Soil Science Society of America Journal 65:1392-1399 (2001)
© 2001 Soil Science Society of America
DIVISION S-1 - SOIL PHYSICS
Influence of a Nonionic Surfactant on the Water Retention Properties of Unsaturated Soils
Ahmet Karagunduza,
Kurt D. Pennell*,a and
Michael H. Youngb
a School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332
b Division of Hydrologic Sciences, Desert Research Institute, 755 E. Flamingo Road, Las Vegas, NV 89119
* Corresponding author (kpennell{at}ce.gatech.edu)
 |
ABSTRACT
|
|---|
Surfactants are widely used in household products, industrial processes and as adjuvants to improve the delivery and effectiveness of agrochemicals. Due to their amphiphilic nature, surfactants tend to accumulate at gas-liquid and solid-liquid interfaces, and thus, have the potential to influence water flow and retention in unsaturated soils. The objective of this study was to investigate the effects of a nonionic surfactant, Triton X-100, on the interfacial properties and capillary pressure-water content relationships of F-70 Ottawa sand and Appling soil. In the presence of surfactant, soil water contents decreased incrementally as the surfactant concentration was increased from 0 g L-1 up to the critical micelle concentration (CMC) of Triton X-100 (0.15 g L-1). Over the same surfactant concentration range, the surface tension of water decreased from 7.2 x 10-2 J m-2 to 3.2 x 10-2 J m-2 while solid-liquid contact angle decreased from 40° to 10°. No further changes in interfacial properties or soil water characteristics were observed at surfactant concentrations above the CMC. The experimental results were used to develop and evaluate alternative scaling approaches to describe concentration dependent changes in soil water characteristics based on the van Genuchten model. A scaling factor that incorporated both surface tension and content angle relationships provided accurate predictions of soil water retention curves over a range of surfactant concentrations. A simplified form of the scaling factor also was developed, on the basis of a single fitting parameter without the need for surface tension and contact angle data. Although further validation of the simplified scaling factor will be required, this approach offers an efficient means to describe the effects of concentration dependent changes in interfacial properties on soil water characteristics.
|
INTRODUCTION
|
|---|
SURFACE-ACTIVE AGENTS (surfactants) are used extensively in detergents, industrial processes, household products, and pesticide formulations. As a consequence of their widespread use and frequent resistance to biodegradation, surfactants may persist in wastewater treatment systems at relatively high concentrations (Wagener and Schink, 1987; Holt and Bernstein, 1992; Zoller, 1994; Moreno et al., 1994). Subsequent land application of wastewater treatment effluent and sludge as a source of irrigation water and nutrients can result in the indirect release of relatively large quantities of surfactant to the environment, particularly in developing countries. In addition, almost all herbicide formulations require some form of adjuvant to improve herbicide handling, delivery, and effectiveness. The most common types of surfactant adjuvants are tallow amine ethoxylates, nonylphenol and octylphenol ethoxylates, alcohol ethoxylates, block polymers, and castor oil ethoxylates (Hochburg, 1996). These surfactants typically comprise 5 to 10% (50 to 100 g L-1) of the formulated herbicide product and 0.1 to 0.5% (1 to 5 g L-1) of the spray tank solution. As a result, the amount of surfactant applied to agricultural lands can be substantial, particularly if repeated herbicide applications are required throughout the growing season.
Due to their amphiphilic properties, possessing both hydrophilic and lipophilic moieties, surfactants tend to accumulate or adsorb at gas-liquid and solid-liquid interfaces. Below the surfactant concentration at which micelles begin to form, the critical micelle concentration (CMC), interfacial adsorption of surfactants typically results in a lowering of both the surface tension of water and the contact angle between the solid and aqueous phases (Rosen, 1989). Therefore, the water holding capacity of soils is likely to decrease in the presence of surfactants. This phenomenon could increase the depth of infiltration during spray events, and create a positive pressure gradient between regions of surfactant-rich and surfactant-free soil water (Tschapek et al., 1981, 1991; Karkare et al., 1993; Karkare and Fort, 1993). Karkare and Fort (1993) reported that soil water content was substantially altered in the presence of surfactants, with higher water saturations observed in surfactant-free soil at the same pressure as soil containing surfactant. These findings indicate that surfactant-induced changes of interfacial properties may be an important factor influencing water flow and retention in unsaturated soils.
