Nonaqueous Phase Liquid Mobilization Research Articles

Influence of aqueous film forming foams on the solubility and mobilization of non-aqueous phase liquid contaminants in quartz sands

At sites where aqueous film forming foams (AFFFs) are used for fire suppression or training activities, interactions between dissolved foam constituents and organic liquids could alter contaminant migration in the subsurface. In this study, batch reactor and column experiments were conducted to investigate the potential for AFFF solutions to enhance the solubility and mobility of three representative non-aqueous phase liquid (NAPLs), JP-4 jet fuel, trichloroethene (TCE), and tetrachloroethene (PCE). For AFFF concentrations up to 5% wt. (50,000 mg/L), aqueous solubilities of TCE and PCE increased by less than 50%, indicating the absence of micellar solubilization. However, NAPL-water interfacial tensions were reduced to less than 1.5 mN/m and resulted in accumulation of up to 2.25 mg/m2 of AFFF at the NAPL-water interface. To assess the potential for AFFF to mobilize residual (entrapped) NAPL at a field application rate of 3% wt. (30,000 mg/L), columns were packed with two size fractions of Ottawa sands (20–30 mesh and 60–80 mesh) that yielded residual NAPL saturations ranging from 11.7 to 17.6%. Following injection of 3 pore volumes of the 3% wt. AFFF solution, partial mobilization of residual NAPL was observed for PCE, TCE, and JP-4, with saturation reductions of 0.7 to 2% in 20–30 mesh and 0.3% to 1.3% in 60–80 mesh Ottawa sand. The columns were then flushed with an ultralow-IFT surfactant solution consisting of 4% wt. 1:1 Aerosol AY/OT, which resulted in nearly complete mobilization of the remaining residual NAPL. When NAPL desaturation curves were expressed in terms of the total trapping number (NT), the threshold value of NT required for NAPL mobilization by 3% wt. AFFF was approximately 2 × 10−5, consistent with previous studies. These findings demonstrate that AFFF solutions can lead to partial mobilization of residual NAPL, and that the total trapping number concept can be used to predict such behavior a priori. In addition, the observed IFT reductions are sufficient to alter NAPL accumulation and redistribution behavior in the subsurface through enhanced spreading above the water table (e.g., JP-4) or penetration into lower-permeability media (e.g., TCE, PCE).

Mobilization of n-hexadecane in porous media using food grade amphiphiles

A model liquid hydrocarbon (n-hexadecane) was used to gain insight on the mobilization of non-aqueous phase liquids (NAPLs) trapped in porous formations, important for both enhanced oil recovery (EOR) and oil spill remediation in porous media. Food-grade surfactants lecithin from soy and tween 80 were compared to commercial dispersant Corexit 9500A, for their ability to mobilize the hydrocarbon originally trapped in a water-filled cryolite porous medium. Red dye was added to the n-Hexadecane to improve visualization, and the aqueous phase incorporated different ratios of the surfactants Lecithin and Tween 80 to seek synergistic benefits when the two surfactants are combined. Visual-microscopic flooding experiments carried on a miniature packed bed produced images which were then analyzed using grayscale image analysis. It was determined that a ratio 40:60 by weight of Lecithin to Tween 80 has the best performance in the mobilization of n-hexadecane from the porous media. Furthermore, any (non-optimal) mixture of lecithin and tween 80 exhibited a better (synergistic) performance in n-Hexadecane recovery from the porous media than when applying either surfactant separately.

