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Synergistic Effects of Organic Contaminants and SoilOrganic Matter on the Soil-Water Partitioning andEffectiveness of a Nonionic Surfactant (Triton X-100)Achara Ussawarujikulchai a , Shonali Laha b & Berrin Tansel ba Faculty of Environment and Resource Studies, Mahidol University, Salaya, Nakorn Pathom,Thailandb Department of Civil and Environmental Engineering, Florida International University,Miami, Florida, USAPublished online: 21 May 2008.

To cite this article: Achara Ussawarujikulchai , Shonali Laha & Berrin Tansel (2008) Synergistic Effects of OrganicContaminants and Soil Organic Matter on the Soil-Water Partitioning and Effectiveness of a Nonionic Surfactant (Triton X-100),Bioremediation Journal, 12:2, 88-97, DOI: 10.1080/10889860802060170

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Bioremediation Journal, 12(2):88–97, 2008Copyright ©c 2008 Taylor and Francis Group, LLCISSN: 1088-9868DOI: 10.1080/10889860802060170

Synergistic Effects of Organic Contaminantsand Soil Organic Matter on the Soil-Water

Partitioning and Effectiveness of a NonionicSurfactant (Triton X-100)

Achara Ussawarujikulchai,1

Shonali Laha,2

and Berrin Tansel21Faculty of Environment andResource Studies, MahidolUniversity, Salaya, NakornPathom, Thailand2Department of Civil andEnvironmental Engineering,Florida International University,Miami, Florida, USA

ABSTRACT Napthalene- and decane-contaminated soils were treated with Tri-ton X-100 (a nonionic surfactant) to characterize the soil-water partitioningbehavior of the surfactant in soils with different organic content. Soil sampleswith different organic content were prepared by mixing sand-mulch mixturesat different proportions. The experimental results indicated that the amount ofsurfactant sorbed onto soil increased with increasing soil organic content andincreasing surfactant concentration. The effective critical micelle concentration(CMC) also increased with increasing organic content in soil. The CMC ofTriton X-100 in aqueous systems without soil was about 0.3 mM and the ef-fective CMC values measured for soil-water-surfactant systems (approximately1:19 soil/water ratio) with 25%, 50%, and 75% mulch content were 0.9, 1.0, and1.7 mM, respectively. Sub-CMC surfactant sorption was modeled accuratelywith both the Freundlich and the linear isotherm. The maximum surfactantsorption onto soil varied from 66% to 82% of added surfactant in the absenceof contaminant. The effective CMC values for Triton X-100 increased to someextent in the presence of contaminants, as did the maximum surfactant sorp-tion. The maximum surfactant sorbed onto the soil with 75% mulch contentincreased from 82% for clean soils, to 95% and 96% for soils samples contam-inated with naphthalene and decane, respectively.

KEYWORDS CMC, organic contaminants, partition coefficient, sorption, soil washing,surfactants

INTRODUCTIONSurfactants have been used in laboratory and field investigations to facilitate

the remediation of soils and aquifers contaminated with hydrophobic organiccompounds (HOCs). HOCs generally partition into the hydrocarbon-like mi-cellar core of surfactant aggregates, giving micelles the capacity to solubilizeHOCs. This characteristic has resulted in significant interest in the use of

Address correspondence to BerrinTansel, Associate Professor,Department of Civil andEnvironmental Engineering, FloridaInternational University, Miami, FL33199, USA. E-mail: tanselb@fiu.edu

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surfactants for the treatment of recalcitrant organic con-taminants in the soil-water environment by facilitat-ing HOC desorption from soil, increasing apparentaqueous phase HOC concentrations, and potentiallyincreasing HOC bioavailability and degradation. Theminimum surfactant concentration at which micellesstart to form is called the “critical micelle concentra-tion” (CMC) (Yalkowsky, 1999). At supra-CMC sur-factant concentrations, any further surfactant additioncontributes to the formation of additional micelles. Inthe presence of soil, significant amounts of surfactantare lost from the aqueous pseudo-phase by partition-ing onto soil; this raises the apparent CMC of thesurfactant.

