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

Enhancing remediation of residual DNAPL in multilayer aquifers: Post-injection of alcohol-surfactant-polymer mixtures

Although polymer-surfactant injection is an effective remediation technology for multilayer aquifers contaminated by Dense Non-Aqueous Phase Liquids (DNAPL), the existence of residual DNAPL after treatment is inevitable. This study evaluates the efficiency of the post-injection of alcohol-surfactant-polymer (ASP) mixtures containing 1-propanol/1-hexanol, sodium dodecylbenzenesulfonate (SDBS), and xanthan in enhancing remediation of residual DNAPL in layered systems. A range of experimental devices, including batch, rheological measurements, centimetric 1D column, and decametric 2D tank experiments, were employed. Batch experiments revealed that the inclusion of 1-hexanol swelled the DNAPL volume due to alcohol partitioning. Conversely, with only 1-propanol present in the alcohol-surfactant (AS) mixture, DNAPL dissolved in the aqueous phase. The co-presence of 1-hexanol along with 1-propanol in AS mixture favored 1-propanol's partitioning into the DNAPL phase. Column experiments, following primary xanthan-SDBS (XS) injections, demonstrated that ASP mixtures with 1-hexanol (regardless of presence of 1-propanol) underwent a mobilization mechanism. DNAPL appeared in the effluent as an organic phase after the post-injection of 0.3 pore-volumes (PV), by a reduction trend in its density. In contrast, mixtures with solely 1-propanol exhibited a solubilization mechanism, with DNAPL dissolving in the aqueous phase and emerging in the effluent after approximately 1 PV. 2D tank experiments visualized mobilization and solubilization mechanisms in multilayered systems. Post-injection of the ASP mixture with solely 1-propanol led to DNAPL solubilization, demonstrated by a dark zone of varied DNAPL concentrations, followed by a clearer white zone indicating significant DNAPL dissolution. Injecting ASP mixture containing both 1-propanol and 1-hexanol mobilized swollen DNAPL ganglia throughout layers, with these droplets coalescing and migrating to the recovery point. The darkness of mobilized droplets was faded as more DNAPL was recovered. The solubilization ASP mixture enhanced the recovery factor by 0.02 while the mobilization ASP mixture led to a 0.08 increase in the recovery factor.

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.

Density-modification displacement using colloidal biliquid aphron for entrapped DNAPL contaminated aquifer remediation

Colloidal biliquid aphron (CBLA) is a strong density modifier for dense nonaqueous phase liquids (DNAPLs). However, the underlying mechanisms responsible for density modification and displacement is not yet clear. Here, a series of batch column and sandbox experiments were conducted to achieve substantial removal and irreversible density reduction of tetrachloroethylene (PCE). The mass of PCE retained in the column and sandbox was less than 1% under suitable injection conditions, and the density of PCE in the effluent was less than that of water (fluctuated in the range of 0.74-0.96 g/cm³). The displacement process was controlled by the high viscosity ratio of CBLA to PCE (52.3). The emulsified and dissolved phase of PCE formed after reaction with CBLA, and the light nonaqueous phase liquid (LNAPL) phase formed after injecting demulsifier solution. The phase analysis played a significant role in monitoring the changes in concentration and density of PCE. The density-modification displacement technique using CBLA reduced the mass of residual PCE by a factor of 165 compared to surfactant flushing, and there was no risk of downward migration of PCE. This study contributes to a better remediation of entrapped DNAPL in contaminated aquifer.

Density-regulated remediation of dense non-aqueous phase liquids using colloidal biliquid aphrons (CBLA): Force model of transport and distribution

Using colloidal biliquid aphrons (CBLAs) for density control has been proved to a promising technology in dense non-aqueous phase liquids (DNAPLs) contaminated aquifer remediation. However, the transport and distribution of CBLAs in aquifer is an urgent issue for actual application in groundwater. Especially considering the fact that CBLAs have a lower density than water. In this work, the role of buoyancy force on CBLA transport in water-saturated sandbox was investigated, and the force model of CBLA in pore space was developed. Furthermore, the density regulation of trichloroethylene (TCE) in sandbox was studied using CBLA. We found that buoyancy plays a significant role compared with other interaction forces in the transport of CBLA, and the sine of the rising angle of CBLA has a significant correlation with the force on CBLA. CBLA at 5 times the volume of TCE displaced the TCE at the bottom of the tank by upward mobility and the maximum concentration dramatically decreased to 31.23 mg/L. These results can be used for predicting the transport of CBLA (as well as other remediation reagents that are less dense than water) in aquifer and are beneficial to the subsequent remediation application of CBLA in actual contaminated sites.

