sorption of nonionic organic compounds in soil-water systems containing a micelle-forming surfactant

Environ. Sci. Techno/. 1995, 29, 903-913

Sorption of Nonionic Organic Compounds in Soil- Water Systems Containing a Micelle-Forming Surfactant S H A O B A I S U N * , + A N D W I L L I A M P . I N S K E E P Department of Plant, Soil & Environmental Sciences, Montana State University, Bozeman, Montana 5971 7-0312

S T E P H E N A . B O Y D Department of Crop & Soil Sciences, Michigan State University, East Lansing, Michigan 48824

In this study, w e examined the effect of a micelle- forming surfactant (Triton X-100) on the sorption of 1,l- bis(p-chlorophenyl)-2,2,2-trichlororethane (p,p’-DDT), 2,2’,4,4‘,5,5’- h exa c hlo ro bi p h enyl (2,2’,4,4‘,5,5’- PC B 1, and 1,2,4-trichlorobenzene (1,2,4-TCB) in a soil-water system. A t aqueous phase Triton X-100 concentrations (CTX) below 200 mg L-l (approximately the critical micelle concentration, cmc, for Triton X-loo), apparent soil-water distribution coefficients (let) for each compound studied increased with increasing CTX. A t CTX values above 200 mg L-l, K9c values decreased with increasing CTX. Below the cmc, surfactant monomers in the aqueous phase are relatively ineffective as a partitioning medium for nonionic organic compounds (NOCs), while the sorbed surfactant molecules increase the sorptive capacity of the solid phase. Above the cmc, however, Surfactant micelles in the aqueous phase begin to compete with the sorbed surfactant as an effective partitioning medium for the poorly water-soluble NOCs (e.g., p,p’-DDT), resulting in a 10-fold decrease in K9c a t a CTX of about 600 mg L-l. Two conceptual models were developed, which adequately described the functional dependence of K9c on CTX.

Introduction The solubility enhancement of nonionic organic compounds (NOCs) by surfactants may represent an important tool in chemical and biological remediation of contami- nated soils and sediments. In aqueous systems, the presence of surfactant micelles or emulsions may enhance the solubility of NOCs by acting as a hydrophobic sorptive phase for the NOCs (1 -4) . However, most environmental remediation efforts involve soil-water or sediment-water systems, where both NOC and surfactant molecules may interact with the solid phase. There is a need therefore to understand the effects of surfactants on the distribution of NOCs in soil-water systems.

The ability of surfactants to enhance the apparent aqueous phase concentrations of NOCs in soil-water systems may result in increased contaminant bioavailability and mobility and hence be useful in developing remediation technologies such as bioremediation and soil washing. The solubility enhancement of NOCs is believed to be governed by a mechanism where the NOC molecules partition into surfactant micelles or emulsions present in the aqueous phase (1, 4) and is mathematically described as:

S*,ISW = 1 + Xe,Ke, (2)

for micelle-forming (eq 1) and emulsion-forming (eq 2) surfactants, respectively, where S*W is the apparent solute solubility at the total surfactant concentration ( X ) , SW is the intrinsic solute solubility in pure water, Xi,, X m c and &m

are the fractional concentrations of surfactant monomers, micelles, and emulsions, respectively, and Kmn, K m c and Kem are the solute partition coefficients between the surfactant monomers and water, micelles and water, and emulsions and water, respectively. For a micelle-forming surfactant (e.g., Triton X-1001, the degree of solubility enhancement differs greatly at surfactant concentrations below and above the critical micelle concentration (cmc) at which the surfactant micelles begin to form in solution. Generally, a significant increase in the solubility of a NOC is observed only at surfactant concentrations above the cmc.

Although the NOC solubility enhancement properties of surfactants in simple aqueous systems are well defined, much less is known about the effects of surfactants on the partitioning of NOCs in soil-water and sediment-water systems. This knowledge is needed for the development of effective and safe environmental remediation technologies utilizing surfactants. Previously, we described the sorption of naphthalene, phenanthrene, and 2,2‘,4,4‘,5,5‘- hexachlorobiphenyl (2,2’,4,4’,5,5’-PCB) in a soil-water system containing petroleum sulfonate oil surfactants (commercially named Petronates) (5). Petronates are unique in that they form stable microemulsions in water and do not exhibit a distinct cmc. As a result, the solubility enhancement of a NOC is a linear function of surfactant

* Corresponding author. + Present address: Department of Civil Engineering and Operations

Research, Princeton University, Princeton, NJ 08544; e-mail address: [email protected]. Fax: (609)258-1270.

