Environ. Sci. Technol. 1994, 28, 1550-1560
Distribution of Nonionic Surfactant and Phenanthrene in a Sediment/Aqueous System David A. Edwards,?Zafar Adeel,* and Rlchard G. Luthy'f* H & A of New York, 189 North Water Street, Rochester, New York 14604, and Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
A nonionic surfactant, Triton X-100, can act either to enhance or to inhibit phenanthrene sorption from bulk solution onto Lincoln fine sand, depending on the bulk solution surfactant concentration. The distribution of phenanthrene between the sand and the bulk solution is characterized by a partition coefficient that can range in value from less than 0.04 to nearly 10 times that in the absence of surfactant. Sorbed Triton X-100 acts to enhance phenanthrene sorption;not only does the sorbed surfactant directly increase the fractional organic carbon content of the sand but also, on a carbon-normalized basis, the sorbed surfactant is much more effective as a sorbent for phenanthrene than is humic matter. Conversely, Triton X-100 micelles in the bulk solution can greatly enhance the solubilization of phenanthrene and, thus, its desorption from the sand. The balance between surfactant sorption and solubilization effects on the sorption of phenanthrene depends on a number of factors, principally the surfactant concentration and the nature of the solid sorbent. Significant differences in surfactant sorption and its effects on the solubilization of phenanthrene are noted between the low organic carbon sand described here and previously described systems with soils of moderate organic carbon content. Introduction
A number of surfactant processes relate to technologies for treating soils or sediments contaminated with hydrophobic organic compounds (HOCs). Such processes include irreversible sorption of cationic surfactants onto soils for enhancing HOC sorption and immobilization (I); surfactant-inducedlowering of interfacial tension between water, nonaqueous phase liquid contaminants, and sediments in order to induce two-phase flow (2); surfactant mediation of HOC bioavailability and/or microbialactivity (3);and micellar solubilization to effect the desorption of HOCs during soil flushing ( 4 , 5 ) . At present, the state of the art is such that these concepts are being developed through laboratory testing, and very little data have been obtained through field studies. Figure 1shows the effects of important abiotic processes in a system containing a nonionic surfactant, a HOC, and a soil of moderate organiccarbon content (6). These effects are illustrated for comparison with analogous effects described subsequently for a low organic carbon sediment system. As illustrated in this figure, the surfactant can exist in a soil/aqueous system as dissolved monomers, sorbed molecules on the soil, or aggregated groups of molecules called micelles. Molecules of HOC in such a system can also exist in several forms; they can be
* Corresponding author. t H & A of New York. t
Carnegie Mellon University.
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Environ. Scl. Technol., Vol. 20, No. 8, 1994
Flgure 1. Schematic diagram of abiotic processes in a soiVaqueous system containing a nonionic surfactant, a hydrophobic organic compound (HOC), and soil humic matter.
solubilized in surfactant micelles, dissolved in the surrounding solution, sorbed directly on the soil, or sorbed in association with sorbed surfactant. The present study analyzesthe results of tests with batch systems containing Triton X-100 nonionic surfactant, Lincoln fine sand, and phenanthrene, where phenanthrene is not present as a separate organic phase. The distribution of HOC in such a system is predominantly affected by the processes of surfactant micellization, surfactant sorption, phenanthrene sorption, and solubilization of the HOC in the surfactant micelles. This study emphasizes the competition between surfactant micelles in the bulk solution and surfactant surface aggregateson the sediment for the uptake of HOC. The results of the study are compared with the results of a similar investigation using soils of moderate organic carbon content (7).
