War. Res. Vol. 26. No. 10, pp. 133%1345, 1992 Printed in Great Britain. All rights reserved
Copyright © 1992Pergamon Press Ltd
SORPTION OF NON-IONIC SURFACTANTS ONTO SOIL ZHONGBAO LXU, DAVID A. EDWARDS and RICHARD G. LUTHY * O Department of CivilEngineering,CarnegieMellon University,Pittsburgh,PA 15213,U.S.A. (First received June 1991; accepted in revised form March 1992)
Almtrat't--Experiments in batch soil/aqueous systems were conducted to evaluate the sorption onto soil of three micelle-forming non-ionic surfactants and one lamellae-forming non-ionic surfactant. Non-ionic surfactant sorption onto soil was assessed using a surface tension technique for aqueous-phase surfactant concentrations less than surfactant monomer saturation. These sorption data were found to fit a Freundlich isotherm. Non-ionic surfactant sorption onto soil was assessed in the presence of surfactant micelles, or surfactant bilayer lamellae, depending on the type of surfactant, with either a spectrophotometric technique or a chemical oxidation technique. Sorption of the micelle-forming non-ionic surfactants onto soil was found to be constant at a value of the bulk solution surfactant concentration exceeding surfactant monomer saturation, i.e. the critical micelle concentration. Sorption of the'lamellae-forming non-ionic surfactant onto soil was found to be an increasing function of the surfactant dose for bulk solution surfactant concentrations exceeding a critical aggregate concentration. Understanding of surfactant sorption onto soil is needed to assess surfactant mobility in soil and surfactant-facilitated transport of organic compounds in soil/aqueous systems. Key words--non-ionic surfactant, sorption, lgepal CA-720, Tergitol NP-10, Triton X-100, Brij 30, soil,
surface tension, CMC, spectrophotometry
INTRODUCTION Surfactants are amphiphilic molecules that, owing to their spatial variations in polarity, can dissolve in solvents, sorb at interfaces and form aggregates called micelles that can solubilize hydrophobic organic compounds (HOCs). An understanding of these properties is important in characterizing the behavior of surfactants and HOCs in laboratory soil/aqueous batch systems (Edwards et al., 1992), as well as in modeling facilitated transport of HOCs by suffactant micelles in the subsurface. Such transport may occur inadvertently (Huling, 1989; Valsaraj and Thibodeaux, 1989), or by design, as in micellar suffactant-assisted remediation of contaminated soil or sediment (Ellis et al., 1984; Nash and Traver, 1986, Rajput et al., 1989; Vigon and Rubin, 1989). In modeling the distribution of H O C in a micellar suffactant solution, with the latter being a single, isotropic and thermodynamically-stable phase (Attwood and Florence, 1983), it is frequently convenient to consider the hydrophohic interiors of the micelles collectively as a pseudophase distinct from the surrounding solution termed the aqueous pseudophase (Valsaraj and Thibodeaux, 1989). In a system of soil and micellar surfactant solution, the organic.coated solid particles of the soil must also be considered for the distribution of surfactant and HOC. Although nearly all surfactant properties and processes are of potential significance in understanding the behavior of surfactants and HOCs in a soil/ *Author to whom all correspondence should be addressed.
aqueous system, surfactant sorption onto soil is a particularly important process that must be considered in experimental and modeling studies. Sorption of surfactant onto soil may result in much surfactant being unavailable for micellar solubilization of HOCs (Edwards et aL, 1992). In addition, the presence of sorbed as well as dissolved surfactant changes HOC sorption behavior (Liu et al., 1991; Edwards et al., 1992). Another reason for investigating surfactant sorption onto soil is that of understanding the transport in soil of the surfactants themselves, as well as their microbial degradation products. The latter may be relatively toxic, as is the case for alkylphenol ethoxylates (Brunner et al., 1988), or relatively non-toxic, as is the case for aikyl ethoxylates (Lewis, 1991). Surfactant sorption
A number of researchers have investigated the sorption of anionic surfactants onto soil (e.g. Fink et al., 1970; Urano et al., 1984; Hand and Williams, 1987; DiToro et al., 1990). Their work has largely been motivated by concern over the presence of anionic suffactants such as alkyibenzenesulfonate (ABS), linear alkylbenzene sulfonate (LAS) and other synthetic anionic detergents in wastewaters and surface waters. Little research, however, has been conducted thus far with regard to the sorption onto soil of non-ionic surfactants, which, because of their lack of charge, would be expected to behave differently than anionic surfactants in sorbing onto soil. Sorption of non-ionic surfactants has been reported for model, homogeneous sorbents, e.g. glass beads or
ZHONGBAOLIU et al.
