Surfactant Effects on the Transport of Air Bubbles in Porous Media M. Zhang 1 and S.E. Bums I Abstract Air sparging is a promising technology for the removal of volatile organic compounds (VOCs) from contaminated groundwater. The sparging process mobilizes contaminants to the vapor phase through mass transfer into air bubbles. Ideally in air sparging, bubbles are pressurized into groundwater through an injection well and through buoyancy, the contaminant-containing gas bubbles migrate to the surface where they can be collected for treatment. While air sparging is being implemented at the field scale for the remediation of VOC contaminated aquifers, there are still difficulties with the remedial process. The most limiting problem is that of nonuniform distribution of stripping gases through the subsurface. Significant channeling of the injected air occurs in soils that have a grain size smaller than approximately four mm (Ji et al. 1993). Because large portions of the aquifer are not exposed to the stripping gas, contaminant removal becomes mass transfer limited, as contaminants must diffuse to the air channels. This paper investigates the use of four different surfactants to enhance the air sparging process through the generation of small diameter air bubbles. Smaller bubbles are desirable in air sparging because they have a large surface area to volume ratio that promotes mass transfer, they are less buoyant resulting in a longer residence time in the system, and they are less prone to channeling. The experimental investigation quantified coalescence, average diameter, and size distributions of air bubbles produced in an aqueous and porous media system. For the bench-scale systems tested in this study, neither the surfactant charge nor the surfactant molecular structure had a significant effect on the physical characteristics of the bubbles generated. All surfactants produced similar average Graduate Research Assistant and Assistant Professor, respectively; Department of Civil Engineering, P.O. Box 400742, Thornton Hall, Charlottesville, VA 22904-4742 (804) 924-6370 (v); (804) 982-2951 (f); sburns@virginia.edu 121
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`122 ENVIRONMENTAL GEOTECHNICS bubble diameters and size distributions in both aqueous systems and in saturated porous media. In all cases, the average diameters of the injected bubbles were significantly reduced in the presence of trace concentrations of surfactants. INTRODUCTION Air sparging is a promising technology for the removal of volatile organic compounds (VOCs) from contaminated soil and groundwater. The sparging process is a stripping technique that mobilizes contaminants to the vapor phase through mass transfer into air bubbles. In air sparging, bubbles are pressurized into groundwater through an injection well with slotted screens. Through buoyancy, the gas migrates toward the surface, stripping VOCs from the groundwater as it percolates through the soil matrix. When the gas reaches the surface, it can be collected for further treatment. Other mechanisms active in the sparging process include biological degradation of contaminants through the introduction of oxygen into the groundwater, a process known as biosparging (Johnson et al. 1993), adsorption and desorption, dissolution, advection, dispersion, and diffusion (Semer and Reddy 1998). While air sparging is currently being implemented at the field scale for the remediation of VOC contaminated aquifers, there are difficulties with the remedial process, and the design and implementation are still largely empirical (Reddy et al. 1995). One of the most limiting problems is that of nonuniform distribution of stripping gases through the subsurface (Kueper and Frind 1988). Significant channeling of the injected air occurs in soils that have a grain size smaller than approximately four mm (Ji et al. 1993; Semer et al. 1998). Because large portions of the aquifer are not exposed to the stripping gas, contaminant removal becomes mass transfer limited as contaminants must diffuse to the air channels (Ahlfeld et al. 1994), which produces a concentration tailing similar to what is observed in other remedial techniques such as pump-and-treat. Another significant factor in the efficiency of air sparging is contaminant availability. Unger et al. (1995) demonstrated that at early times in the sparging process, the direct transfer of nonaqueous phase liquids (NAPLs) to the vapor phase causes a rapid reduction in contaminant mass in the system. However, at later stages of the process, contaminant transfer into the aqueous phase and subsequent diffusion to air channels are the controlling mechanisms (Braida and Ong 1998). The efficiency of air sparging is strongly affected by the air injection system parameters, soil conditions, and contaminant characteristics (Baker and Benson 1996; Reddy and Adams 1998). In addition, the physical characteristics of the stripping gas are a significant factor in the behavior of an air sparging system. Ideally, the air injected during the sparging process will form small diameter bubbles that transport as discrete elements through the system. Decreasing the diameter of stripping bubbles offers several advantages: smaller diameter bubbles have a larger surface area to volume ratio per volume of gas which is favorable for mass transfer, they have decreased buoyancy accompanied