The objective of this study was to examine the syn-ergistic effects of soil organic carbon and organic con-taminants on the partitioning characteristics of TritonX-100, a nonionic surfactant, in aqueous systems. Sur-factant losses through sorption onto soil have receivedconsiderable attention, since the additional surfactantdemand represents significant costs in the surfactant-amended remediation of HOC-contaminated aquifers.Surfactant sorption onto soil generally increases withsurfactant concentration, reaching a maximum valueas the surfactant concentration approaches that neces-sary for micelle formation. This study examines surfac-tant distribution between soil and aqueous phases asa function of the organic carbon content of soil, andquantifies the maximum surfactant sorption capacityof the soils. Furthermore, the effects of the presence oftwo model contaminants (decane and naphthalene) onsurfactant partitioning behavior are also investigated.Because the overall purpose in surfactant-amended re-mediation is to facilitate HOC desorption from soilinto the aqueous pseudo phase, the effects of surfactantconcentration on the solubilization of soil-sorbed con-taminants were also examined for soils with differentorganic content. Several models have been presented inthe literature for surfactant solubilization of HOCs, in-cluding molar solubilization ratios and surfactant-waterpartition coefficients. These models are used to ana-lyze the surfactant solubilization data obtained in thisstudy.

MATERIALS AND METHODSMaterials

Triton X-100, a nonionic octyl-phenyl-ethoxylate sur-factant, was used in this study. Triton X-100 is rep-

resented with the molecular formula C8H17-C6H4-O(CH2CH2O)nH where n is ∼9.5, molecular weight(MW) ∼625, liquid, specific gravity [S.G.] of 1.065 at25◦C, with a CMC value in aqueous solution of approx-imately 0.2 mM. Triton X-100 was selected because of itslow CMC and widespread use in other surfactant stud-ies. Commercially available Triton X-100 was 97% pureand was used as received. Naphthalene and decane wereselected as representative organic contaminants. Naph-thalene (C10H8, MW 128.2, white solid volatile aro-matic hydrocarbon with characteristic mothball odor,S.G. of 1.10 at 20◦C, and log Kow of 3.36) was dis-solved in methanol to a concentration of 40 g L−1 (or0.312 mol L−1). Decane (C10H22, MW 142.3, aliphaticliquid with S.G. of 0.73, and log Kow of 5.58) was alsodissolved in methanol at a concentration of 8% (v/v)corresponding to 58.4 g L−1 (or 0.41 mol L−1). The con-centrations of naphthalene and decane selected in thesetests were based on preliminary solubility tests withnaphthalene and decane in methanol. Methanol wasused as the solvent because it showed no effect on sur-factant solubilization (Edwards et al., 1991; Bramwell,1997). Potassium dichromate (K2Cr2O7), sulfuric acid(H2SO4), ferroin indicator, and ferrous sulfate (FeSO4)were used for determining the soil organic contentby the Walkley-Black method. Mercuric chloride wasused to inhibit biological degradation in the soil-water-surfactant systems. Benzene and acetone were used toclean the platinum-iridium ring after every surface ten-sion measurement. The chemicals were obtained fromFisher Scientific (Pittsburgh, PA, purity >95%), andused as received.

Soil SamplesSoil samples were prepared by mixing commer-

cially available sand and mulch at specified propor-tions. Commercial grade Cypress Mulch and South-down Tropical Play Sand were obtained from the localgarden supplies store (Home Depot, Miami). Mulchwas shredded using a laboratory blender and sievedthrough a standard sieve number 30 (pore size 590 µm).Both mulch and sand were dried using an Isotherm 500model oven (Fisher Scientific) prior to preparation ofthe soil samples. The organic content of the soil wasadjusted by varying the sand to mulch (w/w) ratio.