Mechanisms of irreversible density modification using colloidal biliquid aphron for dense nonaqueous phase liquids in contaminated aquifer remediation

The use of colloidal biliquid aphron (CBLA) as density modifier to reduce the density of dense nonaqueous phase liquids (DNAPLs) irreversibly is an efficient strategy to control the migration of DNAPLs in contaminated aquifers. However, the process and mechanism of the density regulation using CBLA is still not clear and there is still a big gap in the application of CBLA in actual contaminated sites. In this study, we carried out density modification of 5 DNAPLs (nitrobenzene (NB), dichloromethane (DCM), trichloroethylene (TCE), carbon tetrachloride (CTC), perchloroethylene (PCE)) using CBLA and studied the effect of co-existing ions by 3D response surface method. We found that DNAPLs changed to light nonaqueous phase liquids (LNAPLs) and float up after interaction with light organic liquid from CBLA. The density modification process is limited by the demulsificaiton of CBLA and the density of DNAPL itself. Density regulation of DNAPLs followed pseudo-second-order kinetics. The co-existing ions affected the stability of CBLA and the demulsification ability of the demulsifier. Aquifer materials and low temperature did not influence the density control effect of CBLA. This research advances the practical application of density control of DNAPLs using CBLA, and makes important contributions for subsequent combined remediation approach.

The role of surfactants in colloidal biliquid aphrons and their transport in saturated porous medium

In remediation of dense non-aqueous phase liquids (DNAPLs), colloidal biliquid aphrons (CBLAs) could be added to produce a lower density nonaqueous phase which mitigate downward migration of DNAPL to non-polluted aquifers. There is still a big gap in the application of CBLAs in the remediation of actual polluted sites, especially the absence of relevant studies on its transport behavior in the sites, and its structural model has not been fully verified. These two factors could affect the effectiveness of CBLAs in the underground environment and its effect on density control. In this study, we prepared CBLAs with different surfactants and verified the structural model of CBLA based on their particle size distributions and demulsification performance. We studied the effects of particle concentrations, injection velocities, and porous media size on the migration of CBLA using the breakthrough curves and distribution profiles along the column. Experimental results indicated that surface elasticity of CBLAs was inversely proportional to the concentration of the anionic surfactant sodium dodecyl sulphate (SDS), which led to easier demulsification of CBLA with the increase in SDS concentration. This observation was in agreement with the verified structural model of the CBLA which constitute both internal nonionic and external anionic surfactants. Furthermore, CBLA deposition is mainly caused by interception and is not suitable for application in fine media. Low concentration of CBLA and high injection flow rate help CBLA to form a remediation area with a certain radius. This study solved the problem of DNAPLs in contaminated groundwater from the perspective of density regulation, and made contributions towards the development of combined remediation approaches using CBLAs.

Colloidal biliquid aphron demulsification using polyaluminum chloride and density modification of DNAPLs: optimal conditions and common ion effect

Dense non-aqueous phase liquids (DNAPLs) entrapped and pooled in aquifers serve as a long term source of groundwater contamination because of their low solubility and high density. Density modification displacement (DMD) with colloidal biliquid aphrons (CBLAs) is a promising approach to prevent DNAPL downward migration during surfactant-based remediation processes. CBLA demulsification and quick release of internal light organic matter is the key to density modification of DNAPLs. In this work, it is reported for the first time that polyaluminum chloride (PAC) could destabilize CBLAs efficiently. The optimum conditions for demulsification of CBLAs by PAC were obtained; the effects of several specific ions in groundwater on demulsification of CBLAs by PAC were investigated. The results indicated that the CBLA was completely demulsified by PAC within 10 minutes and released light organic matter. It recorded the highest demulsification efficiency when the addition ratio (VPAC/VCBLA) was 2 : 1, concentration of PAC was 0.7 g L-1 and the PVR of CBLAs was 8. Most cations (sodium, magnesium and calcium ions) had inhibitory effects on demulsification of CBLAs by PAC with increasing ion concentration, but iron ions had no effect on it. Sulfate anions showed a stronger inhibitory effect on demulsification of CBLAs by PAC compared to chloride ions. With PAC as the demulsifier, CBLAs could be demulsified efficiently, irreversibly modifying the density of trichloroethylene in 5 minutes.

