ARTICLE pubs.acs.org/IECR
Effects of Surfactants on Solubilization of Perchloroethylene (PCE) and Trichloroethylene (TCE) Sivaram Harendra*,† and Cumaraswamy Vipulanandan† †
Department of Civil and Environmental Engineering, University of Houston, Houston, Texas 77204-4003, United States ABSTRACT: Enhanced solubilization of dense nonaqueous phase liquids (DNAPLs) especially chlorinated hydrocarbons using four types of surfactants was investigated. The solubilization kinetics of perchloroethylene PCE (100 mg/L solubility in water) and trichloroethylene TCE (1000 mg/L) in anionic (sodium dodecyl sulfate (SDS)), nonionic (Triton X-100), cationic (cetyltrimethylammonium bromide (CTAB)), and a biosurfactant (UH biosurfactant) were investigated at room temperature in continuously stirred batch reactors. The size distribution of surfactant micelles were measured using the dynamic light scattering device (DLS). The mean size of SDS, CTAB, Triton X-100, and UH biosurfactant micelles were 4.2 nm, 3.8 nm, 4.5 nm, and 59.1 nm, respectively. Micelle partition coefficients (Km) and molar solubility ratio (MSR) for PCE and TCE in 10 g/L of surfactant solutions have been quantified, and the solubility of PCE and TCE in the surfactant solutions increased by about 10-fold. Solubilization kinetics for PCE and TCE in various surfactant solutions was represented using a hyperbolic relationship. Also the relationship between solubility and interfacial surface tension reduction was investigated. Of the surfactants studied, Triton X-100 had the highest PCE solubilized per gram of surfactant, whereas for TCE, biosurfactant had the highest TCE solubility per gram of surfactant.
1. INTRODUCTION The prevalence of dense nonaqueous organic phase liquids (DNAPLs) at contaminated groundwater sites across the United States is well documented.1,2 DNAPLs, especially chlorinated solvents, present in the subsurface represent a threat to groundwater supply, and removing these materials from the ground has been the focus of considerable attention over the last 20 years.35 In order to remove the DNAPLs from the ground and/or degrade them in industrial wastewaters, it is important to increase their solubility in water.68 Many industries regularly use large quantities of PCE and TCE as solvents. Given this high frequency of use, handling, and transportation, along with past disposal and storage practices, chlorinated DNAPL compounds now represent a significant threat to soil and groundwater resources. These are halogenated organic compounds that have been widely used as an industrial cleaning solution and a universal degreasing agent. It is used as an extraction solvent for greases, oils, fats, waxes, and tars, and as a refrigerant. Major environmental releases of PCE and TCE are from air emissions from metal degreasing plants. Wastewater from metal finishing, paint and ink production, electrical components, and rubber processing industries also contains chlorinated solvents. When present in water in sufficient quantity, PCE and TCE may form dense nonaqueous phase liquids (DNAPLs), which are minimally soluble in water. The relatively slow dissolution of nonaqueous phase liquid contaminants into groundwater has prompted investigation of nonionic and other surfactants as potential solubilizing agents for enhanced dissolution and removal of NAPL from the subsurface.9 A surfactant molecule is amphiphilic, having two distinct structural moieties, one polar and other nonpolar. The polar moiety of the molecule has an affinity for water and other polar substances, while the nonpolar moiety is hydrophobic. As a r 2011 American Chemical Society
result of its amphiphilic nature, a surfactant molecule may dissolve in water as a monomer, adsorb at an interface, or be incorporated with other surfactant molecules as part of a micelle.10 Surfactants have been shown to increase the removal of contaminants from laboratory soil columns. Surfactants increase contaminant removal either by increasing the apparent solubility of the contaminant through solubilization or by mobilizing residual or pooled NAPL through interfacial tension reduction.11 Anionic surfactant (sodium dodecyl sulfate (SDS)), nonionic surfactant (Triton X-100), cationic surfactant (cetyltrimethylammonium bromide (CTAB)), and a biosurfactant (UH biosurfactant)1214 were selected in this study. The above-mentioned surfactants are both widely used in industry and studied by many researchers.1518 Triton X-100 increases the mass transfer coefficient for organic solute desorption from soil contaminated with trichloroethylene (TCE). CTAB was proposed for enhanced sorption/enhanced solubilization groundwater remediation and for removal of hydrophobic organic contaminants (HOCs) from contaminated aquifers.19 The amount of oil that a surfactant can solubilize is related to the reduction of the interfacial tension (IFT), given by ChunHuh equation.21 According to the equation as solubilization ratio increases, the IFT decreases. A surfactant, when present at low concentration in a system, has the property of adsorbing onto the surfaces of the system and altering the surface free energies of the system to a marked degree.20 The term “interface” indicates a boundary between any two immiscible phases, while the term “surface” denotes an interface where one phase is gas. Chemical surfactants are chemically synthesized surfactants, while biosurfactants Received: December 28, 2010 Accepted: March 23, 2011 Revised: March 18, 2011 Published: March 23, 2011 5831