Although the potential effects of surfactants on soil water characteristics have been discussed in the literature, only limited research has been conducted to directly quantify the influence of surfactants on capillary pressure-water saturation relationships. One of the most commonly used soil water retention relationships was developed by van Genuchten (1980):
 | (1) |
where 
and n are fitting parameters, h is the capillary pressure head (m), 
is volumetric water content (m3 m-3), and r and s are the residual and saturated volumetric water contents (m3 m-3), respectively. To account for differences in the surface tension of aqueous solutions, the capillary pressure head term (h) in the van Genucthen (1980) relationship was modified by Smith and Gillham (1994) using the following scaling factor:
 | (2) |
where 
0 and 1 are the respective surface tension values of the reference solution (e.g., water) and the solution of interest (J m-2), and h0 and h1 are the capillary pressure heads reference solution and the solution of interest (m), respectively. The scaling relationship given in Eq. [2] was incorporated into a numerical model to illustrate the potential effects of a 7% n-butanol solution on unsaturated water flow (Smith and Gillham, 1994). More recently, Smith and Gillham (1999) conducted laboratory column experiments and numerical modeling studies to investigate the effects of concentration dependent changes in surface tension on unsaturated water flow and solute transport.
Capillary pressure-saturation relationships for two-phase, organic liquid and water, systems are frequently derived from soil water retention data (air and water) using a modified form of the Leverett (1941) function (e.g., Morrow, 1976; Lenhard and Parker, 1987; Kueper and Frind, 1991).
 | (3) |
where Pc is the capillary pressure (Pa) defined as Pn - Pw with the subscripts n and w referring to the non-wetting and wetting phases, respectively, Se is the effective saturation defined as (Sw - Swr)/(1 - Swr) with the subscript wr referring to the wetting phase residual, is the interfacial or surface tension (J m-2), 
is the contact angle, and the subscripts org, air, and H2O refer to the organic liquid phase, gaseous phase, and aqueous phase, respectively. In practice, the contact angle is often assumed to be zero and thus, cos is equal to unity. However, Demond and Roberts (1991) reported that the value of cosorg/H2O was significantly less than unity for weakly water-wet systems. In addition, Morrow (1975) and Demond and Roberts (1991) modified Eq. [3] to account for the effects of surface roughness and interface curvature on the contact angle. The utility of the modified Leverett (1941) function to describe organic liquid and water pressure-saturation relationships for systems containing a strongly-sorbed cationic surfactant, cetyltrimethylammonium bromide (CTAB), was subsequently demonstrated by Desai et al. (1992) and Demond et al. (1994). In an extension of this work, Lord et al. (1997a)(b) investigated the effects of octanoic acid speciation and concentration on interfacial properties, contact angle and capillary pressure-saturation relationships for two-phase systems (air-water and o-xylene-water). Although the utility of the modified Leverett (1941) scaling factor (Eq. [3]) was demonstrated in these studies, such an approach has not been evaluated using the van Genuchten (1980) relationship (Eq. [1]) for systems containing nonionic surfactants.
Surface tension and contact angle are strongly dependent on surfactant concentration, and have been shown to decline sharply until the CMC of the surfactant has been reached (Rosen, 1989). For this reason, scaling factors used to describe soil water retention characteristics in the presence of surfactants must account for concentration dependent properties of the system, and should be applicable at surfactant concentrations above and below the CMC. Thus, the objectives of this study were to (i) experimentally determine soil water retention characteristics as a function of surfactant concentration; (ii) evaluate the utility of several possible scaling approaches for use with the van Genuchten (1980) equation; and (iii) develop a simplified scaling approach that can be employed in the absence of surface tension and contact angle data. For the experimental phase of this study, soil water characteristics of a reference sand (F-70 Ottawa sand) and Appling soil were determined in the presence of a representative nonionic surfactant (Triton X-100). Surface tension and contact angle data were measured independently over a surfactant concentration range of 0 to 2.0 g L-1. Triton X-100 was selected for study because it is widely used as a detergent and as an adjuvant for pesticides and herbicides, exhibits toxicity toward aquatic species and is persistent in the environment (Narkis and Ben-David, 1985; Ahel et al., 1994; Renner, 1997).
|
MATERIALS AND METHODS
|
|---|
Experimental Materials
A reference quartz sand, F-70 Ottawa sand, and Appling soil were used as the solid phases for the soil water retention experiments. F-70 Ottawa sand (40-270 mesh) was obtained from U.S. Silica (Ottawa, IL), and was used as received. Appling soil was collected from the upper 30 cm of the soil profile, corresponding to the Ap1 and Ap2 horizons, at the University of Georgia Agricultural Experiment Station, located near Eastville, GA. The soil was classified as a loamy coarse sand of the Appling series (clayey, kaolinitic thermic Typic Hapludult). Prior to use, the Appling soil was air-dried and ground to pass a 2 mm (9-mesh) sieve. The specific surface areas of F-70 Ottawa sand and Appling soil were determined to be 1.5 x 102 m2 kg-1 and 3.5 x 103 m2 kg-1, respectively, on the basis of N2 adsorption at 77°K. Total organic carbon (TOC) analysis indicated that F-70 Ottawa sand contained no detectable organic carbon, while Appling soil contained 7.7 g OC kg-1.
