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Comments Comment on Effect of Temperature on Surfactant-Driven Water Movement in Wet “Unsaturated” Sand
We have followed with great interest the work of Karkare, Fort, and colleagues related to the ability of surfactants to move water in unsaturated porous media. In their most recent work, Karkare and Fort1 extend their previous work to include the effect of temperature. They report that the results of the present study support the conclusions of their previous work. Specifically, they maintain that “... criteria for surfactant effectiveness as water movers include (1) water insolubility, (2) high equilibrium spreading pressure at the air-water interface, and (3) formation of a solid condensed monolayer at the equilibrium spreading pressure.” Though they find the above criteria necessary for surfactants to be effective water movers, their previous work2 showed that surfactant-driven flow is a result of capillary pressure gradients caused by surface tension differences between surfactantfree and surfactant-containing regions of the unsaturated porous medium. Smith and Gillham,3,4 Henry et al.,5,6 and Henry and Smith7 present experimental data and transient numerical modeling showing that significant flow can be induced in unsaturated porous media by 1-butanol, a high-solubility, short-chain alcohol which does not form a solid monolayer. The flow behavior in systems containing a high-solubility surfactant such as butanol is considerably different than that in systems containing an insoluble surfactant. Because of the increasing use of surfactants for environmental remediation, and the fact that many organic contaminants may exhibit some degree of surface activity, we feel it is important to emphasize that organic compounds that do not meet the criteria specified by Karkare and Fort1 are capable of inducing significant water movement in unsaturated porous media. The method used by Karkare and Fort to quantify the watermoving effectiveness of surfactants is inadequate for expressing the complex nature of water movement in systems containing soluble surfactants. We seek to draw the attention of interested readers to our work that describes such systems and provides additional tools for the characterization and analysis of those systems. Karkare and Fort1 examined the water-moving characteristics of surfactants by using closed, horizontal, uniformly wetted, unsaturated sand columns (after Tschapek and Boggio8). One-half of the column contained water and surfactant, while the other half was wetted with pure water. Henry et al.5 used the same horizontal column † ‡
University of North Carolina at Wilmington. McMaster University.
(1) Karkare, M. V.; Fort, T. Langmuir 2002, 18, 2190. (2) Karkare, M. V.; Fort, T. Langmuir 1993, 9, 2398. (3) Smith, J. E.; Gillham, R. W. Water Resour. Res. 1994, 30, 343. (4) Smith, J. E.; Gillham, R. W. Water Resour. Res. 1999, 35, 973. (5) Henry, E. J.; Smith, J. E.; Warrick, A. W. J. Hydrol. 1999, 223, 164. (6) Henry, E. J.; Smith, J. E.; Warrick, A. W. J. Hydrol. 2001, 245, 73. (7) Henry, E. J.; Smith, J. E. J. Contam. Hydrol. 2002, 56, 247. (8) Tschapek, M.; Boggio, L. Z. Pflanzenernaehr. Bodenkd. 1981, 144, 30.
Figure 1. Dependence of surface tension on butanol concentration. Modified after ref 9.
technique and studied the water-moving abilities of myristyl alcohol, one of the low-solubility alcohols used by Karkare and Fort,1,2 as well as those of 1-butanol, a high-solubility alcohol. Water with solid monolayer coverage of myristyl alcohol has a surface tension of approximately 27.1 mN/m.2 Henry et al.5 used water with a dissolved butanol concentration of 7 wt %, which has a surface tension of approximately 26 mN/m (Figure 1, modified from ref 9). Though myristyl alcohol and 7% butanol cause similar decreases in surface tension, the criteria of Karkare and Fort1 suggest that surfactantinduced water movement in the butanol system should be minimal since butanol is highly soluble and does not form a solid monolayer. To examine the differences between the flow that is induced by high- and low-solubility alcohols, we present experimental results from Henry et al.5 The 24-h change in gravimetric moisture content for duplicate horizontal sand columns containing either myristyl alcohol or 7% butanol is depicted in parts a and b of Figure 2, respectively. The right-hand side (RHS) of each column initially contained water and surfactant while the lefthand side (LHS) contained only pure water. Figure 2 qualitatively demonstrates that both myristyl alcohol and butanol can move significant quantities of water, though the two systems exhibit distinct differences. The general behavior observed in the myristyl alcohol experiments (Figure 2a) is very similar to that reported by Karkare and colleagues1,2,10,11 for the same surfactant. Surfactant-induced capillary pressure gradients between the LHS and RHS of the column caused water to drain from the RHS and resulted in wetting of the LHS. Drainage was confined to the RHS because myristyl alcohol was not transported by bulk advective flow at concentrations sufficient to extend the zone of depressed surface tension (9) Bikerman, J. J. Physical Surfaces; Loebl, E. M., Ed.; Physical Chemistry, a Series of Monographs 20; Academic Press: New York, 1970. (10) Karkare, M. V.; La, H. T.; Fort, T. Langmuir 1993, 9, 1684. (11) Karkare, M. V.; Fort, T. Langmuir 1996, 12, 2041.