Soil Organic ContentThe organic content of mulch was determined by

gravimetric analysis (i.e., loss of mass by ignition)

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(ASTM D 2974-87 procedure), and the organic carboncontent was measured by the Walkley-Black method(i.e., chemical oxidation of the organic fraction) (ASA,1965).

Aqueous CMC from Surface TensionMeasurements

Surfactant solutions at different concentrations rang-ing from 0.0001% to 2% (v/v) were prepared withde-ionized water. The selected concentration range ex-tended from below CMC to above CMC. Surface ten-sion measurements were conducted with a Fisher Sur-face Tensiometer Model 20 according to ASTM D 1331-89 (Standard Test Method for Surface and InterfacialTension of Solutions of Surface-Active Agent). The sur-face tension measurements were taken at room tempera-ture of 21◦C ± 1◦C. Each sample was tested at least fourtimes to ensure that consistent surface tension readingswere obtained.

Surfactant Sorption ExperimentsBatch sorption studies were conducted using soil

samples mixed with aqueous surfactant solutions insealed 125-ml glass Erlenmeyer flasks. The total massof soil used in each batch test was 4 g and the organiccontent of the soil was adjusted by changing the sand-to-mulch ratio (e.g., 3 g of sand mixed with 1 g of mulchyielded 25% mulch content). Seventy-five milliliters ofsurfactant solution was added to each 4-g soil sampleyielding a soil/water ratio of approximately 1:19 (w/w),and 240 mg/L of mercuric chloride was added as mi-crobial inhibitor. Preliminary mixing experiments withdifferent mixing times (varying between 1 and 48 h)indicated that a mixing time of 2 h was sufficient forthe system to reach equilibrium. Each flask was agitatedwith a Gyrotory Shaker Model G2 (New Brunswick Sci-entific, Edison, NJ) for 2 h and left undisturbed for 36h for separation of the solid phase by gravity. An IECClinical Centrifuge (Fisher Scientific, Pittsburgh, PA)was used to remove residual solids and colloids from thesupernatant after the aqueous phase was decanted fromthe settled sample. The amount of surfactant sorbedonto soil was calculated by considering the differencein the surfactant doses required to produce a given sur-face tension value in the presence and absence of soil.

Surfactant Partitioning in thePresence of Contaminants

Soil samples were contaminated by adding 10 ml ofmethanol-contaminant solution to 4 g soil in a sealed125-ml glass Erlenmeyer flask. This theoretically yieldedcontaminant amounts of 400 mg naphthalene or 584mg decane per flask. The flasks were then placed on anorbital shaker for 2 h to thoroughly mix the contam-inant with soil. Following the agitation, the contami-nated soil samples were opened to allow the methanolto evaporate in a fume hood for 24 h. It is expectedthat some of the added contaminant, especially naph-thalene, volatilized along with the solvent. However,because all the tests were similarly performed, loss ofcontaminant at this stage was not a significant factorfor the comparative evaluation of the samples from dif-ferent reactors. After 24 h, 75 ml of surfactant solutionand 240 mg/L mercuric chloride (to inhibit biodegra-dation) were added to each flask. After mixing for 2h, the flasks were stored for 36 h to allow for separa-tion of soil by gravity. The supernatant from each sam-ple was centrifuged for 15 min and the surface tensionof the supernatant was measured at room temperature.The amount of surfactant sorbed onto the soil sampleswas calculated as before, by comparing the surfactantdosages required to form micelles in the presence andabsence of soils.