Triton X-100 (an octylphenol ethoxylate) was obtained from the Union Carbide Corp. (Houston, TX), and was used as received without purification. The hydrophile-liphophile balance (HLB) of Triton X-100 is 13.5, the average number of ethylene oxide (EO) groups is 9.5, and the average molecular weight is 625 g mole-1 (Rosen, 1989). All aqueous surfactant solutions were prepared with deionized-distilled (DI) water containing 0.5 g L-1 CaCl2 as a background electrolyte and 0.5 g L-1 NaN3 to prevent biological activity. The dynamic viscosity of aqueous solutions, measured using an RS75 rheometer (Haake, Paramus, NJ) at 20°C, ranged from of 1.00 x 10-3 Pa s for distilled water to 1.05 x 10-3 Pa s for a 10 g L-1 solution of Triton X-100. Viscosities determined for the 10 g L-1 Triton X-100 solution were constant for shear rates ranging from 200 to 1000 s-1, which is indicative of Newtonian behavior.
Surface Tension and Contact Angle
The surface tension of aqueous surfactant solutions, ranging in concentration from 0 to 0.75 g L-1 Triton X-100, was determined by the du Nouy ring method. The apparatus consisted of a Cahn DCA 322 dynamic contact analysis system (Thermo-Haake, Paramus, NJ) connected to a personal computer. A platinum-iridium du Nouy ring was immersed in the test solution and then retracted through the gas-liquid interface. The surface tension of the solution was calculated from the force required to pull the ring through the interface, using a correction factor which incorporates the dimensions of the ring and solution density. Prior to use, the platinum-iridium ring was placed in a flame to oxidize contaminants.
Contact angles between aqueous surfactant solutions and solid surfaces were measured using a goniometer microscope equipped with a CCD camera and lens assembly (Ramè-Hart, Inc., Mountain Lakes, NJ). Glass (Superfrost pre-cleaned, Fisher Scientific, Pittsburg, PA) and quartz microscope slides (Technical Glass Products, Inc., Painesville TWP, OH) were utilized as the solid surfaces. Contact angle measurements were obtained using a sessile drop method and a captive air bubble method. For the sessile drop method, a quartz or glass microscope slide was positioned horizontally inside a rectangular quartz chamber (0.075 m length x 0.075 m height x 0.025 cm depth) on two quartz stands (0.01 m in height). The chamber was filled with approximately 10 mL of water and sealed with parafilm to maintain high relative humidity within the chamber, and to minimize evaporative losses. A single drop of aqueous solution was then formed on the microscope slide using a micro syringe (Gilmont, Great Neck, NY). A digitized image of the drop was captured using a personal computer connected to the Goniometer apparatus. The contact angle was obtained from this image by adjusting the cross hair of the telescope to attain tangency with the drop surface. Images were captured after an equilibration period of 5 to 10 min, which was sufficient for the shape of the drop to stabilize. Prior to use, the glass and quartz microscope slides were cleaned with methanol and DI water, soaked in a 5 M HNO3 solution overnight, and rinsed thoroughly with DI water. For the captive air bubble method, the microscope slides were placed in the quartz chamber, submerged in test solution, and allowed to equilibrate for 24 hr. An air bubble was then formed on the underside of the microscope slide and the contact angle was determined following the procedure described above. For each surfactant concentration and slide, contact angle measurements were performed in duplicate or triplicate.
Soil Water Retention Curves
Soil water pressure saturation relationships were determined for F-70 Ottawa sand and Appling soil at surfactant concentrations ranging from 0 to 2.5 g L-1 and 0 to 0.75 g L-1, respectively, using a Tempe cell system (Soil Moisture Equipment Corp., Santa Barbara, CA). Each Tempe cell (0.06 m height x 0.057 m i.d.) contained a porous ceramic plate with a bubbling pressure exceeding 1 atm. The Tempe cells were packed with air-dried soil under vibration in 0.01 m increments. Prior to water saturation, the packed Tempe cells were flushed with CO2 to allow for more rapid dissolution of the entrapped gas phase. The cells were then saturated with aqueous solutions containing Triton X-100. The saturation process was performed using a low-speed piston pump (Model QG-20, Fluid Metering Inc., Syosset, NY) equipped with a stainless steel pump head module at a flow rate of 30 mL hr-1 (8.3 x 10-9 m3 s-1) until the influent and effluent surfactant concentrations were identical. After allowing the cells to equilibrate for one day, several additional pore volumes of surfactant solution were flushed through columns at a flow rate of 30 mL hr-1 (8.3 x 10-9 m3 s-1). Once complete water saturation was achieved, pressure was applied to top of the cells in increments over a pressure range of 0 to 100 kPa (0 to 10 m H2O). A low-pressure regulator, configured in series with a nullmatic-type regulator (Soil Moisture Equipment Corp., Santa Barbara, CA), was used to apply pressure to the cells. Pressures ranging from 0 to 10 kPa (0 to 1 m H2O) were measured with a water manometer, while pressures ranging from 10 to 100 kPa (1 to 10 m H2O) were recorded directly from a pressure meter installed in the manifold. At each pressure increment, the cell was removed from the apparatus and the soil water content was determined gravimetrically by a digital balance with a resolution of ± 0.01 g (Model PB 3002, Mettler Toledo, Columbus, OH). Equilibrium conditions were assumed when the weight difference of the cells over two consecutive days was less than 0.1 g. To determine the moisture content of F-70 Ottawa sand and Appling soil at a pressure of 1500 kPa (150 m H2O), pressure plate measurements were conducted following the methods described by Klute (1986). All soil water retention experiments were performed in duplicate.
