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SOIL PHYSICS

Upward Infiltration into Porous Media as Affected by Wettability and Anionic Surfactants

^a Graduate School of Environmental Science, Okayama Univ., 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
^b Toray Industries, Inc., Chuo-ku, Tokyo 103-8666, Japan

^* Corresponding author (ishi{at}cc.okayama-u.ac.jp).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The influence of surfactants on water infiltration in soil is not fully understood. The objective of this study was to propose a model for evaluating effects of an anionic surfactant on upward infiltration under saturated conditions in porous materials with highly contrasting wettability. The simplified equation for upward infiltration based on Darcy's law is equivalent to the widely used Washburn equation. We experimentally determined upward infiltration of sodium dodecyl sulfate (SDS) solution (0–700 mol m^–3) into 60-cm-long, 2-cm-diameter columns filled with air-dry materials (glass beads, sand, leaf mold, peat moss, or polyethylene particles). In hydrophilic glass beads and sand, the infiltration rate decreased as the SDS concentration increased due to a decrease in solution surface tension (from 72 to 38 mN m^–1). The proposed model could describe the infiltration in all materials and at all concentrations when fitting to the initial parts of the curve of infiltration front vs. time. Contact angles were obtained by fitting the model to the measured height of the infiltration front in the saturated range as a function of time. In columns filled with hydrophobic materials, the infiltration rate increased with SDS concentration, corresponding with a decrease in contact angles from >125 to 69° for polyethylene particles and from 102 to 43° for peat moss. In leaf mold, the infiltration rate decreased as the SDS concentration increased, probably due to swelling. The proposed equation was found useful for calculating saturated hydraulic conductivity and contact angles but limited in the case of swelling porous material.

Abbreviations: SDS, sodium dodecyl sulfate

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Surfactants are used in detergents, shampoos, and chemical fertilizers as an anticaking agent, in agricultural chemicals as an emulsifying agent, and other uses. Surfactants are also used for remediating contaminated soil, enhancing oil recovery (West and Harwell, 1992), and ameliorating soil water repellency (Cisar et al., 2000; Kostka, 2000). Enormous quantities have been used (approximately 15 million Mg of soap and synthetic surfactants in the world per year), and they have been discharged into the environment (Lewis, 1991). Because surfactants degenerate cells (Sakashita, 1979), they strongly affect living organisms and ecosystems (Lewis, 1991). Their overall influence on the soil environment is not fully understood.

Solid surface characteristics are changed when surfactants are adsorbed on solid surfaces (Koopal et al., 1999). Surfactants also decrease water surface tension (Hiemenz, 1986, p. 391–398). Therefore, surfactants influence water and solute movement in soils. The influence of surfactant concentration on unsaturated flow caused by the depression of surface tension has been reported for sands (Karkare and Fort, 1993; Smith and Gillham, 1994, 1999; Henry et al., 1999; Henry and Smith, 2002). Surfactant application increased the infiltration rate into hydrophobic soils consisting of sands coated with organic compounds (Pelishek et al., 1962; Feng et al., 2002). The application of a nonionic surfactant either before or during irrigation increased the dispersion of a hydrophobic sandy loam (Mustafa and Letey, 1969), and this dispersion decreased flow rates in the soil (Miller et al., 1975). Soil hydraulic conductivity decreases when sodium dodecyl sulfate (SDS) is applied due to precipitation of the divalent electrolyte dodecylsulfate (Liu and Roy, 1995). Both nonionic and ionic surfactants induce hydraulic conductivity reduction in loam and sand (Allred and Brown, 1994). Most organic compounds are surface active in aqueous solution and can reduce water surface tension (Henry and Smith, 2002), and may change the contact angle of a porous material (Koopal et al., 1999).

