Emulsion-based recovery of a multicomponent petroleum hydrocarbon NAPL using nonionic surfactant formulations

Surfactants can aid subsurface remediation through three primary mechanisms - solubilization, mobilization and/or emulsification. Among these mechanisms, emulsification in porous media is generally not well studied or well understood; particularly in the context of treating sources containing multicomponent NAPL. The objective of this research was to elucidate the processes responsible for recovery of a multicomponent hydrocarbon NAPL when surfactant solutions are introduced within a porous medium to promote the formation of kinetically-stable oil-in-water emulsions. Emulsifier formulations considered here were selected to offer similar performance characteristics while relying on different families of non-ionic surfactants - nonylphenol ethoxylates or alcohol ethoxylates - for emulsification. The families of surfactants have particular environment relevance, as alcohol ethoxylates are often used where replacement of nonylphenol content is necessary. Results from batch and column studies suggest performance of the two formulations was similar. With both, a synergistic combination of emulsification and mobilization led to recovery of a synthetic gasoline NAPL. The relative contribution of solubilization to the recovery was found to be minor. Moreover, the physical processes associated with emulsification and mobilization acted to limit the amount of preferential recovery (or fractionation) of the multicomponent NAPL.

Chemotaxis Increases the Retention of Bacteria in Porous Media with Residual NAPL Entrapment

Chemotaxis has the potential to decrease the persistence of nonaqueous phase liquid (NAPL) contaminants in aquifers by allowing pollutant-degrading bacteria to move toward sources of contamination and thus influence dissolution. This experimental study investigated the migratory response of chemotactic bacteria to a distribution of residual NAPL ganglia entrapped within a laboratory-scale sand column under continuous-flow at a superficial velocity of 0.05 cm/min. Naphthalene dissolved in a model NAPL 2,2,4,4,6,8,8-heptamethylnonane partitioned into the aqueous phase to create localized chemoattractant gradients throughout the column. A pulse mixture of equal concentrations of Pseudomonas putida G7, a strain chemotactic to naphthalene, and Pseudomonas putida G7 Y1, a nonchemotactic mutant, was introduced to the column and effluent bacterial concentrations were measured with time. Breakthrough curves (BTCs) for the two strains were noticeably different upon visual inspection. Differences in BTCs (compared to nonchemotactic controls) were quantified in terms of percent recovery and were statistically significant ( p < 0.01). Chemotaxis reduced percent recovery in the effluent by 45% thereby increasing the population of bacteria that were retained within the column in the vicinity of residual NAPL contaminants. An increase in flow rate to a superficial velocity of 0.25 cm/min did not diminish cell retention associated with the chemotactic effect.

Chemotaxis Increases the Residence Time of Bacteria in Granular Media Containing Distributed Contaminant Sources

The use of chemotactic bacteria in bioremediation has the potential to increase access to, and the biotransformation of, contaminant mass within the subsurface. This laboratory-scale study aimed to understand and quantify the influence of chemotaxis on the residence times of pollutant-degrading bacteria within homogeneous treatment zones. Focus was placed on a continuous-flow sand-packed column in which a uniform distribution of naphthalene crystals created distributed sources of dissolved-phase contaminant. A 10 mL pulse of Pseudomonas putida G7, which is chemotactic to naphthalene, and Pseudomonas putida G7 Y1, a nonchemotactic mutant strain, were simultaneously introduced into the sand-packed column at equal concentrations. Breakthrough curves obtained from experiments conducted with and without naphthalene were used to quantify the effect of chemotaxis on transport parameters. In the presence of the chemoattractant, longitudinal dispersion of PpG7 increased by a factor of 3, and percent recovery decreased by 43%. In contrast, PpG7 Y1 transport was not influenced by the presence of naphthalene. The results imply that pore-scale chemotaxis responses are evident at an interstitial velocity of 1.8 m/day, which is within the range of typical groundwater flow. Within the context of bioremediation, chemotaxis may work to enhance bacterial residence times in zones of contamination, thereby improving treatment.