Experimental moisture release curve data for each surfactant concentration and soil type were fit to Eq. [1] by a nonlinear, least squares regression procedure (SYSTAT, ver. 5.03). The saturated and residual volumetric water contents, s and r, were determined from the endpoints of each moisture release curve, so that only the parameters and n were obtained from the fitting procedure. A Wilcoxon Rank Sum test procedure (SYSTAT, ver. 5.03) was used to evaluate statistical differences between paired soil water retention curves as a function of surfactant concentration. The ability of several different scaling approaches to describe experimental soil water retention curves as a function of surfactant concentration was evaluated by the Root Mean Square Error (RMSE):
 | (4) |
where, M is the measured value, P is the predicted value and n is the number of measured data points.
Analytical Methods
Aqueous-phase concentrations of Triton X-100 were measured with a Hewlett Packard (HP) Model 1100 high performance liquid chromatograph (HPLC) equipped with an HP Hypersil ODS column (12.5 cm length x 4 mm i.d., 5 µm particle size) and diode array detector. Triton X-100 is UV absorbent and was analyzed at a wavelength of 278 nm. The HPLC system was operated at a flow rate of 60 mL hr-1, with a sample injection volume of 100 µL. The mobile phase initially contained 20% acetonitrile and 80% water, increased to 100% acetonitrile and 0% water over a period of 3 min, remained constant for the next 6 min, and decreased to the initial values during the final 2 min of each analysis. All aqueous surfactant samples were analyzed in duplicate.
|
RESULTS AND DISCUSSION
|
|---|
Surface Tension and Contact Angle
The relationship between the surface tension of water and the concentration of Triton X-100 is shown in Fig. 1
. The measured surface tension decreased from 7.2 x 10-2 to 3.2 x 10-2 J m-2 as the concentration of Triton X-100 was increased from 0 to approximately 0.15 g L-1. Above a surfactant concentration of 0.15 g L-1, the surface tension remained essentially constant. This inflection point corresponds to the CMC of Triton X-100, and indicates the concentration at which surfactant monomers aggregate in solution to form micelles (Rosen, 1989). Above the CMC, the concentration surfactant monomers in solution remains constant, while the number of micelles increases with increasing surfactant concentration. Literature values for the CMC of Triton X-100 range from 0.11 to 0.16 g L-1 (Ross and Olivier, 1959; Kile and Chiou, 1989; Edwards et al., 1991).
Contact angle measurements as a function of Triton X-100 concentration are presented in Table 1 for both glass and quartz slides. In the absence of surfactant and electrolyte, measured contact angles ranged from approximately 36° to 47°, depending on the experimental method and the solid surface. In general, the addition of background electrolyte (0.5 g L-1 NaN3 + 0.5 g L-1 CaCl2) caused a slight increase in the measured contact angle. These findings are consistent with data reported by Wu et al. (1994), who determined the contact angle between DI water and glass powder, on the basis of thin layer wicking, to be 49° and 66° in the absence and presence of electrolyte (0.28 g L-1 CaCl2), respectively. Ethington (1990) also reported contact angles of 29° and 42° for water drops on two different quartz slides. A number of researchers, however, obtained much smaller contact angle values, ranging from 2° to 7°, for water drops and air bubbles on quartz slides (e.g., Desai et al., 1992; Lord et al., 1997b). The larger contact angle values reported herein are most likely the results of differences in the experimental method, cleaning procedure and properties of the solid surface, and indicate that the surfaces of the quartz and glass slides were slightly hydrophobic or weakly water-wetting. This condition may, in fact, be more representative of natural soils, which are likely to contain organic matter and would not be subject to rigorous cleaning. For example, Bachmann et al. (2000) obtained advancing contact angles values ranging from 22° to 94° for a sandy soil, using a modified sessile drop approach in which a glass slide was covered with a single layer of soil particles.
