Kurt D. Pennell

Surfactant enhanced remediation of soil columns contaminated by residual tetrachloroethylene

The ability of aqueous surfactant solutions to recover tetrachloroethylene (PCE) entrapped in Ottawa sand was evaluated in four column experiments. Residual PCE was emplaced by injecting t4C-labeled PCE into water-saturated soil columns and displacing the free product with water. Miscible displacement experiments were conducted before and after PCE entrapment to determine the influence of residual PCE on column dispersivities. The first two column studies involved the injection of a 4% solution of polyoxyethy!ene (POE) (20) sorbitan monooleate, resulting in the removal of 90% and 97% of the residual PCE from 20- 30- and 40- 120-mesh Ottawa sand, respectively. Although micellar solubilization of PCE was the primary mode of recovery in these experiments, ~his process was shown to be rate-limited based on: (a) the disparity between initial steady-state concentrations of PCE in the column effluent and equilibrium vah, es measured in batch experiments; and (b) the increase in effluent concentrations of PCE following periods of flow interruption. In the latter two experiments, surfactant solutions containing mixtures of sodium sulfosuccinates removed > 99% of the residual PCE from soil columns packed with 40-270-mesh Ottawa sand. Approximately 80% of the PCE was mobilized as a separate organic liquid after flushing with < 100 mL of these surfactant solutions. This study demonstrates the capacity of surfactant flushing to enhance the recovery of residual PCE from Ottawa sand and indicates that ultra-low inter- facial tensions (< 0.0Ol dyn cm -I) are not required to achieve significant PCE mobilization when buoyancy forces are important. The potential for displacement of dense nonaqueous- phase liquids as a separate organic phase should, therefore, be evaluated during the selection of surfactant formulations for aquifer remediation.

An Experimental Investigation of Rate-Limited Nonaqueous Phase Liquid Volatilization in Unsaturated Porous Media: Steady State Mass Transfer

Results of one-dimensional soil column experiments are presented to evaluate the factors influencing volatilization of entrapped nonaqueous phase liquids (NAPLs) in unsaturated sandy porous media. Three-phase fluid saturations measured in Ottawa and Wagner sands were found to depend upon porous media grain size and distribution, with residual water and NAPL saturations ranging from 8 to 16% and 4 to 10%, respectively. In general, residual NAPL saturations were 2–3 times less than NAPL entrapped in similar two-phase (organic-water) systems. During volatilization of three single-component NAPLs (styrene, toluene, and tetrachloroethylene), contaminant vapor phase effluent concentrations deviated from local equilibrium values by 10–40% for pore velocities ranging from 0.25 to 1.5 cm/s. In contrast to NAPL dissolution, mass transfer rates were found to decrease with decreasing soil mean grain size. An empirical correlation based on the modified Sherwood number and Peclet number was developed which incorporates the soil mean grain size as a surrogate measure of NAPL distribution. The utility of this model is demonstrated for the prediction of steady state volatilization rates in independent NAPL-porous media systems.

Sorption and Retardation of Organic Contaminants in Subsurface Systems: Effects on Transport and Fate

Sorption processes in subsurface systems are complex, often involving non-linear phase relationships and rate-limited conditions. These processes unquestionably impact reactive solute behavior under typical field scale conditions, and therefore must be considered in attempts to model or otherwise predict contaminant fate and transport in the subsurface. The thoughtful selection of equilibrium and rate models that accurately describe the inherently complex and system-specific dynamics of sorption processes is an imperative for accurate fate and transport modeling.

Detection of Chlorinated Hydrocarbons in Aqueous Surfactant Solutions by Near-IR Raman Spectroscopy

Near-IR Raman spectroscopy is used to detect chlorinated hydrocarbons under surfactant-enhanced solubilization conditions. The Raman bands of tetrachloroethylene (PCE) and 1,2-dichlorobenzene (DCB) in micelle solutions could be observed in the presence of humic acid when a 784-nm diode laser was used. With 532- or 632.8-nm excitation, humic acid fluorescence obscured the Raman signals. For quantification, the integrated area of the carbon-chlorine stretch mode (PCE) or the phenyl ring-breathing mode (DCB) was used. Test results for samples with unknown concentrations based on linear calibration curves were in agreement with results from an accepted gas chromatography method. Detection limits were calculated to be 240 ppm for tetrachloroethylene and 500 ppm for 1,2-dichlorobenzene. Our study has shown the feasibility of this technique for field applications.