Because the liquid–solid contact angle is an index of soil wettability, it has been measured by the capillary rise method using Poiseuille's approximation (Letey et al., 1962), the Washburn equation (Michel et al., 2001; Goebel et al., 2004), or Darcy's law (Emerson and Bond, 1963; Nakaya et al., 1977). The Washburn equation has been commonly used to determine contact angles of powders or porous solids (Hiemenz, 1986, p. 335–338). Capillarity, which is defined by the contact angle, the surface tension, and the equivalent pore radius, is experimentally determined during transient flow in Washburn's method. Equilibrium height is not used to determine the capillarity, because the method is considered during transient flow. The contact angles of the fractions <63 and 63 to 100 µm for sandy soil measured with the sessile drop method compared reasonably well with those measured with the capillary rise method (Bachmann et al., 2000).

Considering their importance, surfactant effects should be properly included in water flow models; however, such effects are neglected in the standard flow models based on capillarity by assuming that the contact angles are zero. The assumption may hold for many natural conditions; however, for hydrophobic porous materials that originate from organic wastes and composts and for anionic surfactants, the contact angle assumption is invalid. Because the Washburn equation is based on Poiseuille's law, saturated hydraulic conductivity does not appear in the equation. When an infiltration equation is based on Darcy's law, the flow can be described with the saturated hydraulic conductivity. Capillary infiltration front vs. time has been given in the Washburn equation (Marmur, 2003). A similar relationship based on Darcy's law has not been obtained, however.

One problem is that analytical techniques and methods that allow simple, standardized evaluation of surfactant effects for diverse soils and porous materials do not exist. The objective of this study was to propose a model for evaluating the effects of an anionic surfactant on upward infiltration under saturated conditions in porous materials with highly contrasting wettability. We propose an equation by capillary action under gravity based on Darcy's law. To determine the effect of an anionic surfactant on infiltration in porous materials, we provide a theoretical explanation of the influences of an anionic surfactant on upward infiltration in glass beads, sand, leaf mold, peat moss, and polyethylene particles.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
We used glass beads (soda-lime glass BZ-01, As One Corp., Osaka, Japan), sand (silica sand, Toyoura Keiseki Kogyo Corp., Yamaguchi, Japan), leaf mold (as sold for general garden use), peat moss (as sold for general garden use), and polyethylene particles (Hi-zex Million 340M, Mitsui Chemicals, Tokyo) for the experiments. Their physical characteristics are listed in Table 1 . The sand, leaf mold, and peat moss were sieved through a 0.85-mm sieve, and the residue remaining on a 0.075-mm sieve was used. The leaf mold and peat moss were crushed before sieving. Because the radius of glass beads and polyethylene particles were near 0.06 and 0.08 mm, they were used without sieving. The glass beads were washed with toluene, heated at 450°C, and washed with 600 mol m^–3 HCl. All materials were washed with 1 mol m^–3 NaCl and pure water, then dried in an oven at 30°C for about 24 h before the experiments. The arithmetic mean radius of each material was obtained from the ratio of the dry mass of each residue on 0.425-, 0.250-, 0.106-, and 0.075-mm sieves (no replication). The density of each of the other materials was determined using a pycnometer. The averages of triplicated data are listed in Table 1. The calculation method for porosity in Table 1 is described next and those for equivalent pore radius and suction are described below.

View larger version (39K): [in this window] [in a new window]   Fig. 1. Schematic diagram of the setup for the upward infiltration experiment.

View larger version (53K): [in this window] [in a new window]   Fig. 2. Height of infiltration front during upward infiltration of sodium dodecyl sulfate (SDS) solution into columns filled with (a) glass beads, (b) sand, (c) leaf mold, (d) polyethylene particles, and (e) peat moss. Symbols are measured values. Solid lines denote the calculated values that were obtained from fitting the measured data. Dotted lines continuing from the solid lines are model predictions outside the valid range using the same parameters as before. The lines are fitted with the measured values for 0, 7, 70, and 700 mol m–3 SDS solutions.

The saturated material column prepared in the swelling experiment was used for the measurement of saturated hydraulic conductivity, which was conducted using the falling-head method (Klute and Dirksen, 1986). The measurement was taken three times for each column. The measured values are given in Table 3 .

	UPWARD INFILTRATION THEORY

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

[1]

where q is the water flux density (m³ m^–2 s^–1), is the volumetric water content (m³ m^–3), v is the pore water velocity (m s^–1), x is the distance of the infiltration front from the inflow boundary of the column (m), t is time (s), k is the saturated hydraulic conductivity (m s^–1), and H is the hydraulic head difference between the inflow boundary and the infiltration front (m).

