Final Report: A Multi-Scale Investigation of Mass Transfer Limitations in Surfactant-Enhanced Aquifer RemediationEPA Grant Number: R825405
Title: A Multi-Scale Investigation of Mass Transfer Limitations in Surfactant-Enhanced Aquifer Remediation
Investigators: Mayer, Alex , Pope, Gary A.
Institution: Michigan Technological University , The University of Texas at Houston
EPA Project Officer: Chung, Serena
Project Period: October 1, 1996 through October 31, 1999 (Extended to October 31, 2000)
Project Amount: $299,792
RFA: Environmental Fate and Treatment of Toxics and Hazardous Wastes (1996) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals
The goal of this research project was to investigate mass transfer between residual nonaqueous phase liquids (NAPLs) and an aqueous phase containing surfactants. The investigation will yield mass transfer relationships for surfactant-enhanced NAPL dissolution. The dependence of mass transfer rates on remediation design variables-surfactant constituents, types, concentrations, and aqueous phase flow rates?will be determined. The work also will attempt to identify the phenomena responsible for mass transfer limitations in surfactant-enhanced NAPL dissolution, such as the viscosity of aqueous surfactant solutions, diffusivities in aqueous-surfactant solutions, mass exchange from NAPL to aqueous solutions, and mass exchange from aqueous solution to micelles.
Surfactant-enhanced solubilization of residual, NAPL contaminants is an emerging, subsurface remediation technology. The potential for nonequilibrium conditions was investigated for surfactant-enhanced solubilization of a NAPL, trichloroethylene (TCE), in a model porous medium. The surfactant formulation consists of an anionic surfactant, sodium dihexyl sulfosuccinate, an alcohol, and an electrolyte in aqueous solution. Batch solubilization experiments were conducted to assess the significance of chemical rate limitations. Surfactant flood experiments were conducted in packed columns with residual TCE. Mass transfer rate coefficients were determined as a function of aqueous phase pore velocity, NAPL volumetric fraction, and surfactant concentration. A correlation for predicting mass transfer rate coefficients as a function of system properties was developed. The mass transfer rate coefficients and correlation were obtained by fitting a transport simulator to the column effluent concentration results. Significant differences are found between the correlation developed here and correlations developed for other NAPL-surfactant systems. The correlation predicts near-linear dependences of mass transfer rates on NAPL volumetric fraction and pore velocity. Using the Damkohler number, the degree of nonequilibrium behavior in surfactantenhanced NAPL solubilization was analyzed for a range of conditions. Nonequilibrium conditions are significant at relatively low NAPL volumetric fractions.
A two-dimensional micromodel and image capture system was applied to observe micro-scale NAPL mobilization and solubilization phenomena. In each experiment, a common residual NAPL field was established, followed by a series of mobilization and solubilization experiments. Mobilization floods included pure water floods with variable flow rates and surfactant floods with variations in surfactant formulations. At relatively low capillary numbers, the surfactant mobilization floods resulted in higher NAPL saturations than for similar capillary number, pure water floods. These differences in macroscopic saturations are explained by differences in micro-scale mobilization processes. The formation of micro-scale dissolution fingers dominated the solubilization of the residual NAPL remaining after the mobilization stage. This produced nonequilibrium macro-scale NAPL solubilization. We observed a macroemulsion phase, which formed spontaneously and persisted during the solubilization stage of the experiments.
We used batch and column experiments to assess the effect of surfactant formulation on the rate of NAPL solubilization. The experimental variables were surfactant type, surfactant concentration, electrolyte concentration, and co-solvent concentration. Model equations were proposed and solved to describe solubilization under the conditions of each type of experiment. Using these models, a solubilization rate constant, kb, and an overall mass transfer rate coefficient, k, were estimated from the batch and column experiments, respectively. The solubilization rate constant was consistently sensitive to surfactant type, surfactant concentration, and electrolyte concentration. The estimated solubilization rate constants varied over two orders of magnitude. The results of the column experiments also were sensitive to the surfactant formulation. Variations in the fitted mass transfer rate coefficient parameter, o, were related to variations in the surfactant formulations. A comparison between the results of the batch and column experiments yields an apparent relationship between o and kb. This relationship suggests that the mass transfer rate coefficient is directly related to the formulation of the surfactant solution.
The practical implication of nonequilibrium mass transfer between NAPL and surfactant solution is that removing a specified mass of NAPL will require more time than that calculated assuming equilibrium. Equilibrium assumptions should be used with caution because this approach will significantly underestimate the amount of time and mass of surfactant required to solubilize a given volume of NAPL. More research is needed to determine the underlying physical and chemical mechanisms that control mass transfer rates in NAPL-surfactant systems, because it may be possible to manipulate physical and chemical conditions to improve surfactant flood efficiencies. For example, larger-scale, physically based mechanisms, such as bypassing, may be more important than small-scale, mass transfer limitations. In this case, the use of mobility control agents during the surfactant flood may increase apparent mass transfer rates, thus increasing the efficiency of the flood. Furthermore, it is appropriate to investigate and apply surfactant formulations that reach chemical equilibrium with a given NAPL quickly, such as the case with the NAPL-surfactant system used here, as evidenced by the batch and early column experimental results.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
|Other project views:||All 31 publications||5 publications in selected types||All 5 journal articles|
||Mayer AS, Zhong LR, Pope GA. Measurement of mass transfer-rates for surfactant-enhanced solubilization of nonaqueous phase liquids. Environmental Science & Technology 1999;33(17):2965-2972.||
||Mayer AS, Carriere PPE, Gallo C, Pennell KD, Taylor TP, Williams GA, Zhong L. Groundwater quality. Water Environment Research 1997;69(4):777-844.||
||Mayer AS, Lenhard RJ. Special issue on multiphase flow and chemical transport introduction/editorial. Advances in Water Resources 1998;21(2):75-76.||
||Zhong LR, Mayer AS, Pope GA. The effects of surfactant formulation on nonequilibrium NAPL solubilization. Journal of Contaminant Hydrology 2003;60(1-2):55-75.||