silica gel (Partyka et al., 1984; Levitz and Van Damme, 1986; Aston et al., 1989). Several published studies relate to sorption of non-ionic surfactants onto natural or artificial soil. Valoras et aL (1969) described the sorption of two commercial non-ionic surfactant soil-wetting agents onto several different soils of unknown organic carbon content. The researchers concluded that the sorption properties of the two non-ionic suffactants were distinct, with each of the two surfactants probably having different effects on infiltration rates, soil wetting properties or plant toxicity; however, the major finding of the study relevant to environmental research was that sorption equilibrium for each surfactant was approached rapidly. For all the soils studied, except for peat, the time required to approximate sorption equilibrium was about an hour. Urano et ai. (1984) evaluated the sorption of two non-ionic surfactants onto several different soils of known organic carbon contents. The surfactants employed were an alkyl ethoxylate surfactant, CtHj30(CHeCH20)6H, denoted here as C6Et, and an alkylphenol ethoxylate surfactant, CgHjgCtH40(CH2CHzO)~0H, denoted as CgPEI0. They found that sorption could be characterized for each surfactant/ soil system by a Freundlich isotherm at surfactant concentrations up to the surfactant CMC, or critical micelle concentration. The researchers found for the three soils of their study having organic carbon contents ranging from 1,7 to 6.0% that sub-CMC surfactant sorption onto soil was proportional to the organic carbon content of the soil, indicating that most of the sorbed surfactant was associated with the organic matter. This finding is supported by data from Aston et al. (1989) showing that non-ionic surfactant is sorbed to a greater extent onto silica dioxide particles that are chemically modified to provide a hydrophobic surface than onto unmodified silica dioxide particles of similar dimensions. The Urano et ai. (1984) study did not provide data for surfactant sorption at values for the bulk solution surfactant concentration greater than the CMC. Vigon and Rubin (1989) studied the sorption of several non-ionic surfactants onto an artificial soil. Only a small number of tests were performed for each surfactant, yielding data over a broad range of surfactant doses. Most of the surfactant doses appear to have been greater than that required for each surfactant to have attained its respective aqueous-phase CMC. Both the sub-CMC and supra-CMC sorption results were presented in the form of percent surfactant sorbed versus surfactant dose. As reported in the present paper for several surfactants, the amount of micelle-forming non-ionic surfactant that sorbs onto soil may plateau at a maximum value that occurs concomitantly with attainment at the aqueous-phase CMC. Hence, the absolute amount of sorbed micelleforming non-ionic surfactant in a soil/aqueous system may be constant for bulk solution surfactant concentrations greater than the CMC, with the result that
the fraction of the total surfactant sorbed in the system decreases as a function of surfactant dose. This type of result is suggested in the figures of Vigon and Rubin (1989). Abdul and Gibson (1991) evaluated sorption onto sandy aquifer material of a commercial alkyl ethoxylate, RO(C2H40)vH where R ffi C,0-12H~,_,s (denoted as Ci0_pET). The surfactant sorption was described by an increasing, curvilinear isotherm. C M C and CA C
At 25°C, most common types of synthetic nonionic surfactants in dilute aqueous solution with concentrations greater than a threshold concentration for aggregation produce a single phase consisting of either spherical, spheroidal or rod-like micelles, surrounded, in each case, by an aqueous solution of surfactant monomers. As described by Mitchell et al. (1983), these same non-ionic surfactants at higher temperatures and/or higher concentrations can produce surfactant aggregate types other than typical miceUes; these aggregates may include vesicles, bilayer lamellae, cubic arrays of ordered spherical micelles or multiple hexagonal arrays of long rods. At 25°C, a relatively small number of commercial surfactant compounds create two-phase systems consisting of a surfactant bilayer iameilae phase and a saturated suffactant monomer phase when the suffactant is present in bulk solution at relatively small concentrations in excess of what is referred to in this paper as the critical aggregate concentration (CAC). Lamellae-forming surfactants produce a two-phase solution instead of a homogeneous, single phase. The bilayer aggregates of lameilae-forming surfactants tend to align themselves in bulk solution, rather than assuming random, distributed positions in the aqueous media as do typical micelles. In addition, the interaggregate interaction dynamics of bilayer lamellae differ substantially from those of micelles in bulk solution (Mitchell et al., 1983). The latter property of lamellae-forming surfactants may significantly influence the sorption of this type of surfactant onto soil. Although lamellae-forming non-ionic surfactants are interesting, they represent only a small fraction of commercially available non-ionic suffactants; the emphasis in this paper is accordingly on the sorption onto soil of non-ionic surfactants that form typical micelles in bulk solution. Micelle-forming non-ionic surfactant molecules in bulk solution in the supra-CMC bulk solution surfactant concentration range are represented both by dissolved monomers with a concentration equal to the CMC, and by surfactant micelles. The concentration of surfactant in micelle form is equal to the difference between C,,~, the bulk solution surfactant concentration, and the CMC. As the study of Urano et aL (1984) is limited to sub-CMC bulk solution surfactant concentration, and since there are difficulties associated with interpreting the results of