When we adopt the conditions of the upward infiltration experiment (Fig. 1) to Eq. [1], we get

[2]

[3]

where L ( = 0.2 m) is the depth of the bottom of the column from the water surface, h is the suction at the infiltration front during infiltration (m), is the surface tension of the solution (N m^–1), is the advancing contact angle (°), is the density of the solution (kg m^–3), g is the gravitational acceleration (9.81 m s^–2), and r is the equivalent pore radius in the material column (m). The value h in Eq. [3] is sometimes denoted as a capillary rise in a tube whose radius is r. Equation [3] was originally derived from the Laplace equation, however. In this equation, h is not a capillary rise but the suction at the infiltration front during infiltration into dry porous materials. The value h can be assumed to be constant when porous materials are saturated under an infiltration front during infiltration (Emerson and Bond, 1963; Hillel, 1980, p. 13–16; Tabuchi, 1995). Solving Eq. [2] under the condition that h, k, and are constant when x = 0 at t = 0 and x = x at t = t, we get

[4]

This equation is limited to applications when porous materials are saturated under an infiltration front during infiltration, because h and are assumed to be constant values and k is the saturated hydraulic conductivity.

It should be noted that, for a cylindrical tube, it can be shown that Eq. [4] is equivalent to the Washburn equation. The average velocity in a cylindrical tube, v, is

[5]

where r is the radius of the cylinder (m) and µ is the viscosity of the liquid (N s m^–2). Substituting Eq. [5] into Eq. [1], we get

[6]

When we consider L = 0 m and substituting Eq. [6] into Eq. [4], we get the following Washburn equation (Marmur, 2003):

[7]

That is, the newly derived upward infiltration Eq. [4] is physically equivalent to the Washburn Eq. [7]. In contrast to Eq. [7], the proposed new Eq. [4] contains the saturated hydraulic conductivity, k. Therefore, k can be determined from Eq. [4] as described below. Validity of the new Eq. [4] could also be evaluated by comparing calculated and measured k.

The advancing contact angle, , can be calculated from Eq. [3] when h and are known. The values h and /k in Eq. [4] were determined by best fitting x and t with the experimental values. The value r in Eq. [3] was derived from the upward infiltration experiment for ethanol and calculated with Eq. [4], provided = 0 for ethanol (Letey et al., 1962); r was calculated with the derived h and Eq. [3]. The calculated r and h for each material are listed in Table 1 and the calculated is listed in Table 2. The h values indicate the suction at the infiltration front when porous materials are saturated under an infiltration front, as defined above.

Saturated hydraulic conductivity, k, in the upward infiltration experiment was evaluated using the derived value /k. In this calculation, porosity, which had already been derived, was used instead of , assuming it was almost equal to .

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Validity of the Upward Infiltration Equation
The proposed upward infiltration Eq. [4] is a Green–Ampt type equation. This type of equation has been found to apply quite satisfactorily for cases of infiltration into initially dry, coarse-textured soils (Hillel, 1980, p. 13–16). For cases of upward infiltration into dry sand, this type of theory agrees well with the experimental data collected when a saturated condition is maintained (Emerson and Bond, 1963; Tabuchi, 1995). When the infiltration front rises to some extent, large pore cells will become unsaturated and small pore cells will be saturated (Emerson and Bond, 1963). Then the suction at the infiltration front, h, the volumetric water content, , and the hydraulic conductivity, k, will change. Because we intend to apply the proposed upward infiltration Eq. [4] to saturated infiltration, the fitted parameters h and /k in Eq. [4] are constant in the calculation during infiltration. This condition cannot be applied to unsaturated infiltration. Saturated infiltration occurs only in the initial stage. Therefore, Eq. [4] can be applied only in the initial stage of the infiltration. We calculated x and t by best fitting them with the measured values of the initial stage using Eq. [4].