0013-936X/95/0929-0903$09.00/0 (E 1995 American Chemical Society VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1903

TABLE 1

Structure and Chemical Properties of Triion X-1 00, p,p'=DDT, 2,2',4,4',5,5'-PCB, and 1,2,4=TCB

Structure

r C H ~ , c C ~ C ( c ~ r ~ c o C H ~ C ~ ~ " ~ t

Triton X-100 (n = 9-10, UV molar absorbtivity Q 275.5 nm = 1.33 x 1 03)

CI, \ A

Q .CH-C-CI Y

CI F' /

Y Y CII CI'

2,2',4,4',5,5'-PCB

c&cl

1,2,4-TCB

MW, g mol" Sw, mg I" + 6 2 4 a I -

3 5 4 . 5 1 0 . 0 C ~ 5 5 ~

3 6 0 . 9 0 . O O l d

5 . 3 4 c

5. 84d

2 . 7 0 '

6 . 3 6 b

6 . 72e

- 4 . 1 4 b * f

4 . 2 6 b

a Data from Union Carbide Chemicals and Plastics Company, Inc. Ref 1, S, at 25 "C. E Ref 13. Ref 14, S, at 25 "C. e Ref 15. 'Value for 1,2,3-trichIorobenzene.

concentration, starting at a surfactant concentration near zero. This is in contrast to conventional micelle-forming surfactants, which often show very little solubility enhancement below the cmc and substantial solubility enhancement above the cmc.

The sorption of NOCs in a surfactant-free soil-water system is believed to be governed by a mechanism where the NOC molecules partition into the soil organic matter phase (6-8). However, when the sorption of a NOC occurs in soil-water systems containing Petronate surfactant, two additional competitive processes (in addition to the intrinsic partitioning into soil organic matter) affect the distribution of NOCs in the aqueous phase and the solid phase: (i) the partitioning of NOCs into aqueous phase surfactant emulsions and (ii) the sorption of NOCs by the sorbed surfactant (5). Depending on the net effect of these two processes, the apparent solute soil-water distribution coefficient (k?) in a system containing surfactant may increase or decrease relative to the intrinsic distribution coefficient ( K ) of the same solute in a surfactant-free system. Both Kand k? k?)

coefficient of p,p’-DDT between surfactant monomers and water (Kmc) is 1.5-2 log units less than that between surfactant micelles and water (Kmn) (1).

In this study, we investigate the sorption of NOCs in soil-water systems containing a micelle-forming surfactant (commercial Triton X-loo), which has a tensiometrically measured cmc of 130 mg L-l in soil-free aqueous systems ( 1 ) . The NOCs examined were 1,l-bis(p-chloropheny1)- 2,2,2-trichlororethane (p,p’-DDT), 2,2’,4,4’,5,5’-hexachlorobiphenyl(2,2’,4,4’,5,5’-PCB), and 1,2,4-trichlorobenzene (1,2,4-TCB), which exhibit awide range ofwater solubilities. Our results show that the apparent distribution coefficient, KI, exhibited a similar pattern as a function of the aqueous phase TritonX- 100 concentrations ( C d for all three solutes studied: KI values increased and were greater than Kwhen CTX increased from zero to approximately 200 mg L-l, followed by a decline in KI upon further increases in C ~ X greater than 200 mg L-l. Two conceptual models, which accurately describe the functional dependence of KI values on CTx, are also presented.

Materials and Methods Commercial Triton X-100. Triton X-100 was purchased from Aldrich Chemical Co. (Milwaukee, WI) and used without further treatment. Chemically, TritonX-100 is an octylphenoxy polyethoxyethanol nonionic surfactant, (CH~)~C-CHZ-C(CHZ)Z-C~H~-(O-CHZ-CH~)~-OH, which has an average n of 9.5, an average molecular weight of 624, and a density of 1.070 g cm’. Commercial Triton X-100 also contains a small amount of polyethylene glycol (less than 3%). The U V molar absorptivity (at 275.5 nm) in aqueous solution is 1.33 x lo3 (data from Union Carbide Chemicals and Plastics Company, Inc.).

Solutes. 14C-Labeled 1,2,4-TCB, p,p’-DDT, and 2,2’,4,4’,5,5’-PCB were obtained from Sigma Chemical Co. (St. Louis, MO). Table 1 lists the chemical structures, molecular weights, water solubility, log I(om (Kom is the soil organic matter normalized solute sorption coefficient), and log KO, (KO, is the solute octanol-water partitioning coefficient).

Soil. The soil used in this study was an Oshtemo silt loam (B horizon), which contains 0.10% organic C (ca., 0.17% soil organic matter), 89% sand, 5% silt, and 6% clay. The soil was air-dried for 48 h and sieved (<2 mm) before use.

Surface Tension of Soil-Water System Containing Triton X-100. Samples were prepared in duplicate by mixing 1.0 g of soil, Triton X-100 stock solution (1000 mg L-l), and distilled water in 25-mL Corex centrifuge tubes. Initial Triton X-100 concentrations ranged from 0 to 1000 mg L-’ with an interval of 100 mg L-l, and the total solution volume of each sample was 25.0 mL. These samples were shaken for 24 h at room temperature (23 “C) and then centrifuged at a relative centrifugal force (RCF) of approximately 7500g for 20 min. Supernatant (20 mL) was separated from each sample and placed in a surface tension measuring dish. After 30 min, the surface tension was determined using a Surfacetensiomat Model 20 (Fisher Scientific, Pittsburgh, PA).