Biodegradation and cometabolic modeling of selected beta blockers during ammonia oxidation

Accurate prediction of pharmaceutical concentrations in wastewater effluents requires that the specific biochemical processes responsible for pharmaceutical biodegradation be elucidated and integrated within any modeling framework. The fate of three selected beta blockers-atenolol, metoprolol, and sotalol-was examined during nitrification using batch experiments to develop and evaluate a new cometabolic process-based (CPB) model. CPB model parameters describe biotransformation during and after ammonia oxidation for specific biomass populations and are designed to be integrated within the Activated Sludge Models framework. Metoprolol and sotalol were not biodegraded by the nitrification enrichment culture employed herein. Biodegradation of atenolol was observed and linked to the activity of ammonia-oxidizing bacteria (AOB) and heterotrophs but not nitrite-oxidizing bacteria. Results suggest that the role of AOB in atenolol degradation may be disproportionately more significant than is otherwise suggested by their lower relative abundance in typical biological treatment processes. Atenolol was observed to competitively inhibit AOB growth in our experiments, though model simulations suggest inhibition is most relevant at atenolol concentrations greater than approximately 200 ng·L(-1). CPB model parameters were found to be relatively insensitive to biokinetic parameter selection suggesting the model approach may hold utility for describing pharmaceutical biodegradation during biological wastewater treatment.

Assessment of quantitative structural property relationships for prediction of pharmaceutical sorption during biological wastewater treatment

In this study, we critically examined the available data related to pharmaceutical (PhAC) sorption in biological treatment processes. Using these data, we developed and assessed single and polyparameter quantitative structural activity models to better understand the role of sorption in PhAC attenuation. In contrast to other studies, our analysis suggests that values of the sorption coefficient (KD) are poorly correlated to single parameter models employing logKOW or the apparent partition coefficient (i.e., KOW corrected to the experimental pH). Results from the development of polyparameter models suggest that the range of functional moieties typically incorporated in PhAC molecules offers a diverse set of interactions between PhAC and sludge surface (e.g., hydrogen bonding, electrostatic interactions, and hydrophobic interactions). Of particular importance is the role of dissociation and resulting charge(s) of a PhAC in solution. Results demonstrate that when developing predictive models it is advantageous to separate PhACs based upon the charge of the dominant species at the experimental pH. Yet, use a single model for PhACs which are negatively charged and uncharged may have practical utility. Performance of the polyparameter models, however, was found to plateau with a pred-R(2) between 0.50 and 0.60, even when six statistically relevant predictors are included. This outcome suggests that effective predictive models for PhAC sorption cannot include solely PhAC descriptors, rather they must incorporate critical properties related to the sorbent (i.e., mixed liquor) surface.

Kinetic limitations on tracer partitioning in ganglia dominated source zones

Quantification of the relationship between dense nonaqueous phase liquid (DNAPL) source strength, source longevity and spatial distribution is increasingly recognized as important for effective remedial design. Partitioning tracers are one tool that may permit interrogation of DNAPL architecture. Tracer data are commonly analyzed under the assumption of linear, equilibrium partitioning, although the appropriateness of these assumptions has not been fully explored. Here we focus on elucidating the nonlinear and nonequilibrium partitioning behavior of three selected alcohol tracers - 1-pentanol, 1-hexanol and 2-octanol in a series of batch and column experiments. Liquid-liquid equilibria for systems comprising water, TCE and the selected alcohol illustrate the nonlinear distribution of alcohol between the aqueous and organic phases. Complete quantification of these equilibria facilitates delineation of the limits of applicability of the linear partitioning assumption, and assessment of potential inaccuracies associated with measurement of partition coefficients at a single concentration. Column experiments were conducted under conditions of non-equilibrium to evaluate the kinetics of the reversible absorption of the selected tracers in a sandy medium containing a uniform entrapped saturation of TCE-DNAPL. Experimental tracer breakthrough data were used, in conjunction with mathematical models and batch measurements, to evaluate alternative hypotheses for observed deviations from linear equilibrium partitioning behavior. Analyses suggest that, although all tracers accumulate at the TCE-DNAPL/aqueous interface, surface accumulation does not influence transport at concentrations typically employed for tracer tests. Moreover, results reveal that the kinetics of the reversible absorption process are well described using existing mass transfer correlations originally developed to model aqueous boundary layer resistance for pure-component NAPL dissolution.