The measured data and the calculated values for the upward infiltration experiments are shown in Fig. 2. The height of the infiltration front in Fig. 2 denotes the distance from the fixed water table as described in Fig. 1. Solid lines denote the calculated values that were obtained from fitting the measured data. Dotted lines continuing from the solid lines are model predictions outside the valid range using the same parameters as before. The dotted lines deviate from the data, probably due to unsaturated conditions. The lines (Fig. 2) are fitted to the measured values for 0, 7, 70, and 700 mol m^–3 SDS solutions. Figure 2 shows that the results for the contrasting soil materials and the different SDS solution concentrations could be well described (i.e., valid for the solid lines only) with Eq. [4]. Standard deviations of the heights of infiltration fronts and correlation ratios of the heights with time for solid lines were calculated from measured values. Each standard deviation was calculated with the deviations of the measured heights from the calculated values. Standard deviations were <1 cm. Correlation ratios were higher than 0.96, except for leaf mold at 700 mol m^–3 and polyethylene particles at 21 mol m^–3; the correlation ratio of the former is 0.888 and that of the latter is 0.669.

By using a Green–Ampt type of model, Emerson and Bond (1963) and Tabuchi (1995) indicated that, for cases of upward infiltration into dry sand, a linear relationship existed between the rate of rise of the infiltration front and the reciprocal of the infiltration distance while a saturated condition was maintained. The infiltration front rises to some extent, however, and large pore cells will become unsaturated and small pore cells will be saturated (Emerson and Bond, 1963). Then the suction at the infiltration front, h, the volumetric water content, , and the hydraulic conductivity, k, will change. Because we intended to apply the proposed upward infiltration Eq. [4] to saturated infiltration, the fitted parameters h and /k in Eq. [4] were constant during infiltration in the calculation (see below). This condition cannot be applied to unsaturated infiltration. Saturated infiltration occurs only in the initial stage. Therefore, Eq. [4] can be applied only in the initial stage of infiltration. The good agreements in solid lines and the deviations in dotted lines correspond to the initial saturated flow condition and the latter unsaturated condition, respectively (Fig. 2).

A physical parameter such as saturated hydraulic conductivity must be checked, however, to see whether the infiltration equation is of value. Measured saturated hydraulic conductivities are compared with calculated values obtained with the proposed Eq. [4] in Table 3. The calculated k values at 0 mol m^–3 are similar to the measured values for all porous materials except about 33% of difference for glass beads; for polyethylene particles, k values could not be obtained because water was not able to infiltrate. At 700 mol m^–3, calculated and measured values are almost identical for glass beads, sand, and polyethylene particles; however, for peat moss, the calculated values are about 50% lower, and for leaf mold they are 45 times larger than the measured value. Therefore, the calculated line for leaf mold at 700 mol m^–3 is wrong.

The large difference between measured and calculated hydraulic conductivities for leaf mold was probably caused by swelling, an effect that is not captured in the proposed model. The expansion ratio, the ratio of increased thickness to the initial thickness of the material column in the swelling experiment, for 700 mol m^–3 SDS solution was 4.3% larger than for pure water; the initial thickness was 10.0 cm, saturated thickness with water was 10.4 cm, and saturated thickness with 700 mol m^–3 SDS solution was 10.83 cm. The difference is much larger than that noted with the other materials (Table 3). When the soil swells, larger pores tend to become smaller and soil permeability decreases.

The measured saturated hydraulic conductivities at 700 mol m^–3 for the other materials were smaller than those at 0 mol m^–3 because the liquid viscosity of 700 mol m^–3 SDS solution is 2.1 times larger than that of pure water; the saturated hydraulic conductivity is inversely proportional to the viscosity, as shown in Eq. [6]. Conversely, saturated hydraulic conductivity values were similar among SDS solutions at 70 mol m^–3 because these solutions have similar viscosities (Table 2). The calculated k values for glass beads, sand, and peat moss at 700 mol m^–3 were also smaller than those at 0 mol m^–3. In contrast, for leaf mold, k values were similar at 0, 3.5, and 7 mol m^–3, and larger for SDS concentrations >70 mol m^–3. These results basically indicate that the proposed model captures these processes. For increasing SDS concentrations, saturated hydraulic conductivity is predicted to be a constant value if water saturation, water flow paths (i.e., no swelling), and liquid viscosity remain the same. For leaf mold at 21 and 70 mol m^–3, however, the value did not remain constant. This result indicates that pore structure changes when concentration exceeds 7 mol m^–3, and that the swelling of leaf mold probably affected the pore structure. In fact, upward infiltration became considerably slower at concentrations >7 mol m^–3 (Fig. 2c). For hydrophobic materials (polyethylene particles and peat moss), the calculated k values vary and show no clear trend among different SDS concentrations, probably indicating differences in water saturation and water flow paths, because infiltration into smaller pores may become difficult when the obtuse contact angle becomes larger.