Sorption of TritonX-100. A batch equilibrium method was employed to measure the sorption isotherm of Triton X-100 in the Oshtemo soil-water system. Samples were prepared in triplicate by mixing 1.00 g of soil, Triton X- 100 stock solution (1000 mg L-l), and distilled water in 25-mL

Corex centrifuge tubes. The initial Triton X-100 concentrations for these samples were 0, 50, 150, 200, 250, and 300, then up to 1000 mg L-l with an interval of 100 mg L-I.

The total solution phase volume of each sample was 25.0 mL. The samples were shaken for 24 h at room temperature (23 “C) and then centrifuged at a RCF of approximately 7500g for 20 min. The supernatant of each sample was carefully separated from the solid phase using a disposable Pasteur pipet and then analyzed using a Hitachi U-2000 spectrophotometer at a wavelength of 274.8 nm, which produces a completely isolated peak. Another wavelength used to confirm the results was 222.8 nm, which gives a peak with greater absorbance but is close to the nontrans- parent region of water.

Sorption of Solutes in Soil-Triton-Water Systems. The soil-water distribution coefficients of 1,2,4-TCB, p,p’- DDT, and 2,2‘,4,4‘,5,5‘-PCB in the presence (KI) or absence (4 ofTritonX-100 were derived from the sorption isotherms determined using batch equilibrium experiments. The sorption isotherms were obtained in the Oshtemo soil- watersystemscontainingO-lOOOmgL-l initialTritonX-100 concentration, with an interval of 100 mg L-l. For each sorption isotherm at a fixed initial Triton X- 100 concentration, three samples (in duplicate) were prepared at different initial solute concentrations, ranging from 0.0483 to 0.145 mg L-’ for 1,2,4-TCB, 0.0157-0.0471 mg L-’ for p,p-DDT, and 0.01 14-0.0228 mg L-l for 2,2’,4,4’,5,5’-PCB. (To prevent undesirable crystallization of solute, the initial p,p’-DDT and 2,2‘,4,4’,5,5‘-PCB concentrations, which were greater than their water solubilities, were achieved by multiple instances of adding solute into the sample at a subsaturation concentration everyO.5 h duringthe early hours of shaking.) Each sample contained 1.00 g of soil and 25.00 mL of total solution volume, sealed in a 25-mL Corex centrifuge tube, and was shaken for 24 h at room temperature (23 “C). Preliminary studies indicated no significant difference in sorption of 2,2’,4,4‘,5,5’-PCB between 24 and 48 h. The samples were centrifuged at a RCF of approximately 7500g for 20 min to separate supernatant from the solid phase. A total of 1.00 mL of the supernatant was mixed with 5 mL of ScintiSafe Plus 50% and analyzed using a Packard Tri- Carb 2200CA liquid scintillation analyzer to determine the equilibrium concentration of the solute. The equilibrium solute concentration in the soil phase was calculated by difference. The solid phase was analyzed using a Biological Oxidizer OX300 (R. J. Harvey Instruments Corporation) to estimate the solute recovery for each compound used. A solute recovery greater than 93% was achieved for all compounds.

Results and Discussion The sorption isotherm of Triton X-100 on Oshtemo soil was nonlinear, exhibiting a plateau in sorbed TritonX-100 at equilibrium Triton X-100 concentrations (Cnc) greater than 200 mg L-l (Figure 1). Because of the difficulty in obtaining an accurate fit of our TritonX-100 sorption data to any simple models (Le., Freundlich or Langmuir), the experimentally determined equilibrium Triton X-100 concentrations in the aqueous phase and in the soil phase were used directly in our subsequent efforts to interpret and model the solute sorption data in soil-water-surfactant systems. A comparison of the changes in surface tension with increasing C, between the Oshtemo soil- water mixture and a pure water system (soil-free) (1) suggests that the presence of soil caused a more gradual

VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1905

12000

Tm 10000 x

P

5

.. - .- 8 8000

c 0

6 6000

.- c E Q) 0 E 8

z 8 4000

E 0 c .- c’ 2000

0

Sorption Isotherm 4

’* Surface Tension in Soil-Free System (Kile & Chiou, 1989) K ‘11;

~...-......-..”’

Surface Tension in Oshtemo-Water System

O.@. . - - *

46

44

42

r 40

-I (0 3

3 P w

36 3

38Q.

3 Ir

34

32

30 0 1 00 200 300 400 500 600 700

Triton X-lo0 Concentration in Solution (CTX), mg I-’ FIGURE 1. Triton X-100 sorption isotherm (squares) and surface tension (closed circles) in the Oshtemo soil-water system. Also included is a replot of surface tension (open circles) in a soil-free Triton X-100 solution (1).