Determination of ContaminantConcentrations in Aqueous Phase by

SPME/GC/FIDSolid phase microextraction (SPME) was used to ex-

tract the contaminant from the aqueous supernatant,and the extract was analyzed by gas chromatographycoupled with a flame ionization detector (GC/FID).Polydimethylsiloxane (PDMS) 100-µm fiber was usedfor the SPME method. The GC was an HP 5890 Se-ries II (Hewlett Packard); the temperatures used at theinjection and detection ports were 250◦C and 280◦C,respectively. A DB-5 column with a constant columnhead pressure of 10 psi was used (Agilent J&W, PaloAlto, CA). EZ Chrom Elite software was used for anal-ysis of the results. SPME assemblies (Supelco, Belle-fonte, PA), including SPME holder for manual use and100-µm PDMS fiber, were used for naphthalene anddecane extraction. A manual SPME sampling stand for4-ml vials was used while extracting the analytes and

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a Corning Heat/Stir Plate was used to heat the sam-ples during extraction. The heating temperature rangewas 40◦C to 550◦C and the stirring range was 60 to12000 rpm. For the SPME extraction, 4-ml screw-topclear vials preassembled with PTFE/silicon-lined phe-nolic caps were used. All the equipment and parts usedfor SPME extraction process were obtained from Su-pelco. GC/FID (Hewlett-Packard Series II) was used forSPME fiber desorption and GC analysis to analyze theconcentration of naphthalene and decane in samples.

RESULTS AND DISCUSSIONThe organic content of mulch was determined to be

94% by the ignition method and the fraction of organiccarbon (foc) was determined to be 26% by the Walkley-Black method (i.e., about 24% of the organic contentconsisted of carbon). Although mulch is primarily com-prised of organic matter (94%), the term percent mulchis used rather than percent organic content in presenta-tion of the results.

The CMC values were determined as the intersec-tion of the two linear portions of the plots of mea-sured surface tension versus the logarithm of surfac-tant concentration. From the experimental measure-ments, the CMC of Triton X-100 in aqueous solu-tion was determined to be between 3.1 × 10−4 and4.2 × 10−4 mol L−1. Table 1 summarizes the CMCvalues determined in this and earlier studies. Thereported CMC values ranged from 6.9 × 10−5 to3.3 × 10−4 mol L−1. The variations in reported valuesby different researchers are due to differences in surfac-

TABLE 1 Summary of the CMC Values Measured for Triton X-100 in Aqueous Solution

Average Average Surfacemolecular MW tension tests

Chemical name formula Symbol (g/mol) CMC (M) References

2.07 × 10−4 Kile and Chiou, 19891.7 × 10−4 Liu et al., 19921.7 × 10−4

Octylphenylethoxylate; 3.18 × 10−4 Zheng and Obbard, 2002Polyoxyethylene (9.5) C8H17-C6H4- 2 × 10−4

octylphenol; O(CH2CH2O)9.5H C8PE9.5 625 (3–3.3) × 10−4 Liu et al., 1991; Laha and Luthy, 1992Octylphenylethoxylate with

average n = 9.5;2.24 × 10−4

Octylphenoxypolyethoxyethanol 6.89 × 10−5 Li and Chen, 20023.07 × 10−4

3.92 × 10−4 This study4.21 × 10−4

tant purity, aqueous solution compositions, measure-ment technique used, and temperature of the aqueoussystems.

Figure 1 compares the surface tension measurementsfor supernatant samples from aqueous surfactant solu-tions in the presence and absence of soil. Significantlymore surfactant was required to produce micelles in thepresence of soil in comparison to those aqueous surfac-tant solutions with no soil. In addition, the CMC forTriton X-100 increased with increasing soil organic con-tent. For systems with soil containing 25% mulch, theTriton X-100 dose required to reach CMC was 9.2 ×10−4 mol L−1; soil with 50% mulch reached CMC at1.0 × 10−3 mol L−1 Triton X-100, and soil with 75%mulch reached CMC at 1.7 × 10−3 mol L−1. The num-ber of moles of surfactant sorbed per gram soil, Qsurf,was estimated from Equation 1 (Zheng and Obbard,2002; Liu et al., 1992) using the measured surface ten-sion values (σ ) corresponding to the surfactant doses(i.e., Csurf,soil and Csurf,a in mol L−1) in the presenceand absence of soil.