Micro-scale displacement of NAPL by surfactant and microemulsion in heterogeneous porous media

Industrial processes such as remediation of oil-contaminated aquifers and enhanced oil recovery (EOR) often utilize chemical additives to increase the removal of non-aqueous phase liquids (NAPLs) from subsurface formations. Although the majority of crude oils are classified as LNAPLs, they often contain heavy molecules (DNAPLs) such as asphaltenes that tend to adsorb on minerals and alter their wettability. Effective additives are therefore those that can reduce the threshold capillary pressure, thus mobilizing LNAPL inside pore spaces and solubilizing DNAPL from rock surfaces. Nonionic surfactants in brine have often been injected to oil or contaminated aquifer formations in order to enhance NAPL displacement through IFT reduction. Recent studies revealed that surfactant-based microemulsions have a higher tendency to alter the wettability of surfaces, compared to surfactants alone, leading to more effective NAPL removal. However, the impact of these additives on pore-scale displacement mechanisms and multi-phase fluid occupancy in porous media is, to date, still unclear. In this study, x-ray microtomography experiments were performed to investigate the impact of surfactants and microemulsions on the mobilization and solubilization of NAPL in heterogeneous rocks. Saturation profiles indicated that an incremental NAPL removal was attained by addition of microemulsion to brine, compared with surfactant. Residual cluster size distributions revealed that microemulsions could break up large clusters into smaller disconnected ones, improving their mobilization in the rock. In-situ contact angle measurements showed that microemulsions could reverse the wettability of rough contaminated surfaces to a higher extent than surfactants. Unlike surfactant alone, the surfactant-solvent blend in the carrier fluid of microemulsions was able to penetrate rough grain surfaces, particularly those of dolomite cement, and desorb asphaltenes in the form of small-emulsified NAPL droplets, which were eventually washed away by the continuous flow process. The greater wettability alteration caused by microemulsions resulted in a lower threshold capillary pressure, which in turn promoted the mobilization of NAPL ganglia more than surfactant alone.

Optimizing Ethanol Enhanced NAPL Remediation Using Evolutionary Algorithms

In the past decades, subsurface non-aqueous phase liquid (NAPL) contamination has been recognized as one of the most widespread and challenging environmental problems. Thus, researchers have focused their efforts on developing and testing the efficiency of remediation methodologies, able to address the unique nature of these contaminants. Recently, in-situ flooding techniques for the accelerated removal of NAPLs trapped in the subsurface have been proposed, where additives are injected together with water upgradient of the NAPL-contaminated area in order to alter the physio-chemical properties of the contaminants, such as interfacial tension, and enhance their solubilities. In this work, the efficiency of ethanol enhanced NAPL remediation is addressed. To this end, a non-linear, multi-objective optimization strategy is developed by combining a multiphase flow simulation model with evolutionary algorithms. Two conflicting optimization objectives are considered: minimizing operation cost and maximizing remediation efficiency, while preventing uncontrolled NAPL mobilization. More specifically, the first objective involves the operation cost of the procedure, which is directly proportional to the pumping rate, duration and ethanol volume used. The second represents the environmental considerations of the problem that, in this work, are described by the maximization of free product removal and the prevention of DNAPL vertical spreading.

Development of a Laboratory Procedure to Evaluate the Consolidation Potential of Soft Contaminated Sediments

Abstract Consolidation settlement of non-aqueous phase liquid (NAPL) contaminated sediments may trigger NAPL mitigation. The consolidation potential and resulting NAPL mobilization of the sediments should be evaluated in the laboratory; however, due to the highly compressible and weak nature of riverbed sediments, it is usually not possible to conduct conventional consolidation tests on sediment specimens. In this study, a triaxial setup was modified to work effectively under low stresses. Kaolinite was used to represent the soil solid phase and Soltrol 130 (a type of mineral oil) was used to represent the NAPL. Both oil-wetted and water-wetted regimes were analyzed. Hexane Extraction and moisture content tests results confirmed the final fluid amounts in the specimen obtained by measuring the effluent volume during consolidation. The results of the tests show that approximately 0.1 g of NAPL per 1 g of soil solids is unlikely to be mobilized by consolidation. The developed procedure could also be employed to define the mobile and immobile fractions of NAPL and the expected compression of contaminated sediments. The volume of NAPL in excess of the retained residual can be used to design NAPL collection systems or to size layers of NAPL sorbent materials such as organo-clays.