TABLE 2

Experiamtally Measured Appreat Soil- Water Distibution Coefficients (P) d p,p’=BDT, 2,2’,4,4’,5,5’-PC6, and 1,2,dTCB as a Function of Triton X-100 Aqneous Concentrations (Cn)

N1 (standard error, r *)

GX (ma 1-l)

0.0 24.3 64.3 105.7 128.8 158.6 207.1 294.9 375.9 496.0 613.4

p,p’-DDT

456 (19.0,0.985) 716 (9.2, 0.999) 856 (9.2, 0.999) 1268 (11.9, 0.999) 1479 (14.5, 0.999) 1496 (83.9, 0.974) 1078 (28.6,0.994) 224 (9.4,0.985) 71.6 (1.7, 0.995) 48.9 (0.4, 0.999) 43.3 (0.9, 0.997)

2,2,4,4‘,5,5’-PCB

493 (13.7, 0.992) 613 (16.2, 0.993) 833 (1 4.0,0.997) 1034 (3.9, 1.000) 1196 (16.6,0.998) 1161 (26.4,0.993) 869 (23.8, 0.993) 182 (1.7,0.999) 82.2 (0.7, 0.999) 46.2 (0.7, 0.998) 32.8 (0.2, 1.000)

1,ZA-TCB

6.54 (0.17, 0.994) 7.45 (0.03, 1.000) 9.82 (0.39,0.988) 13.2 (0.10, 1.000) 20.2 (0.43,0.996) 27.1 (0.55,0.997) 31.8 (0.69,0.996) 34.3 (0.71, 0.996) 28.2 (0.31,0.999) 23.7 (0.26,0.999) 18.4 (0.20, 0.999)

(transitional) change in the surface tension of the aqueous phase. Thus, the apparent cmc of Triton X-100 in the Oshtemo-water system was not as well-defined as in the soil-free system reported previously (I).

Sorption isotherms of 1,2,4-TCB, 2,2’,4,4’,5,5’-PCB, and p,p’-DDT on the Oshtemo soil at different Cz’s were linear over the concentration range tested as observed previously for numerous NOCs and soils (Figure 2) (6-8). The apparent solute soil-water distribution coefficients, K* (equal to the slopes of these isotherms), were significantly affected bythe additionofTritonX-100 (Table2). All solutes shared a similar sorptive behavior pattern in the presence of increasing C z (Figure 3). That is, relative to the corresponding intrinsic Kvalue in a surfactant-free system, K. increased when CTX increased from zero to approximately

200 mg L-l, followed by decreases ink? at Cmvalues greater than 200 mg L-I.

The percent increase in K* at lower CTX values (0-200 mg L-l) appeared to be proportional to the solute water solubility (SW); that is, the higher the SW, the greater the percent increase in K.. For 1,2,4-TCB (SW = 18 mg L-l), p,p’-DDT (SW = 0.0055 mg L-I), and 2,2’,4,4’,5,5’-PCB (SW = 0.001 mg L-l), the maximum KI values were 520%, 330%, and 240% of the corresponding Kvalues, respectively. The percent decrease in K. at higher CTX values (’200 mg L-I), however, was inversely proportional to the solute water solubility. For instance, the K* values of 1,2,4-TCB, p,p’- DDT, and 2,2’,4,4’,5,5’-PCB at a Cm of 613.4 mg L-’ were 280%, 9.5%, and 6.6% of their corresponding K values, respectively. It is noteworthy that for a relatively water-

906 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 4,1995

1.2

1 .o

0.8

0.6

0.4

0.2

0.0 0.000 0.004 0.008 0.012 0.01 6 0.020

0.6 I i

0.5

0.4

0.3

0.2

0.1

0.0

CTX, mg I-’

o 24.3 v 64.3 A 105.7 M 128.8 x 158.6

207.1 e 294.9 v 375.9 A 496.0 M 613.4

0.002 0.004 0.006 0.008 0.01 0 L . 0.000

2.4

2.0

1.6

1.2

0.8

0.4

0.0 0.000 0.024 0.048 0.072 0.096 0.120

Equil. NOC Concentration in Solution, mg I-’ FIGURE 2. Sorption isotherms of p,p’-DDT, 2,2,4,4‘,5,5’-PCB, and 1,2,4-TCB in the Oshtemo soil-water system containing different aqueous Triton X-100 concentrations (&). Each measured data point is an average of duplicates.

soluble solute, such as 1,2,4-TCB, the addition of Triton X-100 (up to 613.4 mg L-l) did not reduce but increased the amount of sorbed solute (yielded a higher I?) in the soil- water system over the whole Cm range studied (Figure 3). The solubility enhancement of NOCs by Triton X- 100 in the aqueous phase has been well documented (1). Thus, the increase in K* at lower Cm’s is apparently caused by the strong sorption of Triton X- 100 by the soil phase (Figure 1) and the functionality of the sorbed Triton X-100 as a

sorptive phase for the solutes. This behavior is analogous to the partitioning phases produced by residual petroleum, PCB oils, or Petronates in soils (59,lO). A more quantitative description of the relationship between K* and CTX is given later in our discussion.