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Industrial & Engineering Chemistry Research are surface-active compounds that are produced by various microorganisms.13,14 The presence of the hydrophobic group causes the distortion of liquid water structure, resulting in an increase in the free energy of the system. The increase in free energy implies less work is needed to bring a surfactant molecule to the surface than to bring a water molecule to the surface, resulting in excess surfactant molecules on the surface. The presence of surfactant molecules on the surface decreases the work needed to create new surface area, thus decreasing the surface tension. On the other hand, the presence of the hydrophilic group prevents the surfactant from being expelled completely from the solvent as a separate phase, since that would require dehydration of the hydrophilic group. Depending upon the nature of the hydrophilic group, surfactants are classified.2027
2. OBJECTIVE The overall objective of this study was to investigate the solubilization of PCE and TCE in various types of surfactants. The specific objectives were as follows: 1) Investigate the effect of various surfactants (anionic, cationic, nonionic, and biosurfactant) on the rate and total solubilization of PCE and TCE and in water solution. 2) Model the kinetics of PCE and TCE solubilization at room temperature. 3. THEORY Two very different technologies have been proposed that utilize surfactants to remove dense nonaqueous phase liquids (DNAPLs) from porous media. One is based on the increased solubilization that occurs in the presence of surfactant micelles and the other one mobilization of residual liquids trapped in porous media by capillary forces.18 Mobilization technologies are limited to sites where the flow of mobilized liquid can be controlled, and the potential for migration through layers that previously acted as capillary barriers is extremely small. Solubilization technologies generally pose less risk with regard to uncontrolled migration and are less complex to design. Thus there is a need for more information on surfactants that are capable of solubilizing DNAPLs and removing them as single phase microemulsions. Addition of medium chain length alcohols to surfactant solutions often enhances solubilization of chlorinated solvents and aids in the formation of microemulsion. A nanoemulsion is an optically transparent dispersion of liquid droplets ( CTAB (CMC=0.4 g/L) > UH biosurfactant (CMC=0.7 g/L) > SDS (CMC=2.3 g/L). The nonionic and cationic surfactants had greater potential for solubilizing PCE than anionic surfactant. The solubilization of TCE in various concentration of surfactant is shown in Figure 3. The maximum solubility was reached within 30 min as shown in Figure 4. Typical solubility (S)time (t) relationship for PCE and TCE with surfactant solutions at room temperature is shown in Figure 5. The apparent solubility of TCE increased linearly with the surfactant concentration above CMC. Of the surfactants used, for a surfactant concentration of 10 g/L, UH biosurfactant solubilized up to 8250 mg/L compared to SDS, Trion X-100, and CTAB that solubilized 5000 mg/L, 7110 mg/L, and 6500 mg/L respectively. The solubilization of TCE by SDS and Triton X-100 has been reported up to 10 g/L of surfactants.12 In the present study Triton X-100 solubilized more TCE compared to SDS surfactants unlike the trend reported.12 5833
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Table 1. Solubilization Kinetic Parameters A and B for PCE and TCE A
B
R2
chlorinated solvent
surfactant
PCE
UH biosurfactant
0.0040
0.00078
0.98
PCE PCE
CTAB SDS
0.0034 0.0047
0.00076 0.00081
0.98 0.97
PCE
Triton
0.0030
0.00074
0.99
TCE
UH biosurfactant
0.00115
0.00121
0.996
TCE
CTAB
0.000314
0.000142
0.996
TCE
SDS
0.000216
0.000187
0. 990
TCE
Triton
0.000161
0.000139
0.999
Based on preliminary investigation (where S > 0, dS/dt > 0, and d2S/dt2 < 0 (tf¥ Sf limiting value)) solubilization kinetics of PCE/TCE was represented by hyperbolic relationships rather than exponential or bilinear relationships as follows t S¼ A þ Bt
ð1Þ
Figure 6. Variation of Km with surfactant concentration (X-X0) for PCE and TCE.