Efficient remediation of crude oil-contaminated soil using a solvent/surfactant system

Crude oil contaminated soil has been widely recognized to constitute a major environmental issue due its adverse effects on human health and ecological safety. The main objective of this study is to explore the possibility of using an ex situ solvent/surfactant washing technique for the remediation of crude oil-contaminated soil. Three organic solvents (methanol, acetone, and toluene) and one surfactant (AES-D-OA) were employed to form three kinds of solvent/surfactant systems, and utilized to evaluate the desorption performance of crude oil from soil. Natural soil, crude oil-contaminated soil, and after-remediation soil were characterized by SEM, EDX, FT-IR, and contact angle. The ability of solvent/surfactant systems to remove crude oil from soil was determined as a function of solvent polarity, mass ratio of solvent to surfactant, temperature, and ionic strength. The removal of crude oil by the toluene/AES-D-OA system was found to be more effective than the other systems. At a high toluene ratio, more than 97% of crude oil could be removed from contaminated soil. Crude oil removal efficiency was also found to increase with rising temperature or increasing ionic strength appropriately. Experimental results suggested that, compared to conventional surfactant-aided remediation, the combined utilization of surfactant and solvent achieved superior results for crude oil removal because of their similar compositions and structures in terms of aromaticity and polarity.

Efficient remediation of crude oil-contaminated soil using a solvent/surfactant system

Crude oil contaminated soil has been widely recognized to constitute a major environmental issue due its adverse effects on human health and ecological safety. The main objective of this study is to explore the possibility of using an ex situ solvent/surfactant washing technique for the remediation of crude oil-contaminated soil. Three organic solvents (methanol, acetone, and toluene) and one surfactant (AES-D-OA) were employed to form three kinds of solvent/surfactant systems, and utilized to evaluate the desorption performance of crude oil from soil. Natural soil, crude oil-contaminated soil, and after-remediation soil were characterized by SEM, EDX, FT-IR, and contact angle. The ability of solvent/surfactant systems to remove crude oil from soil was determined as a function of solvent polarity, mass ratio of solvent to surfactant, temperature, and ionic strength. The removal of crude oil by the toluene/AES-D-OA system was found to be more effective than the other systems. At a high toluene ratio, more than 97% of crude oil could be removed from contaminated soil. Crude oil removal efficiency was also found to increase with rising temperature or increasing ionic strength appropriately. Experimental results suggested that, compared to conventional surfactant-aided remediation, the combined utilization of surfactant and solvent achieved superior results for crude oil removal because of their similar compositions and structures in terms of aromaticity and polarity.

Water CAs and EDX analysis of (a) natural soil, (b) crude oil-contaminated soil, and (c) after-remediation soil.

The influence of cosolvent and heat on the solubility and reactivity of organophosphorous pesticide DNAPL alkaline hydrolysis

The presented research concerned the compatibility of cosolvents with in situ alkaline hydrolysis (ISAH) for treatment of organophosphorous (OPP) pesticide contaminated sites. In addition, the influence of moderate temperature heat increments was studied as a possible enhancement method. A complex dense non-aqueous phase liquid (DNAPL) of primarily parathion (~50 %) and methyl parathion (~15 %) obtained from the Danish Groyne 42 site was used as a contaminant source, and ethanol and propan-2-ol (0, 25, and 50 v/v%) was used as cosolvents in tap water and 0.34 M NaOH. Both cosolvents showed OPP solubility enhancement at 50 v/v% cosolvent content, with slightly higher OPP concentrations reached with propan-2-ol. Data on hydrolysis products did not show a clear trend with respect to alkaline hydrolysis reactivity in the presence of cosolvents. Results indicated that the hydrolysis rate of methyl-parathion (MP3) decreased with addition of cosolvent, whereas the hydrolysis rate of ethyl-parathion (EP3) remained constant, and overall indications were that the hydrolysis reactions were limited by the rate of hydrolysis rather than NAPL dissolution. In addition to cosolvents, the influence of low-temperature heating on ISAH was studied. Increasing reaction temperature from 10 to 30 °C provided an average rate of hydrolysis enhancement by a factor of 1.4-4.8 dependent on the base of calculation. When combining 50 v/v% cosolvent addition and heating to 30 °C, EP3 solubility was significantly enhanced and results for O,O-diethyl-thiophosphoric acid (EP2 acid) showed a significant enhancement of hydrolysis as well. However, this could not be supported by para-nitrophenol (PNP) data indicating the instability of this product in the presence of cosolvent.

