Environ. Sci. Technol. 2000, 34, 3977-3981
Optimization of the Surfactant/ Alcohol Formulations for the Remediation of Oily Contaminated Porous Media T R U O N G H O N G T I E N , * ,† MEHDI BETTAHAR,‡ AND SHINSUKE KUMAGAI‡ Department of Civil Engineering, School of Engineering and Research Center for Advanced Waste and Emission Management, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Accurate selection of a surfactant/alcohol formulation for mobilization of organic contaminants from soils requires consideration of oil volume fraction (θO) as a major parameter. This study examines the influence of θO on behavior of systems containing brine, sodium dodecyl benzenesulfonate, n-pentanol, and n-alkanes. A batch salinity scan technique was employed to investigate the change of systems with the change of θO. The results reveal that the effect of θO is significant; with increasing θO, the systems change from Winsor II f Winsor III f Winsor I. The batch experimental data were then used to develop a correlation for optimum formulation with corrected term, e.g. f(A) alcohol function to take into account the effect of θO. At constant concentration of n-pentanol, the correlation exhibits a linear dependence of logarithm of optimal salinity (S*) on θO. The importance of considering θO as a major parameter in the optimum correlation was verified through the column experiments and good agreement was obtained. A surfactant formulation selected based on the developed correlation for θO equal to degree of oil saturation (SO) possessed the highest potential for n-dodecane recovery among other formulations. These findings will be valuable in the design of groundwater remediation scenarios when surfactant and alcohol are used for remediation of crude oils.
Introduction The displacement or mobilization of residual nonaqueous phase liquids (NAPLs) from the porous media under normal flow regimes requires ultralow (99% was used as cosurfactant for producing middle-phase microemulsions. Three saturated hydrocarbons (alkanes), n-decane, n-dodecane, and n-tetradecane with purity >99%, and diesel oil representing a mixture of hydrocarbons were used as contaminants. All contaminants were used for the batch experiments, but only n-dodecanes a representative of the saturated hydrocarbons found in gasoline and jet fuel (15)swas selected as a testing contaminant for column experiments. n-Dodecane has a density of 0.75 g/cm3, an aqueous solubility of 3.7 µg/L, and a dynamic viscosity of 1.6 kg/ms (15, 16). The surface tension of n-dodecane is 24.9 dyn/cm, while the IFT between ndodecane and water is 52.8 dyn/cm (16). The contaminants were dyed with oil-red-O, so that appearance of the different phases could be observed quantitatively. All these chemicals were purchased from Tokyo Kasei Kogyo Company, except the diesel oil, which was purchased from a gasoline station. Sodium chloride with purity of 99% was used as electrolyte, and water was distilled for preparation of the aqueous solutions. The porous medium is composed of 0.5-mm glass beads, which have a specific surface area of 76 cm2/cm3, a porosity of 0.373, and a hydraulic conductivity of 0.023 cm/s. The glass beads were washed several times by distilled water prior to use. VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Phase diagram S - θO for n-decane at 20 g/L of n-pentanol concentration.