where S is the concentration of PCE or TCE solubilized in time t, and parameters A and B depend on the type and concentration of surfactant. Parameter A represents the initial rate solubilization, and parameter 1/B represented the ultimate solubilization. Both parameters A and B were determined for both PCE and TCE (Table 1). The hyperbolic relationship can be rearranged to a linear form. If the data satisfied eq 1, then the assumption of hyperbolic is acceptable, and the parameters A and B can be determined. In Figure 5, the relationship in eq 1 was verified for the various surfactants. Linear relationship with coefficient of correlation (R) = 0.98 or higher indicated that hyperbolic relationships can be used to represent the solubilization kinetics of PCE or TCE in various surfactant solutions. Equation 1 could be further modified by rearranging the terms and representing the terms as follows S t ¼ Sult t50 þ t
ð2Þ
where Sult is the maximum solubility of PCE or TCE under the testing condition, and t50 is the time taken to solubilize 0.5 Sult. In case of PCE, of the surfactants investigated up to a surfactant concentration of 10 g/L (Figure 3), Triton X-100 solubilized up to 1250 mg/L which was the maximum compared to other surfactants. CTAB, UH biosurfactant, and SDS solubilized 1210 mg/L, 1148 mg/L, and 1107 mg/L of PCE respectively. Based on the modeling t50 (eq 2) for Triton, CTAB, UH biosurfactant, and SDS were 4.03, 4.25, 5.13, and 5.88 min, respectively (Figure 4). It is observed that higher the t50 the lower was the amount of PCE solubilized by the surfactants. In case of TCE, based on the modeling, t50 (eq 2) for Triton, CTAB, UH biosurfactant, and SDS were 1.3, 2.7, 0.8, and 4.4 min, respectively (Figure 4). 5.2. Partition Coefficient (Km). The solubility of PCE and TCE increased linearly with the surfactants concentration above CMC. Enhancement in solubility is related to micelles in the solution. The micelle partition coefficient Km represents the PCE and TCE in the micelles to the PCE and TCE in solution and can be defined as12 KmðxÞ ¼
Y Y0 for ðX > X0 Þ Y0
ð3Þ
Figure 7. MSR of PCE and TCE in different surfactants.
where Y is the molar concentration of solubilized PCE and TCE at a given molar concentration of surfactant (X). The solubility of PCE increased linearly with the surfactants concentration above CMC. Y0 is the molar concentration of solubilized PCE at the CMC of the surfactant (X0). In case of PCE, the micelle partition coefficient (Km) was 7918 for Triton X-100 which was the highest compared to the other surfactants. The Km values for CTAB, UH biosurfactant, and SDS were 1241, 532, and 165, respectively (Figure 6). For TCE, the micelle partition coefficient (Km) was 325 for Triton X-100 which was the highest compared to the other surfactants. The Km values for UH biosurfactant, CTAB, and SDS were 181, 167, and 99, respectively (Figure 6). 5.3. Molar Solubilization Ratio (MSR). The solubilization capacity of a surfactant can be represented as molar solubilization ratio (MSR).12 The molar solubilization ratio (MSR) is represented by MSR ¼
Y Y0 X X0
ð4Þ
Hence MSR is the slope of the solubility relationship above the CMC of the surfactants. Commonly used units to present 5834
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Figure 8. Relationship between MSR and CMC of various surfactants. Figure 10. Surfactants þPCE/TCE variation with surface tension (ST) and interface tension (IT).