Sorption of non-ionic surfactants onto soil the other surfactant sorption studies previously referred to, it appears that there is little quantitative information available regarding the nature of supraCMC non-ionic surfactant sorption onto soil. Such information would have obvious significance with regard to supra-CMC surfactant-facilitated transport of HOC in soil systems. The purpose of this paper is to begin to provide such results, and specifically to assess the supra-CMC (or supra-CAC) as well as sub-CMC (or sub-CAC) sorption behavior of four non-ionic surfactants: three of which form micelles in aqueous solution, and one of which at its CAC at 25°C forms with water a two-phase suffactant bilayer lamellar solution (Mitchell et al., 1983). MATERIALS AND METHODS The experiments of this study include three different methods for determining surfactant sorption in a soil/ aqueous system: surface tension, spectrophotometry and chemical oxidation. The first method permits measurement of surfactant sorption for aqueous-phase surfactant concentrations less than the CMC or CAC; the second and third methods permit measurement of surfactant sorption for bulk solution surfactant concentrations greater than the CMC or CAC. In the first method, surface tension is measured as a function of surfactant concentration in an aqueous system; the surface tension of the supernatant of a soil/aqueous system is then measured as a function of surfactant dose, defined as the number of moles of surfactant added to the system divided by the aqueous volume in liters. A similar technique has been used to measure sorption of surfactant onto modified silica dioxide beads (Seng and Sell, 1977). In the second method, the amount of azo dye incorporated in bulk solution is measured by spectrophotometry in supernatant as a function of the amount of surfactant added to a soil/aqueous system; this method allows calculation of supra-CMC sorption or supra-CAC sorption, if such exists. For the third method, the chemical oxygen demand (COD) was measured in supernatant as a function of the amount of surfactant added to a soil/ aqueous system. Four different non-ionicsurfactants were employed in the batch test samples for surface tension measurement: three alkylphenol ethoxylate types, and one alkyi ethoxylate type, as described in Table I. The aikylphenol ethoxylate surfactants, CsPE,.s, C,PE,2, and C,PEI0.5, are micelle-forming surfactants in dilute solution at 25°C. The alkyi ethoxylate suffactant of this study, C,2E~, is a lamellae-forming surfacrant; however, other ethoxylate surfactants in the 12-aikylgroup homologous series having more than four ethylene oxide groups form micelles in dilute solution at 25°C (Mitchell et al., 1983). The non-ionic surfactants of this study were obtained directly from either the manufacturer or a chemical distributor and were used without further purification. As with all commercial ethoxylate surfactants, the number of ethylene oxide groups given for each surfactans is an average value, with the actual number of OE groups for a particular surfactant having a statistical distribution (Mukerjee and Mysels, 1971; Kile and Chiou, 1989).
The soil of this study was an undisturbed, subhumid, gra~land Morton soil, similar to the llarnes-Hamerly Association soil described by Mihelcic and Luthy (1988). The soil was air-dried and screened through a U.S. standard No. 10 mesh (2 mm) sieve to remove coarser soil fragments. The soil was tested by the Walkley-Black method (ASA, 1965) and found to have a fractional organic carbon content of 0.0096. Surface tension measurement Each surface tension experiment involved eight to ten batch test samples in 250 ml Erlcnmeyer flasks. Batch test samples consisted of deionized water, soil, surfactant and mercuric chloride, the latter added to inhibit bacterial degradation of surfactant. A series of stock surfactant solutions were prepared for each surfactant, such that after addition of the appropriate volumes to each batch test, the resultant bulk solution surfactant concentration values would range from less than the CMC to approx. 