Qsurf = (Csurf,soil − Csurf,a) ·(

Va

Wsoil

)

= Csurf,sorb ·(

Va

Wsoil

)(1)

where Csurf,soil and Csurf,a are the surfactant doses (inmol L−1) required to produce a surface tension valueof σ in the presence and absence of soil, respectively;Va is the volume of the aqueous solution (L), Wsoil

is the mass of soil (g), Qsurf is the number of molesof surfactant sorbed per mass of soil (mol g−1), and

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FIGURE 1 Comparison of surface tension measurements for surfactant solutions with and without soil.

Csurf,sorb is the concentration of sorbed surfactant givenby the difference of the above two (Csurf,sorb = Csurf,soil

− Csurf,a).

Sub-CMC Surfactant SorptionFigure 2 presents the surfactant sorption isotherms

estimated based on surface tension measurements forsurfactant doses up to the effective CMC. The sorptiondata presented in Figure 2 were modeled using the Fre-undlich and linear sorption isotherms. The regressionequations obtained for the Freundlich and linear sorp-

FIGURE 2 Surfactant sorption onto soils at sub-CMC surfactant levels based on surface tension measurements.

tion isotherms applied for measured surfactant sorptionprior to the onset of micellization are also providedin Figure 2. The Freundlich isotherm coefficients wereclose to one, and both models represented the measuredsurfactant sorption data adequately with R2 values con-sistently greater than 0.99.

Supra-CMC Surfactant SorptionFor surfactant concentrations above the CMC, the

maximum value of surfactant sorption (Qmax) by soilsamples was estimated using the following equation

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TABLE 2 Surfactant Dose Required for Micelle Formation and the Corresponding Amounts of Surfactant Sorbed onto Soils with DifferentOrganic Content

Surfactant concentration Surfactant sorbedat CMC (CMCeff) onto Soil

Qmax

% Mulch in soil log [M] mol/L mol/L % mol/g soil

No soil −3.51 3.09 × 10−4 N/A N/A N/A25 −3.04 9.12 × 10−4 6.03 × 10−4 66% 1.13 × 10−5

50 −2.99 1.02 × 10−3 7.14 × 10−4 70% 1.34 × 10−5

75 −2.77 1.70 × 10−3 1.39 × 10−3 82% 2.60 × 10−5

(Zheng and Obbard, 2002):

CMCeff = CMC + Qmax

(Wsoil

Va

)(2)

It was assumed that micelles are not sorbed and thatTriton X-100 sorption plateaus to a maximum value atthe effective CMC. Table 2 presents the CMCeff val-ues obtained for Triton X-100 in the soil-water systemsused in this study with a soil-to-water ratio of approx-imately 1:19. The CMCeff values for the aqueous sys-tems with soils containing 25%, 50% and 75% mulchwere 9.1 × 10−4 mol L−1, 1 × 10−3 mol L−1 and 1.7 ×10−3 mol L−1, respectively. These CMCeff values wereused in Equation (2) to estimate the maximum Triton X-100 sorption onto soil (Qmax). Table 2 also presents theamount of surfactant sorbed onto soil and the percentsurfactant lost by sorption as well as the maximum sur-factant sorption capacity (Qmax) for each soil mixture.The surfactant sorbed onto soil is calculated as the dif-ference between the surfactant doses required to achievemicellization in the presence and absence of soil. Thepercent loss of surfactant due to sorption represents amaximum since the underlying assumption is that sur-factant micelles are not sorbed onto soil. This presumesthat there is no further surfactant sorption at surfactantdoses beyond the effective CMC as indicated by pre-vious research. The values for Qmax listed in the lastcolumn of Table 2, employ the actual soil-water com-position used in the experiments—i.e., 75 ml surfactantsolution with 4 g soil—to compute the sorbed surfactantconcentration. Qmax values increased with increasingorganic content. The maximum surfactant adsorptiononto a soil (Qmax) is a constant for a particular soil andis independent of the soil/water ratio used; however,the measured effective CMC increased with increasingsoil-to-water ratio. Since the soils with higher organic

content tend to adsorb more surfactant monomers, theamount of surfactant left in solution to form micellesis reduced.