The hydraulic conductivity and correlation ratio results show that the upward infiltration equation is most appropriate for glass beads and sand. It is also appropriate for leaf mold at 7 mol m^–3, peat moss at 0 and 700 mol m^–3, and polyethylene particles at 700 mol m^–3.

Calculation of Contact Angle
The calculated advancing contact angles derived with the upward infiltration equation are listed in Table 2. Several contact angles were omitted from Table 2 for cases in which the upward infiltration equation is not appropriate. Those for leaf mold at 21, 70, and 700 mol m^–3 were omitted because the pore structures changed during infiltration due to swelling. That for peat moss at 70 mol m^–3 was omitted because it could not be calculated: cos in Eq. [3] became >1.0 in calculation. The contact angle, , can be calculated from Eq. [3] when h and are known (see above). The value of h is determined by a best fit of the measured values. The values used in these calculations, along with the resulting (shown with no parentheses or dagger), for each SDS concentration are listed in Table 2. For calculations using the surface tension of pure water ( = 72 mN m^–1), a dagger was added to the resulting . For calculations made with both the for pure water and that corresponding to the SDS concentration, parentheses are added to values in Table 2.

When SDS is adsorbed on a material, the concentration of the infiltration front is smaller than the influent concentration. The influence on must be considered in this case. Because SDS has a hydrophobic tail, adsorption by hydrophobic interaction occurs in organic materials. Therefore, the influence was considered for leaf mold, polyethylene particles, and peat moss. The influence must be considered for sand because it has 0.5% organic matter. In fact, the measured heights of the infiltration front for 0, 3.5, 7, and 21 mol m^–3 solutions were almost the same for sand (Fig. 2b). For similar infiltration rates, the SDS concentration at the infiltration front was supposed to become almost 0 mol m^–3, because SDS was adsorbed on the sand in the bottom part of the column during infiltration. For leaf mold, the measured heights of the infiltration front for 0 and 3.5 mol m^–3 solutions were almost the same (Fig. 2c) due to the effect of adsorption. Therefore, = 72 mN m^–1 was also used for calculating for the 3.5 mol m^–3 solution. In leaf mold for the 7 mol m^–3 solution, the result was the same when either = 72 or 39 mN m^–1 was used, because was 90°. For both peat moss and polyethylene particles, both surface tensions of pure SDS concentrations and = 72 mN m^–1 were used because the concentrations at the infiltration fronts were unknown. The expected contact angle ranges are listed in parentheses in Table 2 in these cases. In addition, the calculated k values varied or showed no clear trend among different SDS concentrations (see above). These results indicate differences in water saturation and the water flow path. Therefore, these calculated values are not very reliable.

The methods of contact angle calculation that use this upward infiltration equation and the Washburn equation are not highly reliable because water flow in porous materials is not a simple tube and the advancing contact angle of a low-surface-tension liquid is not always 0° (Siebold et al., 2000); however, there is no better method than these (Hiemenz, 1986, p. 335–338).

