ESTIMATING INTERNAL MASS-TRANSFER RATES IN SOILS USING SCINTILLATION FLUID EXTRACTION

This work examines the mass transfer resistance associated with the se paration of sorbed organic contaminants, tetrachloroethene (PCE) and t richloroethene (TCE), from the Borden sand and Moffett aquifer solids, respectively. The experimental method entails extracting the sorbed s olute directly into scintillation fluid. Desorption rate data are inte rpreted using a pore diffusion model, in spherical coordinates, to det ermine effective pore diffusivity values (D-p). An apparent increase i n 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, estima ted D-p values were of the same order of magnitude for the two methods . This result suggests that the solvent extraction technique is subjec t to mass transfer resistances that are similar to those encountered i n 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 a n aqueous diffusion mechanism. Corresponding results for a Borden frac tion were less certain, but suggested a more severe mass transfer resi stance.