Grantee Research Project Results
Final Report: Dissolution Dynamics of Multiple Component NAPLs In Aqueous and Surfactant/Cosolvent Systems
EPA Grant Number: R827112Title: Dissolution Dynamics of Multiple Component NAPLs In Aqueous and Surfactant/Cosolvent Systems
Investigators: Brusseau, Mark
Institution: University of Arizona
EPA Project Officer: Aja, Hayley
Project Period: September 1, 1998 through August 31, 2001 (Extended to December 20, 2002)
Project Amount: $362,453
RFA: Exploratory Research - Physics (1998) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Air , Safer Chemicals
Objective:
The overall objective of this research project was to enhance our understanding of the dissolution behavior of multiple-component non-aqueous phase (immiscible) organic liquids (NAPLs) in subsurface systems. The specific objectives of this research project were to: (1) investigate the effect of NAPL composition-dependent factors on dissolution of multiple-component NAPLs; (2) investigate the effect of NAPL mass-transfer limitations on dissolution of multiple-component NAPLs; and (3) investigate the effect of NAPL composition and mass-transfer limitations on the enhanced dissolution of multiple-component NAPLs caused by solubilization agents.
Organic chemicals in the form of NAPLs occur in the subsurface at numerous contaminated sites and can act as long-term sources of groundwater and vadose-zone contamination. Effective risk assessment at these sites depends on the ability to accurately characterize the mass transfer of components between the NAPLs and the aqueous and vapor phases. This mass transfer is influenced by a number of processes. Although the factors controlling dissolution of single-component NAPLs have been examined in some detail, the dissolution behavior of multiple component NAPLs is not as well understood. For example, the influence of temporally variable factors, such as mole fraction and mixture non-ideality on the magnitude and rate of mass transfer, is unclear. Furthermore, additional work is needed to elucidate the processes controlling dissolution of NAPL mixtures in the presence of enhanced-solubilization agents (such as surfactants, alcohols, and other cosolvents), the use of which is becoming increasingly popular for subsurface remediation. To adequately characterize the human-health risks associated with organic contaminants present in the subsurface, and to design effective remediation schemes for such contamination, it is essential to gain a better understanding of the dissolution behavior of multiple-component NAPLs.
Summary/Accomplishments (Outputs/Outcomes):
We have achieved accomplishments along several fronts, as outlined below.
Objective 1: Investigate the Effect of NAPL Composition-Dependent Factors on Dissolution of Multiple-Component NAPLs. As a part of this project, we concluded several sets of experiments to investigate the ideality of aqueous dissolution behavior for multiple-component NAPL systems. Much of this work was conducted in conjunction with experiments performed using solubility enhancing agents (as part of Objective 3). The results of these experiments are thus described under Objective 3.
Complex mixtures of NAPLs have been found in the vadose zone of many contaminated sites. The behavior of multiple-component NAPLs in water-unsaturated systems has not been investigated to the same degree as for water-saturated systems. We conducted a study to examine the behavior of a multiple-component NAPL under water-unsaturated conditions. A site in Tucson, AZ, contaminated by a mixture of tetrachloroethene (PCE) (15-20 weight percent) and diesel fuel, was used as a real-world site model for this work. The purpose of this study was to characterize the mobilization potential, phase-partitioning behavior, and dynamic vapor removal of this mixture. The results indicate that changes in the composition of the NAPL, with respect to PCE concentration, do have an impact on the physical properties (density, viscosity, surface tension, and interfacial tension) and phase partitioning behavior of the NAPL. However, the variations in composition are not likely to increase the mobilization potential of the NAPL, based on capillary and bond number calculations. Comparison of results obtained from batch phase-partitioning experiments to predictions based on Raoult's Law indicates that the diesel-PCE mixture essentially behaves as an ideal mixture for both aqueous and vapor partitioning. Two-dimensional vapor flushing experiments were conducted to characterize the removal behavior of PCE and selected diesel constituents from the diesel-PCE mixture, as induced by vapor extraction. Ideal (Raoult's Law) behavior was observed. A manuscript describing the results of this work is in preparation.
Objective 2: Investigate the Effect of Mass-Transfer Limitations on Dissolution of Multiple Component NAPLs. Long-term elution tailing of organic contaminants often is observed when water or air is used to flush contaminated porous media. This tailing, which has a significant impact on site cleanup, has been attributed to several factors. These include nonlinear, rate-limited sorption/desorption, diffusive mass transfer associated with physically heterogeneous porous media, and rate-limited dissolution of immiscible organic liquids. Characterization of tailing behavior through the quantitative analysis of multiple coupled factors is necessary to enhance our understanding of contaminant transport. The purpose of this study was to investigate the transport and elution behavior of trichloroethene (TCE) in a naturally heterogeneous (poorly sorted) aquifer material, with a specific focus on characterizing and quantifying the relative contributions of rate-limited immiscible-liquid dissolution, and non-linear, rate-limited sorption/desorption to low-concentration elution tailing. A comparison of TCE elution behavior for systems with and without immiscible-liquid phase present suggests that the low-concentration elution tailing observed in the former experiments is associated primarily with non-linear, rate-limited sorption/desorption. The dissolution, transport, and elution of TCE was successfully simulated using a mathematical model we developed that combines independent, coupled descriptions of rate-limited dissolution and non-linear, rate-limited sorption/desorption. Specifically, immiscible-liquid dissolution was described using a first-order mass transfer approach, with a temporally variable dissolution rate coefficient. Sorption/desorption was described using an approach incorporating a continuous distribution of rate-limited domains. The results of this study indicate that multiple processes contributed to TCE elution behavior when immiscible-liquid phase was present, and that a multi-process model was required to accurately simulate the measured data. This work is presented in a manuscript accepted for publication (Johnson, et al., 2003). We have extended this work by developing a mathematical model that incorporates a continuous distribution of initial dissolution rate coefficients, as well as a continuous distribution of rate-limited sorption domains. This approach is used to explicitly account for the impact of local-scale heterogeneity on immiscible-liquid distribution and dissolution. A manuscript presenting this new model is in preparation.