Phase Transfer of Palladized Nanoscale Zerovalent Iron for Environmental Remediation of Trichloroethene

Palladium-doped nanoscale zerovalent iron (Pd-NZVI) has been shown to degrade environmental contaminants such as trichloroethene (TCE) to benign end-products through aqueous phase reactions. In this study we show that rhamnolipid (biosurfactant)-coated Pd-NZVI (RL-Pd-NZVI) when reacted with TCE in a 1-butanol organic phase with limited amounts of water results in 50% more TCE mass degradation per unit mass of Pd-NZVI, with a 4-fold faster degradation rate (kobs of 0.413 day(-1) in butanol organic phase versus 0.099 day(-1) in aqueous phase). RL-Pd-NZVI is preferentially suspended in water in biphasic organic liquid-water systems because of its hydrophilic nature. We demonstrate herein for the first time that their rapid phase transfer to a butanol/TCE organic phase can be achieved by adding NaCl and creating water-in-oil emulsions in the organic phase. The significant enhancement in reactivity is caused by a higher electron release (3e(-) per mole of Fe(0)) from Pd-NZVI in the butanol organic phase compared to the same reaction with TCE in the aqueous phase (2e(-) per mole of Fe(0)). XPS characterization studies of Pd-NZVI show Fe(0) oxidation to Fe(III) oxides for Pd-NZVI reacted with TCE in the butanol organic phase compared to Fe(II) oxides in the aqueous phase, which accounted for differences in the TCE reactivity extents and rates observed in the two phases.

Partition behavior of surfactants, butanol, and salt during application of density-modified displacement of dense non-aqueous phase liquids

Density-modified displacement (DMD) is a recent approach for removal of trapped dense NAPL (DNAPL). In this study, butanol and surfactant are contacted with the DNAPL to both reduce the density as well as release the trapped DNAPL (perchloroethylene: PCE). The objective of the study was to determine the distribution of each component (e.g., butanol, surfactant, water, PCE) between the original aqueous and PCE phases during the application of DMD. The results indicated that the presence of the surfactant increased the amount of n-butanol required to make the NAPL phase reach its desired density. In addition, water and anionic surfactant were found to partition along with the BuOH into the PCE phase. The water also found partitioned to reverse micelles in the modified phase. Addition of salt was seen to increase partitioning of surfactant to BuOH containing PCE phase. Subsequently, a large amount of water was solubilized into reverse micelles which lead to significantly increase in volume of the PCE phase. This work thus demonstrates the role of each component and the implications for the operation design of an aquifer treatment using the DMD technique.

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.

Effect of temperature on cosolvent flooding for the enhanced solubilization and mobilization of NAPLs in porous media

This paper examines the potential for enhanced NAPL recovery from the subsurface through the combined application of hot water and cosolvent flushing. Batch experiments were conducted to determine the effect of temperature on fluid properties and the multiphase behavior of the ethanol-water-toluene system and to assess the impact of temperature on the capillary, Bond and total trapping numbers and on flooding stability. Column flooding experiments were also conducted to evaluate toluene NAPL recovery efficiency for different ethanol contents and flushing solution temperatures. The ethanol content considered ranged from 20 to 100% by mass, while the flushing solution temperatures were varied from 10 to 40°C. It is shown that small variations in the system temperature can strongly influence the solubilization, mobilization and stability of the multiphase system, but that the impact of temperature on the enhanced NAPL recovery is also dependent on the ethanol content of the flushing solution. The impact of hot water on NAPL recovery was most pronounced at intermediate ethanol contents (40-60% by mass) where the increase in system temperature led to enhanced NAPL solubilization as well as NAPL mobilization. This study demonstrates that coupling of hot water with in situ cosolvent flooding is a potentially effective remedial alternative that can optimize NAPL recovery while reducing the amount of chemicals injected into the subsurface.