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`ENVIRONMENTAL GEOTECHNICS 123 by increased residence time, and they exhibit decreased channeling through saturated porous media. Burns and Zhang (1999) quantified the effects of several system parameters on the size and size distributions of bubbles produced in bench-scale column studies. Bubble size and size distribution was extremely sensitive to pressure, with both parameters increasing as the injection pressure was increased. Orifice injector type was also a significant control on the bubbles generated, with single opening injectors creating a more uniform distribution of bubbles than multiple orifice injectors. The presence of a particulate media in the system led to increased bubble coalescence and the formation of larger diameter bubbles than was observed in purely aqueous systems. These effects were more pronounced as the grain size was reduced. Finally, trace concentrations of surfactant in the system were found to reduce the particulate media effect, generating a uniformly small bubble size and size distribution in the presence or absence of a particulate media. Surfactants are surface-active solutes that tend to concentrate at the interface of two phases. Typically, surfactants are characterized by a hydrophilic end and a hydrophobic end, and the addition of a surfactant to an aqueous system alters the interfacial tension at the air-water interface which results in smaller diameter bubbles. The presence of a surfactant coating on a bubble will also increase drag (Chhabra and Kee 1992) which will increase the residence time in the system, and will reduce the coalescence of bubbles (Bischof et al. 1993; Jeng et al. 1986; Sadhal et al. 1997), both of which are advantageous in stripping operations. While there are data in the literature that demonstrate that surfactant coatings can decrease the rate of mass transfer of contaminants into the vapor phase (Clift et al. 1978), other studies have indicated that at low concentrations, surfactants will accumulate at the trailing edge of a bubble translating vertically through an aqueous system, leaving the top cap of the bubble relatively surfactant free and available for mass transfer (Oguz and Sadhal 1988; Quintana 1990). Additional studies have shown that the mass transfer of oxygen is not impeded by certain surfactant coatings (Ju et al. 1991), or is more strongly controlled by the interfacial surface area (Bischof et al. 1993; Molder et al. 1998). In addition, surfactants have been shown to enhance the desorption of contaminants from mineral surfaces and organic matter (Deitsch and Smith 1995), which suggests that they may be beneficial in facilitating more rapid clean up times in the air sparging process. STUDY OBJECTIVES This work presents the results of an experimental investigation performed to evaluate the effect of surfactant charge and surfactant molecular structure on the formation and transport of gas bubbles in aqueous systems and saturated porous media. Specifically, the study compares the results of experiments performed with trace concentrations of nonionic, anionic, and cationic surfactants added to the system. The surfactants chosen for study also represented a variety of molecular structures.
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`124 ENVIRONMENTAL GEOTECHNICS EXPERIMENTAL METHODS Materials All water used in the experimental study was deionized, organic free (Bamstead, Nanopure). Four surfactants were used in the experiments: one nonionic surfactant: t-octylphenoxypolyethoxyethanol (Triton X- 100; C6H17C6H40(CH2CH20)9.sH); two anionic surfactants: sodium dodecyl sulfate (SDS; CH3(CH2)IIOSO3Na) and sodium dodecylbenzenesulfonic acid (SDA; C12H25C6I-4SO3Na); and one cationic surfactant: decyltrimethylammonium bromide (DTMA; C13H30BrN). All surfactants were obtained from Sigma Aldrich Chemical Company and were used as received. A syringe needle with an inside diameter of 0.114 mm (Fisher Scientific, Gauge 26S) was used as the air injection orifice in all experiments. Air was injected into the base of the test cell through Tygon tubing connected to the injection orifice. The injection pressure was controlled using a pressure regulator (Fairchild Industrial Products Company, Model 10). Highly idealized porous media were used in the study: uniform spherical silica beads (14.5 mm or 27.0 mm diameter). All experiments were performed in a rectangular glass test chamber (45 mm by 295 mm by 260 mm) with flat cell walls in order to prevent optical distortion during the imaging experiments. Image Acquisition and Processing All image collection and processing in the experiments was performed using a long-distance microscope (Questar, model 1) in combination with a color CCD camera (Sanyo model, VCC-3972), a method that has proven successful in the quantification of air bubble characteristics in both aqueous and saturated soil systems (Bums et al. 1998; Burns et al. 1997). A complete description of the experimental procedure is given in Bums and Zhang (1999). For the tests in porous media, the particles were packed into the test cell at a porosity of approximately 40%. Approximately three inches of water was left above the packed test cell and images were taken as the bubbles emerged from the soil column into the water headspace. RESULTS AND DISCUSSION A series of experiments was performed to quantify the effects of trace concentrations of surfactant on the formation of air bubbles in an aqueous medium. A baseline experiment was performed in deionized water, and the average diameter and size distributions of bubbles produced in pure water at an injection pressure of 9.0 kPa were quantified. Four additional experiments were performed using a different surfactant in each experiment: Triton X100, SDS, SDA, and DTMA were added to the system at a concentration of 1.6 x 10 4 M. The injection pressure in the surfactant-enhanced experiments was also 9.0 kPa. Figure 1 shows the results of experiments performed in both the aqueous and aqueous/surfactant systems.