5 times the CMC. Each batch test sample in an Erlenmeyer flask contained 18mg of mercuric chloride, the appropriate amount of surfactant stock solution, 1.0-20.0 g of soil, and a sufficient volume of deionized water such that after its addition to the batch test sample, the total sample fluid volume was 90 ml. Each flask was sealed with a rubber stopper covered by Parafilm '*M". The sealed Erlenmeyer flasks were mounted on a wrist action shaker for at least 24 h and shaken for 7 rain periods at 30 rain intervals in order to periodically resuspend the soil and facilitate attainment of equilibrium. Tests involving surfactant solubilization of polycyclic aromatic hydrocarbons in soil/aqueous suspensions have previously demonstrated that periodic agitation at 30 rain intervals resulted in surfactant attaining sorption equilibrium within 24 h (Liu et al., 1991), Sorption equilibrium was further ensured prior to measurement of surface tension by letting the soil particles in the batch test quiescently settle for about 36 h or more, after which the supernatant was free of significant amounts of suspended soil particles. All of the glassware used in the surface tension experiments was cleaned with chromic acid and rinsed three times with tap water and deionized water, and oven-dried at 95°C; the pipettes and volumetric flasks were air-dried. 20 ml of the settled supernatant was carefully removed in a temperature-controlled laboratory at 24-25°C by volumetric pipette and placed in a beaker, and the solution was allowed to equilibrate for 20 min or more. The surface tension of the supernatant at 24-25°C was measured with a Fisher Tensiomat Model 21 tensiometer. This instrument employed a clean platinum-iridium ring that was suspended in the supernatant from a torsion balance, and the force in dynes per centimeter required to pull the ring free from the supernatant was measured. Each sample of supernatant was tested with at least three readings to ensure that consistent values were obtained. The averaged value was corrected to account for the geometry of the experimental apparatus. The ring was cleaned between each measurement with an acetone rinse followed by a heating of the ring until it glowed red in a gas flame. Spectrophotometry In this technique, each batch test sample was prepared in a 50 ml centrifuge tube with 6.25 g soil, 9 mg of mercuric chloride, a precalculated volume of surfactant stock and
Table I. Non-ionicsurfactantsemployedin this study Surfactant Average molecularformula S y m b o l AverageMW (g/tool) Igepal CA-720 CsHI~-C6H4-O(CH2CH2OlI2H CBPE12 735 Tersitol NP-10 CgH,9-C6H4-O(CH2CH20)Io.sH CgPE,o.5 682 Triton X-100 C,H ,7-C6H4-O(CH2CH20)9.sH C,PF_~.5 628 Brij 30 CI~H~(OCH2CH2)4OH C,2E4 363
ZHONGBAOLzu el al.
sufficient deionized water to bring the total fluid volume to 45 ml. The volumes of surfactant stock were selected such that the resultant bulk solution surfactant concentrations would represent a range of values exceeding the CMC. The centrifuge tubes were sealed with Teflon-lined septa and secured with open-port screw caps. The samples were equilibrated for 24 h by rotating on a tube rotator for a period of 15 min at 30 min intervals, after which the samples were removed and set aside for about 12 h to allow the solids in the samples to settle. The samples were then centrifuged for 30 min to further reduce solids in suspension. 20 and 5 mi aliquots of supernatant were withdrawn from each sample by syringe and placed in other centrifuge tubes. The pH of the withdrawn supernatant was adjusted to 7 to ensure correspondence between the color of the azo dye in solution and that of calibration standards. The 5 ml sample was used as a reference. Several grams of the azo dye p-dimethylaminoazobenzene were added to the 20 ml sample of supernatant; this amount of dye was sufficient to provide an excess after equilibration. The tubes containing the supernatant were sealed and placed in a reciprocating water bath at 25°C for about 72 h after which they were left to stand for 3-4 h prior to sampling to allow settling of unsolubilized azo dye. The samples were then expressed through a preequilibrated 0.22 #m Teflon filter, and the filtrate was placed in a Bausch & Lomb Spectronic 88 spectrophotometer for absorbance measurement at 400 nm. Readings at 400 nm were also made for the reference sample with surfactant but without dye to permit corrections for background absorbance. The corrected absorbance values were employed in creating calibration curves for each of the non-ionic surfactants. Figure I shows calibration curves for Brij 30 0.5
o.,. , ' ° /
Surfactant Concentration (mol/L) (b)
~ 0 C
Surfactant ConGentration (tool/L) Fig. 2. Calibration curve showing chemical oxygendemand (COD) as a function of the bulk solution non-ionic surfactant concentration for Brij 30. (Cm2E4)and Triton X-100 (CsPEg.s) expressed as absorbance due to the presence of azo dye versus the bulk solution surfactant concentration. Igepal CA-720 exhibited similar relationships between absorhance and bulk solution surfactant concentration. The calibration curves showed that above the CMC or CAC of each suffactant there is a linear correlation between the amount of absorhance and the surfactant dose. In some instances it was diff~mlt to express the surfactant/dye solution through the 0.22/~m filter, as was the case for the higher bulk solution suffactant concentrations; in these cases the surfactant/dye solution was diluted with surfactant solution, and the absorbance was corrected by a dilution factor.