View this table:
[in this window]
[in a new window]
|
Table 1. Contact angle values for deionized water, the reference electrolyte solution and Triton X-100 solutions.
|
|
In the presence of Triton X-100 the contact angle decreased from approximately 40° to 10° as the surfactant concentration increased from 0.001 to 0.15 g L-1, regardless of the experimental method. At concentrations greater than 0.15 g L-1, which corresponds to the measured CMC of Triton X-100, further reductions in contact angle were not observed. These results are consistent with data reported by Warr et al. (1984), who observed a sharp reduction in contact angle with increasing concentration of nonylphenol ethoxylate on methylated (hydrophobic) quartz plates. For untreated (hydrophilic) quartz plates, the measured contact angle increased gradually from 12° to 25° as the surfactant concentration was increased to the CMC (Warr et al., 1984). The observed changes in contact angle with surfactant concentration were attributed to the formation of an adsorbed surfactant monolayer on the quartz surface; the hydrophobic surfactant moiety oriented outward for untreated (hydrophilic) quartz and the hydrophilic surfactant moiety oriented outward for methylated (hydrophobic) quartz. Similar conceptual models were presented by Clunie and Ingram (1983) to describe the adsorption of nonionic surfactants on polar and nonpolar surfaces.
Several other researchers, however, have observed an increase and subsequent decrease in the contact angle as the concentration of Triton X-100 increased from 0 g L-1 to the CMC. For example, Gonzalez and Travalloni-Louvisse (1989) found that the contact angle between quartz and Triton X-100 solutions increased from 20° at 0.0001 g L-1, remained constant at 38° from 0.002 to 0.02 g L-1, and then decreased to 10° at a concentration of approximately 0.1 g L-1. Li and Gu (1985) observed a similar relationship between concentration of Triton X-100 and contact angle, which increased to a maximum value of 34° at 0.05 g L-1 and then decreased to 5° at 1.0 g L-1. At low surfactant concentrations, adsorption of Triton X-100 was attributed to interactions between hydrophilic surfactant moiety and silanol or hydoxyl groups on the quartz surface. In this scenario, the hydrophobic tail of the adsorbed surfactant monomer would be oriented outward, thereby increasing the contact angle with water. When the concentration of Triton X-100 was further increased, the authors proposed that interactions between the hydrophobic moieties of surfactant monomers led to the formation of an adsorbed surfactant bilayer. Under these conditions, the hydrophilic moiety of the surfactant would be oriented outward, thereby reducing the contact angle with water. These experimental data and proposed explanations are consistent with contact angle data obtained in the presence of cationic surfactants (e.g., Desai et al., 1992).
Soil Water Retention Curves
Soil water retention curves for F-70 Ottawa sand and Appling soil measured at Triton X-100 concentrations ranging from 0 to 2.5 g L-1 and 0 to 0.75 g L-1, respectively, are shown in Fig. 2
. Duplicate capillary pressure-soil water saturation experiments (Tempe cells) were conducted for each soil and surfactant concentration, although average values are plotted in Fig. 2 due to the large number of data points. In the presence of Triton X-100, soil water retention curves for both F-70 Ottawa sand and Appling soil were significantly different (P < 0.05) from those obtained for the reference electrolyte solution, with the exception of F-70 Ottawa sand at a surfactant concentration of 0.05 g L-1. For example, at a negative pressure head of 0.23 m H2O (2.3 kPa) the volumetric water content of F-70 Ottawa sand decreased by 45.5% at a surfactant concentration of 0.750 g L-1, compared with water content values obtained in the absence of surfactant (Fig. 2a). Similar behavior was observed for Appling soil, although the reductions in volumetric water content were not as dramatic as those observed for F-70 Ottawa sand (Fig. 2b). In all but two cases, soil water retention curves determined in the presence of Triton X-100 were significantly different (P < 0.1) from one another at surfactant concentrations below the CMC (0.15 g L-1). For surfactant concentrations above the CMC, however, no significant differences (P > 0.05) in soil water retention curves were observed, with the exception of F-70 Ottawa sand at a surfactant concentration of 1.5 g L-1.
View larger version (26K):
[in this window]
[in a new window]
|
Fig. 2. Soil water retention curves for (A) F-70 Ottawa sand and (B) Appling soil as a function of Triton X-100 concentration.
|
|
These results demonstrate that the presence of Triton X-100 significantly altered soil water retention properties of both F-70 Ottawa sand and Appling soil. The observed changes in soil water characteristics correspond to independently measured reductions in surface tension and contact angle as a function of increasing Triton X-100 concentration below the CMC. To account concentration dependent changes in surface tension and contact angle on soil water characteristics, a scaling factor similar in form to Eq. [3] was employed:
 | (5) |
where the subscripts 0 and 1 refer to the reference solution, in this case DI water + 0.5 g L-1 NaN3 + 0.5 g L-1 CaCl2, and the aqueous surfactant solution, respectively. The pressure head scaling factor given in Eq. [5] was substituted for the pressure head (h) in the van Genuchten (1980) equation (Eq. [1]), to obtain the following capillary pressure-volumetric water content expression.