In a previous study, we reported a mathematical model that accurately described the effect of some emulsion- forming surfactants (commercial Petronates) on the sorption of naphthalene, phenanthrene, and 2,2‘,4,4’,5,5’-PCB

VOL. 29. NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 907

1600

1400

m

a ;t, 1000

4 1200

2

k

v!

t

800 Q

9 600 Q

0 400

- ri

2 r

200

0

35

30

25

5 20 3,

:N e -L

15 -I 0 m

10

5

0 0 100 200 300 400 500 600 700

cTX, mg I" FIGURE 3. Plot of the apparent soil-water distribution coefficients (P) of p,@-DDT, 2,2',4,4',5,5'-PCB, and 12,4-TCB as a function of the equilibrium concentration of Triton X-100 in the aqueous phase (h). Standard errors of P values are listed in Table 2.

in Oshtemo soil-water systems (5):

(3)

where Ksemiom is the solute partition coefficient between sorbed Petronate and natural soil organic matter (which is equivalent to the NOC partition coefficient between sorbed Petronate and water divided bythe NOC partition coefficient between soil organic matter and water), Xsem/om is the fractional concentration of sorbed Petronate normalized for natural soil organic matter, Kern and zrn are defined in eq 2. Conceptually, this model (eq 3) accounts for nuo competitive sorptive phases in addition to the native soil organic matter: (i) the surfactant emulsions in the aqueous phase, which would act to increase the apparent solute concentration in the aqueous phase and thus reduce KI (the denominator in eq3), and (ii) the soil-sorbed surfactant, which would act to increase the solute concentration in the soil phase and thus increase KI (the numerator in eq 3). The difference between the apparent P and the intrinsic Kfor a give solute depends on the net effects of both sorbed and aqueous phase surfactant fractions.

A micelle-forming surfactant (such as commercial Triton X- 100) may exist in two forms when dissolved in an aqueous phase: monomers and micelles. At very low surfactant concentrations, the solution contains only surfactant monomers. Surfactant micelles begin to form in the aqueous phase at a certain concentration (the cmc), and above that concentration the solution contains both monomers and micelles. As sorptive phases for NOCs, especially for poorly water-soluble NOCs, surfactant monomers and micelles exhibit significantly different effective-

ness, which is indicated by the magnitude of Kmn and Kmc, respectively (1). Thus, the effect of aqueous phase surfactant on the distribution of a given NOC, i.e., the &&& term in eq 2, should be separated into two terms: one accounting for monomers and the other for micelles. Based on this conceptual model, the effect of a micelle-forming surfactant on the solute soil-water distribution can be expressed as:

where is the solute partition coefficient between sorbed Triton X-100 and native soil organic matter, and Xs/om is the fractional concentration of sorbed TritonX- 100 per unit mass of native soil organic matter. Kmn, Kmc, Xmn, and X,, were defined in eq 2. The definition of Kslom here actually represents the effectiveness of the sorbed Triton X-100 as a sorption medium for a given NOC, relative to the native soil organic matter ( i e . , Ks/om = Ks/&m, where Ks is the partition coefficient of a given NOC between the sorbed surfactant and water) (5).

Experimental data for K1 as a function of CTX were used to fit the model presented in eq 4, utilizing a nonlinear least-square algorithm. The values ofXs/om were obtained using experimentally determined equilibrium concentrations of Triton X- 100 in the soil phase (Figure 1). The values of X,, and X,, were calculated using the following expressions:

xaaq = &In + &C

kn = Xaq and GC = 0 when Xaq 5 X,,, (6)

908 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 4, 1995

TABLE 3

Fitted Values of KdO,,,, log Ks,B Critical Micelle Concentration (cmc), and Triton X=100 Concentrations at lower (Cl) and Upper (Cu) MiceNization Boundaries, Using Simple (eq 4) and Transitional (eq 9) Models Describing Sorption of NOCs in Soil- Water Systems Containing Triton X.100 Surfactant

simple model (eq 4) transitional model (eq 9) Kvom log IG* cmc (mg 1-l) z Kvom log IG &(ma 1-’1 G(mg L-’) z

p,p’-DDT 2.55 5.75 218 0.950 2.55 5.75 181 510 0.964 2,2‘,4,4‘,5,5’-PCB 3.03 6.32 232 0.951 3.03 6.32 183 123 0.962 1,2,4-TCB 0.699 2.54 360 0.921 a Kslom = KJK,,, where K, is the solute partition coefficient between sorbed Triton X-100 and water, and Ksiom is the solute partition coefficient

between sorbed Triton X-100 and native soil organic matter.

&, = X,,, and &, = Xaq - X,,, when Xaq > X,,, (71

where Xq is the total fractional concentration of Triton X-100 in the aqueous phase, and &mc is defined as the fractional concentration of Triton X-100 in the aqueous phase at the cmc (Le., the value of Xaq at the crnc). Values of Xmn and X,, are dependent on the magnitude of Xaq relative to the cmc. Although we could use an independent estimate of the cmc of TritonX-100 from aqueous solution studies (e.g., ref 11, we chose to treat the cmc ( i e . , &,,I as an unknown parameter fitted with the least-square nonlinear algorithm. This decision was based on our observa- tion that the cmc of Triton X-100 was less resolved in the presence of soil than in pure water (Figure 1).