Chemical oxygen demand (COD) was measured in solutions with different non-ionic surfactant concentrations. The procedure followed the closed reflux, titrimetric method, 5220C (APHA, 1989). A 2ml sample of the equilibrated solution as described in the spectrophotometry methods section was pipetted into a vial containing premeasured quantities of acid, catalyst and chloride compensator as prescribed in the method. A measured excess of potassium dichromate was added to the vial and the sample was refluxed for 2 h at 150°C in a Hach Model 16500 microCOD, aluminum-block apparatus. The sample fluid was permitted to cool, and was titrated against ferrous ammonium sulfate standard solution. The measured COD values were plotted against corresponding suffactant doses to establish a calibration curve as shown in Fig. 2. The COD was measured in supernatant solutions that were prepared by diluting measured volumes of filtered (0.22/~m) supernatant from soil/aqueous systems. Blank solutions comprising soil/aqueous suspensions in absence of surfactant were used to correct for residual soil-derived COD. RESULTS AND DISCUSSION
Chemical oxygen demand
C)imethylarninoazobenzene / 0.4' Solubil~ ¢0 .O
Triton X-lO0 (GSREg.S)
Surfactant Concentration (mol/L) Fig. 1. Calibration curves showing spectrophotometric abSorbance readings of dimethylaminoazobenzene (azo) dye solubilizate as a function of the bulk solution non-ionic surfactant concentration for Brij 30 (a) and Triton X-100
The appropriate method needed to evaluate the amount of surfactant sorbed on soil in a soil/aqueous system may depend on the type of the surfactant tested and the range of the bulk solution surfactant concentrations of the samples. A surface tension technique was found in this study to be effective for assessing the sorption of both miceile-forming surfactants and the iameilac-forming surfactant at values of C.,~f less than the CMC or CAC, respectively. For solutions of micellc-forming surfactants having values of C,,rf greater than the CMC, the
Sorption of non-ionic surfactants onto soil 80
65 ¸ soX~_
ID v 45
t~ 1= " U)
Igepal CA-720 7O
(g/mL) o 0.011 " 0.0137
~ ~\~, No Soil-..4
25 20 . , -S.O -S.5 -5.0
. . -4.5
. . -4.0
Log (Surfactant concentration, tool/L) Fig. 3. Plot of surface tension as a function of the logarithm of the bulk solution C-~PE,0.5 non-ionic surfactant concentration in an aqueous system with the critical micelle concentration (CMC) indicated at the point of inflection. spectrophotometric technique with an azo dye was employed successfully. F o r solutions o f the lamellae-forming surfactant, the C O D m e t h o d gave satisfactory results. The s u b - C M C and supraC M C techniques prove to be c o m p l e m e n t a r y and c o r r o b o r a t e each other, particularly in the case o f the micelle-forming surfactants.
Figure 3 shows a standard plot of surface tension in dynes per centimeter plotted against the logarithm of surfactant concentration in moles per liter for aqueous solutions of CgPEI0.S. The surface tension curve is comprised of two linear segments; the inflection point represents the CMC, or CAC, depending on the type of surfactant, at which concentration the bulk solution is saturated with surfactant monomers. At greater bulk solution surfactant concentrations, the surface tension remains constant. The surfactant in excess of the CMC or CAC in bulk solution is manifested in the form of surfactant micelles or aggregates (Martin et al., 1969; Rosen, 1989). Measured CMC or CAC values for all four surfactants are given in Table 2 where they are shown to compare well with CMC or CAC values from the literature for homogeneous surfactants. In Figures 4 and 5 the surface tension is plotted against the logarithm of the surfactant dose for several soil/aqueous systems with increasing soil/ water weight-to-volume ratios, represented by the curves on the right. It is apparent from these figures that the greater the soil/water weight-to-volume ratio,
Log (Surfactant dose, tool/L)
Fig. 4. Plot of surface tension as a function of the logarithm of the CsPEt2 non-ionic surfactant dose in aqueous and soil/aqueous systems of varying soil/water weight-to-volume ratios. For a given system, the initiation of micelle formation is indicated by the minimal surfactant dose at which the surface tension ceases to decline.
