Grantee Research Project Results
Final Report: Evaluation of Sub-micellar Synthetic Surfactants versus Biosurfactants for Enhanced LNAPL Recovery
EPA Grant Number: R827015C031Subproject: this is subproject number 031 , established and managed by the Center Director under grant R827015
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: HSRC (1989) - Northeast HSRC
Center Director: Sidhu, Sukh S.
Title: Evaluation of Sub-micellar Synthetic Surfactants versus Biosurfactants for Enhanced LNAPL Recovery
Investigators: Sabatini, David A. , McInerney, Michael , Knox, Robert
Institution: University of Oklahoma
EPA Project Officer: Aja, Hayley
Project Period: November 1, 2004 through January 31, 2006 (Extended to March 30, 2006)
RFA: Integrated Petroleum Environmental Consortium (IPEC) (1999) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Targeted Research
Objective:
The release of light nonaqueous phase liquids (LNAPLs) into the subsurface is a well-documented environmental concern. Past practices have led to extensive LNAPL groundwater contamination, mostly in the form of petroleum products. Potential for further contamination also is a concern when considering the presence of LNAPL sources in the subsurface (e.g., underground storage tanks and oil/gas pipelines). Surfactant enhanced aquifer remediation has emerged as one of the most technically effective and cost-competitive technologies for remediation of LNAPL contamination. This technology has applications to filling stations, refineries, and even active well sites. Although advances in surfactant chemistry dramatically have improved LNAPL removal efficiencies, the key to further improvements in the economic competitiveness of surfactant-based technologies is to reduce the mass of surfactant needed to recover the free-phase LNAPLs. Previous work by the investigators found that some biologically produced surface active agents, biosurfactants, can remove a large percentage of residual hydrocarbon from sand-packed columns at biosurfactant concentrations 10- to 100-fold lower than typically used for surfactant-enhanced LNAPL mobilization. The objective of this research project was to assess the relative technical and economic efficiency of synthetic surfactants versus biosurfactants for recovery of LNAPL contamination.
Summary/Accomplishments (Outputs/Outcomes):
Surfactant flushing of contaminated subsurface environments has emerged as one of the most technically effective and cost-competitive technologies for remediation of LNAPL contamination. Surfactants (surface active agents), commonly known as soaps or detergents, are amphiphilic molecules that have both water-like and oil-like regions to their molecule. Because they are amphiphilic, surfactants are surface active molecules, meaning that when they are placed in water-oil or water-air systems, they accumulate at the interface with their water-like region in the polar water phase and their oil-like region in the nonpolar oil or less polar air phase. In this way, both regions of the molecule are in a preferred phase and the free energy of the system is minimized.
When the aqueous surfactant concentration exceeds a certain level, surfactant molecules self-aggregate into spherical structures known as micelles, which contain 50 or more surfactant molecules. Micelles form when the surfactant concentration exceeds the critical micelle concentration (CMC). Micelle formation is unique to surfactant molecules and differentiates them from alcohols, which do not form such aggregates. Surfactant micelles increase the aqueous concentration of low-solubility organic compounds by providing a hydrophobic region into which organic compounds can partition. The micelle concentration increases with increasing surfactant concentrations above the CMC. The apparent solubility of the contaminant increases correspondingly. Surfactant concentrations well above the CMC (e.g., 10 to 20 times the CMC or more) are used to maximize contaminant solubility and extraction efficiency. The use of a single surfactant to enhance solubility is called solubilization. Although this is a fairly straightforward approach to enhancing NAPL dissolution, it may not be the most efficient approach. By using a mixture of surfactants, the water-NAPL interfacial tension (IFT) is reduced dramatically, which further improves the solubility of NAPL. Intentionally lowering the water-LNAPL interfacial tension to displace entrapped NAPL is a process called mobilization.