Independently measured or estimated K Kmnr and Kmc values were used as input parameters to eq 4. The intrinsic soil-water distribution coefficients (fl were experimentally determined for each solute in the absence of Triton X- 100. Log Kmc values of 6.15 and 3.82 for p,p’-DDT and 1,2,3-TCB (which would be approximately the same for 1,2,4-TCB), respectively, were reported in an independent study (1). A log Kmc value of 6.53 for 2,2’,4,4’,5,5’-PCB was estimated using a linear-extrapolation approach described in our previous study (3, based on the linear relationship of log Ko,andlogK,, and two known points (p,p’-DDT and 1,2,4- TCB). Values of Kmn for 1,2,4-TCB and 2,2’,4,4’,5,5’-PCB have not been measured independently. However, for poorly water-soluble NOCs such as p,p’-DDT, log Kmc is approximately 1.44 times greater than log Kmn. Conse- quently, alog K,,value of 4.53 was assumed for 2,2’,4,4’,5,5’- PCB. Kile and Chiou (1) reported that the solubility enhancement of 1,2,3-TCB by Triton X- 100 was negligible at Triton X-100 concentration below the cmc. Conse- quently, we assumed that K,, was equal to zero for 1,2,4- TCB. In summary, eqs 5-7 and independently measured or estimated K Kmn, and Kmc values were used to limit the unknown fitted parameters to the cmc and Ks/om (Table 3).

The two-parameter (cmc and Ks/om) fit of eq 4 accurately described the functional dependence of K* on CTX for all three solutes (Figure 4). As C ~ X increased from 0 to approximately 200 mg L-l, K* increased 3-5 times compared to the intrinsic K. This is consistent with a period of rapid increase in the sorbed TritonX- 100 (Figure 1) prior to the formation of TritonX-100 micelles in solution, where

than 200 mg L-l, the sorbed TritonX-100 reaches a plateau ( ie . , Ks/omXs/om = constant), and Triton X-100 micelles in the aqueous phase become an important partition phase with increasing C,, ( ie . , the K,,X,, term in eq 4 becomes more important). As a result, K* values decrease, and for p,p’-DDT and 2,2’,4,4‘,5,5’-PCB, the K* values decrease significantly below their respective intrinsic Kvalues. The

Ks/omXs/om =- K m A m n and KmcXmc = 0. At Cm values greater

fitted values of Kslom (Table 3) were 2.55 and 3.03 for p,p‘- DDT and 2,2‘,4,4‘,5,5‘-PCB, respectively, indicating that, for NOCs with lowwater solubility, the sorbed TritonX-100 is more effective per unit mass as a partitioning medium than is the native soil organic matter. The fitted Ks/om for 1,2,4-TCB (which has a much higher water solubility than p,p’-DDT and 2,2’,4,4’,5,5’-PCB) is smaller than 1 (0.6991, suggesting that the native soil organic matter is a more effective sorptive phase than sorbed Triton X-100. Fitted

values were used to calculate K,, which is the NOC partition coefficient between sorbed TritonX- 100 and water (K, = Ks/om&m) (Table 3). The calculated Ks values increased with decreasing water solubilities of the NOCs used in this study.

Fittedvalues of the cmc (Table 3) using the simple model (eq 4) ranged from 218 to 360 mg L-’ for the three solutes studied and were higher than the tensiometrically measured cmc reported by Kile and Chiou (1) for a soil-free solution (130 mg L-l). Our independent measurement of the surface tension in the Oshtemo soil-water system containing Triton X-100 (Figure 1) shows a more gradual change in surface tension near the cmc (from 80 to 300 mg L-l) in the presence of soil than in pure water. The cmc determined using conventional ( e g , tensiometric) methods may indicate the surfactant concentration where surfactant micelles begin to form in solution, but may not necessarily correspond to the surfactant concentration where surfactant micelles begin to function as a highly effective sorptive medium for NOCs. For example, it has been pointed out that the change in surface tension of a surfactant solution on exceeding the cmc is generally less than a factor of 3 ( I ] ) , whereas the associated change in the apparent water solubility of p,p’- DDT can be more than 2 orders of magnitude (1). In addition, Triton X-100 is a molecularly heterogeneous surfactant (n = 9- 10, Table 11, that may exhibit preferential sorption of molecules with different n values. This would contribute to a shift in the cmc of Triton X-100 in soil- water systems compared to pure water systems. Conse- quently, the interaction of surfactant molecules with a sorbing phase (soil) changes the apparent cmc at which significant solubility enhancement occurs. In this specific case, the cmc values of Triton X-100 which correspond to formation of an effective partitioning phase for p,p’-DDT and 2,2‘,4,4‘,5,5‘-PCB fall between 218 and 232 mg L-l, respectively, in the presence of the soil compared to 130 mg L-’ in pure water systems (1). In essence, the fitted cmc values reported here (Table 3) might suggest a more reliable surfactant concentration where Triton X-100 micelles begin to function as a highly effective sorptive phase for p,p’-DDT, 2,2’,4,4’,5,5’-PCB, and 1,2,4-TCB in the Oshtemo soil-water system.

VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 909

1600

1400 m

k 1200 v! ;r: -?*

h- 1000

Y

(v U C Q

800 Q

j ,

% 2

Q

9 600 0

c c 400

200

0 0 100 200 300 400 500 600 700

CTX, mg I” FIGURE 4. Comparison of the measured F values of p,p’-DOT, 2,2,4,4‘,5,5’-PCB, and 12,CTCB with the corresponding fitted F values (simple model, eq 4) as a function of Cot.