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`ENVIRONMENTAL GEOTECHNICS 125 Figure 1. Effect of Surfactant Charge on Bubbles Produced in an Aqueous System The baseline experiment performed in the aqueous system clearly demonstrates that the characteristics of the bubbles produced by the needle injection orifice are fairly uniform, with a narrow range in the size distribution of bubbles produced. Because the formation of bubbles in an aqueous system is a surface controlled phenomenon, it follows that the physical characteristics produced using a single injection orifice would be fairly uniform. In contrast, the characteristics of bubbles produced using a heterogeneous surface (e.g. a frit or diffuser) tend to be far less uniform, with a wide range in bubble sizes (Bums and Zhang 1999). In each case, the addition of the surfactant to the system reduced the average bubble diameter to less than one-half the diameter that is produced in a surfactant-free system. All four surfactants produced similar distributions, with the anionic surfactants (SDS and SDA) producing the smallest average diameters with statistically identical distributions. The cationic surfactant (DTMA) produced a slightly larger average bubble diameter, while the nonionic surfactant (Triton X-100) produced the largest average diameter, with a wider distribution in size. The presence of the surfactant in the system has two significant effects on the formation and transport behavior of the stripping bubbles. First, the interfacial tension at the air/water interface is significantly reduced, which leads to the formation of smaller diameter bubbles than would appear in surfactant-free
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`126 ENVIRONMENTAL GEOTECHNICS systems. Additionally, surfactants provide a barrier to coalescence, making transport as discrete bubbles more probable. Experiments were performed to evaluate the effect of a particulate media on the formation and transport of air bubbles. In these experiments, the test cell was packed with uniform silica beads, with diameters of either 14.5 mm or 27.0 mm. While the particulate fraction was a highly idealized representation of a porous media, it still provided useful insight into the formation and aggregation behavior of bubbles transporting through porous media. The test column was packed with the beads to a porosity of approximately 40%, and five experiments were performed: one in a surfactant-free system, and four using each of the surfactants (surfactant concentration = 1.6 x 10-4 M). The presence of a particulate media in the system has several notable effects on the average size and size distribution of bubbles produced. Figure 2 shows the results of experiments performed under four experimental conditions: aqueous with no surfactant added, 14.5 mm diameter beads saturated (no surfactant added), and 27.0 mm diameter beads saturated (no surfactant added). The results of an aqueous experiment performed with a 1.6 x 10 -4 M concentration of Triton X-100 are shown for comparison purposes. Clearly, the presence of a solid phase leads to a significant increase in both the average diameter and size distribution of bubbles produced. In addition, the average size increases and the size distribution broadens as the grain size becomes finer. These trends were observed in the simulated soils tested here, as well as in natural soil samples (unpublished research results). The most significant increase in bubble size occurred in the largest twenty per cent of the distribution, indicating that a significant volume of the stripping gas was occupied in very large diameter bubbles. The production of large diameter bubbles is undesirable because the surface area available for mass transfer is lower per given volume of gas when compared to smaller diameter bubbles. Figures 3 and 4 show the results of experiments performed in porous media to quantify the effect of surfactant charge on the formation and movement of air bubbles through porous media. The column was packed with either 14.5 mm or 27.0 mm diameter beads, and replicates were performed in aqueous and surfactant systems. It is most interesting to note that the presence of surfactant in the system, even at the trace quantities present in these experiments, eliminated the porous media effect, producing uniformly small distributions of bubbles. In all experiments performed in a surfactant- enhanced particulate media, the bubble characteristics showed little deviation from the characteristics of bubbles produced in a purely aqueous media. As in the aqueous systems, the anionic and cationic surfactants produced similar sizes and distributions, while the nonionic surfactant produced a similar, but slightly larger average bubble size and wider distribution. These data indicate that the charge and molecular structure of the surfactant had little effect on the physical characteristics of the bubbles produced.
