Environ. Sci. Techno/. 1995, 29, 903-913
Sorption of Nonionic Organic Compounds in Soil- Water Systems Containing a Micelle-Forming Surfactant SHAOBAI SUN*,+AND WILLIAM P. INSKEEP Department of Plant, Soil & Environmental Sciences, Montana State University, Bozeman, Montana 5971 7-0312
STEPHEN A. BOYD Department of Crop & Soil Sciences, Michigan State University, East Lansing, Michigan 48824
In this study, w e examined the effect of a micelleforming surfactant (Triton X-100) on the sorption of 1,lbis(p-chlorophenyl)-2,2,2-trichlororethane (p,p’-DDT), 2,2’,4,4‘,5,5’- h exa c hlo ro bi ph enyl (2,2’,4,4‘,5,5’- PC B1, and 1,2,4-trichlorobenzene (1,2,4-TCB) in a soil-water system. At 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. At CTXvalues 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 contaminated 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*Wis the apparent solute solubility at the total surfactant concentration ( X ) , SWis 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 Kemare 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 Engineeringand 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,
+
Structure
and 1,2,4=TCB
Sw, mg I"
MW, g mol"
I -
r C H ~ , c C ~ C ( c ~ r ~ c o C H ~ C 6 2 4~a ~ " ~ t
Triton X-100 (n = 9-10, UV molar absorbtivity Q 275.5 nm = 1.33 x 103) CI, \
A
Q
Y
.CH-C-CI
F'
354.5
1
CII
5.34c
6.36b
0. OOld
5. 84d
6 . 72e
4.26b
CI
/
360.9
Y
0.0C~55~
Y CI'
2,2',4,4',5,5'-PCB
c&cl
2.70'
-4. 14b*f
1,2,4-TCB a Data from Union Carbide Chemicals and Plastics Company, Inc. Ref 1, S, at 25 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 competitiveprocesses (inaddition 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? are partition coefficients, defined as the ratio of the solute concentration in the soil phase relative to the equilibrium solute concentration in the aqueous phase. A net increase in the aqueous phase solute concentration results in a smaller k? than K,and a net decrease in the aqueous phase solute concentration results in a greater K* value than K. 904
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4 , 1 9 9 5
"C.E Ref 13. Ref
14,
S, at 25 "C.e Ref 15. 'Value for
Our previous study demonstrated that differences between K1 and Kat a given surfactant concentration are a function of the water solubilities of the NOCs (5). As water solubility decreased, a greater decrease of the K1 values from K was observed (at Petronate aqueous concentrations ranging from 0 to approximately 170 mg L-l). For instance, the K1 of 2,2',4,4',5,5'-PCB (SW= 0.001 mg L-l) decreased by a factor of over 200 from the intrinsic value of 478 (in the Petronate-free system) to 2.36 (in the system containing 170.3 mg L-' Petronate L), while the k? of phenanthrene (SW= 1.6 mg L-I) only changed from the intrinsic value of 13.3 to 6.02 over the same Petronate L concentration range. For a relatively water-soluble compound such as naphthalene (SW= 31.7 mg L-l), a slight increase of k? was obtained. A model that quantitatively describes the effect of Petronates on sorption of NOCs based on the relative effectiveness of the sorbed Petronate and aqueous Petronate emulsions functioning as sorptivemedia for NOCs was also reported (5). As compared to Petronates, micelle-formingsurfactants may exhibit different and more complex effects on NOC sorption coefficients. Near the cmc, there is likely to be variability in the effectiveness of the surfactant as a sorptive medium for NOCs in the solution phase, indicated by noticeable differences between K,, (for monomers) and K,, (for micelles) values. For instance, the partition