Biosurfactant production traditionally has been viewed as a mechanism to enhance hydrocarbon biodegradation by increasing the apparent aqueous solubility of the hydrocarbon. There are several biosurfactants, however, that generate low interfacial tensions between the hydrocarbon and the aqueous phases required to mobilize residual hydrocarbon. In particular, the lipopeptide biosurfactant produced by Bacillus species and the rhamnolipid produced by various Pseudomonas species reduce the interfacial tension between certain hydrocarbon and aqueous phases to very low levels (< 0.01 mN/m). In addition, the critical micelle concentrations are low, 20-50 mg/l, indicating that the biosurfactants are effective even at very low concentrations. Thus, surfactant enhanced subsurface technology is one of several innovative technologies that is being evaluated widely for remediation of subsurface NAPL spills. Recent advances have helped to make this technology economically viable even when using higher concentrations (1,000 to 40,000 mg/L) of synthetic surfactants. Recent work with biosurfactants, however, shows that these materials can produce similar removal efficiencies with much lower surfactant concentrations (approximately 1 to 2 orders of magnitude lower). Thus, the economics of this technology may be even more favorable using biosurfactants, while also improving the environmental friendliness of implementing this technology (biosurfactants versus synthetic surfactants).
The objective of this research is to assess the relative technical and economic efficiency of synthetic surfactants versus biosurfactants used to recover free-phase LNAPLs. Laboratory soil column tests were conducted to identify optimal solutions of synthetic surfactant and biosurfactant solutions for mobilizing LNAPLs. Until now, the interfacial activity and efficacy of recovering residual hydrocarbon only has been studied with individual biosurfactant compounds. Our work evaluated not only the efficacy of individual biosurfactants but also whether mixtures of different lipopeptides and/or rhamnolipids have enhanced interfacial activities.
Biosurfactants potentially could replace or be used in conjunction with synthetic surfactants to provide for more cost-effective subsurface remediation. To design effective biosurfactant/surfactant formulations, information about the surface-active agent and the targeted NAPL is required. With this information, we hypothesized that it is possible to formulate biosurfactant/surfactant mixtures that provide the appropriate hydrophobic/hydrophilic conditions to generate ultralow IFT against LNAPLs. Secondly, we hypothesized that mixtures of biosurfactants and/or synthetic surfactants will have enhanced properties, making them more effective than individual biosurfactants or synthetic surfactants for removal of entrapped LNAPLs. First, we tested the efficacy of biosurfactants from individual bacterial strains and mixtures of biosurfactants from different bacterial strains with and without synthetic surfactants for enhanced interfacial activity. One type of biosurfactant that has proven effective in recovering entrapped oil from sand or sandstone laboratory model systems is the lipopeptide biosurfactants made by various Bacillus species. Multiple regression analysis showed that the interfacial activity of various lipopeptides against toluene depended on the fatty acid composition present in the lipopeptide. Specifically, the relative proportions of 3-hydroxy-fatty acids with carbon chain lengths of 14, 15, 16, and 18. A heterogeneous fatty acid composition was more effective than a homogeneous composition in lowering the IFT against toluene. The multiple regression model allowed us to predict the interfacial activity against toluene for different lipopeptide biosurfactants for their fatty acid composition.
To test the hypothesis that biosurfactant mixtures provide the appropriate hydrophobic/hydrophilic conditions to achieve ultralow interfacial tensions (< 0.1 mN/m), lipopeptide biosurfactants with the more hydrophilic, rhamnolipid biosurfactant were prepared. Toluene has an equivalent alkane number (EACN) of 1 and is relatively hydrophilic compared to hydrocarbons with higher EACN. Because of the hydrophilic nature of toluene, we hypothesized that a mixture of a hydrophilic biosurfactant (rhamnolipid) and a more hydrophobic biosurfactant (lipopeptide) would be required to achieve ultralow IFT. The IFT against toluene decreased to ultralow levels when the rhamnolipid was mixed with lipopeptides with appropriate hydrophobic/hydrophilic fatty acid composition. Alone, neither the lipopeptide nor the rhamnolipid biosurfactants produced ultralow interfacial tensions against toluene. Hexane and decane have EACN values of 6 and 8, respectively, and are thus more hydrophobic than toluene. To obtain ultralow IFT against these hydrocarbons, the surfactant mixture must contain relatively hydrophobic surfactants. This prediction was tested by using mixtures of lipopeptide biosurfactants with the more hydrophobic synthetic surfactant, C12, C13-8PO sulfate. This mixture had ultralow IFT against hexane and decane. In general, we found that lipopeptide biosurfactants with a heterogeneous fatty acid composition or mixtures of lipopeptide and rhamnolipid biosurfactants effectively lowered the IFT against hydrophilic LNAPL. Conversely, mixtures of lipopeptide biosurfactants with more hydrophobic synthetic surfactants effectively lowered the IFT against hydrophobic LNAPL.
