Artificial aging of phenanthrene in porous silicas using supercritical carbon dioxide

Expedited artificial aging is described and demonstrated using a novel system that circulates a solution of supercritical carbon dioxide and a hydrophobic organic sorbate (phenanthrene) through a closed loop containing a porous substrate. Unlike traditional methods used to simulate the natural aging process, our approach allows for real-time monitoring of sorption equilibria, and the process is highly accelerated due to the unique physical properties of supercritcal carbon dioxide. The effectiveness of the system to simulate aging was demonstrated with a series of experiments in which three silicas with varying particle and pore sizes were loaded with phenanthrene. Batch aqueous desorption experiments were used to evaluate the extent of the aging process. For the two types of particles containing the largest pores (i.e., mean diameters of 202 and 66 A), 95% and 86%, respectively, of the phenanthrene was released to the aqueous fraction within 3 h. In contrast, only 16% of the phenanthrene was released from particles having a mean pore diameter of 21 A after 24 h. These results were confirmed by the results from an aqueous column desorption experiment. Confounding factors that might contribute to slow aqueous desorption such as the hydration state of the particles' surfaces, the chemical form of the loaded phenanthrene, and the organic carbon content were investigated and/or normalized for all three particle types. Consequently, we were able to attribute the slow desorption behavior and the presence of the resistant fraction in the 21 A silica to pore effects. With properly designed experiments, the results of this study suggest that the supercritical fluid system could be extended to the study of contaminant aging and bioavailability in natural soils and sediments.