Influence of Wettability on Infiltration of Pure Water and 700 mol m^–3 Sodium Dodecyl Sulfate Solution
The measured heights of the infiltration front for pure water (Fig. 3 ) were in the descending order of glass beads > sand > leaf mold > peat moss > polyethylene particles; the smaller advancing contact angle of the material, the higher the infiltration front (Table 2). This result indicates that the influence of contact angle or wettability on the infiltration rate was larger than the differences in average particle size (Table 1). At an early stage, the heights of the infiltration front for glass beads were smaller than those for sand, probably because, for the glass bead columns, the pore sizes corresponding with the measured k values were considerably smaller than those of the sand column. As expected, we found that pure water infiltrated well into the hydrophilic and poorly into the hydrophobic materials. Pure water did not infiltrate into columns filled with polyethylene particles even at 20-cm water head. Nakaya et al. (1977) indicated that the height of the infiltration front decreased with an increase in coated humic acid on quartz sand. Because the coating of humic acid increases hydrophobicity, their result agrees with our result. The measured height for peat became >0 cm after 200 min, even though the calculated contact angle was 99°. This result indicates that the peat surface at the infiltration front became hydrophilic during measurement. Michel et al. (2001) demonstrated the change from hydrophilicity to hydrophobicity during desiccation of peat. Our result corresponds to their result.

View larger version (33K): [in this window] [in a new window]   Fig. 3. Height of infiltration front during pure water upward infiltration into columns filled with glass beads, sand, leaf mold, polyethylene particles, or peat moss.

View larger version (30K): [in this window] [in a new window]   Fig. 4. Height of infiltration front during 700 mol m–3 sodium dodecyl sulfate (SDS) solution upward infiltration into columns filled with glass beads, sand, leaf mold, polyethylene particles, or peat moss.

For leaf mold, the height of the infiltration front also decreased with increasing SDS concentration (Fig. 2c). The contact angle for 0 mol m^–3 was 82°, slightly acute. Therefore, the decrease in surface tension with the concurrent increase in SDS concentration might have caused the decrease in height up to the 7 mol m^–3 SDS solution.

For hydrophobic materials (polyethylene particles and peat moss), the heights of the infiltration fronts increased with increasing SDS concentration because of the increase in suction, except for 700 mol m^–3 SDS solution (Fig. 2d and 2e). The suction increased with the increase in SDS concentration, because the surface tension decreased with the increase in SDS concentration and cos is negative when is obtuse; the relationship between suction and surface tension is given in Eq. [3]. The influence of suction on the height of the infiltration front is depicted in Fig. 5 . The solid lines are the calculated values with the same (=110°) and a different . In the calculations with Eq. [4], /k = (/k)₇₀₀(0.89/1.89) was used. The value of (/k)₇₀₀ is the value obtained when the calculated values were fitted with the measured values at 700 mol m^–3 SDS solution. The data for 700 mol m^–3 SDS solution should be most suitable because the material became wettable. Therefore, (/k)₇₀₀ was chosen. The solution viscosity was modified, however, to 0.89 Pa s by multiplying by 0.89/1.89 to compare with the data for 0 to 70 mol m^–3 SDS solutions. The calculated result indicates that the height increased with the decrease in surface tension. The change in surface tension cannot completely explain the increased height, however; the measured increase is much larger than the calculated increase.

View larger version (25K): [in this window] [in a new window]   Fig. 5. Height of infiltration front during upward infiltration into columns filled with polyethylene particles. Solid lines are the calculated curves with different surface tensions and the same advancing contact angle ( = 110°). Numbers in the legend are sodium dodecyl sulfate (SDS) concentrations.

For the hydrophobic materials (polyethylene particles and peat moss), the effect of contact angles are significant. The contact angles for these materials became smaller with an increase in SDS concentration, which caused the increase in height of the infiltration front. The materials became wettable with increasing SDS concentration because the surfactant is amphipathic. In Fig. 6 , the measured heights of the infiltration fronts and those calculated with = 38 mN m^–1, in addition to the different advancing contact angle in the polyethylene particles, are shown. In the calculations with Eq. [4], /k = (/k)₇₀₀(0.89/1.89) was also used to compare the data for 0 to 70 mol m^–3 SDS solutions, as mentioned above. The result clearly shows that the influence of the contact angle on the height was significant. In fact, the obtained advancing contact angles for 700 mol m^–3 SDS ( = 69° for polyethylene particles, = 43° for peat moss) are much smaller than those for 0 mol m^–3 SDS ( > 125° for polyethylene particles, = 102° for peat moss). Pelishek et al. (1962) showed a similar effect of the wetting agent on infiltration in materials whose contact angles with pure water were 72 and 83°. We also show that a decrease in surface tension with an increase in the SDS concentration increased the height of the infiltration front.