Although the fit of eq 4 to the experimental data adequately described the functional dependence of K” on CTX, the model overestimated K* at low CTX’S (e200 mg L-l)

and underestimated K* at high Cm’s (’200 mg L-l). The model also predicted an abrupt decrease in K* at the cmc, whereas the experimentally measured data failed to exhibit such behavior. In eq 4, Ks/om is treated as a constant over the entire range of CTX’S. However, several factors may cause the sorbed surfactant to function as a less effective (indicated by a smaller Ks/om value) partitioning medium at very low CTX’S. First, there may be a threshold quantity of sorbed surfactant necessary to function effectively as a NOC partitioning phase (10). Secondly, the sorbed surfactant molecules may exist in two forms, surface monomor and surface micelle (12). At low CTX values, surfactant molecules are likely sorbed as monomers. As Cnincreases, it is likely that micelles form on the sorbing surface either via sorption of aqueous micelles or via the association of aqueous monomers and adsorbed monomers (12). In either case, the role of sorbed phase surfactant may change as a function of CTX, reflecting the change in affinity of NOCs for sorbed surfactant monomers and sorbed surfactant micelles. Consequently, eq 4 may be improved by splitting the Xs/omKs/om term into two terms, one for surface monomers and one for surface micelles.

An alternative approach to improve the simple model defined in eq 4 involves an examination of the published data on solubility enhancement ofp,p’-DDT byTritonX-100 ( I ) , which suggests a gradual transition in solubility enhancement near the cmc, consistent with the sequential

micellization of surfactant molecules of different aggrega- tion numbers. The Kmn and Kmc values are generally derived from the slope of tangents on the curve of apparent solubility (S*w) vs surfactant concentration in solution (Cd, below and above the cmc, respectively. However, as CTX ap- proaches and exceeds the cmc, there is a gradual transition from Kmn to Kmc, Le., the slope of the tangents in this transitional region (denoted Ktr) is a dependent variable of CTX, rather than a constant. The exact mathematical dependence of Ktr on CTX is not clear. However, the S*W- Cm curve of p,p’-DDT in the presence of Triton X-100 exhibits a quadratic-lie shape around the cmc (11, thus we may assume that Ktr is a linear function of CTX (k, the slope of a tangent at a given point on the function curve is the derivative of the function at that point and d(x2)/dx = 2x). Given Kmn and Kmc as the lower and upper boundaries, respectively, Kt, (as a function of CT~) can be defined as:

where CL is the surfactant concentration of the lower boundary of the transitional region, and Cu is the surfactant concentration of the upper boundary of the transitional region. Therefore, the simple model (eq 4) can be rewritten to include the effect of K,, on kl:

910 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995

XI

where X,, is the fractional concentration of aqueous Triton X-100 at any point in the transitional region. Values of X,,, X,,, and X,, can be calculated based on the total fractional concentration of Triton X-100 in the aqueous phase (Kq), the fractional concentration at the lower boundary (Xd andthe fractionalconcentrationattheupper boundary (X,,):

X,,," = Xas, & = 0, and kc = 0, when Aq 5 X, (10)

&, =X,, 4, =Aq - X,, and&,, = 0, when A,, > X, and A. < Xu (11)

I

a function of Kq is illustrated in Figure 5a. The shaded areas under the curves represent contributions ofX,.K,,,., X,,(K,,,, + K,,)/2 and X,,,,K,, to the denominator in eq 9. Individual contributions ofX,,K,,, X,,K,,, and Xt,(K,. + K,,)/2 as well as the cumulative sum of these three terms (i.e.,the denominatorineql)) asafunctionofQ,areplotted in Figure 5b.

Inclusion of a transitional region near the cmc in the model (eq 91 improved the fit of experimentally measured x1 values as a function of C, (Figure 6). The transitional model was fit using three parameters [G., GI. and K,inm),

, / /

- consequently, there is one additional fitted parameter

, .~ . .~ ,. ~~ compared to the simple model described in eq 4. Fitted X,,," = X,, X,, = XI, - XI and X,,,c = &q - Xu,

values of the apparent cmc for p,p'-DDT and 2,2',4,4',5,5'- PCB using the simple model (eq 4) were 218 and 231 mg L-I, respectively. These cmc values are bracketed by the

wnenn, z A,, LILJ

The importance of solute partition coefficients for the various aqueous Triton X-100 phases (Kmn, K,r, and K,J as

VOL. 29. NO. 4.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY m 9-1

1 600

3 1200 14001 m 8

m

;a, %

lo00 cy U c n

800 4

j ,

Q

0 600

c 5 400 il 2

200

/ /

0

8

m DDT Measured 0 PCB Measured 1 - - PCB Fit - DDT Fit

0 100 200 300 400 500 600 700 Cm, mg I-’