The interfacial properties of the rhamnolipid biosurfactant against several hydrocarbons and the efficiency of rhamnolipid biosurfactant and synthetic surfactant mixtures to improve the interfacial activity of the surfactant system against these hydrocarbons were further investigated. Ultralow IFT of 0.03 mN/m for toluene was observed for toluene with a 0.01 w/w percent concentration of the rhamnolipid and 3 w/w percent NaCl. The IFT for hexane, decane, and hexadecane was higher than 0.5 mN/m and remained fairly constant regardless of the NaCl concentration. These data show that the rhamnolipid is hydrophilic. The rhamnolipid formed microemulsions with toluene and, at a fixed rhamnolipid concentration, the microemulsion transitioned from a Winsor Type I to III to II microemulsion as the NaCl concentration increased. The IFT decreased to a minimum within the Type III region and then increased with further increases in salinity. The point at which the IFT between the middle phase and the excess water phase is the same as the IFT between the middle phase and the excess oil phase is called optimum formulation, and the electrolyte concentration at this condition is called optimal salinity (S*), which was about 12 percent NaCl for the rhamnolipid. Ultralow IFT (≤ 0.1 mN/m) was achieved within this three-phase region. At a fixed electrolyte concentration, the volume of the middle phase increased with increasing rhamnolipid concentration. The solubilization parameter value (milliliters of oil solubilized per gram of surfactant) with 1.0 w/w% rhamnolipid was 15.49 mL of toluene per gram of rhamnolipid. Temperature did not have a significant impact on the phase behavior of the rhamnolipid. The concentrations of the rhamnolipid where the first and second sharp reductions in IFT occurred are the CMC (0.001 w/w% or 0.019 mM) and critical microemulsion concentrations (0.01 w/w% or 0.1884 mM), respectively. The CMC of the rhamnolipid is lower than the CMC of most of conventional synthetic ionic surfactants. This is an advantage as the rhamnolipid concentration required in most applications is expected to be much lower than that of synthetic ionic surfactants.
Because the rhamnolipid biosurfactant proved to be relatively hydrophilic, we hypothesized that mixtures of rhamnolipid biosurfactants and synthetic surfactants would produce ultralow IFT against more hydrophobic hydrocarbons than those generated by the rhamnolipid alone. Three alkyl propoxylated sulfate synthetic surfactants were tested in mixtures with rhamnolipid. The alkyl propoxylated sulfates were C12, C13-8PO sulfate; C16-10.7PO sulfate; and C16-18PO-2EO sulfate (in the order of increasing hydrophobicity). Four hydrocarbons, toluene, hexane, decane, and hexadecane, were studied as they represent a wide range of hydrophobicity and EACN. To find desired bio/surfactant formulation for each hydrocarbon, the properties of both bio/surfactants and hydrocarbons were measured. The results showed that hydrocarbons of different EACN required surfactant formulations tailored to provide hydrophobic/hydrophilic conditions and that the addition of a more hydrophobic synthetic surfactant was further reduced the IFT against a more hydrophobic hydrocarbon. The mixtures were found to be able to decrease the IFT for all hydrocarbons by one to two orders of magnitude and, in some cases, ultralow IFT values of less than 0.1 mN/m were obtained, which is highly desirable for environmental remediation and enhanced oil recovery.