View larger version (27K): [in this window] [in a new window]   Fig. 6. Height of infiltration front during upward infiltration into columns filled with the polyethylene particles. Solid lines are the calculated curves with different advancing contact angles and the same surface tension ( = 38 mN m–1). Numbers in the legend are sodium dodecyl sulfate (SDS) concentrations.

In leaf mold, the saturated hydraulic conductivity decreased when the SDS solution at a higher concentration infiltrated. The swelling of the material induced a decrease in hydraulic conductivity. The increases in the heights of the infiltration fronts for 21 and 70 mol m^–3 SDS were restricted in the initial stages, and that for 700 mol m^–3 was restricted in all stages (Fig. 2c). These trends are probably due to the low saturated hydraulic conductivity caused by the swelling. The heights for 21 and 70 mol m^–3 SDS increased at the later stage, probably because the concentrations were too low for enough adsorption to generate sufficient swelling and maintain low hydraulic conductivity. The swelling mechanism for leaf mold can probably be attributed to an electrostatic repulsive force caused by the adsorbed anionic surfactant; however, more research is needed to clarify this point.

Hydraulic conductivity reductions for loam (Allred and Brown, 1994) and silty clay loam (Liu and Roy, 1995) soils caused by SDS have been reported by other researchers. Liu and Roy (1995) suggested that the reduction was mainly caused by the Ca surfactant precipitation. In our experiment, however, the precipitation with divalent cations such as Ca did not occur because divalent cations had been removed before the experiment.

Conversely, the measured hydraulic conductivity of peat moss for 700 mol m^–3 did not become much smaller than that for 0 mol m^–3, although both peat moss and leaf mold are organic soils. The expansion ratios for peat moss differed little between 0 and 700 mol m^–3 SDS, while those for leaf mold differed quite noticeably (Table 3). The different results for leaf mold and peat moss probably arise from the differences in adsorption and structural stability.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The question was how anionic surfactants could potentially influence water flow in soils and porous materials of different solid surface properties. We proposed a simplified model for describing upward infiltration under saturated conditions that is based on Darcy's law and equivalent to the Washburn equation. The model was used for evaluating the effects of an anionic surfactant on infiltration in columns filled with porous materials with highly contrasting wettability. The infiltration as a function of time could be explained with the proposed model for the organic (leaf mold, peat moss, and polyethylene particles) materials and especially well for the inorganic porous materials (glass beads and sand) by using the appropriate surface tensions and contact angles for SDS concentrations ranging between 0 and 700 mol m^–3. The effects of anionic surfactants could be distinguished for porous materials of different wettability.

For the hydrophilic porous materials (glass beads and sand), the infiltration rate decreased as the SDS concentration increased due to the decrease in surface tension at the infiltration front following adsorption of SDS from the solution. For the hydrophobic porous materials (polyethylene particles and peat moss), the infiltration rate increased with SDS concentration mainly connected with a decrease in the contact angle and surface tension. For leaf mold, interactions between SDS solution and the solids were causing swelling and limiting the application of the model and data evaluation. The solid surface characteristics of leaf mold should be separately studied and considered in future investigations with respect to interaction with an anionic surfactant.

The relatively simple experiments and the proposed upward infiltration model can be useful for the systematic analysis of wettability and surfactant effects for soils of differing and complex properties; the influence of surfactants on infiltration may vary in a wide range. The study indicates that a better process-based understanding of how surfactants affect the physicochemical nature of the surface of solid soil particles is needed for improved technical use, for instance, of organic waste materials and surfactants in applications such as soil remediation.

	ACKNOWLEDGMENTS

 
We thank Mitsui Chemicals for the donation of the polyethylene particles. This research was funded by the Grant-in-Aid for Scientific Research (KAKENHI, 14560198), Japan Society for the Promotion of Science.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
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Received for publication October 25, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS UPWARD INFILTRATION THEORY RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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