Liquid-liquid mass transfer of partitioning electron donors in chlorinated solvent source zones

A combination of batch and column experiments evaluated the mass transfer of two candidate partitioning electron donors (PEDs), n-hexanol (nHex) and n-butyl acetate (nBA), for enhanced bioremediation of trichloroethene (TCE)-dense nonaqueous phase liquid (DNAPL). Completely mixed batch reactor experiments yielded equilibrium TCE-DNAPL and water partition coefficients (KNW) for nHex and nBA of 21.7 ± 0.27 and 330.43 ± 6.7, respectively, over a range of initial PED concentrations up to the aqueous solubility limit of ca. 5000 mg/L. First-order liquid-liquid mass transfer rates determined in batch reactors with nBA or nHex concentrations near the aqueous solubility were 0.22 min(-1) and 0.11 min(-1), respectively. Liquid-liquid mass transfer under dynamic flow conditions was assessed in one-dimensional (1-D) abiotic columns packed with Federal Fine Ottawa sand containing a uniform distribution of residual TCE-DNAPL. Following pulse injection of PED solutions at pore-water velocities (vp) ranging from 1.2 to 6.0 m/day, effluent concentration measurements demonstrated that both nHex and nBA partitioned strongly into residual TCE-DNAPL with maximum effluent levels not exceeding 35% and 7%, respectively, of the applied concentrations of 4000 to 5000 mg/L. PEDs persisted at effluent concentrations above 5 mg/L for up to 16 and 80 pore volumes for nHex and nBA, respectively. Mathematical simulations yielded KNW values ranging from 44.7 to 48.2 and 247 to 291 and liquid-liquid mass transfer rates of 0.01 to 0.03 min(-1) and 0.001 to 0.006 min(-1) for nHex and nBA, respectively. The observed TCE-DNAPL and water mass transfer behavior suggests that a single PED injection can persist in a treated source zone for prolonged time periods, thereby reducing the need for, or frequency of, repeated electron donor injections to support bacteria that derive reducing equivalents for TCE reductive dechlorination from PED fermentation.

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.

Phase partitioning modeling of ethanol, isopropanol, and methanol with BTEX compounds in water

This study investigates the equilibrium phase partitioning behavior of ethanol, isopropanol, and methanol in a two-phase liquid-liquid system consisting of water and an individual BTEX (Benzene, Toluene, Ethylbenzene, and Xylenes) compound. A previously developed computer program is enhanced to generate ternary phase diagrams for analysis of each three-component cosolvent-nonaqueous phase liquid (NAPL)-water mixture combination. The required activity coefficients are estimated using the UNIFAC (Universal Quasichemical Functional group Activity Coefficient) model. The UNIFAC-derived ternary phase diagrams generally show good agreement against published experimental data, and similar phase partitioning behavior is observed for every BTEX compound in the presence of the same cosolvent. Furthermore, a set of laboratory experiments is conducted to determine the maximum single-phase water content for every mixture combination considered in this study where the volume composition of the cosolvent and the NAPL components is a blend of 85% alcohol and 15% BTEX compound. Comparison of experimentally-derived maximum single-phase water contents against UNIFAC-derived results shows good agreement for mixtures containing ethanol and methanol, but relatively poor agreement for mixtures containing isopropanol.

Surfactant-enhanced remediation of organic contaminated soil and water

Surfactant based remediation technologies for organic contaminated soil and water (groundwater or surface water) is of increasing importance recently. Surfactants are used to dramatically expedite the process, which in turn, may reduce the treatment time of a site compared to use of water alone. In fact, among the various available remediation technologies for organic contaminated sites, surfactant based process is one of the most innovative technologies. To enhance the application of surfactant based technologies for remediation of organic contaminated sites, it is very important to have a better understanding of the mechanisms involved in this process. This paper will provide an overview of the recent developments in the area of surfactant enhanced soil and groundwater remediation processes, focusing on (i) surfactant adsorption on soil, (ii) micellar solubilization of organic hydrocarbons, (iii) supersolubilization, (iv) density modified displacement, (v) degradation of organic hydrocarbon in presence surfactants, (vi) partitioning of surfactants onto soil and liquid organic phase, (vii) partitioning of contaminants onto soil, and (viii) removal of organics from soil in presence of surfactants. Surfactant adsorption on soil and/or sediment is an important step in this process as it results in surfactant loss reduced the availability of the surfactants for solubilization. At the same time, adsorbed surfactants will retained in the soil matrix, and may create other environmental problem. The biosurfactants are become promising in this application due to their environmentally friendly nature, nontoxic, low adsorption on to soil, and good solubilization efficiency. Effects of different parameters like the effect of electrolyte, pH, soil mineral and organic content, soil composition etc. on surfactant adsorption are discussed here. Micellar solubilization is also an important step for removal of organic contaminants from the soil matrix, especially for low aqueous solubility organic contaminants. Influences of different parameters such as single and mixed surfactant system, hydrophilic and hydrophobic chain length, HLB value, temperature, electrolyte, surfactant type that are very important in micellar solubilization are reviewed here. Microemulsion systems show higher capacity of organic hydrocarbons solubilization than the normal micellar system. In the case of biodegradation of organic hydrocarbons, the rate is very slow due to low water solubility and dissolution rate but the presence of surfactants may increase the bioavailability of hydrophobic compounds by solubilization and hence increases the degradation rate. In some cases the presence of it also reduces the rate. In addition to fundamental studies, some laboratory and field studies on removal of organics from contaminated soil are also reviewed to show the applicability of this technology.