Encapsulation of nZVI particles using a Gum Arabic stabilized oil-in-water emulsion

Stabilization of reactive iron particles against aggregation and sedimentation is a critical engineering aspect for successful application of nZVI (nanoscale zero valent iron) within the contaminated subsurface environment. In this work we explore the stability and reactivity of a new encapsulation approach that relies upon Gum Arabic to stabilize high quantities of nZVI (∼ 12 g/L) in the dispersed phase of a soybean oil-in-water emulsion. The emulsion is kinetically stable due to substantial repulsive barriers to droplet-droplet induced deformation and subsequent coalescence. Sedimentation time scales were found to be on the order of hours (τ=4.77 ± 0.02 h). Thus, the use of Gum Arabic represents an advance in stabilizing nZVI-in-oil-in-water emulsions. nZVI within the emulsion was shown to be reactive with both TCE degradation and H(2) production observed. Degradation rates were observed to be on the same order of magnitude as those reported for less stable, aqueous suspensions of nZVI. TCE consumption within the emulsion was described with an equivalent aqueous phase rate coefficient of ∼ 5 × 10(-4)L(aq)/m(2)h.

Degradation product partitioning in source zones containing chlorinated ethene dense non-aqueous-phase liquid

Abiotic and biotic reductive dechlorination with chlorinated ethene dense non-aqueous-phase liquid (DNAPL) source zones can lead to significant fluxes of complete and incomplete transformation products. Accurate assessment of in situ rates of transformation and the potential for product sequestration requires knowledge of the distribution of these products among the solid, aqueous, and organic liquid phases present within the source zone. Here we consider the fluid-fluid partitioning of two of the most common incomplete transformation products, cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC). The distributions of cis-DCE and VC between the aqueous phase and tetrachloroethene (PCE) and trichloroethene (TCE) DNAPLs, respectively, were quantified at 22 °C for the environmentally relevant, dilute range. The results suggest that partition coefficients (concentration basis) for VC and cis-DCE are 70 ± 1 L(aq)/L(TCE DNAPL) and 105 ± 1 L(aq)/L(PCE DNAPL,) respectively. VC partitioning data (in the dilute region) were reasonably approximated using the Raoult's law analogy for liquid-liquid equilibrium. In contrast, data for the partitioning of cis-DCE were well described only when well-parametrized models for the excess Gibbs free energy were employed. In addition, available vapor-liquid and liquid-liquid data were employed with our measurements to assess the temperature dependence of the cis-DCE and VC partition coefficients. Overall, the results suggest that there is a strong thermodynamic driving force for the reversible sequestration of cis-DC and VC within DNAPL source zones. Implications of this partitioning include retardation during transport and underestimation of the transformation rates observed through analysis of aqueous-phase samples.

Iron-mediated trichloroethene reduction within nonaqueous phase liquid

Aqueous slurries or suspensions containing reactive iron nanoparticles are increasingly suggested as a potential means for remediating chlorinated solvent nonaqueous phase liquid (NAPL) source zones. Aqueous-based treatment approaches, however, may be limited by contaminant dissolution from the NAPL and the subsequent contaminant transport to the reactive nanoparticles. Reactions occurring within (or at the interface) of the NAPL may alleviate these potential limitations, but this approach has received scant attention due to concerns associated with the reactivity of iron within nonaqueous phases. Results presented herein suggest that iron nanoparticles are reactive with TCE-NAPL and exhibit dechlorination rates proportional to the concentration of (soluble) water present within the NAPL. Reactivity was assessed over a 12-day period for five water contents ranging from 0.31 M to 4.3M, with n-butanol used to enhance water solubility in the NAPL. Rates of dechlorination were generally slower than those reported for aqueous-phase dechlorination, but were not observed to slow over the course of the 12-day period. The lack of observed deactivation may indicate the potential that highly efficient (with respect to utilization of available electrons) dechlorination reactions can be engineered to occur within nonaqueous liquids. These results suggest a need for subsequent investigations which focus on understanding the mechanisms of the reactions occurring within NAPL, as well as those assessing the utility of controlling both the iron and water content within a NAPL source zone.

