You are here:
EVALUATION OF SUB-MICELLAR SYNTHETIC SURFACTANTS VERSUS BIOSURFACTANTS FOR ENHANCED LNAPL RECOVERY
Impact/Purpose:
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 fifty 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. While 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 is dramatically reduced 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 has traditionally been viewed as a mechanism to enhance hydrocarbon biodegradation by increasing the apparent aqueous solubility of the hydrocarbon. However, there are several biosurfactants that generate low interfacial tensions between the hydrocarbon and the aqueous phases required to mobilize residual hydrocarbon. In particular, the lipopeptide biosurfacta 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 fifty 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. While 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 is dramatically reduced 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 has traditionally been viewed as a mechanism to enhance hydrocarbon biodegradation by increasing the apparent aqueous solubility of the hydrocarbon. However, there are several biosurfactants that generate low interfacial tensions between the hydrocarbon and the aqueous phases required to mobilize residual hydrocarbon. In particular, the lipopeptide biosurfact
Description:
Biosurfactants could potentially 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 interfacial tension (IFT) against light non-aqueous phase liquids (LNAPL). 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 LNAPL. 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 % concentration of the rhamnolipid and 3 w/w % 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% 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 critical micelle concentration (CMC) (0.001 w/w % or 0.019 mM) and critical microemulsion concentrations (CμC) (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 since the rhamnolipid concentration required in most applications is expected to be much lower than that of synthetic ionic surfactants.
Since 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 since they represent a wide range of hydrophobicity and EACN. In order 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 interfacial tension (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 were then 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. Due to 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 % for toluene and hexane. However, the hydrocarbon removal efficiency for decane and hexadecane was only 40 % 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 interfacial tension (IFT) against light non-aqueous phase liquids (LNAPL). 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 LNAPL. 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 in order 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 % 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.