Estimating internal mass transfer rates in soils using scintillation fluid extraction

This work examines the mass transfer resistance associated with the separation of sorbed organic contaminants, tetrachloroethene (PCE) and trichloroethene (TCE), from the Borden sand and Moffett aquifer solids, respectively. The experimental method entails extracting the sorbed solute directly into scintillation fluid. Desorption rate data are interpreted using a pore diffusion model, in spherical coordinates, to determine effective pore diffusivity values (D_p). An apparent increase in observed desorption rates for long-term samples is discussed in the framework of a potential experimental artifact induced by a cosolvent effect. The results from the scintillation fluid extraction method are less reproducible than those from an aqueous purge-and-trap protocol (used previously for the same solute-sorbent systems). However, estimated D_p values were of the same order of magnitude for the two methods. This result suggests that the solvent extraction technique is subject to mass transfer resistances that are similar to those encountered in aqueous systems. The method is tested over a range of temperatures, and results are interpreted with respect to mechanism validation. For a Moffett fraction, temperature dependent results were indicative of an aqueous diffusion mechanism. Corresponding results for a Borden fraction were less certain, but suggested a more severe mass transfer resistance.