Predicting DNAPL mass discharge from pool-dominated source zones

Models that link simplified descriptions of dense non-aqueous phase liquid (DNAPL) source zone architecture with predictions of mass flux can be effective screening tools for evaluation of source zone management strategies. Recent efforts have focused on the development and implementation of upscaled models to approximate the relationship between mass removal and flux-averaged, down-gradient contaminant concentration (or mass flux) reduction. The efficacy of these methods has been demonstrated for ganglia-dominated source zones. This work extends these methods to source zones dominated by high-saturation DNAPL pools. An existing upscaled mass transfer model was modified to reproduce dissolution behavior in pool-dominated scenarios by employing a two-domain (ganglia and pools) representation of the source zone. The two-domain upscaled model is parameterized using the initial fraction of the source zone that exists as pool regions, the initial fraction of contaminant eluting from these pool regions, and the flux-averaged down-gradient contaminant concentration. Comparisons of model predictions with a series of three-dimensional source zone numerical simulations and data from two-dimensional aquifer cell experiments demonstrate the ability of the model to predict DNAPL dissolution from ganglia- and pool-dominated source zones for all levels of mass recovery.

Oil-in-water emulsions for encapsulated delivery of reactive iron particles

Treatment of dense nonaqueous phase liquid (DNAPL) source zones using suspensions of reactive iron particles relies upon effective transport of the nano- to submicrometer scale iron particles within the subsurface. Recognition that poor subsurface transport of iron particles results from particle-particle and particle-soil interactions permits development of strategies which increase transport. In this work, experiments were conducted to assess a novel approach for encapsulated delivery of iron particles within porous media using oil-in-water emulsions. Objectives of this study included feasibility demonstration of producing kinetically stable, iron-containing, oil-in-water emulsions and evaluating the transport of these iron-containing, oil-in-water emulsions within water-saturated porous media. Emulsions developed in this study have mean droplet diameters between 1 and 2 microm, remain kinetically stable for > 1.5 h, and possess densities (0.996-1.00 g/mL at 22 degrees C) and dynamic viscosities (2.4-9.3 mPa x s at 22 degrees C and 20 s(-1)) that are favorable to transport within DNAPL source zones. Breakthrough curves and post-experiment extractions from column experiments conducted with medium and fine sands suggest little emulsion retention (< 0.20% wt) at a Darcy velocity of 0.4 m/day. These findings demonstrate that emulsion encapsulation is a promising method for delivery of iron particles and warrants further investigation.

Evaluation of trichloroethene recovery processes in heterogeneous aquifer cells flushed with biodegradable surfactants

The ability of two biodegradable surfactants, polyoxyethylene (20) sorbitan monooleate (Tween 80) and sodium dihexyl sulfosuccinate (Aerosol MA), to recover a representative dense non-aqueous-phase liquid (DNAPL), trichloroethene (TCE), from heterogeneous porous media was evaluated through a combination of batch and aquifer cell experiments. An aqueous solution containing 3.3% Aerosol MA, 8% 2-propanol and 6 g/l CaCl(2) yielded a weight solubilization ratio (WSR) of 1.21 g TCE/g surfactant, with a corresponding liquid-liquid interfacial tension (IFT) of 0.19 dyn/cm. Flushing of aquifer cells containing a TCE-DNAPL source zone with approximately two pore volumes of the AMA formulation resulted in substantial (>30%) mobilization of TCE-DNAPL. However, a TCE mass recovery of 81% was achieved when the aqueous-phase flow rate was sufficient to displace the mobile TCE-DNAPL toward the effluent well. Aqueous solutions of Tween 80 exhibited a greater capacity to solubilize TCE (WSR=1.74 g TCE/g surfactant) and exerted markedly less reduction in IFT (10.4 dyn/cm). These data contradict an accepted empirical correlation used to estimate IFT values from solubilization capacity, and indicate a unique capacity of T80 to form concentrated TCE emulsions. Flushing of aquifer cells with less than 2.5 pore volumes of a 4% T80 solution achieved TCE mass recoveries ranging from 66 to 85%, with only slight TCE-DNAPL mobilization (<5%) occurring when the total trapping number exceeded 2 x 10(-5). These findings demonstrate the ability of Tween 80 and Aerosol MA solutions to efficiently recover TCE from a heterogeneous DNAPL source zone, and the utility of the total trapping number as a design parameter for a priori prediction of DNAPL mobilization and bank angle formation when flushing with low-IFT solutions. Given their potential to stimulate microbial reductive dechlorination at low concentrations, these surfactants are well-suited for remedial action plans that couple aggressive mass removal followed by enhanced bioremediation to treat chlorinated solvent source zones.