FIGURE 6. Comparison of the measured K+ values of p,p’-DDT and model, eq 9) as a function of &. fitted CL and Cu values for each solute, respectively, in the transitional model (Figure 6). The relatively wide transitional region (Le., CU - CL), which covers about 300 mg L-’ for p,p’-DDT, appears to be in agreement with the surface tension data obtained for the soil-water system studied (Figure 1) and may reflect the effects of a sorbing surface on preferential sorption of surfactant molecules with different n numbers. The wider transitional region for 2,2’,4,4’,5,5’-PCB of 540 mg L-’ appears larger than observed from the surface tension vs Cm plot (Figure l), which may be due to the uncertainty of estimated values of Kmn and Kmc used as input parameters to the model. Nevertheless, the improved predictive capability of the transitional model at higher Cmvalues is important because the effectiveness of micelle-forming surfactants in solu- bilizing soil-bound contaminants is only realized at Cm values above the cmc.

Conelusions Although the water solubility enhancement of NOCs by surfactants has been widely accepted, addition of micelle- forming surfactants into aqueous systems containing soil or sediment results in more complex effects on the apparent soil-water distribution coefficient (K.) of contaminants. At low surfactant aqueous concentrations (Cn cmc), the observed K. values of a given NOC are generally greater than the intrinsic soil-water distribution coefficient ( K ) in a surfactant-free system. This is due to sorption of the NOC by the soil-bound surfactant phase. At higher CTX’S

2‘,4,4,5,5’-PCB with the corresponding fitted K+ values (transitional

(’cmc), K. decreases due to the formation of aqueous surfactant micelles which effectively compete with the solid phase as a sorptive medium for poorly water-soluble NOCs. Two mathematical models (eqs 4 and 9) conceptualized to account for the competitive effects of the soil-bound surfactant molecules and surfactant micelles (as well as monomers) in the aqueous phase were presented. These models accurately provided a qualitative description of the functional dependence of K* on Cm. The improved model using a transitional region near the surfactant cmc (accounting for sequential surfactant micellization) may be employed to quantitatively predict the K.-Cmrelationship for the sorption of NOCs in soil-water systems containing Triton X-100 surfactant. An accurate prediction of the distribution of NOCs in soil-surfactant-water systems is very important for designing remediation technologies that utilize surfactants, such as soil washing. Our study demonstrates that adequate predictions can be achieved knowing (i) properties of the NOC, such as K, Kmn, Kmc, and K,, which can be either experimentally measured or estimated using other correlated properties (Kow and Kom, etc.) and (ii) properties of the surfactant (e.g., sorption of the surfactant in the system). Although in a soil-free system surfactants generally enhance the apparent aqueous concentration (solubility) of poorly-soluble NOCs in soil-free systems, our study showed that this may not be the case in a soil-surfactant-water system; therefore, selection of appropriate surfactant concentrations becomes critical in soil-washing remediation using surfactants.

912 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995

Ackaewlelgrnents We thank Dr. Cary T. Chiou and an anonymous reviewer for several excellent comments during the review process. This research was supported by the U.S. Environmental Protection Agency Cooperative Agreement CR822268-01- 0, the Michigan State Institute of EnvironmentalToxicology, the Michigan Agricultural Experiment Station, the Western Regional Pesticide Impact Assessment Program (Coopera- tive Agreement 92-6998-131, and the Montana Agricultural Experiment Station.

Literature Cited Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1989, 23, 832. Edwards, D. A.; Luthy, R. G.; Liu, Z. Environ. Sci. Technol. 1991, 25, 127. Jafvert, C. T. Environ. Sci. Technol. 1991, 25, 1039. Kile, D. E.; Chiou, C. T.; Helbum, R. S. Environ. Sci. Technol. 1990, 24, 205. Sun, S.; Boyd, S. A. Environ. Sci. Technol. 1993, 27, 1340. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science 1979, 206, 831. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. WaterRes. 1979, 13, 241.

Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227. Boyd, S. A,; Sun, S. Environ. Sci. Technol. 1990, 24, 142. Sun, S.; Boyd, S. A. J. Environ. Qual. 1991, 20, 557. Schwartz, A. M.; Perry, 7. W. Surface Active Agents-Their Chemistry and Technology; Robert E. Krieger Publishing Co.: Huntington, NY, 1978. Gu, T.; Zhu, B.-Y.; Rupprecht H. Progr. ColloidPolym. Sci. 1992, 88, 74-85. Chiou, C. T. In Reactionsand movement of organic chemicals in soil; Sawhney, B. L., Brown, K., Eds.; SSSA Special Publication, ASA and SSW Madison, WI, 1989; p 25. Sklarew, D. S.; Girvin, D. C. In Reviews of Environmental Contamination and Toxicology; Springer-Verlag: NewYork, 1987;

Chiou, C. T.; Freed, V. H.; Schmedding, D. W. Environ. Sci. Technol. 1977, 11, 475.

pp 1-37.

Received for review May 13, 1994. Revised manuscript received September 29, 1994. Accepted December 27, 1994.@

ES940295E

@Abstract published in Advance ACS Abstracts, February 1, 1995.

VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 913

sorption of nonionic organic compounds in soil-water systems containing a micelle-forming surfactant

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