A Distributed Reactivity Model for Sorption by Soils and Sediments. 6. Mechanistic Implications of Desorption under Supercritical Fluid Conditions

Abstract

Phenanthrene desorption isotherms for five dry soil materials were measured in supercritical carbon dioxide (SC CO2) at up to 12 different temperature (40−60 °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-shale materials at higher SC CO2 pressures is postulated to increase the sorption capacity of the SOM phase, largely offsetting the increased capacity of the solven...