Experimental and numerical validation of the total trapping number for prediction of DNAPL mobilization

The total trapping number (N(T)), quantifying the balance of gravitational, viscous, and capillaryforces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual,flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 x 10(-3)-1.10 x 10(-2)), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing.

Coupling aggressive mass removal with microbial reductive dechlorination for remediation of DNAPL source zones: a review and assessment

The infiltration of dense non-aqueous-phase liquids (DNAPLs) into the saturated subsurface typically produces a highly contaminated zone that serves as a long-term source of dissolved-phase groundwater contamination. Applications of aggressive physical-chemical technologies to such source zones may remove > 90% of the contaminant mass under favorable conditions. The remaining contaminant mass, however, can create a rebounding of aqueous-phase concentrations within the treated zone. Stimulation of microbial reductive dechlorination within the source zone after aggressive mass removal has recently been proposed as a promising staged-treatment remediation technology for transforming the remaining contaminant mass. This article reviews available laboratory and field evidence that supports the development of a treatment strategy that combines aggressive source-zone removal technologies with subsequent promotion of sustained microbial reductive dechlorination. Physical-chemical source-zone treatment technologies compatible with posttreatment stimulation of microbial activity are identified, and studies examining the requirements and controls (i.e., limits) of reductive dechlorination of chlorinated ethenes are investigated. Illustrative calculations are presented to explore the potential effects of source-zone management alternatives. Results suggest that, for the favorable conditions assumed in these calculations (i.e., statistical homogeneity of aquifer properties, known source-zone DNAPL distribution, and successful bioenhancement in the source zone), source longevity may be reduced by as much as an order of magnitude when physical-chemical source-zone treatment is coupled with reductive dechlorination.

Pilot-scale demonstration of surfactant-enhanced PCE solubilization at the Bachman Road site. 2. System operation and evaluation

A pilot-scale demonstration of surfactant-enhanced aquifer remediation (SEAR) was conducted during the summer of 2000 at the Bachman Road site in Oscoda, MI. Part two of this two-part paper describes results from partitioning and nonpartitioning tracer tests, SEAR operations, and post-treatment monitoring. For this field test, 68 400 L of an aqueous solution of 6% (wt) Tween 80 were injected to recover tetrachloroethene-nonaqueous phase liquid (PCE-DNAPL) from a shallow, unconfined aquifer. Results of a nonreactive tracer test, conducted prior to introducing the surfactant solution, demonstrate target zone sweep and hydraulic control, confirming design-phase model predictions. Partitioning tracer test results suggest PCE-DNAPL saturations of up to 0.74% within the pilot-scale treatment zone, consistent with soil core data collected during site characterization. Analyses of effluent samples taken from the extraction well during SEAR operations indicate that a total of 19 L of PCE and 95% of the injected surfactant were recovered. Post-treatment monitoring indicated that PCE concentrations at many locations within the treated zone were reduced by as much as 2 orders of magnitude from pre-SEAR levels and had not rebounded 450 days after SEAR operations ceased. Pilot-scale costs ($365 900) compare favorably with design-phase cost estimates, with approximately 10% of total costs attributable to the intense sampling density and frequency. Results of this pilot-scale test indicate that careful design and implementation of SEAR can result in effective DNAPL mass removal and a substantial reduction in aqueous concentrations within the treated source zone under favorable geologic conditions

Pilot-scale demonstration of surfactant-enhanced PCE solubilization at the Bachman Road site. 1. Site characterization and test design