Surfactant Solubilization of HOCsand the Effects of Organic

Contaminants on SurfactantPartitioning Characteristics

To examine the effects of the presence of organiccontaminants on the behavior of surfactants in soil-water systems, batch surfactant sorption studies wereconducted using two model organic contaminants: thearomatic HOC naphthalene and the aliphatic HOCdecane. Surface tension measurements were conductedon such systems to determine if the presence of con-taminants affects surfactant sorption and the measuredeffective CMC. Because the contaminants were appliedto the soil, it is not possible to comment on the effect ofHOC in the absence of soil. The pseudoaqueous phaseconcentrations of the HOCs were also measured as afunction of the added surfactant dose.

Figure 3a and b present the sorption isotherms for thesurfactant Triton X-100 onto the soil samples tested inthe presence of naphthalene and decane, respectively.These isotherms are all in the sub-CMC range of surfac-tant doses and exhibit Freundlich characteristics similarto those observed earlier for surfactant sorption in theabsence of contaminant (Figure 2). Table 3 summarizesthe effective CMC values measured for the soil-watersystems in the presence and absence of contaminant.The measured effective CMC values increased with in-creasing organic content (expressed as mulch content)regardless of the presence or absence of organic contam-inants. The presence of the contaminants also increasedthe measured effective CMC, especially for the higherorganic content soil. For example, for the soil-water

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FIGURE 3 (a) Surfactant sorption onto soils at sub-CMC surfactant levels in the presence of the contaminant naphthalene (based onsurface tension measurements). (b) Surfactant sorption onto soils at sub-CMC surfactant levels in the presence of the contaminant decane(based on surface tension measurements).

system with 50% mulch, the effective CMC was1.02 mM in the absence of contaminant; this increasedto 3.16 mM and 2.51 mM, in the presence of naphtha-lene and decane, respectively. Assuming that maximum

TABLE 3 Surfactant Dose Required for Micelle Formation (CMCeff) and the Corresponding Amounts of Surfactant Sorbed onto Soilswith Different Organic Content as Affected by the Presence of Contaminants Naphthalene and Decane

effective CMC (CMCeff), mol/L Maximum surfactant sorbed (Qmax), mol/g and %

No Contaminated Contaminated No Contaminated Contaminated% Mulch contaminant with naphthalene with decane contaminant with naphthalene with decane

25 9.12 × 10−4 9.50 × 10−4 1.58 × 10−3 1.13 × 10−5 1.20 × 10−5 2.38 × 10−5

66% 67% 80%50 1.02 × 10−3 3.16 × 10−3 2.51 × 10−3 1.34 × 10−5 5.35 × 10−5 4.13 × 10−5

70% 90% 88%75 1.71 × 10−3 6.31 × 10−3 7.94 × 10−3 2.60 × 10−5 1.13 × 10−4 1.43 × 10−4

82% 95% 96%

surfactant sorption occurs at the effective CMC, thesedata were used to compute Qmax, and a comparison ofthe maximum sorption of surfactant onto soil in thepresence and absence of contaminant is also included

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FIGURE 4 (a) Micellar solubilization with contaminant naphthalene (for 25%mulch, MSR = 0.164; log Km = 4.5; for 50% mulch, MSR =0.164; log Km = 4.5; for 5% mulch, MSR = 0.165; log Km = 4.5). (b) Micellar solubilization with contaminant decane (for 25% mulch, MSR= 0.666; log Km = 7.92; for 50% mulch, MSR = 0.567; log Km = 7.87; for 5% mulch, MSR = 0.464; log Km = 7.81).

in Table 3. The results suggest that the presence ofnaphthalene or decane may increase the maximumamount of surfactant sorbed onto soil (Qmax). For ex-ample, about 70% of the added surfactant was sorbedonto soil for the 50:50 mulch/sand mixture in the ab-sence of contaminant, whereas this increased to 90%and 88% in the presence of naphthalene and decane,respectively.