 | (6) |
The relationship between the surface tension-contact angle scaling factor (Eq. [5]) and the concentration of Triton X-100 below the CMC is shown graphically in Fig. 3
. Contact angle data obtained for captive air bubbles on glass microscope slides were used to calculate the scaling factors shown in Fig. 3. Although a slightly non-linear relationship was observed, as a first approximation, the scaling factor was assumed to be linear over a surfactant concentration range of 0 g L-1 to the CMC (0.15 g L-1). By definition, the pressure head scaling factor is equal to unity in the absence of surfactant, and thus, the scaled van Genuchten (1980) relationship (Eq. [6]) can be expressed as:
 | (7) |
where C is the aqueous concentration of Triton X-100 below the CMC, and is equal to the CMC at concentrations equal to and above the CMC, and ß is the slope of the linear regression shown in Fig. 3. As can be seen from Eq. [7], the term (1 + ßC) is equal to at a surfactant concentration 0 g L-1, and is equal to CMC at the CMC. Thus, if water retention characteristics of a soil are known at the CMC of a surfactant, ß can be estimated by fitting the experimental data to Eq. [7]. A ß value of 4.47 was obtained from the linear regression shown in Fig. 3.
View larger version (16K):
[in this window]
[in a new window]
|
Fig. 3. Relationship between the surface tension-contact angle scaling factor and the concentration of Triton X-100.
|
|
Comparison of Scaling Approaches
Measured soil water retention curves were described using both un-scaled and scaled van Genuchten (1980) relationships. For the reference case (Case I), each soil water retention curve was fit to the van Genuchten (1980) equation to obtain unique values of and n (data shown in Table 2). In the second approach (Case II), the effects of surfactant concentration were ignored and soil water retention curves were based on the values of and n obtained for the reference electrolyte solution. This approach neglected any dependence of surface tension and contact angle on surfactant concentration, and represents a worst-case scenario in which a single water retention relationship was applied, regardless of surfactant concentration. In the third approach (Case III), the pressure head was scaled using only the measured surface tension ratio (0/1), in a manner similar to that employed by Smith and Gillham (1994)(1999) for a 7% butanol solution. In the fourth approach (Case IV), both the surface tension and contact angle relationships were incorporated in the scaling factor as shown in Eq. [5]. The final approach (Case V) was based on the use of a single scaling factor, ß, determined from soil water retention curves obtained in the absence of surfactant and at the CMC of the Triton X-100 (Eq. [7]).
Comparisons between measured data and predicted soil water retention curves for F-70 Ottawa sand at surfactant concentrations below (0.075 g L-1) and above (0.75 g L-1) the CMC of Triton X-100 are shown in Fig. 4a and 4b
, respectively. When a scaling factor was not included (Case II), predictions of soil water content were much larger than the measured values. In contrast, the use of a scaling factor that included only the surface tension ratio (Case III) resulted in underestimation of soil water content values. Incorporating the combined surface tension-contact angle scaling factor (Case IV) resulted in greatly improved fits to the experimental data. Predictions of soil water characteristics using the final approach (Case V), in which a single scaling factor was employed, were similar to those obtained for Case IV, even though Case V required one less fitting parameter.
View larger version (37K):
[in this window]
[in a new window]
|
Fig. 4. Predicted and measured soil water retention curves for F-70 Ottawa sand at Triton X-100 concentrations of (A) 0.075 g L-1 and (B) 0.75 g L-1.
|
|
Results obtained for Appling soil (Fig. 5a and 5b)
were similar to those observed for F-70 Ottawa sand, where predictions based on Case II and Case III led to overestimation and underestimation of measured soil water contents, respectively. The degree of underestimation for Appling soil (Case III), however, was much less than that observed for F-70 Ottawa sand. Case IV and Case V predictions for Appling soil were virtually identical, and could not be distinguished (Fig. 5b). These findings demonstrate that the single fitting parameter (ß), Eq. [7], provided excellent fits to the experimental data without the need for concentration dependent surface tension and contact angle data.
View larger version (36K):
[in this window]
[in a new window]
|
Fig. 5. Predicted and measured soil water retention curves for Appling soil at Triton X-100 concentrations of (A) 0.075 g L-1 and (B) 0.75 g L-1.
|
|
Results of the RMSE analysis, based on measured and predicted pressure-saturation data for F-70 Ottawa sand and Appling soil, are presented in Fig. 6a and 6b
, respectively, for all surfactant concentrations considered. As expected, the reference approach (Case I), in which van Genuchten equation was directly fit to each experimental data set, yielded the lowest RMSE values. In general, Case II (no scaling) and Case III (scaling with surface tension only) yielded the highest RMSE values. In most cases, use of the combined surface tension-contact angle scaling factor (Case IV) gave lower RMSE values than were observed for Case II and III. Furthermore, the RMSE values obtained for Case IV and Case V (single fitting parameter, ß) were similar, indicating that the latter approach provided accurate predictions of soil water characteristics for these surfactant-soil systems.