Effects of initial saturation on properties modification and displacement of tetrachloroethene with aqueous isobutanol

Packed column experiments were conducted to study effects of initial saturation of tetrachloroethene (PCE) in the range of 1.0-14% pore volume (PV) on mobilization and downward migration of the non-aqueous phase liquid (NAPL) product upon contact with aqueous isobutanol ( approximately 10 vol.%). This study focused on the consequences of swelling beyond residual saturation. Columns were packed with mixtures of neat PCE, water and glass beads and waterflooded to establish a desired homogeneous residual saturation, and then flooded with aqueous isobutanol under controlled hydraulic conditions. Results showed a critical saturation of approximately 8% PV for these packed column experimental conditions. At low initial PCE saturations (<8% PV), experimental results showed reduced risk of NAPL-product migration upon contact with aqueous isobutanol. At higher initial PCE saturations (>8% PV), results showed NAPL-product mobilization and downward migration which was attributed to interfacial tension (IFT) reduction, swelling of the NAPL-product, and reduced density modification. Packed column results were compared with good agreement to theoretical predictions of NAPL-product mobilization using the total trapping number, N(T). In addition to the packed column study, preliminary batch experiments were conducted to study the effects of PCE volumetric fraction in the range of 0.5-20% on density, viscosity, and IFT modification as a function of time following contact with aqueous isobutanol ( approximately 10 vol.%). Modified NAPL-product fluid properties approached equilibrium within approximately 2 h of contact for density and viscosity. IFT reduction occurred immediately as expected. Measured fluid properties were compared with good agreement to theoretical equilibrium predictions based on UNIQUAC. Overall, this study demonstrates the importance of initial DNAPL saturation, and the associated risk of downward NAPL-product migration, in applying alcohol flooding for remediation of DNAPL contaminated ground water sites.

Dip-angle influence on areal DNAPL recovery by co-solvent flooding with and without pre-flooding

A two-dimensional (2D) laboratory model was used to study effects of gravity on areal recovery of a representative dense non-aqueous phase liquid (DNAPL) contaminant by an alcohol pre-flood and co-solvent flood in dipping aquifers. Recent studies have demonstrated that injection of alcohol and co-solvent solutions can be used to reduce in-situ the density of DNAPL globules and displace the contaminant from the source zone. However, contact with aqueous alcohol reduces interfacial tension and causes DNAPL swelling, thus facilitating risk of uncontrolled downward DNAPL migration. The 2D laboratory model was operated with constant background gradient flow and a DNAPL spill was simulated using tetrachloroethene (PCE). The spill was dispersed to a trapped, immobile PCE saturation by a water flood. Areal PCE recovery was studied using a double-triangle well pattern to simulate a remediation scheme consisting of an alcohol pre-flood using aqueous isobutanol ( approximately 10% vol.) followed by a co-solvent flood using a solution of ethylene glycol (65%) and 1-propanol (35%). Experiments were conducted with the 2D model oriented in the horizontal plane and compared to experiments at the 15 degrees and 30 degrees dip-angle orientations. Injection was applied either in the downward or upward direction of flow. Experimental results were compared to theoretical predictions for flood front stability and used to evaluate effects of gravity on areal PCE recovery. Sensitivity experiments were performed to evaluate effects of the alcohol pre-flood on PCE areal recovery. For experiments conducted with the alcohol pre-flood and the 2D model oriented in the horizontal plane, results indicate that 89-93% of source zone PCE was recovered. With injection oriented downward, results indicate that areal PCE recovery was 70-77% for a 15 degrees dip angle and 57-59% for a 30 degrees dip angle. With injection oriented upward, results indicate that areal PCE recovery was 57-60% at the 30 degrees dip angle, which was similar to PCE recovery for injection in the downward flow direction. Lower areal PCE recovery at greater dip angles in either direction of flow was attributed to DNAPL swelling and migration, flood front instabilities and bypassing of the displaced fluid past the extraction wells during the alcohol pre-flood. Additional results demonstrate that the use of an alcohol pre-flood can be beneficial in improving DNAPL recovery in the horizontal orientation, but pre-flooding may reduce areal recovery efficiency in dip-angle orientations. This study also demonstrates the use of theoretical perturbation (fingering) analysis in predicting NAPL recovery efficiency for flooding processes in remediating aquifers with dip angles.