A pilot-scale demonstration of surfactant-enhanced aquifer remediation (SEAR) was conducted to recover dense nonaqueous phase liquid (DNAPL) tetrachloroethene (PCE) from a sandy glacial outwash aquifer underlying a former dry cleaning facility at the Bachman Road site in Oscoda, MI. Part one of this two-part paper describes site characterization efforts and a comprehensive approach to SEAR test design, effectively integrating laboratory and modeling studies. Aquifer coring and drive point sampling suggested the presence of PCE-DNAPL in a zone beneath an occupied building. A narrow PCE plume emanating from the vicinity of this building discharges into Lake Huron. The shallow unconfined aquifer, characterized by relatively homogeneous fine-medium sand deposits, an underlying clay layer, and the absence of significant PCE transformation products, was judged suitable for the demonstration of SEAR. Tween 80 was selected for application based upon its favorable solubilization performance in batch and two-dimensional sand tank treatability studies, biodegradation potential, and regulatory acceptance. Three-dimensional flow and transport models were employed to develop a robust design for surfactant delivery and recovery. Physical and fiscal constraints led to an unusual hydraulic design, in which surfactant was flushed across the regional groundwater gradient, facilitating the delivery of concentrations of Tween 80 exceeding 1% (wt) throughout the treatment zone. The potential influence of small-scale heterogeneity on PCE-DNAPL distribution and SEAR performance was assessed through numerical simulations incorporating geostatistical permeability fields based upon available core data. For the examined conditions simulated PCE recoveries ranged from 94to 99%. The effluent treatment system design consisted of low-profile air strippers coupled with carbon adsorption to trap off-gas PCE and discharge of treated aqueous effluent to a local wastewater treatment plant. The systematic and comprehensive design methodology described herein may serve as a template for application at other DNAPL sites.

Stimulated microbial reductive dechlorination following surfactant treatment at the Bachman Road site

A pilot-scale demonstration of surfactant-enhanced aquifer remediation (SEAR) was conducted in July 2000 at the Bachman Road site located in Oscoda, MI. The Bachman aquifer is a shallow, relatively homogeneous, unconfined aquifer formation composed primarily of sandy glacial outwash with relatively low organic carbon content (0.02 wt %). A 6 wt % aqueous solution of Tween 80 (a nonionic, food-grade surfactant) was flushed through a localized dense nonaqueous phase liquid (DNAPL) source zone to recover approximately 19 L of tetrachloroethene (PCE). Post-treatment monitoring revealed PCE concentrations were reduced by up to 2 orders of magnitude within the source zone, and there was no evidence of concentration rebound after more than 450 d. Concentrations of PCE dechlorination products (trichloroethene, cis-1,2-dichloroethene) 450 d after SEAR operations ceased were more than 2 orders of magnitude greater than pretreatment values, suggesting stimulation of native dechlorination activity. Post-treatment monitoring detected increased concentrations of volatile fatty acids generated from the fermentation of residual-level Tween 80 surfactant. These field data suggest that Tween 80 not only induced and maintained anaerobiosis but also provided reducing equivalents to reductively dechlorinating populations present in the oligotrophic Bachman aquifer. Experience from this site supports application of staged treatment strategies that couple SEAR and microbial reductive dechlorination to enhance mass removal and reduce contaminant mass flux emanating from treated source zones.

Refinement of the density-modified displacement method for efficient treatment of tetrachloroethene source zones