the greater is the amount of surfactant that must be added to the system in order to decrease the surface tension by a given amount. Upon the addition of sufficient surfactant such that the aqueous-phase CMC is attained, the surface tension attains a nearly constant, minimum value, signifying surfactant monomer saturation in the aqueous phase in equilibrium with surfactant sorbed on soil. The presence of impurities in commercial surfactants, however, often causes a slight depression in the surface tension values in the vicinity of the CMC (Mukerjee and Mysels, 1971), as evident in Figs 4 and 5. Also, it is possible that some of the impurities affecting the surface tension in these systems may have been present as soluble soil components rather than components of the surfactants themselves. The amount of surfactant sorbed on soil at any aqueous-phase surfactant concentration can be calculated with a technique that employs data from the surface tension plots for both an aqueous system and a soil/aqueous system. The abscissa for a selected data point on the surface tension curve for the aqueous system without soil gives an aqueous-phase surfactant concentration, C,,,f. The corresponding ordinate, a particular value of the surface tension ~, is noted, and the same value of ~ is then located on the surface tension plot for the soil/aqueous system. The abscissa on this plot that corresponds with this value of ~ yields a value for DLo, the bulk surfactant dose in the soil/aqueous system that produces a surface tension value of ~ in the supernatant. The
Table 2. Measured CMC values for non-ionic surfactants from surface tension experiments in the absence of soil Surface tension Reported Surfactant Symbol tests CMC(M) CMC(M ) Reference 18epal CA-720 C.PEI2 2.3 x 10 -+ ~ 6 × 10 -4 Mukerjee and Mysels (1971) Tergitol NP-10 CvPEi0.s 5.4 × 10-s ~ 8 × 10 -5 Mukerj~ and My~.I$ (19"/I) Triton X-100 CsPEg.5 1.7 x 10-4 2 x 10 -4 Kile and Chiou (1989) (3-3.3) x 10 4 Rosen (1989) Brij 30 C,zE4 2.3 x 10 5 6.5 × 10 -5 Rosen (1989) CAC
ZHONGSAO LIU et al. .,,d.0'
Triton X-100 (CAPE9S)
Soil/Water Ratio ,=(g/mL) o 0.03,1
._o c 40. k-
"J -3.5 :
LOg (Aqueous surfactant concentration, tool/L)
Log (Surfactant dose, tool/L) Fig. 5. Plot of surface tension as a function of the logarithm of the CsPF-~.5 non-ionic surfactant dose in aqueous and soil/aqueous systems of varying soil/water weight-to-volume ratios. For a given system, the initiation of miceile formation is indicated by the minimal surfactant dose at which the surface tension ceases to decline.
Fig. 7. Sub4~MC Freundlich sorption isotherm for CgPE,0.s and CsPE,2 non-ionic surfactants developed by the surface tension technique. The logarithm of the number of grams of surfactant sorbed per gram of soil is plotted as a function of the logarithm of the number of moles of surfactant dissolved per liter of solution.
difference between this value of DLo and the selected value of C,,~ is equal to C,o~,, the number of moles of surfactant sorbed per liter of solution, evaluated at the particular bulk solution surfactant concentration. The product of C,oeo and the ratio of v, to w~, the volume of the aqueous solution in liters divided by the weight of the soil in grams, yields a value for Q,~, the number of moles of surfactant sorbed per gram of soil:
the other three non-ionic surfactants of this study at values of C,=~ less than the CMC or CAC, as illustrated in Figs 7 and 8. The Freundlich isotherms are of the form (2s = K" C TM (2)
where g is a measure of sorption capacity and 1/n is an indicator of the curvature of the isotherm. Values for log K and n are summarized in Table 3. It is apparent that the three micelle-forming surfactants Q,.~ = (D,,o - c,°,~)(v./W,o,,) = C, orb(V./W~). (i) have values of n greater than unity; in contrast, the Surfactant sorption may also be expressed as Qs, the sole lamellae-forming surfactant studied has a value number of grams of non-ionic surfactant sorbed per of n less than unity. It is not known at present if these gram of the soil. The different expressions for suffac- results suggest a characteristic trend for micelletant sorption are useful in different applications. forming and lamellae-forming surfactant sorption Figure 6 shows an arithmetic plot of Qs for G~PE~0.s onto soil, or whether factors other than micelle-formplotted against C,,rr for values of C,,,r less than the ing ability, such as the hydrophile-lipophile balance CMC. It is evident that the curve is non-linear. (HLB) number, determine whether the values of n are However, when the log of (2= is plotted against the log less than or greater than unity. The amount of of C=,~ in a Freundlich isotherm plot, a linear fit is sorption onto soil for each of the non-ionic surfacobtained, as is shown in Fig. 7. The same is true for tants at their respective CMC or CAC values is A
o Bdi30 " Trit°nx'lO0
/° ..o.o. N'-'° ,,.,o.,,.
0e+0 le-S 2e-5 3a-5 4e-S Se-5 6e-5 Aqueous Surfactant Concentration (rnol/L)
Fig. 6. Sub-CMC sorption isotherm for ~PEi0.5 non-ionic surfactant developed by the surface tension technique. The number of grams of surfactent sorbed per gram of soil is plotted as a function of the number of moles of surfactant dissolved per liter of solution.
. . . . . . . . . -5.5
LOg (Aqueous surfactant concentration, mollL)
Fig. 8. Sub-CMC and sub-CAC Freundlich sorption isotherm developed, respectively, for CsPE~.s and C,2E4 non-ionic surfactants by the surface tension technique. The logarithm of the number of grams of surfactant sorbed per gram of soil is plotted as a function of the logarithm of the number of moles of surfactant dissolved per liter of solution.