Figure 4a presents the surfactant solubilization dataobtained for naphthalene for the three soil mixturestested. The linear nature of the solubilization curvesindicates that surfactant solubilization of naphthaleneis a micellar phenomenon. The slopes indicated in

Figure 4a represent the molar solubilization ratio (MSR)values for naphthalene in the presence of Triton X-100;the MSR values are not significantly affected by the or-ganic content of soil (i.e., the MSR value is about 0.164for all the soils tested). Similarly, Figure 4b presentsthe solubilization data for decane. For decane contami-nated soil samples, the MSR values were seen to be sig-nificantly affected by the soil composition: with MSRfor decane in Triton X-100 decreasing from 0.666 insoil with 25% mulch, to 0.567 in 50% mulch soil, and0.464 in 75% mulch soil. The average MSR for decanein Triton X-100 was estimated at 0.566. The MSR valuesare an indication of the effectiveness of a surfactant at

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solubilizing a particular compound, and the measuredMSR suggest that Triton X-100 may be better at solubi-lizing decane than naphthalene.

The micelle-water partition coefficient, Km, was cal-culated from the experimentally determined values ofMSR values using the following relationship (Edwardset al., 1991):

Km = MSR1 + MSR

·( 1

0.01805 CCMC

)(3)

where CCMC is the apparent solubility of the HOC (inmol L−1) at CMC which may be approximated as theaqueous solubility of the HOC (Sa).

The estimated MSR and log Km values for naphtha-lene and decane in the soil-water mixtures are presentedin Figure 4. In order to estimate the aqueous phase molefraction of HOC, the estimated solubility of the com-pound in water is required. The solubilities of naphtha-lene and decane are reported as –log Sa values of 3.61and 6.57, respectively (Schwarzenbach et al., 1993). Thisyields solubilities of 2.45 × 10−4 mol L−1 for naphtha-lene, and 2.69 × 10−7 mol L−1 for decane. The mea-sured pseudoaqueous phase concentrations for naph-thalene and decane at CMC were 2.64 × 10−4 mol L−1

and 7.83 × 10−4 mol L−1, respectively. The measured

FIGURE 5 Summary of partitioning characteristics of Triton X-100 in aqueous systems tested.

sub-CMC HOC solubilities suggest that while there ap-peared no significant sub-CMC solubilization of naph-thalene, there was considerable non-micellar solubiliza-tion for decane–its solubility increasing by 3 orders ofmagnitude. Using the reported aqueous solubilities, theaverage log Km value for naphthalene was calculated as4.50, and the log Km value for decane was computedas 7.87. The log Km calculated for naphthalene is closeto the 4.64 value reported by Edwards et al. (1991) fornaphthalene solubilized by Triton X-100 in aqueous so-lution without soil. However, when the measured sub-CMC concentration of the HOC is considered ratherthan the reported aqueous solubilities; the log Km val-ues are 4.47 and 4.40, respectively, for naphthalene anddecane solubilized in Triton X-100. The remarkable dis-crepancy in the log Km for decane is a consequenceof the enhanced sub-CMC solubility. Figure 4 presentsthe log Km computed using reported aqueous solubilityvalues rather than measured sub-CMC concentrations.These values were preferred since they yielded moreconsistent ratios between log Km and log Kow for the twoHOCs.

The alternate approach proposed by Jafvert et al.(1994) and Zhou and Zhu (2005) to describepartitioning of the organic solutes between surfactantmicelles and the water phase was also used.