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`ENVIRONMENTAL GEOTECHNICS 127 Figure 2. Bubble Size Distributions in Particulate Media Figure 3. Surfactant Charge Effects in Particulate Media (14.5 mm)
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`128 ENVIRONMENTAL GEOTECHNICS Figure 4. Surfactant Charge Effects in Particulate Media (27.0 mm) Figure 5 shows the results of a series of experiments performed to quantify the critical concentration of surfactant required to achieve the desired reduction in average bubble diameter. Experiments were performed using the anionic surfactant SDS added at concentrations ranging from 0.5 x 10 .4 M to 1.6 x 10.4 M; concentrations that are significantly lower than the critical micelle concentration for the surfactant. At the lowest value of surfactant concentration, there is little effect on the average bubble diameter or size distribution produced in the column studies. However, as surfactant concentration is increased, the average bubble size decreases and the size distribution broadens until a critical concentration of surfactant is added (1.6 x 10.4 M). Additional increases in the concentration of surfactant did not produce significant variations in the characteristics of the bubbles produced.
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`ENVIRONMENTAL GEOTECHNICS 129 Figure 5. Surfactant Critical Concentrations (SDS) CONCLUSIONS The addition of surfactants to air sparging operations can produce bubbles with physical characteristics that are better suited to stripping operations. This study quantified the effect of four different surfactants, with a range of charge and molecular structures, on the average bubble size and size distributions produced in both aqueous and saturated porous media. While the nonionic surfactant produced a slightly larger average bubble diameter and size distribution, the differences among the surfactants were essentially negligible. The beneficial effects of surfactant addition (e.g. reduced average diameter and size distribution) were achieved in all aqueous and particulate systems tested. Surfactant charge was not determined to have a significant effect on the idealized porous media used in this study; however, it is not clear if this is true in the case of natural soil samples with significant surface charges. Experiments to quantify these effects in natural soils are ongoing. Additional studies are underway to quantify the effect of surfactant addition on the transfer of mass to the vapor phase as well.
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`130 ENVIRONMENTAL GEOTECHNICS ACKNOWLEDGEMENTS The authors thank Carl T. Herakovich for the use of his long distance microscope and James E. Danberg for his assistance with experimental setup. The University of Virginia provided partial financial support for the support of this research. This support is gratefully acknowledged. REFERENCES Ahlfeld, D. P., Dahmani, A., and Ji, W. (1994). "A Conceptual Model of Field Behavior of Air Sparging and Its Implications for Application." Ground Water Monitoring and R emediation, 14(4), 132-139. Baker, D. M., and Benson, C. H. "Review of Factors Affecting In Situ Air Sparging." Non-Aqueous Phase Liquids (NAPLs) in Subsurface Environments, Washington, D.C., 292-310. Bischof, F., Sommerfeld, M., and Durst, F. "Behavior of Fine Dispersed Bubbles in Solutions with Surfactants." Fluids Engineering Conference, Washington, D.C., 131-136. Braida, W. J., and Ong, S. K. (1998). "Air Sparging: Air-Water Mass Transfer Coefficients." Water Resources Research, 34(12), 3245-3253. Bums, S. E., Yiacoumi, S. Z., Frost, J. D., and Tsouris, C. "Application of Digital Image