Recently, it has been reported that first-order dissolution rate coefficients obtained from analysis of immiscible-liquid dissolution experiments may be dependent on the length of the immiscible-liquid source zone. Specifically, the first-order dissolution rate coefficient has been observed to be smaller for longer source zones. This phenomenon would be a critical aspect of determining/estimating dissolution rate coefficients for characterizing immiscible-liquid dissolution behavior. The purpose of this study was to investigate whether this phenomenon is inherent to the dissolution process or is rather because of the nonideal phenomena, such as dissolution fingering. The influence of source-zone length on the dissolution of residual-phase TCE was investigated by conducting dissolution experiments using columns of four lengths (5 cm, 15 cm, 25 cm, and 80 cm). A one-dimensional model coupled to an optimization program was calibrated to the measured data to obtain optimized values for the initial dissolution rate coefficient. The model incorporates a function to account for changes in the rate coefficient associated with changes in pore-water velocity and immiscible-liquid saturation with time. Similar dissolution rate coefficients were obtained for all experiments, irrespective of column length. These results indicate that there is no apparent source-zone scale dependence for immiscible-liquid dissolution under the conditions of these experiments. The results of this study are presented in a manuscript currently in review (Tick, et al., 2003).
Objective 3: Investigate the Effect of NAPL Composition and Mass-Transfer Limitations on the Enhanced Dissolution of Multiple-Component NAPLs Caused by Solubilization Agents. Remediation of NAPLs by conventional pump-and-treat methods (i.e., water flushing) generally is considered to be ineffective because of low water solubilities of NAPLs and because of mass-transfer constraints. Chemical flushing techniques, such as surfactant flushing, can greatly improve NAPL remediation primarily by increasing the apparent solubility of NAPL contaminants. NAPLs at hazardous waste sites often are complex mixtures. However, the equilibrium and nonequilibrium mass-transfer characteristics between NAPL mixtures and aqueous surfactant solutions are not fully understood. This study investigated the equilibrium solubilization behavior of two- and three-component NAPL mixtures (containing akylbenzenes) in biosurfactant solutions. NAPL solubilization was found to be ideal in water (i.e., obeys Raoult's Law), but solubilization in biosurfactant solutions was observed to be nonideal. Specifically, the relatively hydrophobic compounds in the mixture experienced solubility enhancements that were greater than those predicted by ideal enhanced-solubilization theory, while the solubility enhancements for the relatively hydrophilic compounds were less than predicted. The degree of non-ideality is shown to be a nonlinear function of the NAPL-phase mole fraction. Empirical relationships based on the NAPL-phase mole fraction and/or micelle-aqueous partition coefficients, measured in single-component NAPL systems, are developed to estimate values for the multi-component partition coefficients. Empirical relationships that incorporate both the NAPL-phase mole fraction and single-component partition coefficients yield much-improved estimates for the multi-component partition coefficient. A set of column experiments was conducted to examine the solubilization/dissolution behavior of this NAPL under dynamic flow conditions. The results were consistent with those obtained for the batch experiments. A similar set of batch and column experiments also were conducted using a non-surfactant solubilization agent (cyclodextrin). As a part of these studies, a mathematical model was used to quantitate dissolution and transport behavior. Dissolution-related parameters obtained from batch and single-component-NAPL column experiments were used as input for simulating the results of the multiple-component-NAPL experiments. A portion of this work is presented in two published papers (Boving, et al., 2000 and McCray, et al., 2001). Manuscripts presenting additional results are in preparation.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 11 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Johnson GR, Zhang Z, Brusseau ML. Characterizing and quantifying the impact of immiscible-liquid dissolution and nonlinear, rate-limited sorption/desorption on low-concentration elution tailing. Water Resources Research 2003;39(5):1120-1120. |
R827112 (Final) |
not available |
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McCray JE, Bai GY, Maier RM, Brusseau ML. Biosurfactant-enhanced solubilization of NAPL mixtures. Journal of Contaminant Hydrology 2001;48(1-2):45-68. |
R827112 (2000) R827112 (2001) R827112 (Final) |
not available |
Supplemental Keywords:
groundwater, soil, chemical transport, risk assessment, chemicals, solvents, NAPL, remediation., RFA, Scientific Discipline, Water, Geographic Area, Waste, Ecosystem Protection/Environmental Exposure & Risk, Physics, Environmental Chemistry, Remediation, Restoration, State, chemical mixtures, Aquatic Ecosystem Restoration, Engineering, Chemistry, & Physics, Groundwater remediation, groundwater recharge, NAPL, aquifer flushing, Utah (UT), mass transfer, dissolution dynamics, aquifer remediation design, alternative cleanup standards, soil and groudwater remediation, aquatic ecosystems, groundwater contamination, surfactants, cosolvent systems, NAPLsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.