Rhamnolipid/synthetic surfactant formulations then were tested
to determine if they had deleterious properties that would impair their use
in field situations. Surfactant properties such as phase separation,
precipitation, foam stability, and adsorption were tested for these surfactant
mixtures before they were used in column experiments. Neither phase separation
nor precipitation was observed at the operating conditions. These surfactant
formulations showed low foam stability (< 100 mm) as compared to conventional
synthetic surfactants and had negligible adsorption to the porous sand matrix. Because
of the low IFT produced by mixtures, we hypothesized that mixtures of rhamnolipid
and synthetic surfactants would have higher hydrocarbon removal efficiencies
than the rhamnolipid alone in sand-packed columns with residual hydrocarbon
saturations. Three alkyl propoxylated sulfate synthetic surfactants were
tested in mixtures with rhamnolipid biosurfactant: C12, C13-8PO sulfate;
C16-10.7PO sulfate; and C16-18PO-2EO sulfate. Four hydrocarbons were
studied: toluene, decane, hexane, and hexadecane. In previous batch
studies, we found rhamnolipid/synthetic surfactant mixtures produced ultralow
IFT values for all four hydrocarbons. The rhamnolipid/C12, C13-8PO sulfate
mixture and the rhamnolipid/C16-10.7PO sulfate mixture achieved high hydrocarbon
removal efficiency, greater than 85 percent for toluene and hexane. The
hydrocarbon removal efficiency for decane and hexadecane, however, was only
40 percent with the rhamnolipid/C16-18PO-2EO sulfate mixture even though the
IFTs were ultralow. In the future work, the use of a polymer to increase
viscosity and improve mobility control may be needed to improve the removal
efficiency for decane and hexadecane.
In summary, we found that it is possible to formulate biosurfactant/synthetic
surfactant mixtures that provide the appropriate hydrophobic/hydrophilic conditions
to generate ultralow IFT against LNAPLs. Mixtures of biosurfactants and/or
synthetic surfactants demonstrated enhanced interfacial properties making them
more effective than individual biosurfactants or synthetic surfactants for
removal of entrapped LNAPLs. Information on the relative hydrophobicity/hydrophilicity
of the biosurfactants and synthetic surfactants were essential in tailoring
formulations effective against specific hydrocarbons. An important finding
was that the fatty acid composition of lipopeptides is important for interfacial
activity. The problem is that the fatty acid composition varies from
batch to batch so it is important that the fatty acid composition be determined
for each batch to formulate effective surfactant formulations. The rhamnolipid/synthetic
surfactant mixtures produced ultralow IFT values for all four hydrocarbons
tested. The rhamnolipid/C12, C13-8PO sulfate mixture and the rhamnolipid/C16-10.7PO
sulfate mixture achieved high hydrocarbon removal efficiency, greater than
85 percent for toluene and hexane. Problems with phase separation, precipitation,
adsorption, and foaming for the rhamnolipid/synthetic surfactant mixtures were
negligible. The data indicate that the addition of biosurfactants reduced
the amount of synthetic surfactant required for interfacial activity, suggesting
that biosurfactant/synthetic surfactant mixtures may be an economic approach
for subsurface remediation. These results thus encourage the further
evaluation and eventual field testing of biosurfactant-based systems for surfactant-enhanced
aquifer remediation.
Supplemental Keywords:
LNAPL, light non-aqueous phase liquids, surfactants, biosurfactants, hydrocarbons, groundwater contamination, groundwater remediation, surfactant-based technologies, synthetic surfactants, rhamnohipdis, toluene,, RFA, Scientific Discipline, Waste, TREATMENT/CONTROL, Waste Treatment, Remediation, Environmental Chemistry, Hazardous Waste, Environmental Monitoring, Ecological Risk Assessment, Hazardous, Environmental Engineering, hazardous waste management, hazardous waste treatment, risk assessment, advanced treatment technologies, petroleum contaminated soil, biosurfactant, petroleum contaminants, cleanup, remediation technologies, LNAPL, petrochemical waste, treatment, hazadous waste streams, hydrocarbons, technology transfer, aqueous waste streams, aquifer remediationRelevant Websites:
None.