A novel method to remediate dense nonaqueous phase liquid (DNAPL) source zones that incorporates in situ density conversion of DNAPL via alcohol partitioning followed by displacement with a low interfacial tension (IFT) surfactant flood has been developed. Previous studies demonstrated the ability of the density-modified displacement (DMD) method to recover chlorobenzene (CB) and trichloroethene (TCE) from heterogeneous porous media without downward migration of the dissolved plume or free product. However, the extent of alcohol (n-butanol) partitioning required for in situ density conversion of high-density NAPLs, such as tetrachloroethene (PCE), could limit the utility of the DMD method. Hence, the objective of this study was to compare the efficacy of two n-butanol delivery approaches: an aqueous solution of 6% (wt) n-butanol and a surfactant-stabilized macroemulsion containing 15% (vol) n-butanol in water, to achieve density reduction of PCE-NAPL in two-dimensional (2-D) aquifer cells. Results of liquid-liquid equilibrium studies indicated that density conversion of PCE relative to water occurred at an n-butanol mole fraction of 0.56, equivalent to approximately 5 ml n-butanol per 1 ml of PCE when in equilibrium with an aqueous solution. In 2-D aquifer cell studies, density conversion of PCE was realized using both n-butanol preflood solutions, with effluent NAPL samples exhibiting density reductions ranging from 0.51 to 0.70 g/ml. Although the overall PCE mass recoveries were similar (91% and 93%) regardless of the n-butanol delivery method, the surfactant-stabilized macroemulsion preflood removed approximately 50% of the PCE mass. In addition, only 1.2 pore volumes of the macroemulsion solution were required to achieve in situ density conversion of PCE, compared to 6.4 pore volumes of the 6% (wt) n-butanol solution. These findings demonstrate that use of the DMD method with a surfactant-stabilized macroemulsion containing n-butanol holds promise as an effective source zone remediation technology, allowing for efficient recovery of PCE-DNAPL while mitigating downward migration of the dissolved plume and free product.

Use of a surfactant-stabilized emulsion to deliver 1-butanol for density-modified displacement of trichloroethene

A novel surfactant-enhanced aquifer remediation technology, density-modified displacement (DMD), has been developed to minimize risk of dense non-aqueous-phase liquid (DNAPL) downward migration during displacement floods. The DMD method is designed to be implemented using horizontal flushing schemes, with in situ DNAPL density conversion accomplished by the introduction of a partitioning alcohol (e.g., 1-butanol) in a predisplacement flood (preflood). Subsequent NAPL displacement and recovery is achieved by flushing with a low-interfacial-tension (low-IFT) surfactant solution. The efficiency of the DMD method may be enhanced for heavier DNAPLs, such as trichloroethene (TCE), by increasing alcohol delivery and the extent of partitioning during the preflood. The objective of this study was to evaluate the use of a macroemulsion, consisting of 4.7% (vol) Tween 80 + 1.3% (vol) Span 80 + 15% (vol) 1-butanol to achieve efficient in situ density conversion of TCE (relative to that obtained with use of an aqueous preflood solution) prior to low-IFT displacement and recovery from a two-dimensional aquifer cell. The cell was configured to represent a heterogeneous unconfined aquifer system with an overall NAPL saturation between 2% and 3%. After flooding with approximately 1.2 pore volumes of the macroemulsion, a low-IFT solution consisting of 10% (vol) Aerosol MA + 6% (vol) 1-butanol + 15 g/L NaCl + 1 g/L CaCl2 was introduced to displace and recover NAPL. Visual observations and quantitative measurements of effluent fluids demonstrated that in situ density conversion and displacement of TCE-NAPL was successful, with effluent NAPL densities ranging from 0.97 to 0.99 g/mL. For the experimental system employed herein, 93% recovery of the introduced TCE mass was realized after flushing with a combined 2.4 pore volumes of the density conversion and low-IFT solutions. These results demonstrate the increased efficiency of the DMD method when surfactant-based emulsions are used to enhance 1-butanol delivery and partitioning behavior.

Density-modified displacement for DNAPL source zone remediation: density conversion and recovery in heterogeneous aquifer cells