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Kmc = Smic

Sa Csurf,mic= (Cmic − CCMC)

(Csurf − CMC)· 1Sa

= MSRSa

(4)where Csurf,mic is the concentration of surfactant in mi-cellar form (mol L−1), and Smic and Sa are the molarconcentrations of the solubilizate in the micellar andaqueous phases, respectively. The Kmc values estimatedusing average MSR values and reported aqueous solu-bilities were 671 and 2.1 × 106 L moL−1 for naphtha-lene and decane, respectively. The corresponding Kmc

values estimated using measured sub-CMC concentra-tions were 622 and 722 L moL−1, respectively. The MSRand micelle-water partition coefficients (Km and Kmc)are useful descriptors for the solubilizing capability ofsurfactants and the partitioning of HOC between mi-celles and water.

CONCLUSIONSThe amount of surfactant Triton X-100 sorbed onto

soil increased with increasing surfactant concentration,and the effective CMC value increased with the in-creasing soil organic content. Figure 5 provides a sum-mary of partitioning characteristics of Triton X-100 inaqueous systems tested. The CMC of surfactant Tri-ton X-100 in aqueous system in the absence of soilwas about 3.1 × 10−4 mol L−1 and the effective CMCvalues (CMCeff) measured for soil-water-surfactant sys-tems (approximately 1:19 soil/water ratio) with 25%,50% and 75% mulch content were 9.12 × 10−4, 1.02× 10−4, and 1.71 × 10−4 mol L−1, respectively. Sub-CMC surfactant sorption could be modeled adequatelywith either a Freundlich or linear isotherm. The maxi-mum surfactant sorption onto soil varied from 66% to82% of added surfactant in the absence of contaminant.As presented in Table 3, the CMCeff values for TritonX-100 increased in the presence contaminants, as did

the maximum surfactant sorption. The maximum sur-factant sorbed onto the 75% mulch soil increased from82% in the absence of contaminant, to 95% and 96% inthe presence of naphthalene and decane, respectively.

REFERENCESASA. 1965. Methods of Soil Analysis, Part 2: Chemical and Microbiological

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and interfacial tension of solution of surface-active agent.Bramwell, D.-A. P. 1997. Surfactant Solubilization and Subsequent

Bioavailability of Phenanthrene. MS thesis, Florida InternationalUniversity.

Edwards, D. A., R. G. Luthy, and Z. Liu. 1991. Solubilization of polycyclicaromatic hydrocarbons in micellar nonionic surfactant solutions. En-viron. Sci. Technol. 25:127–133.

Jafvert, C., P. L. van Hoof, and J. Heath 1994. Solubilization of non-polarcompounds by non-ionic surfactant micelles. Water Res. 28:1009–1017.

Kile, D. E., and C. T. Chiou. 1989. Water solubility enhancements ofDDT and trichlorobenzene by some surfactants below and abovethe critical micelle concentration. sEnviron. Sci. Technol. 23:832–838.

Laha, S., and R. G. Luthy. 1992. Effects of nonionic surfactants on thesolubilization and mineralization of Phenanthrene in soil-water sys-tems. Biotechnol. Bioeng. 40:1367–1380.

Li, J. L., and B. H. Chen. 2002. Solubilization of model polycyclic aromatichydrocarbons by nonionic surfactants. Chem. Eng. Sci. 57:2825–2835.

Liu, Z., D. A. Edwards, and R. G. Luthy. 1992 Sorption of non-ionic sur-factants onto soil. Water Res. 26:1337–1345.

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Schwarzenbach, R. P., P. M. Gschwend, and D. M. Imboden. 1993. Envi-ronmental Organic Chemistry. New York: John Wiley and Sons.

Yalkowsky, S. H. 1999. Solubility and Solubilization in Aqueous Media.Washington, DC: American Chemical Society; New York: OxfordUniversity Press.

Zheng, Z., and J. P. Obbard. 2002. Evaluation of an elevated non-ionicsurfactant critical micelle concentration in a soil/aqueous system.Water Res. 36:2667–2672.

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