View larger version (60K):
[in this window]
[in a new window]
|
Fig. 6. Root Mean Square Error (RMSE) values based on predicted and measured soil water retention data for (A) F-70 Ottawa sand and (B) Appling soil.
|
|
|
CONCLUSIONS
|
|---|
The presence of a nonionic surfactant (Triton X-100), even at relatively low concentrations, strongly influenced interfacial properties and soil water characteristics of F-70 Ottawa sand and Appling Soil. When the van Genuchten (1980) relationship obtained in the absence of surfactant was employed without a concentration-dependent scaling factor (Case II), predicted soil water contents were substantially larger than values measured in the presence of surfactant. A scaling approach based only on changes in surface tension as a function of surfactant concentration (Case III) did not substantially improve the predictive capability of the van Genuchten (1980) relationship, and resulted in underestimation of measured soil water contents. However, the use of a scaling factor that incorporated surface tension and contact angle dependence on surfactant concentration (Case IV) accurately described soil water retention curves for F-70 Ottawa sand and Appling soil over a wide range of surfactant concentrations. In the absence of surface tension and contact angle data, a simplified scaling factor was developed on the basis of a single fitting parameter (ß), derived from pressure-water saturation data measured in the absence of surfactant and at the CMC of the surfactant. The simplified scaling approach provided an efficient means to predict soil water characteristics in the presence of Triton X-100, and may be applicable to other solute-soil systems subject to concentration dependent changes in surface tension and contact angle.
|
ACKNOWLEDGMENTS
|
|---|
The authors thank Ms. Lee Ogden, Daniel B. Warnell School of Forest Resources at the University of Georgia, for conducting the 1500 kPa pressure plate analysis, and the Union Carbide Corporation for supplying Triton X-100. Funding for this research was provided in part by the Herty Foundation, Research and Development Center, Traditional Industries Program in Pulp and Paper. The content of this publication does not necessarily represent the views of the Foundation and no endorsement should be inferred.
Received for publication June 12, 2000.
|
REFERENCES
|
|---|
- Ahel, M., W. Giger, and C. Schaffner. 1994. Behavior of alkylphenol polyethoxylate surfactants in the aquatic environment-II. Occurance and transformation in rivers. Water Res. 28:1143–1152.
- Bachmann, J., R. Horton, R.R. van der Ploeg, and S. Woche. 2000. Modified sessile drop method for assessing initial soil-water contact angle in sandy soil. Soil Sci. Soc. Am. J. 64:564–567.[Abstract/Free Full Text]
- Clunie, J.S., and B.T. Ingram. 1983. Adsorption of nonionic surfactants. p. 105–152. In G.D. Parkitt and C.H. Rochester (ed.) Adsorption from Solution at the Solid-Liquid Interface. Academic Press, New York.
- Demond, A.H., and P.V. Roberts. 1991. Effect of interfacial forces on two-phase capillary pressure-saturation relationships. Water Resour. Res. 27:423–437.
- Demond, A.H., F.N. Desai, and K.F. Hayes. 1994. Effect of cationic surfactants on organic liquid-water capillary pressure-saturation relationships. Water Resour. Res. 30:333–342.
- Desai, F.N., A.H. Demond, and K.F. Hayes. 1992. Influence of surfactant sorption on capillary pressure-saturation relationships. p. 133–148. In D. Sabatini and R. Knox (ed.) Transport and Remediation of Subsurface Contaminants: Colloidal, Interfacial and Surfactant Phenomena. American Chemical Society, Washington, D.C.
- Edwards, D.A., R.G. Luthy, and A. Lui. 1991. Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions. Environ. Sci. Technol. 25:127–133.
- Ethington, E.F. 1990. Interfacial contact angle measurements of water, mercury and 20 organic liquids on quartz, calcite, biotite, and Ca-montmorillonite substrates. USGS-OFR 90-409.
- Gonzalez, G., and A.M. Travalloni-Louvisse. 1989. The effect of Triton X-100 and ethanol on the wettability of quartz. Langmuir 5:26–29.
- Hochberg, E.G. 1996. The market for agricultural pesticide inert ingredients and adjuvants. p. 203–208. In C.L. Foy and D.W. Pritchard (ed.) Pesticide Formulation and Adjuvant Technology, CRC Press, Boca Raton, FL.
- Holt, M.S., and S.L. Bernstein. 1992. Linear Alkylbenzenes in sewage sludges and sludge amended soils. Water Res. 26:613–624.
- Karkare, M.V., H.T. La, T. Fort. 1993. Criteria for effectiveness of surfactants as water moving agents in unsaturated wet sand. Langmuir 9:1684–1690.
- Karkare, M.V., and T. Fort. 1993. Water movement in unsaturated porous media due to pore size and surface tension induced capillary pressure gradients. Langmuir 9:2398–2403.
- Keuper, B.H., and E.O. Frind. 1991. Two-phase flow in heterogeneous porous media. 2. Model application. Water Resour. Res. 27:1059–1070.
