2) at up to 12 different temperature (40-60 degrees C) and pressure (120-310 atm) conditions for each soil. The Freundlich model adequately described all 40 isotherms obtained. At each condition, sorption capacity increased with increasing organic matter content with a positive dependence on oxygen content. Trends in desorption with SC CO2 temperature and pressure were qualitatively consistent with variations in phenanthrene solubility. A diagenetically altered shale material displayed desorption increases similar in magnitude to solubility changes, but increased pressure stimulated desorption from surface soils far less than expected from solubility considerations. Swelling of amorphous macromolecular soil organic matter (SOM) in the non-share materials at higher SC CO2 pressures is postulated to increase the sorption capacity of the SOM phase, largely offsetting the increased capacity of the solvent phase that accompanies pressure increases. The results extend application of the distributed reactivity model (DRM) to nonaqueous solvents and dry soil systems and lend additional support to the mechanistic importance of rigid or glassy SOM domains in the sorption of non-ionic organic chemicals.