Figure 9. a) FTIR analysis of 10 g/L UH biosurfactant solution, b) FTIR analysis of PCE solubilized in UH biosurfactant, and c) FTIR analysis of TCE solubilized in UH biosurfactant.
solublization relationships are mg/L (solubilized compounds) and mg/L (surfactant). Since the relationship between enhanced solubility (YY0) and surfactant concentration above CMC (X-X0) (micelle concentration) was linear, hence MSR was a constant for each surfactant. In the case of PCE, Triton had the maximum solubility ratio of 0.34 compared to the other surfactants. The MSR values for CTAB, UH biosurfactant, and SDS were 0.21, 0.16, and 0.16, respectively (Figure 7). The relationship between MSR and CMC of PCE is shown in Figure 7. As the CMC increased the hydrophobity decreased and the molar solubility ratio (MSR) decreased. In the case of TCE the MSR values for CTAB, UH biosurfactant, Triton X-100, and SDS were 1.42, 1.50, 2.79, and 0.88, respectively (Figure 7). As compared to trichloroethylene, MSR values of PCE with all the surfactants were lower but showed similar decreasing trend with increasing CMC (Figure 8). Combining eq 3 and eq 4 will result in the following relationship Km ¼ MSR
X X0 Y0
ð5Þ
Since MSR was a constant for each surfactant, the micelle partition coefficient (Km) was not a constant but linearly related
to the excess surfactant concentration (X-X0) in moles as shown in Figure 6 for the surfactants investigated in this study. 5.4. Fourier Transform Infrared (FTIR) Spectroscopy. Infrared spectroscopy was used to identify the active functional groups in the biosurfactant molecule that enhance the solubilization of PCE and TCE. FTIR of UH biosurfactant is shown in Figure 9. The functional groups identified were very broad OH stretch bond (31603660 cm1) which overlaps the CH absorption, CC (2125 cm1), OCO asymmetric stretch (near 1643 cm1), and OCO symmetric stretch (near 1433 cm1).4 The CCl bond of PCE was in the range of 785540 cm1 far right to the active groups of the UH biosurfactant.4 FTIR analysis was done on UH biosurfactant after the solubilization of PCE (Figure 9). The bond structures of OH stretch, CC, OCO asymmetric, and OCO symmetric were all shifted to the right with reduced wave numbers indicating interaction with PCE. The OH stretch bond had shifted from 2950 cm1 to 3160 cm1 a shift to the right of 210 cm1. Similarly CC and OCO have shifted from 2125 cm1 and 1643 cm1 to 2095 cm1 and 1600 cm1. Hence the CC was shifted to the right by 30 cm1, and the OCO was shifted to the right by 43. In the case of TCE, the OH stretch bond had shifted from 2950 cm1 to 3060 cm1, a shift to the right of 110 cm1. Similarly CC and OCO have shifted from 2125 cm1 and 1643 cm1 to 2104 cm1 and 1650 cm1. Hence the CC was shifted to the left by 21 cm1, and the OCO was shifted to the right by 7 (Figure 9). Previous study with lead and UH-biosurfactant interaction showed the peaks shifting to the left with an increase in wave numbers (Kim and Vipulanandan 2006). 5.5. Interfacial Tension. The cohesive forces between liquid molecules are responsible for surface tension. When two partially miscible liquids are brought into contact the interface thus formed possesses free surface energy which is numerically equal to the interfacial tension. As one of the reasons for the increased solubility of chlorinated compounds in surfactant solutions is the reduction of surface tension and interfacial tension. This was shown in Figure 10. The surface tension and interfacial tension of UH biosurfactant and SDS were observed by varying the concentrations 5835
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a) Surfactants increased the solubility of PCE and TCE in water. Of the surfactants used the Triton X-100 solubilized the maximum amount of PCE per unit weight of surfactant as compared to the other surfactants used in this study, whereas UH biosurfactant solubilized the maximum amount of TCE compared to other surfactants. The rate of solubilization of PCE and TCE by various surfactants has been quantified and represented by a hyperbolic relationship. As the CMC increased the hydrophobity decreased and the molar solubility ratio (MSR) decreased. b) Using FTIR, the active groups in the UH biosurfactant that enhanced the solubilization of PCE and TCE was identified. The wavenumber of the OH, CC, and OCO peaks were reduced (shifted to the right) after solubilizing PCE. For TCE, the CC was shifted to the left, and the OCO and OH were shifted to the right. c) The micelle partition coefficient (Km) was not a constant but was linearly related to the excess surfactant concentration (in moles) because MSR was a constant for each of the surfactants investigated in this study.