- Kile, D.E., and C.T. Chiou. 1989. Water solubility enhancements of DDT and trichlorobenzene by some surfactants below and above the critical micelle concentration. Environ. Sci. Technol. 23:832–838.
- Klute, A. 1986. Water Retention: Laboratory Methods. p. 635–660. In A. Klute (ed.) Methods of soil analysis. Part 1. Physical and mineralogical methods. 2nd ed. Agron. Monogr. No. 9. ASA and SSSA, Madison, WI.
- Lenhard, R.J., and J.C. Parker. 1987. Measurement and prediction of saturation-pressure relationships in three-phase porous media systems. J. Contam. Hydrol. 1:407–424.
- Leverett, M.C. 1941. Capillary behavior in porous solids. Trans. Am. Inst. Min. Metall. Pet. Eng. 142:152–169.
- Li, W., and T. Gu. 1985. Equilibrium contact angles as a function of the concentration of nonionic surfactants on quartz plate. Colloid Polymer Sci. 263:1041–1043.
- Lord, D.A., K.F. Hayes, A.D. Demond, and A. Salehzadeh. 1997a. Influence of organic acid solution chemistry on subsurface transport properties. 1. Surface and interfacial tension. Environ. Sci. Technol. 31:2045–2051.
- Lord, D.A., A.D. Demond, A. Salehzadeh, and K.F. Hayes. 1997b. Influence of organic acid solution chemistry on subsurface transport properties. 2. Capillary pressure-saturation. Environ. Sci. Technol. 31:2052–2058.
- Morrow, N.R. 1975. The effects of surface roughness on contact angle with special reference to petroleum recovery. J. Can. Petrol. Technol. 14:42–53.
- Morrow, N.R. 1976. Capillary pressure correlations for uniformly wetted porous media. J. Can. Petrol. Technol. 15:49–69.
- Moreno, A., J. Ferrer, F.R. Bevia, D. Prats, B. Vazques, D. Zarzo. 1994. LAS monitoring in a lagoon treatment plant. Water Resour. 28:2183–2189.
- Narkis, N., and B. Ben-David. 1985. Adsorption of non-ionic surfactants on activated carbon and mineral clay. Water Resour. 19:815–824.
- Renner, R. 1997. European bans on surfactant trigger transatlantic debate. Environ. Sci. Technol. 31:316A–320A.
- Rosen, M.J. 1989. Surfactants and Interfacial Phenomena. John Wiley & Sons, New York.
- Ross, S., and J.P. Olivier. 1959. A new method for the determination of critical micelle concentration of un-ionized associated colloids in aqueous or in non-aqueous solution. J. Phys. Chem. 63:1671–1674.
- Smith, J.E., and R.W. Gillham. 1994. The effect of concentration-dependent surface tension on the flow of water and transport of dissolved organic compounds: A pressure head-based formulation and numerical model. Water Resour. Res. 30:343–352.
- Smith, J.E., and R.W. Gillham. 1999. Effect of solute concentration-dependent surface tension on unsaturated flow: Laboratory and sand column experiments. Water Resour. Res. 35:973–982.
- Tschapek, M., C. Wasowski, and R.M. Torres Sanchez. 1981. Tangential force and water migration in unsaturated disperse material. Colloids and Surfaces 3:295–298.
- Tschapek, M., C. Wasowski, and S. Falasca. 1991. Boundary water (solution) as a medium for spreading of surfactants. Colloids and Surfaces 55:1–8.
- van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898.[Abstract/Free Full Text]
- Wagener, S., and B. Schink. 1987. Anaerobic degradation of nonionic and ionic surfactants in enrichment cultures and fixed bed reactors. Water Resour. 21:615–622.
- Warr, C.G., P. Scales, F. Grieser, J.R. Aston, D.R. Furlong, and T.W. Healy. 1984. Polydisperse non-ionic surfactants: Their solution chemistry and effect on the wettability of solid surfaces. p. 1329–1338. In K.L. Mittal and B. Lindman (ed.) Surfactants in Solution, Volume 2. Plenum Press, New York.
- Wu, W., R.F. Giese, Jr., and C.F. van Oss. 1994. Linkage between x–potential and electron donicity of charged polar surfaces. 1. Implications for mechanisms of flocculation of particle suspensions with plurivalent counterions. Colloids Surfaces 89:241–252.
- Zoller, U. 1994. Non-ionic surfactants in reused water: Are activated sludge/soil aquifer treatments sufficient? Water Resour. 28:1625–1629.
This article has been cited by other articles:
|
|
|
|
 
E. J. Henry, E. J. Henry, and J. E. Smith
Surfactant-Induced Flow Phenomena in the Vadose Zone: A Review of Data and Numerical Modeling
Vadose Zone J.,
May 1, 2003;
2(2):
154 - 167.
[Abstract]
[Full Text]
[PDF]
|
|
|
TOP
ABSTRACT
INTRODUCTION