Low interfacial tension (IFT) displacement (mobilization) of nonaqueous phase liquids (NAPLs) offers potential as an efficient remediation technology for contaminated aquifer source zones. However, displacement of dense NAPLs (DNAPLs) is problematic due to the tendency for downward migration and redistribution of the mobilized DNAPL. To overcome this limitation, a density-modified displacement method (DMD) was developed, which couples in situ density conversion of DNAPLs via alcohol partitioning with low IFT NAPL displacement and recovery. The objective of this work was to evaluate the DMD method for two representative DNAPLs, chlorobenzene (CB) and trichloroethene (TCE). Laboratory-scale experiments were conducted in a two-dimensional (2-D) cell, configured to represent a heterogeneous unconfined aquifer system containing low permeability lenses. After release and redistribution of either CB- or TCE-NAPL, the 2-D aquifer cells were flushed with a 6% (wt) n-butanol aqueous solution to achieve DNAPL to light NAPL conversion, followed by a low IFT surfactant solution consisting of 4% (4:1) Aerosol MA/Aerosol OT + 20% n-butanol + 500 mg/L CaCl2. Visual observations and experimental measurements demonstrated that in situ density conversion and immiscible displacement of both CB and TCE were successful. Effluent NAPL densities ranged from 0.96 to 0.90 g/mL for CB and from 0.95 to 0.92 g/mL for TCE, while aqueous phase densities remained above 0.96 g/L. Density conversion of CB and TCE was achieved after flushing with 1.2 and 4.9 pore vol of 6% n-butanol solution, respectively. Recoveries of 90% CB and 85% TCE were realized after flushing with 1.2 pore vol of the low IFT surfactant solution, which was followed by a 1 pore vol posttreatment water flood. Surfactant and n-butanol recoveries ranged from 75 to 96% based on effluent concentration data. The observed minimal mobilization during the n-butanol density conversion preflood and near complete mobilization during the low IFT displacement flood were consistent with total trapping number calculations. The results reported herein demonstrate the potential efficiency of the DMD technology as a means of DNAPL source zone restoration.

Density-modified displacement for dense nonaqueous-phase liquid source-zone remediation: density conversion using a partitioning alcohol

Entrapped and pooled dense nonaqueous-phase liquids (DNAPLs) often persist in aquifers and serve as a long-term source of groundwater contamination. To address the problematic nature of DNAPL remediation, a surfactant-enhanced aquifer remediation (SEAR) technology, density-modified displacement (DMD), has been developed which significantly reduces the risk of downward migration of displaced DNAPLs. The DMD method is designed to accomplish DNAPL density conversion through the introduction of a partitioning alcohol, n-butanol (BuOH), in a predisplacement flood using conventional horizontal flushing schemes. Subsequent displacement and recovery of the resulting LNAPL is achieved by flushing with a low-interfacial tension surfactant solution. The objective of this study was to investigate density conversion of two representative DNAPLs, chlorobenzene (CB) and trichloroethene (TCE). A series of batch experiments was performed to assess changes in NAPL composition, density, and phase behavior as a function of BuOH mole fraction. Experimental results were used to develop contaminant/BuOH/water ternary phase diagrams and to elucidate regions of contrasting NAPL density. UNIQUAC calculations are presented to support measured compositional and phase behavior data. Density conversion of CB and TCE, relative to water, occurred at NAPL BuOH mole fractions of 0.38 and 0.50, respectively. Significant incorporation of water into the organic phase was observed at relatively high BuOH mole fractions and was shown to limit changes in NAPL composition and density. Interfacial tensions between CB-NAPL and TCE-NAPL and a 6% (by wt) BuOH aqueous solution were found to decrease with increasing NAPL BuOH mole fraction, although in both cases the measured values remained above 2.5 dyn/cm. Total trapping number calculations suggest that, in most aquifer formations, density conversion can be achieved without premature NAPL displacement using a 6% (by wt) BuOH aqueous solution.

Implications of alcohol partitioning behavior for in situ density modification of entrapped dense nonaqueous phase liquids

Surfactant-based remediation techniques have the potential to be very effective for removing dense nonaqueous-phase liquids (DNAPLs) from contaminated sites. However, a risk associated with surfactant-based remediation of DNAPLs is the potential for unwanted downward mobilization of the DNAPL contaminants, making them more difficult to remove from the subsurface. The work described here examines the use of hydrophobic alcohol solutions to reduce the densities of entrapped DNAPLs, converting them to light nonaqueous-phase liquids (LNAPLs). Results of partitioning studies are presented for alcohol-DNAPL systems, in the absence and presence of surfactants. Results indicate that alcohol concentrations near saturation are necessary for conversion of DNAPLs to LNAPLs--particularly for high-density DNAPLs such as trichloroethylene (TCE) and tetrachloroethylene (PCE). Although surfactants can increase the mass of alcohol that can be delivered to a contaminated zone, they appear to change the partitioning equilibrium such that higher alcohol concentrations are required to achieve the same result. Results of this work indicate the importance of minimizing dilution during density modification applications and suggest the concept of using an alcohol macroemulsion flood for density conversion. Implications of the results of this work for remediation system design are discussed.