FIGURE 1. Set up for surfactant/alcohol flushing experiment. Phase Behavior Experiments. Batch studies on phase behavior of the brine/surfactant/alcohol/oil systems were conducted in 110-mL glass bottles capped to prevent volatilization losses. In each bottle, 50 mL of an aqueous phase containing brine, surfactant, and n-pentanol was contacted with an oil phase with variable volumes by shaking and allowing the mixture to stand until phase volumes became time-independent for at least 1 week in a 15 °C room. This temperature was selected for the experiment because it is close to the groundwater temperature range (17). All concentrations are expressed in grams per liter (g/L) of aqueous phase before contacting with oil phase. Different types of oil were used as contaminants, with a constant concentration of SDBS (10 g/L) and variable n-pentanol concentrations. For each WOR, the classical Winsor type I-III-II transitions were obtained by varying the salt concentration (called a salinity scan) while keeping other variables constant. The appearance and disappearance of middle phases were verified by visual observation. The interfacial tensions were measured using the spinning drop method (18). Column Experiments. The column apparatus was made by glass (length, L ) 50 cm, inside diameter, ID ) 10 cm) equipped with two removable end caps (Figure 1). Both end caps were fitted with the steel support meshes to avoid the flow of glass beads from the column. Because in reality most reservoirs are assumed water wet (19), in this experiment the glass beads initially were wetted by water and then mixed with n-dodecane in an amount equal to 10% of the pore volume. The column was then packed with the contaminated glass beads under vibration and fitting the column with the top end cap containing a steel mesh. At the start of the experiment, approximately 10 pore volumes of distilled water were pumped through the bottom of the column to ensure that the glass beads were completely saturated and no free mobile dodecane was displaced from the column. Following the water flow, the aqueous solution containing brine, surfactant, and n-pentanol was pumped in an upward mode with a flow rate of about 30 mL/min to displace the entrapped n-dodecane from the column. This value is equal to the flow rate which would be produced if the column is flooded from the top under a hydraulic gradient of 0.28. The column effluent was collected in glass bottles and immediately capped. About 0.02-1 mL of emulsion taken from each bottle was diluted in 2-propanol and analyzed by gas chromatography (GC-14B, Shimadzu Corporation, Kyoto, Japan) equipped with a packed column and flame ionization detector (FID). 3978
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It is worth noting here that there may be a difference between the described procedure of mixing oil with porous medium and that in a real site (detailed explanation is given in a subsequent section). This may affect the oil mobilization behavior but may not affect the results of verification. This is because the main purpose of the column experiments is only to compare the relative potentials for oil recovery between the formulations optimized at different θO. Thus, the results of verification might depend only on the amount of residual oil (SO) entrapped in the soil. Of course, there is a method of contaminating the soil with NAPL similar to a real site (15, 20); however, it is complex and not necessary to use in this study.
Results and Discussion Effect of WOR on the Phase Behavior. For convenience, the term oil volume fraction or oil fraction (θO), defined as a ratio of the oil volume (VO) to the total volume (VT) of the system, is introduced. Based on this definition, θO is the same as the degree of the oil saturation (SO), that is the volume ratio between oil and total pore space in the porous media. The parameters θO and WOR are related by the following expression
θO ) VO/VT ) 1/(1+WOR)
(1)