Pore Water Characteristics Following a Release of Neat Ethanol onto Pre‐existing NAPL

AbstractNeat ethanol (75.7 L) was released into the upper capillary zone in a continuous‐flow, sand‐packed aquifer tank (8.2 m3) with an average seepage velocity of 0.75 m/day. This model aquifer system contained a residual nonaqueous phase liquid (NAPL) that extended from the capillary zone to 10 cm below the water table. Maximum aqueous concentrations of ethanol were 20% v/v in the capillary zone and 0.08% in the saturated zone at 25 and 30 cm downgradient from the emplaced NAPL source, respectively. A bench‐scale release experiment was also conducted for a similar size spill (scaled to the plan area). The concentrations of ethanol in ground water for both the bench‐ and pilot‐scale experiments were consistent with advective–dispersive limited mass transfer from the capillary to the saturated zone. Concentrations of monoaromatic hydrocarbons and isooctane increased in the pore water of the capillary zone as a result of both redistribution of residual NAPL (confirmed by visualization) and enhanced hydrocarbon dissolution due to the cosolvent effect exerted by ethanol. In the tank experiment, higher hydrocarbon concentrations in ground water were also attributed to decreased hydrocarbon biodegradation activity caused by preferential microbial utilization of ethanol and the resulting depletion of oxygen. These results infer that spills of highly concentrated ethanol will be largely confined to the capillary zone due to its buoyancy, and ethanol concentrations in near‐source zone ground water will be controlled by mass transfer limitations and hydrologic conditions. Furthermore, highly concentrated ethanol releases onto pre‐existing NAPL will likely exacerbate impacts to ground water, due to NAPL mobilization and dissolution, and decreased bioattenuation of hydrocarbons.

Visualization of surfactant‐enhanced nonaqueous phase liquid mobilization and solubilization in a two‐dimensional micromodel

Surfactant‐enhanced aquifer remediation is an emerging technology for aquifers contaminated with nonaqueous phase liquids (NAPLs). A two‐dimensional micromodel and image capture system were applied to observe microscale NAPL mobilization and solubilization phenomena. In each experiment a common residual NAPL field was established, followed by a series of mobilization and solubilization experiments. Mobilization floods included pure water floods with variable flow rates and surfactant floods with variations in surfactant formulations. At relatively low capillary numbers (Nca < 10−3) the surfactant mobilization floods resulted in higher NAPL saturations than for similar Nca pure water floods. These differences in macroscopic saturations are explained by differences in microscale mobilization processes. Sqlubilization of the residual NAPL remaining after the mobilization stage was dominated by the formation of microscale dissolution fingers, which produced nonequilibrium macroscale NAPL solubilization. A macroemulsion phase also was observed to form spontaneously and persist during the solubilization stage of the experiments.

Field test of high molecular weight alcohol flushing for subsurface nonaqueous phase liquid remediation

A pilot scale field test of non‐aqueous phase liquid (NAPL) removal using high molecular weight alcohols was conducted at Operable Unit 1, Hill Air Force Base, Utah. Petroleum hydrocarbons and spent solvents were disposed of in chemical disposal pits at this site, and these materials are now present in the subsurface in the form of a light non‐aqueous phase liquid (LNAPL). This LNAPL is a complex mixture of aromatic and aliphatic hydrocarbons, chlorinated solvents, and other compounds. The field experiment was performed in a 5 m by 3 m confined test cell, formed by driving interlocking sheet pile walls through the contaminated zone into an underlying clay. The test involved the injection and extraction of about four pore volumes (1 pore volume=7000 L) of a mixture of 80% tert‐butanol and 15% n‐hexanol. The contaminants were removed by a combination of NAPL mobilization and enhanced dissolution, and the results of postflood soil coring indicate better than 90% removal of the more soluble contaminants (trichloroethane, toluene, ethylbenzene, xylenes, trimethylbenzene, naphthalene) and 70–80% removal of less soluble compounds (decane and undecane). The results of preflood and postflood NAPL partitioning tracer tests show nearly 80% removal of the total NAPL content from the test cell. The field data suggest that a somewhat higher level of removal could be achieved with a longer alcohol injection.