Progress and Final Reports:
Original AbstractMain Center Abstract and Reports:
R827015 HSRC (1989) - Northeast HSRC Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R827015C001 Evaluation of Road Base Material Derived from Tank Bottom Sludges
R827015C002 Passive Sampling Devices (PSDs) for Bioavailability Screening of Soils Containing Petrochemicals
R827015C003 Demonstration of a Subsurface Drainage System for the Remediation of Brine-Impacted Soil
R827015C004 Anaerobic Intrinsic Bioremediation of Whole Gasoline
R827015C005 Microflora Involved in Phytoremediation of Polyaromatic Hydrocarbons
R827015C006 Microbial Treatment of Naturally Occurring Radioactive Material (NORM)
R827015C007 Using Plants to Remediate Petroleum-Contaminated Soil
R827015C008 The Use of Nitrate for the Control of Sulfide Formation in Oklahoma Oil Fields
R827015C009 Surfactant-Enhanced Treatment of Oil-Contaminated Soils and Oil-Based Drill Cuttings
R827015C010 Novel Materials for Facile Separation of Petroleum Products from Aqueous Mixtures Via Magnetic Filtration
R827015C011 Development of Relevant Ecological Screening Criteria (RESC) for Petroleum Hydrocarbon-Contaminated Exploration and Production Sites
R827015C012 Humate-Induced Remediation of Petroleum Contaminated Surface Soils
R827015C013 New Process for Plugging Abandoned Wells
R827015C014 Enhancement of Microbial Sulfate Reduction for the Remediation of Hydrocarbon Contaminated Aquifers - A Laboratory and Field Scale Demonstration
R827015C015 Locating Oil-Water Interfaces in Process Vessels
R827015C016 Remediation of Brine Spills with Hay
R827015C017 Continuation of an Investigation into the Anaerobic Intrinsic Bioremediation of Whole Gasoline
R827015C018 Using Plants to Remediate Petroleum-Contaminated Soil
R827015C019 Biodegradation of Petroleum Hydrocarbons in Salt-Impacted Soil by Native Halophiles or Halotolerants and Strategies for Enhanced Degradation
R827015C020 Anaerobic Intrinsic Bioremediation of MTBE
R827015C021 Evaluation of Commercial, Microbial-Based Products to Treat Paraffin Deposition in Tank Bottoms and Oil Production Equipment
R827015C022 A Continuation: Humate-Induced Remediation of Petroleum Contaminated Surface Soils
R827015C023 Data for Design of Vapor Recovery Units for Crude Oil Stock Tank Emissions
R827015C024 Development of an Environmentally Friendly and Economical Process for Plugging Abandoned Wells
R827015C025 A Continuation of Remediation of Brine Spills with Hay
R827015C026 Identifying the Signature of the Natural Attenuation of MTBE in Goundwater Using Molecular Methods and "Bug Traps"
R827015C027 Identifying the Signature of Natural Attenuation in the Microbial
Ecology of Hydrocarbon Contaminated Groundwater Using Molecular Methods and
"Bug Traps"
R827015C028 Using Plants to Remediate Petroleum-Contaminated Soil: Project Continuation
R827015C030 Effective Stormwater and Sediment Control During Pipeline Construction Using a New Filter Fence Concept
R827015C031 Evaluation of Sub-micellar Synthetic Surfactants versus Biosurfactants for Enhanced LNAPL Recovery
R827015C032 Utilization of the Carbon and Hydrogen Isotopic Composition of Individual Compounds in Refined Hydrocarbon Products To Monitor Their Fate in the Environment
R830633 Integrated Petroleum Environmental Consortium (IPEC)
R830633C001 Development of an Environmentally Friendly and Economical Process for Plugging Abandoned Wells (Phase II)
R830633C002 A Continuation of Remediation of Brine Spills with Hay
R830633C003 Effective Stormwater and Sediment Control During Pipeline Construction Using a New Filter Fence Concept
R830633C004 Evaluation of Sub-micellar Synthetic Surfactants versus Biosurfactants for Enhanced LNAPL Recovery
R830633C005 Utilization of the Carbon and Hydrogen Isotopic Composition of Individual Compounds in Refined Hydrocarbon Products To Monitor Their Fate in the Environment
R830633C006 Evaluation of Commercial, Microbial-Based Products to Treat Paraffin Deposition in Tank Bottoms and Oil Production Equipment
R830633C007 Identifying the Signature of the Natural Attenuation in the Microbial Ecology of Hydrocarbon Contaminated Groundwater Using Molecular Methods and “Bug Traps”
R830633C008 Using Plants to Remediate Petroleum-Contaminated Soil: Project Continuation
R830633C009 Use of Earthworms to Accelerate the Restoration of Oil and Brine Impacted Sites
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.