To investigate the effect of θO on the system phase behavior, salinity scans were conducted for a range of θO while maintaining all other variables constant. The maximum θO was selected not greater than 0.5 since the typical residual saturation of NAPL may occupy only between 5 and 40% of the pore volume (15, 21). The typical results for the systems with n-decane at n-pentanol concentration of 20 g/L are shown in Figure 2. The dashed curves run from lower left to upper right are the boundaries between Winsor I or Winsor II and Winsor III systems. The solid line through the center of the three-phase region connecting the points, at which the water-oil IFTs are minimum, is called as the trace of optimal salinity (S*). The definition of the optimal salinity here is similar to that of Kunieda and Shinoda (10) and West and Harwell (22). As seen from Figure 2, the effects of θO and salinity on system phase behavior are inverse. Upon increasing the salinity, the phase behavior changes from Winsor I f Winsor III f Winsor II, but with increasing θO, change of phase behavior is from Winsor IIf Winsor III f Winsor I. It is also seen that the salinity needed to produce the three-phase system is increased with increasing θO. These observed phenomena can be explained as follows: let KC be the distribution coefficient of n-pentanol between oil and aqueous phases
KC ) CO/CA
(2)
where CO and CA are the equilibrium concentrations of n-pentanol in oil and aqueous phases. Upon increasing θO in the system, i.e., increasing the oil volume, while keeping the aqueous volume constant, CA will be decreased due to the additional partition of n-pentanol into the oil phase. Since KC is constant for a given temperature, CO is also decreased. The n-pentanol can be considered as both cosurfactant to increase the hydrophobic nature of aqueous phase and co-oil to decrease hydophobicity of the oil phase (23). Thus, a decrease in concentration of n-pentanol in both phases creates more dissimilarity between these phases, which in turn promotes the system to become o/w microemulsion (or Winsor I). This also means that with increasing θO, the salinity required to produce the middlephase microemulsion is also increased. Because maximizing oil recovery occurs at the point of minimum IFT between aqueous and nonaqueous phases, instead of dealing with a range of sanilities for which middle phases exist, only the trace of the optimal salinity (Figure 2) is focused in this study. The S* is a function of many variables, such as the surfactant structure, the temperature, the alcohol type and concentration, and the oil type and fraction. Fixing θO of 0.2 and considering the effect of these variables independently, Salager et al. (8) had found that S* can be correlated with other variables of the middle-phase microemulsion by the following empirical correlation
ln S* ) K(ACN) + f(A) - σ + aT(T - 25)
(3)
where K is a constant depending on the surfactant type; f(A) is a function of heavy alcohol (n-pentanol or heavier) and its concentration in aqueous solution; σ is a parameter characteristic of surfactant; ACN is the alkane carbon number, e.g. the number of carbons in the hydrocarbon chain; aT is the temperature coefficient; and T is the current temperature. The significance of eq 3 is that this correlation links the variables that produce an optimal formulation for systems containing anionic surfactants, sodium chloride, water, alkanes, and various alcohols. For crude oil and mixture of pure hydrocarbons, an equivalent alkane carbon number (EACN) can be assigned as in eq 4 (24)
(EACN)m ) ΣXi(EACN)i
(4)
where (EACN)m and (EACN)i are the equivalent alkane carbon number for the oil mixture and component i, respectively and Xi is the mole fraction of the component i in the mixed oil phase, that is, ΣXi ) 1. The substitution of (EACN)m into eq 3 results in
ln S*m ) K(EACN)m + f(A) - σ + aT(T-25)
(5)
It is apparent from eqs 3 and 5 that the term θO has not been considered in the function of S*. However, in this present study, the results of these batch experiments show that the partitioning of the surfactant/alcohol mixture is quite affected by θO and that a correlation has to be provided to keep threephase behavior condition at any θO. Figure 3 shows the logarithm of the S* as a function of θO. Interestingly, all curves are straight but not parallel lines. Using all the model parameters determined at WOR ) 4, the optimal salinity, S*x at any oil fraction, θO ) x can be related with S*0.2 at θO ) 0.2 (or WOR ) 4) as in eq 6 * * ln S*x ) ln S0.2 + ∆ ln S* ) ln S0.2 +
∆ ln S* (x - 0.2) (6) x - 0.2
Setting ka ) ∆ ln S*/(x-0.2), eqs 3 and 5 will be
FIGURE 3. ln S* versus θO for n-decane with different concentrations of n-pentanol.