10.1021/la026619r CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003
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Figure 2. (a) Experimental moisture content profiles for the myristyl alcohol (MA) system and (b) experimental moisture content profiles for the 7% butanol system. The soil columns were initially at a uniform moisture content and contained pure water in the left half and surfactant (myristyl alcohol or butanol) in the right half.
into the LHS. In contrast, the 7% butanol system exhibited a zone of drainage that propagated into the LHS as the zone of depressed surface tension propagated with the advance of the butanol solute front. The additional drainage that occurred in the LHS near the center of the column resulted in higher moisture contents at the left end of the column than those that were seen in the myristyl alcohol system. Another interesting feature of the butanol system that was not seen in the myristyl alcohol system is the gradual change in moisture content between approximately 3 and 7 cm along the column. This gradual change was caused by solute dispersion, which resulted in a surface tension gradient within the butanol solute front. To quantify a surfactant’s water moving effectiveness, Karkare et al.10 determined the amount of water moved within the column after 24 h and referred to that as the “specific water movement”. This value was calculated as the mass of water gained in the half of the column that was originally surfactant-free divided by the dry mass of sand in the column. With this method, the average amount of water moved in the myristyl alcohol experiments of Henry et al.5 was 0.019 (grams of water moved/grams of sand). Because the amounts of water that drained from the RHS of the butanol and myristyl alcohol sand columns in parts a and b of Figure 2 appear to be approximately the same, one might expect the calculated specific water movement for the butanol and myristyl alcohol systems to be the same. If the specific water movement for the butanol experiments is calculated using the method of Karkare and Fort (mass of water gained by the LHS divided by the mass of sand in the column), the average value is 0.018 (grams of water moved/grams of sand). This value is close to the value calculated for the myristyl alcohol system. However, the similar values for the myristyl alcohol and butanol experiments are misleading because they ignore the fact that the underlying flow behavior in the two systems was fundamentally different. The drainage of the RHS of the butanol system was actually less than that in the myristyl alcohol system, and a portion of the water that wetted the LHS of the butanol column came from drainage within the LHS. Additionally, the change in moisture content within the LHS of the butanol system varied spatially and had an average maximum change in water content (+7.2%) that was greater than the essentially uniform average change
in moisture content (+4.0%) seen in the myristyl alcohol system. The high mobility of butanol, relative to myristyl alcohol, also has an additional effect on flow. Though not shown here, the hysteretic unsaturated flow and transport modeling of Henry et al.6 predicts that backflow will occur in the butanol column, whereas Karkare et al.10 found that the myristyl alcohol system is stable for at least 2 months. The backflow in the butanol system is a result of gradual changes in surface tension that occur due to butanol diffusion after the large initial water displacement. We feel that the use of the “specific water movement” as a descriptor of flow and as an indicator of surfactants that are “effective water movers” fails to adequately capture the full nature of surfactant-induced flow and is inappropriate for use with some surfactants. When shortchain alcohols or other soluble surfactants are used, the quantitative description of water movement could be improved by redefining the descriptor to include information about the magnitude of spatial variations in moisture content. The need for categorizing surfactants using these types of descriptors could be precluded through the use of numerical models to simulate surfactant-induced flow since the models have been shown to be capable of capturing the flow behavior in both low- and high-solubility systems.5,6 For these modeling endeavors, concentrationand temperature-dependent surface tension data, such as those collected by Karkare, Fort, and others, will provide required input parameters, while the results from the flow experiments will be necessary for model calibration and validation of the simulation results. A final point to consider when examining the water moving characteristics of a surfactant is the degree to which the surfactant-induced flow perturbation will propagate. Though the experiments of Karkare and colleagues1,2,10,11 and Henry et al.5 demonstrate that an insoluble surfactant such as myristyl alcohol can impact flow within an area on the order of 10-20 cm, it is questionable whether an insoluble surfactant would have a significant impact on larger-scale flow. The propagation of the zone of depressed surface tension will be limited because the surfactant resides primarily at the air-water interface and is not transported into previously surfactantfree regions at concentrations high enough to produce solid monolayer coverage, a necessary condition for flow in insoluble surfactant systems.10 In contrast, soluble surfactants such as butanol, which can depress surface
Comments
tension over a wide range of concentrations and are readily transported by bulk flow, can impact flow and transport within a large volume of an unsaturated porous medium. Henry and Smith7 present the results from two-dimensional (2D) flow experiments related to the effect of dissolved butanol on flow in a synthetic aquifer with a shallow unsaturated zone. The 2D experiment used a laboratory-scale aquifer model that was 2.44 × 1.53 × 0.108 m. Butanol solution (7 wt %) was slowly applied at a constant rate at a point source located on the soil surface. After 8.4 days of butanol application, a surfactant-induced drainage zone that originated below the point source had propagated approximately 1.8 m in the direction of ambient groundwater flow. The advance of the butanol solute front resulted in drainage from initial moisture contents of approximately 25% (by volume) to moisture contents between 7 and 11% within a large portion of the
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tension-saturated zone above the water table. The drainage was due to surfactant-induced flow and occurred despite the fact that 7% butanol solution was applied at a constant rate at the point source. Eric J. Henry*,† and James E. Smith‡
Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, North Carolina 28403, and School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1 Received September 27, 2002 In Final Form: January 30, 2003 LA026619R
