Parameters from single‐component isotherm models were used in multicomponent isotherm models to predict the aqueous phase sorption of trichloroethylene (TCE) in the presence of tetrachloroethylene (PCE) in four zeolites, Tenax, and three natural solids. The Langmuir, the Polanyi‐Dubinin, and the Freundlich or the Langmuir‐Freundlich isotherm models were used to simulate single‐component sorption in zeolites. The Langmuir two‐site, the Polanyi‐Dubinin two‐site, and the Freundlich or the Langmuir‐Freundlich isotherm models were used to simulate single‐component sorption in Tenax and natural solids. Two‐site models have been used previously to model sorption in soils and sediments, and they combine an adsorption component (e.g., Langmuir) with a linear partitioning component. By using parameters from the different single‐component isotherm models, the multicomponent Langmuir, the ideal adsorbed solution theory, and the Polanyi theory were each used to predict multicomponent sorption. In general, the ability to predict TCE sorption in the presence of PCE depended more on the choice of the single‐component model than the multicomponent model, and better results were obtained when the Freundlich or the Langmuir‐Freundlich isotherm was used for single‐component sorption. This suggests that the more mechanistically based Langmuir and Polanyi‐type models may not adequately describe the distribution of adsorption sites in some model and natural solids. The Freundlich or the Langmuir‐Freundlich model, although empirical, has greater flexibility in characterizing sorbent heterogeneity and results in better multicomponent model predictions. However, this last statement is tenuous, because more solids must be tested against various model combinations.
A 13-year study of tritium transport through a field-scale earthen liner was conducted by the Illinois State Geological Survey to determine the long-term performance of compacted soil liners in limiting chemical transport. Two field-sampling procedures (pressure-vacuum lysimeter and core sampling) were used to determine the vertical tritium concentration profiles at different times and locations within the liner. Profiles determined by the two methods were similar and consistent. Analyses of the concentration profiles showed that the tritium concentration was relatively uniformly distributed horizontally at each sampling depth within the liner and thus there was no apparent preferential transport. A simple one-dimensional analytical solution to the advective–dispersive solute transport equation was used to model tritium transport through the liner. Modeling results showed that diffusion was the dominant contaminant transport mechanism. The measured tritium concentration profiles were accurately modeled with an effective diffusion coefficient of 6×10−4mm2/s, which is in the middle of the range of values reported in the literature.
Magnetic resonance imaging (MRI) was used to determine the effects of pore-scale heterogeneity on the dissolution of a nonaqueous phase liquid (NAPL) in water-saturated flow-through columns (1.2 cm in diameter) packed with either ∼500 or ∼1000 micron diameter angular silica gel (referred to as SG500 and SG1000, respectively). Columns were contaminated with 1,3,5-trifluorobenzene at residual saturation and then purged with water at a constant Darcy velocity of 1.83 m/day. Three-dimensional 19F images were acquired every 2−5 h at an imaging resolution of 59 × 234 × 234 μm3. Imaging results show that the specific NAPL surface area (at) is linearly related to the NAPL volumetric fraction (θn) and that the constant of proportionality between these parameters is determined by the blob size and geometry distribution. Overall (expressed as the modified Sherwood number, Sh') and intrinsic (expressed as the apparent Sherwood number, Shapt) mass transfer rate coefficients were calculated. Values of Sh' and Shapt for SG500 were approximately three times less than those for SG1000. For both solids, Sh' first increased or stayed the same and then decreased with decreasing θn, while Shapt generally increased with decreasing θn. These results suggest that during dissolution new flow paths were created (i.e., bypass zones were eliminated) as NAPL dissolved, decreasing the fraction of NAPL−water interfaces adjacent to pores filled with stagnant water and the average diffusion length scale. Since at for SG500 was dominated by less spherical multipore blobs (as opposed to more spherical singlets for SG1000), these results also suggest that the extent of flow bypassing (and the average diffusion length scale) increases in systems with more irregular blobs. These results are important because Sh' correlations and a “sphere” dissolution model do not account for transient changes in the fraction of NAPL surface area that contributes to dissolution or for the effect of initial blob size and geometry distribution on this fraction.