ln S* ) K(ACN) + f(A) - σ + aT(T - 25) + ka(x - 0.2)
(7)
ln S*m ) K(EACN)m + f(A) - σ + aT(T - 25) + ka(x - 0.2) (8) Details on the method and results of determining the parameters K, σ, f(A), and aT has been given in the works of Salager et al. (8). The straight line feature exhibited in Figure 3 is quite useful in practice since its slope is the ka value, which is constant for each concentration of n-pentanol. The plots of ka versus concentrations of n-pentanol shown in Figure 4 indicates that ka is increased with increasing n-pentanol concentration and is essentially independent of the oil type. In a free n-pentanol system, changing θO does not change the system behavior, so ka is equal to zero. This means that the approach, in which θO is not a controllable or selectable parameter in selection of an optimum formulation for soil remediation with NAPL as adapted in many works (5, 7, 8, 14) and is valid only in this case. For the systems containing heavy alcohols, the effect of θO must be taken into account in the optimum correlations as described in eqs 7 and 8, respectively. When applying the microemulsion for soil remediation, in general, temperature, oil type, and its content are prescribed; what is wanted is to find an efficient mixture of surfactant and alcohol to produce an optimum middle-phase microemulsion. Equations 7 and 8 allow one to estimate the values of adjustable variables (f(A) and ka), needed to obtain an optimum formulation, which can yield ultralow IFT between water and oil. The correlations also allow one to predict the change of phase behavior according to the change of oil saturation in the soil. However, to use the eqs 7 and 8 for the selection of an optimum surfactant formulation for soil remediation, it is necessary to verify these results through the column experiments. This is because the conditions applied in the batches (in absence of the porous media and under equilibrium and static regimes) are not the same as that in the porous media (under dynamic and nonequilibrium conditions). Column Experiments. The column tests were specifically designed to verify the importance of considering θO as a major parameter in the selection of an optimum formulation for mobilizing oil at a certain residual saturation (SO) from the soils. For doing so, the glass beads were contaminated with a fixed amount of n-dodecane (SO ) 0.1). Three mixtures containing the same concentrations of surfactant (10 g/L) and n-pentanol (30 g/L), but different salinities (8.55, 6.6, and 3 g/L), were used as testing formulations. These salinities were calculated using eq 7 for θO1 ) 0.2 (> SO), θO2 ) 0.1 () SO), and θO3 ) 0.02 (< SO), respectively. VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Plot of ka versus n-pentanol concentrations for different types of oil.
FIGURE 5. Cumulative recovery of 10%-entrapped n-dodecane (a) and mass of n-dodecane displaced (b) from the column (V represents the discharged surfactant volume and Vp is the pore volume of the porous medium). Results of the column experiments show that a greater part (more than 90%) of entrapped n-dodecane could be removed after injection of only 1.37, 1.05, and 1.77 Vp of the solutions with the salinities of 8.55, 6.6, and 3 g/L, respectively (Figure 5a). To remove this amount of n-dodecane by using conventional pump and treat method, however, more than 20 × 106 Vp of water is needed. This value is estimated based on the aqueous solubility of the n-dodecane (3.7 µg/L). Among these three formulations, it is clear that the formulation optimized at θO1 (> SO) is less suitable since it needs highest salt concentration but yields lowest efficiency. Although the final recovery (93.8%) of n-dodecane obtained by using the formulation at θO2 () SO) is less than that (96.8%) with the formulation at θO3 (< SO), the amount of surfactant/ alcohol solution needed to achieve this final level is reduced by nearly 2 times. This will decrease the cost of remediation significantly. Furthermore, when more than 90% of ndodecane could be recovered, the final residual saturation of n-dodecane is only 1.82 g per kg of the glass beads: this is an appropriate situation for implementing the biodegradation stage (25). Given this, the formulation with θO2 () SO) should be the optimum one among these three formulations. This means that the results of the batch experiments are in agreement with that of the column experiments. Because 3980