Sorption of non-ionic surfactants onto soil Table 3, Values of Freundlich isotherm coeff¢ients Io8 K and n for non-ionic suffactants
lgepal CA-720 Tergitol NP-10 Triton X-100 Brij 30
C,PEm2 CgPEio.s CsPEg.s CI2E4
0.058 0.41 0.86 7.79
1.79 1.67 1.34 0.47
0.11114" substantial. The fractional organic carbon content of the soil of 0.0096, is increased to the range of 0.017-0.02 as a result of surfactant sorption. An advantage of measuring surfactant sorption with the surface tension technique is that it is non-intrusive, allowing direct assessment of the interaction between surfactant and soil. The method requires no separation of equilibrated phases, other than simple gravimetric settling to provide a supernatant that can be used for surface tension measurement. Suspended colloids and microparticulates do not introduce inherent measurement error in this technique. In contrast, conventional soil sorption tests in which apparent liquid-phase solute concentrations are measured may give erroneous results owing to the presence of particle-associated sorbate in the bulk solution.
Supra-CMC sorption In a soil/aqueous system at 25°C in which the bulk solution surfactant concentration exceeds the suffactant monomer solubility, different phenomena occur for the micelle-forming surfactants and the lamellaeforming surfactant. If a surfactant forms micelles, then the micelles, which do not sorb onto soil, may represent all or nearly all of the surfactant added to the system in excess of the CMC. The smallest surfactant dose that corresponds to the minimum, plateau value of surface tension for the soil/aqueous system therefore provides, after subtracting the CMC and multiplying by the ratio of v, to w~l, a specific 0.014
0.002 0.000 0.000
S u r f ~ tension
A Azodye .
. . 0.001
. . 0.002
Solution Surfactant Concentration (tool/L)
Fig. 9. Sorption isotherm for C~PE,.s miceile-forming nonionic surfactant developed for sub-CMC and supra-CMC bulk solution surfactant concentrations by the surface tension and spectrophotometric techniques, respectively. The number of grams of surfactant sorbed per gram of soil is plotted as a function of the number of moles of surfactant dissolved per liter of solution, showing a maximum, plateau value of sorption attained at the CMC.
(CgPE12) o 6
Surface tension Azo dye
Solution Surfactant Concentration (rnol/L)
Fig. 10. Sorption isotherm for CsPE,2 micelle-forming nonionic surfactant developed for sub-CMC and supra-CMC bulk solution surfactant concentrations by the surface tension and spectrophotometric techniques, respectively. The number of grams of surfactant sorbed per gram of soil is plotted as a function of the number of moles of surfactant dismlved per liter of solution, showing a maximum, plateau value of sorption attained at the CMC.
value for Q,.~ that is equal to Qm., the maximum, plateau value of sorption in moles per gram for the micelle-forming surfactant on that particular soil, Using the spectrophotometric technique, Qs, the product of Q,~ and the surfactant molecular weight, was measured at various bulk solution surfactant concentrations. The amount of surfactant sorbed in each case was evaluated by subtracting from the surfactant dose the bulk solution surfactant concentration determined from corrected azo dye spectrophotometric measurements and the calibration curve created for use with this technique. Sorption data for C,PF_~.5 and CgPEI2 are shown, respectively, in Figs 9 and 10, along with surface tension data that corroborate the spectrophotometric data. These figures show that a maximum, plateau value of sorption is attained for the two non-ionic, micelleforming surfactants. Data from additional experiments having different soil/water weight-to-volume ratios demonstrate that the maximum number of moles of surfactant sorbed per gram of soil is a constant, independent of the soil/water weight-to-volume ratio for each micelleforming surfactant at all the soil/water weight-tovolume ratios studied. These results are displayed in Fig. ! 1, in which C,oeo,expressed in moles per liter, is plotted against the soil/water weight-to-volume ratio, expressed in grams per liter. The slope of the plot gives Q . ~ with units ofmol per gram. Values for Q,,~ are given for the three micelle-forming suffactants in Table 4. For a system of soil and micellar surfactant solution, a knowledge of the value of Qm,~ permits calculation of the concentration of surfactant in micelle form in the system, an important parameter in predicting surfactant solubilization of HOCs (Edwards et al., 1992). If a non-ionic surfactant is a lamellae-former at 25°C, then the surfactant bilayer lamellae phase and
ZHONOBkO LIu el el.
o Triton X-lO0 (CSPE9.S) a IgelpalCA-720 (CSPE121
Brij 30 ( C 1 2 E 4 )
o Surfacetension A COD a Azo dye
0.001 o~ 0.000
Soil / Water Ratio (g/L) Fig. 11. Plot of the number of moles of suffactant sorbed per liter of solution as a function of the soil/water weight-tovolume ratio in grams per liter for the micelle-forming non-ionic surfactants CsPF_~.s,C_~PE,z,and C.~PE,0.sat their
respective aqueous-phase CMCs. The slope of each curve yields a value for Q,m and demonstrates that Qm, is a constant over the range of soil/water weight-to-volume ratios tested.