Cosolvent-enhanced chemical oxidation of perchloroethylene by potassium permanganate

A laboratory study was conducted to examine cosolvent-enhanced in-situ chemical oxidation (ISCO) of perchloroethylene (PCE) using potassium permanganate (KMnO4). The conceptual basis for this new technique is to enhance permanganate oxidation of dense non-aqueous phase liquids (DNAPLs) with the addition of a cosolvent, thereby increasing DNAPL solubility while avoiding mobilization. Among 17 cosolvent candidates screened, tertiary butyl alcohol (TBA) and acetone were the most stable in the presence of KMnO4, both of which increased PCE aqueous solubility significantly, and therefore are suitable to be used as cosolvent in this study. Batch experiments indicated that the second-order rate constant for PCE oxidation by potassium permanganate was 0.043+/-0.002 M(-1) s(-1) in the purely aqueous (no cosolvent) solution. In the presence of 20% cosolvent (volume fraction=fc=0.2), the rate constant decreased to 0.036+/-0.003 M(-1) s(-1) with TBA and to 0.031+/-0.002 M(-1) s(-1) with acetone. However, in the presence of free-phase PCE, chloride ion concentration from PCE oxidation in acetone/water solutions (fc=0.2) was about twice that in aqueous solutions, indicating that the increase in PCE solubility more than compensated for the decrease in reaction rate constant, such that the oxidation efficiency of PCE was increased with cosolvent. A complete chlorine mass balance was observed in the aqueous system, whereas approximately 70% was obtained in TBA/water or acetone/water (fc=0.2). In soil columns containing residual DNAPL and subjected to isocratic flushing with step-wise increases in f(c) cosolvent, TBA at fc=0.2 resulted in PCE mobilization, whereas acetone at fc<or=0.5 did not. Therefore, although both TBA and acetone exhibit similar solubility enhancements, acetone may be a better solvent choice for use in in-situ remediation of DNAPL 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.

TCE recovery mechanisms using micellar and alcohol solutions: phase diagrams and sand column experiments

Forty-one phase diagrams and fifteen sand column experiments were conducted to evaluate the efficiency of three types of washing solutions to recover trichloroethylene (TCE) at residual saturation and to identify the recovery mechanisms involved. This study demonstrates that: (1) an alcohol and a surfactant combination is more efficient than an alcohol used alone in water; (2) the prediction of the dominant recovery mechanism from the tie line slopes in phase diagram is accurate and can be reproduced in sand column experiments; and (3) TCE recovery efficiency in sand column experiments is generally well represented by the position of the miscibility curve in phase diagrams in the low concentration range. However, the miscibility curve alone is not sufficient to exactly predict the TCE recovery mechanisms involved. Tie line slopes and the critical tie line have to be taken into consideration to select the active matter as well as its concentration and to predict the dominant recovery mechanism in sand column experiments. The sand column experiments quantified the recovery efficiency of each solution and identified the proportion of the recovery mechanisms (mobilisation vs. solubilisation). Washing solutions with an active matter concentration above the critical tie line caused dominating mobilisation. Mobilisation was also dominant when the active matter of the washing solution partitioned into the organic phase and the active matter concentration was below the critical tie line. Solubilisation and emulsification were dominant for washing solutions containing active matter, which principally partitioned into the aqueous phase and an active matter concentration below the critical tie line.