Figure 11. The micelle size (diameter) distribution of surfactants with and without solubilized PCE.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (713)890-2503. Fax: (713)743-4260. E-mail: ps_harendra@ yahoo.com.
’ ACKNOWLEDGMENT This study was supported by the Center for Innovative Grouting Materials and Technology (CIGMAT) at the University of Houston with funding from the Texas Hazardous Waste Research Center and Texas Higher Education Coordinating Board (THECB). The contents do not necessarily reflect the views and policies of the funding agencies.
Figure 12. The micelle size (diameter) distribution of UH biosurfactant with and without solubilized PCE.
of surfactant solutions. The UH biosurfactant’s surface tension (ST) and interfacial tension (IT) is less compared to those of SDS as shown in Figure 10. The UH biosurfactant concentration varied from 0 to 10 g/L. The surface tension and interfacial tension of UH biosurfactant reduced from 72 to 24 and 16 to 9, respectively whereas the surface tension and interfacial tension of SDS reduced from 72 to 37 and 25 to 12, respectively. This is one of the reasons where UH biosurfactant has more solubility compared to SDS. This argument was verified in the above solubility studies. The mean size of SDS, CTAB, Triton X-100, and UH biosurfactant micelles were 4.2 nm, 3.8 nm, 4.5 nm, and 59.1 nm, respectively. After PCE was solubilized in these surfactants, the size of the micelles increased to 6.4 nm, 6.0 nm, 6.8 nm, and 85.7 nm, respectively (Figure 11 and Figure 12). Except for the biosurfactant, micelle size (diameter) change in all other surfactants was very similar which was around 2 nm.
6. CONCLUSIONS In this study, solubilization of PCE and TCE in various surfactant solutions was investigated in continuous stirred batch reactors. Based on the results the following can be concluded.
’ NOMENCLATURE CTAB- cetyltrimethylammonium bromide SEM- scanning electron microscopy EDS- energy dispersive X-ray spectroscopy XRD- X-ray diffraction UH biosurfactant- University of Houston biosurfactant DNAPL- dense nonaqueous phase liquid SDS- sodium dodecyl sulfate PCE- tetrachloroethylene TCE- trichloroethylene Km- micelle partion coefficient CMC- critical micelle concentration MSR- molar solubilzation ratio DNAPL- dense nonaqueous phase liquid ST- surface tension IT- interfacial tension S- solubility t- time t50- time taken to solubilize half of the maximum solubility Sult- ultimate solubility parameter A- the initial rate solubilization parameter 1/B- ultimate solubilization ’ REFERENCES (1) Abriola, L. M.; Drummond, C. D.; Hahn, E. J.; Hayes, K. F.; Kibbey, T. G.; Lemke, D. L.; Pennel, K. D.; Petrovskis, E. A.; Ramsburg, 5836
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