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 nontransparent 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‘-PCBin the presence (KI)or absence (4ofTritonX-100were derived from the sorption isotherms determined using batch equilibrium experiments. The sorption isotherms were obtained in the Oshtemo soilwatersystemscontainingO-lOOOmgL-linitialTritonX-100 concentration, with an interval of 100 mg L-l. For each Materials and Methods sorption isotherm at a fixed initial Triton X- 100 concentraCommercial Triton X-100. Triton X-100 was purchased tion, three samples (induplicate) were prepared at different from Aldrich Chemical Co. (Milwaukee, WI) and used initial solute concentrations, ranging from 0.0483 to 0.145 without further treatment. Chemically, TritonX-100 is an mg L-’ for 1,2,4-TCB,0.0157-0.0471 mg L-’ for p,p-DDT, octylphenoxy polyethoxyethanol nonionic surfactant, and 0.0114-0.0228 mg L-l for 2,2’,4,4’,5,5’-PCB. (To prevent (CH~)~C-CHZ-C(CHZ)Z-C~H~-(O-CHZ-CH~)~-OH, which has undesirable crystallization of solute, the initial p,p’-DDT an average n of 9.5, an average molecular weight of 624, and 2,2‘,4,4’,5,5‘-PCB concentrations, which were greater and a density of 1.070 g cm’. Commercial Triton X-100 than their water solubilities, were achieved by multiple also contains a small amount of polyethylene glycol (less instances of adding solute into the sample at a subsaturation than 3%). The U V molar absorptivity (at 275.5 nm) in concentration everyO.5 h duringthe early hours of shaking.) aqueous solution is 1.33 x lo3 (data from Union Carbide Each sample contained 1.00 g of soil and 25.00 mL of total Chemicals and Plastics Company, Inc.). solution volume, sealed in a 25-mL Corex centrifuge tube, Solutes. 14C-Labeled 1,2,4-TCB, p,p’-DDT, and and was shaken for 24 h at room temperature (23 “C). 2,2’,4,4’,5,5’-PCBwere obtained from Sigma Chemical Co. Preliminary studies indicated no significant difference in (St. Louis, MO). Table 1 lists the chemical structures, sorption of 2,2’,4,4‘,5,5’-PCB between 24 and 48 h. The molecular weights, water solubility,log I(om (Komis the soil samples were centrifuged at a RCF of approximately 7500g organic matter normalized solute sorption coefficient), and for 20 min to separate supernatant from the solid phase. log KO, (KO,is the solute octanol-water partitioning A total of 1.00mL of the supernatant was mixed with 5 mL coefficient). of ScintiSafe Plus 50% and analyzed using a Packard TriSoil. The soil used in this study was an Oshtemo silt Carb 2200CA liquid scintillation analyzer to determine the loam (B horizon), which contains 0.10% organic C (ca., equilibrium concentration of the solute. The equilibrium 0.17% soil organic matter), 89% sand, 5% silt, and 6%clay. solute concentration in the soil phase was calculated by The soil was air-dried for 48 h and sieved ( X, and A. < Xu (11)
X,,," = X,, X,, = XI, - XI and X,,,c = &q - Xu, , . wnenn, z
.~ A,, .~,. ~~
LILJ
The importance of solute partition coefficients for the various aqueous Triton X-100phases (Kmn,K,r,and K,J as
/
/
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 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
+
-
VOL. 29. NO. 4.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY m 9-1
1600
m
83 m
;a,
140 1
-
1200
% cy
1 m DDT Measured 0 PCB Measured -DDT Fit - PCB Fit
/
/ 0
lo00
U
c
n
4
8
800
Q
0
600
j,
c
5
400
il
2
200
0
100
200
400 Cm,mg I-’
300
FIGURE 6. Comparison of the measured K+ values of p,p’-DDT and model, eq 9) as a function of &. fitted CLand 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 solubilizing 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 micelleforming 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 ( C n 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 912 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995
500
600
700
2‘,4,4,5,5’-PCB with the corresponding fitted K+ values (transitional
(’cmc), K. decreases due to the formation of aqueous surfactantmicelles 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 surfactantin the system). Although in a soil-freesystem 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.
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 CR822268-010, the Michigan State Institute of EnvironmentalToxicology, the MichiganAgriculturalExperiment Station,the Western Regional Pesticide Impact Assessment Program (Cooperative Agreement 92-6998-131, and the MontanaAgricultural 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; pp 1-37. Chiou, C. T.; Freed, V. H.; Schmedding, D. W. Environ. Sci. Technol. 1977, 11, 475.
Received for review May 13, 1994. Revised manuscript received September 29, 1994. Accepted December 27, 1994.@
ES940295E @Abstractpublished in Advance ACS Abstracts, February 1, 1995.
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