the enveloping suffactant monomer phase that are created when the bulk solution surfactant concentration exceeds the CAC may result in apparent surfactant sorption behavior that differs from that of typical micelle-forming non-ionic surfactants. This is the case for the lamellae-forming suffactant of this study. Evaluating surfactant sorption for this surfactant entailed measuring the amount of COD in supernatant from a soil/aqueous system and employing the calibration curve to obtain a value for the bulk solution surfactant concentration to calculate the number of moles of surfactant sorbed per liter of solution. A plot of supra-CAC sorption data derived from the COD measurements is shown in Fig. 12 along with spectrophotometry data and sub-CAC surface tension data. It is apparent from Fig. 12 for values of C,,rf greater than the CAC that the amount of surfactant sorbed is greater than that sorbed at the CAC, ~0.01 g/g. For this lamellae-forming surfactant, the amount of surfactant sorbed at the CAC does not appear to represent an upper boundary for surfactant sorption, in contrast to the situation for the micelle-forming surfactants. In order to account for supra-CAC sorption, it is envisioned that there may be interactions between the surfactant bilayer lameilae in solution and the sorbed surfactant bilayer assemblages on the soil, or the soil itself, resulting in some fraction of the surfactant added in excess of the CAC undergoing apparent sorption onto soil. Similar phenomena are known to occur in concentrated Table 4. Maximum mrption coeffcient Q,== for non-ionic alkylphenol ethoxylate surfactants Surfactant
lppal CA-720 Tergitol NP-10 Triton X-100
Sorption coeffment Q.,~ (mol/g) 1.40 x 10 -s 1.13 x 10 s 1.90 x 10-5
Solution Surfactant Concentration (reel/L)
Fig. 12. Sorption isotherm for Ct2E4 lamellae-forming nonionic surfactant developed for sub-CAC bulk solution surfactant concentrations by the surface tension technique and for supra-CAC bulk solution surfactant concentrations by the COD and azo dye techniques. The number of grams of surfaetant sorbed per gram of soil is plotted as a function of the number of moles of surfactant dissolved per liter of solution, showing supra-CAC sorption occurring in excess of that attained at the CAC (2.3 x 10-5 rnol/l). solution, with the solution to form a attractive forces forces upon close
lamellar entities separating out of separate phase due to interlamellar exceeding interlamellar repulsive packing (Mitchell et al., 1983).
CONCLUSIONS AND APPLICATIONS
Insights regarding non-ionic surfactant sorption onto soil are obtained from the results of surface tension, spectrophotometry and chemical oxygen demand (COD) experiments in batch test soil/ aqueous systems. A substantial fraction of non-ionic surfactant in a soil/aqueous system can sorb onto soil. At 25°C, the several micelle-forming non-ionic surfactants tested in this study exhibit sorption behavior that differs markedly from that of a non-ionic suffactant tested that forms bilayer lamellar aggregates. At bulk solution surfactant concentrations less than the surfactant critical micelle concentration (CMC) or the critical aggregate concentration (CAC), respectively, surface tension experiments can provide sorption data for each type of non-ionic surfactant. Sorption for each of the four non-ionic surfactants in the sub-CMC or sub-CAC concentration range can be characterized with a Freundlich isotherm. Spectrophotometric or COD experiments in soil/aqueous systems can provide sorption data for non-ionic surfactants having bulk solution concentrations equal to or greater than surfactant monomer solubility. Although sorption for each of the micelleforming surfactants in the supra-CMC bulk solution surfactant concentration range is constant, sorption for the lamellae-forming surfactant tested in this concentration range appears to be at least initially an increasing function of the surfactant dose, with sorption occurring in excess of that attained at the CAC. Ethoxylate surfactants may act as complexing agents for divalent cations by ion-dipole interaction, the
Sorption of non-ionic surfactants onto soil effects o f which may include species and concentration-dependent swelling and shrinking o f micelles and phase changes o f lamellae. The potential impacts o f these phenomena on supra-CMC sorption measurement techniques need evaluation. Sorption o f non-ionic surfactant onto soil may increase the fractional organic carbon content of the soil, thereby modifying its sorptive characteristics. Non-ionic surfactant sorption may also retard the transport o f the surfactant relative to that of a carrier fluid flowing through a soil, as well as decrease the amount of surfactant available for micellar solubilization and enhanced transport o f hydrophobic organic compounds in porous soil media. Sorption of non-ionic surfaetant onto soil may also affect the bioavailability o f the surfactant for microbial degradation. Additional work is needed to understand the underlying processes affecting non-ionic surfactant sorption onto natural materials, including nonequilibrium kinetic effects. Such information will aid the quantification and prediction o f the behavior of non-ionic surfactants in physico-chemical fate and transport models. Acknowledgements--This work was sponsored by the U.S. Environmental Protection Agency, Office of Exploratory Research under grant number R-816113-01-0. We express our appreciation to Annette M. Jacobson for her useful comments.
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