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the screening process for obtaining an optimum middlephase microemulsion with crude oils is very time-consuming, the developed correlations will significantly reduce laboratory works and thus the cost associated with design of surfactantenhanced subsurface remediation systems. Interpretation. The results of the IFT measurements by spinning drop method show that the IFT between aqueous phase and n-dodecane is minimum (0.0023 dyn/cm) at salinity of 6.6 g/L, whereas it is 0.0179 and 0.079 dyn/cm at salinities of 3 and 8.55 g/L, respectively. The values of IFTs reported here are averages of 15 single measurements. The error of the single measurement depends on the magnitude of the IFT. It ranges between (12% and (27% for tensions of 0.1 and 0.003 dyn/cm, respectively. Because of the lowest water-oil IFT produced by the formulation with salinity of 6.6 g/L, it is the optimal one. Two other formulations with salinities of 3 and 8.55 g/L are referred to as underoptimized and overoptimized, respectively. The observed phenomena can also be interpreted quantitatively using the phase behavior diagram shown in Figure 2. At θO ) 0.1, the system with salinity of 6.6 g/L exhibits Winsor III type, while the systems with salinities of 3 and 8.55 g/L exhibit Winsor I and II types. For this reason, the formulation with salinity of 6.6 g/L would yield lowest wateroil IFT, therefore possessing highest potential for oil recovery. Figure 5b shows that the volumes of the surfactant/alcohol solution needed to reach the points, at which the n-dodecane appeared in the column effluent, are different for the three formulations. Although the initially saturated water in the column prior to surfactant flushing was 0.9 Vp, measurable quantities of n-dodecane appeared in the effluent after only 0.75 and 0.6 Vp for the salinities of 6.6 and 8.55 g/L, respectively. These data indicate that the mobilized ndodecane formed a bank of free product that was displaced from the column prior to surfactant breakthrough (26, 27). It was also observed during the experiments that there was a preferential flow of the aqueous phase through the n-dodecane bank. This allows the surfactant/alcohol solution together with solubilized n-dodecane to reach the column effluent before complete displacement of water. At the salinity of 3 g/L, the n-dodecane appeared in the effluent after nearly 0.9 Vp of the surfactant/alcohol solution had passed through the column indicating that at the moment of n-dodecane appearance in the effluent, almost all initially saturated water was displaced by the surfactant/alcohol solution. The displacement of the n-dodecane as free product ceased after ∼1.82, 1.15, and 1.6 Vp of the surfactant solution had passed through the column for the salinities of 3, 6.6, and 8.55 g/L, respectively. After these points, only solubilized n-dodecane was observed in the effluent, and cumulative n-dodecane recovery slightly increases and reaches almost constant values of 91.8%, 93.8%, and 96.8%. This means that in comparison with the salinity of 8.55 g/L, the surfactant/ alcohol formulations at salinities of 3 and 6.6 g/L were able to mobilize the additional 5% and 2% of n-dodecane as free products, respectively. It is due to the fact that behind the n-dodecane bank, there still remains very small amount of n-dodecane, which is much less than initial SO. Obviously, the formulation with lower salinity would have greater potential to mobilize this remaining n-dodecane, because it makes the system closer to the optimum state. As mentioned previously that the method of contaminating the glass beads in this study might not be true in a real contaminated site. The packing of glass beads after mixing them with water and oil (full area of glass beads came in direct contact with water and oil) might result in less degree of pore continuity of the porous medium. This might be the best for the oil displacement process since surfactant solution could access almost all of the pores and therefore could mix with almost all of the oil droplets inside the column. But in
the real site, soil structure has been formed before the presence of water and oil, so creating high degree of pore continuity. For these reasons, only a fraction of the total surface area of the soil particles comes in direct contact with water and oil. Furthermore, because of the high degree of pore continuity and pore geometry, the surfactant solution might not have access to all pores; therefore, it might not have contact with all of the oil droplets. Another difference is that a uniform distribution of the oil created in the experiment might not occur in the real site because of the soil heterogeneity. Due to this reason, the formulation optimized at θO ) SO might form only one type of the system (Winsor III), which in turn results in a uniform displacement of the oil through entire column. However, in the real site, three system types (Winsor I, III, II) might simultaneously occur at different soil places having SO, which are less, equal, and higher θO taken for the batch, respectively. As a result, nonuniform displacement of the oil would take place, so decreasing the efficiency of the surfactant formulation. To minimize this effect, it is recommended that the value of θO in eqs 7 and 8 should be taken as an average of SO over the entire contaminated zone.
Acknowledgments This research was supported by the Ministry of Education, Science, Sport and Culture, Japan (Monbusho). The authors are indebted to the late Professor U. Matsubayashi for his consistent contributions to this research. Special thanks are also given to Mr. Y. Inoue, Ms. E. Katayama, and Ms. V.T.H. Minh, for their assistance with the experiments.