Analysis for Size Distribution Measurements of Microbubbles." Engineering Foundation Conference on Imaging Technologies: Techniques and Applications in Civil Engineering, Davos, Switzerland, 100-107. Bums, S. E., Yiaeoumi, S. Z., and Tsouris, C. (1997). "Microbubble Generation for Environmental and Industrial Separations." Separations and Purification Technology, 11,221-232. Bums, S. E, and Zhang, M. (1999). "Digital Image Analysis to Assess Microbubble Behavior in Porous Media." Journal of Computing in Civil Engineering, 13(1 ), 43-48. Chhabra, R. P., and Kee, D. D. (1992). "Transport Processes in Bubbles, Drops, and Particles.", Hemisphere Publishing Corporation, New York, 255. Cliff, R., Grace, J. R., and Weber, M. E. (1978). Bubbles, Drops, and Particles, Academic Press, New York. Deitsch, J. J., and Smith, J. A. (1995). "Effect of Triton X-100 on the Rate of Trichloroethene Desorption from Soil to Water." Environmental Science and Technology, 29(4), 1069-1080. Jeng, J. J., Jer, R. M., and Yang, Y. M. (1986). "Surface Effects and Mass Transfer in Bubble Column." Industrial and Engineering Chemistry, Process Design and Development, 25(4), 974-978. Ji, W., Dahmani, M. A., Ahlfeld, D. P., Lin, J. D., and Hill, E. (1993). "Laboratory Study of Air Sparging: Air Flow Visualization." Ground Water Monitoring and Remediation, 13(4), 115-126.
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`ENVIRONMENTAL GEOTECHNICS 131 Johnson, R. L., Johnson, P. C., McWhorter, D. B., Hinchee, R. E., and Goodman, I. (1993). "An Overview of In Situ Air Sparging." Ground Water Monitoring and Remediation, 13(4), 127-136. Ju, L.-K., Lee, J. F., and Armiger, W. B. (199t). "Effect of the Interfacial Surfactant Layer on Oxygen Transfer Through the Oil/Water Pahse Boundary in Perfluorocarbon Emulsions." Biotechnology and Bioengineering, 37(6), 505-511. Kueper, B. H., and Frind, E. O. (1988). "An Overview of Immiscible Fingering in Porous Media." Journal of Contaminant Hydrology, 2, 95-1 I0. Molder, E., Tenno, T., and Nigu, P. (1998). "Influence of Surfactants on Oxygen Mass-Transfer Through the Air-Water Interface." Critical Reviews in Analytical Chemistry, 28(2), 75-80. Oguz, H. N., and Sadhal, S. S. (1988). "Effects of Soluble and Insoluble Surfactants on the Motion of Drops." Journal of Fluid Mechanics, 194, 563-579. Quintana, G. C. (1990). "Effect of Surface Blocking on Mass Transfer from a Stagnant Cap Drop." International Journal of Heat and Mass Transfer, 33(12), 2631-2640. Reddy, K. R., and Adams, J. A. (1998). "System Effects on Benzene Removal from Saturated Soils and Ground Water Using Air Sparging." Journal of Environmental Engineering, 124(3), 288-299. Reddy, K. R., Kosgi, S., and Zhou, J. (1995). "A Review of In-Situ Air Sparging for the Remediation of VOC-Contaminated Saturated Soils and Groundwater." Hazardous Waste & Hazardous Materials, 12(2), 97-118. Sadhal, S. S., Ayyaswamy, P. S., and Chung, J. N. (1997). Transport Phenomena with Drops and Bubbles, Springer, New York. Semer, R., Adams, J. A., and Reddy, K. R. (1998). "An Experimental Investigation of Air Flow Patterns in Saturated Soils During Air Sparging." Geotechnical and Geological Engineering, 16, 59-75. Semer, R., and Reddy, K. R. (1998). "Mechanisms Controlling Toluene Removal from Saturated Soils During In Situ Air Sparging." Journal of Hazardous Materials, 57, 209-230. Unger, A. J. A., Sudicky, E. A., and Forsyth, P. A. (1995). "Mechanisms Controlling Vacuum Extraction Coupled with Air Sparging for Remediation of Heterogeneous Formations Contaminated by Dense Nonaqueous Phase Liquids." Water Resources Research, 31(8), 1913- 1925.
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