Literature Cited (1) Miller, C. A.; Hwan, R. N.; Benton, W. J.; Fort, T., Jr. J. Colloid Interface Sci. 1977, 61(3), 554-568. (2) Sjoblom, J. Emulsions and Emulsion Stability; Marcel Dekker: 1996; pp 223-227. (3) Touhami, Y.; Hornof, V.; Neale, G. H. J. Colloid and Surfaces A: Physicochem. Eng. Aspects. 1998, 132, 61-74. (4) Graciaa, A.; Lachaise, J., Sayous, J. G.; Grenier, P.; Yiv. S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93(2), 474-486. (5) Shiau, B. J.; Sabatini, D. A.; Harwell, J. H.; Vu, D. Q. Environ. Sci. Technol. 1996, 30(1), 97-103. (6) Rosen, M. J. Surfactant and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989; pp 232-235.
(7) Bettahar, M.; Schafer, G.; Baviere, M. Environ. Sci. Technol. 1999, 33(8), 1269-1273. (8) Salager, J. L.; Morgan, J. C.; Schechter, R. S.; Wade, W. H.; Vasquez, E. Soc. Pet. Eng. J. 1979, 19, 107-115. (9) Salager, J. L.; Minana, M.; Perez, M.; Remirez, M.; Rojas, C. I. J. Dispersion Sci. Technol. 1983, 4(3), 313-329. (10) Kunieda, H. Shinoda, K. Bull. Chem. Soc. Jpn. 1982, 55, 17771781. (11) Kahlweit, M. J. Phys. Chem. 1995, 99, 1281-1284. (12) Shinoda, K.; Arai, H. J. Colloid Interface Sci. 1967, 25, 429-431. (13) Kunieda, H.; Ishikawa, N. J. Colloid Interface Sci. 1985, 107(1), 122-128. (14) Ducreux, J.; Baviere, M.; Bocard, C.; Monin, N.; Muntzer, P.; Razakarisoa, O.; Arnaud, C. In Proceedings of the 5th Annual Symposium on Grounwater and Soil Remediation; Toronto, Canada, October 2-6, 1995. (15) Pennel, K. D.; Abriola, L. M.; Weber, W. J., Jr. Environ. Sci. Technol. 1993, 27(12), 2332-2340. (16) Demond, A. H.; Lindner, A. S. Environ. Sci. Technol. 1993, 27(12), 2318-2331. (17) Baran, J. R.; Pope, G. A.; Wade, W. H.; Weerasooriya, V. Environ. Sci. Technol. 1996, 30(7), 2143-2147. (18) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106(20), 86068615. (19) Bear, J. Dynamics of fluids in porous media; American Elsevier: 1972; pp 443-444. (20) Powers, S. E.; Abriola, L. M.; Weber, W. J., Jr. Water Resour. Res. 1994, 30(2), 321-332. (21) Brown, C. L.; Pope, G. A.; Abriola, L. M.; Sepehrnoori, K. Water Resour. Res. 1994, 30(11), 2959-2977. (22) West, C. C.; Harwell, J. H. Environ. Sci. Technol. 1992, 26(12), 2324-2330. (23) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir. 1994, 10, 11341139. (24) Cayias, J. L.; Schechter, R. S.; Wade, W. H. Soc. Pet. Eng. J. 1976, 16, 351. (25) Baviere, M.; Bocard, C.; Ducreux, J.; Monin, N.; Muntzer, P.; and Razakarisoa, O. SPE International Symposium on Oilfield Chemistry; Texas, February 19-21, 1997; pp 481-492. (26) Pennell, K. D.; Jin, M.; Abriola, L. M.; Pope, G. A. J. Contaminant Hydrol. 1994, 16, 35-53 (27) Bettahar, M.; Ducreux, J.; Schafer, G.; Dorpe, F. V. J. Transport Porous Media 1999, 37(3), 255-276.
Received for review December 14, 1999. Revised manuscript received May 31, 2000. Accepted June 15, 2000. ES991379W
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