Background
For an aqueous or a solid/aqueous system containing a HOC, the presence of nonionic surfactant micelles in the bulk solution of the system results in the partitioning of the HOC between two different bulk solution compartments, commonly referred to as pseudophases. The micellar pseudophase consists of the hydrophobic interiors of surfactant micelles collectively, whereas the aqueous pseudophase consists mainly of the water that surrounds the micelles and dissolved surfactant monomers (8, 9). Micellesform only when Csurf,the bulk solution surfactant concentration, exceeds the surfactant critical micelle concentration (CMC). The two pseudophases that result after micelle formation can be shown to behave in many respects as separate phases. The micellar pseudophase, being hydrophobic, solubilizes HOCs and can greatly increase the apparent solubility of HOCs in the bulk solution. In an aqueous system in the absence of solids, 0013-936X/94/0928-1550$04.50/0
0 1994 American Chemical Society
solubilization depends primarily on the composition and concentration of surfactant, the composition and concentration of HOC, and the temperature of the system (IO). The distribution of solubilized HOC in the micellar pseudophase and dissolved HOC in the aqueous pseudophase can be described for either aqueous systems or solid/ aqueous systems with amole fraction partition coefficient, Km (8): K,,, = Xm/Xa
(1)
where Xmdenotesthe mole fraction of HOC in the micellar pseudophase and Xa denotes the mole fraction of HOC in the aqueous pseudophase. Values of Km for several HOCs are provided in the case of nonionic surfactants in ref 11 and in the case of an anionic surfactant in ref 9. Another important compartment for the distribution of both nonionic surfactant and HOC in systems with soil or sediments is the solid. Surfactant may sorb onto the mineral surfaces of solids and/or partition into organic coatings that may be present on the solids. Mechanisms for nonionic surfactant adsorption onto inorganic surfaces are described in refs 10 and 12-17. Examples of nonionic surfactant partitioning into liquid organic phases, which may in some respects be analogous to partitioning into humic matter, are described in refs 18and 19. The extent of surfactant sorption by adsorption onto, and/or partitioning into, the solids in a system can be characterized by the parameter Qsurf, which has units of moles of sorbed surfactant per gram of solid. Sorbed surfactant can enhance the capacity of a solid to act as a sorbent for HOC (I, 6, 20). For soils and sediments, the most common measure of HOC sorption capacity is the HOC solid/water distribution coefficient, Kd (L/g). Statistical correlations for Kd generally employ a parameter foc,the fractional organic carbon content of the solid, which has units of grams of organic carbon per gram of bulk solid. In previous work with micelle-forming surfactants and soils, Edwards et al. (6) proposed for correlation thatto,*, the effective fractional organic carbon content of a solid after surfactant sorption, could be expressed as
foe*
foc
+ tQsurfMWsdcarbon
(2)
where MWsudis the molecular weight of the surfactant (g/mol), fcarbon is the weight fraction of carbon in the surfactant (g/g), and t is an effectiveness factor characterizing the carbon-normalized capacity of sorbed nonionic surfactant to function, relative to humic matter, as a sorbent for HOC. A value of unity for t was found to result in good model results for surfactant solubilization of HOCs in systems containing nonionic surfactant and soils of moderate organic carbon content (7). This indicates that, on a carbon-normalized basis, sorbed nonionic surfactant on certain soils may be only as effective as humic matter as a sorbent for HOCs. No published studies are available regarding values of the effectiveness factor E for nonionic surfactant sorbed on sediments of low organic carbon content. A modified value of the HOC partition coefficient applied to the distribution of HOC between the solid and the aqueous pseudophase alone may be estimated for micellar systems as (6) Kd,cmc
= Kd(Sw/Scmc) (foc*/foc)
(3a)
= (mol of HOC sorbed/g of solid)/(mol of HOC in aqueous CMC solution/L) (3b)
where Kd,cmc is the HOC solid/aqueous-pseudophase partition coefficient in the presence of micellar surfactant solution (L/g), Swis the HOC solubility limit in water (mol/L), and Scmcis the HOC solubility limit in a monomeric surfactant solution at the CMC (mol/L). An estimated value of Kd,cmc, as obtained from eq 3, is particularly useful for characterizing HOC sorption in systems for which there is no additional surfactant sorption beyond that occurring at the CMC. Kd,cmc is a constant for such systems at all micellar bulk solution surfactant concentrations. The phase distribution of HOC between the bulk surfactant solution and a solid can be characterized by F, the fraction of HOC residing in the bulk solution in the presence of surfactant micelles. A value for F at any surfactant dose can be approximated well by the following expression for a soil or sediment system containing a HOC with a & value greater than about lo4(e.g.,phenanthrene):
F = A / ( A+ B )
(44
= mol of HOC in the aqueous and micellar solution/ total mol of HOC in the system (4b)
where A = 1 + KmVwCmic, and B = Kd,cmcWsed/Uaq* vwis the molar volume of water (0.01805 L/mol at 25 "C), Cmjc is the concentration of surfactant in micellar form in the bulk solution (mol/L), Wsed is the weight of the soil or sediment (g), and uaq is the bulk solution volume (L). A more general equation, applicable to HOCs with lower Kd values, is given in ref 6. Eqs 2-4 together constitute a model for predicting the distribution of sorbed and solubilized HOC in a solid/ aqueous system in which nonionic surfactant micelles are present. One means for describing such a distribution is to plot F versus D,,the surfactant dose (mol/L), given as the total surfactant mass added to the system divided by the total liquid volume of the system. Since some of the surfactant mass added to asolid/aqueous system is sorbed on the solids, the bulk solution surfactant concentration in such a system is always less than the surfactant dose. Significant solubilization of HOC does not occur in most cases until the surfactant dose is greater than Dcm,,the surfactant dose at which the CMC is attained in the bulk solution. Another potentially useful parameter for describing the distribution of HOC in a solid/aqueous system with surfactant micelles is &,surf, the ratio of the number of moles of sorbed HOC per gram of solid to the number of moles of HOC either dissolved or solubilized in the bulk solution per liter of solution. This parameter can be evaluated directly by experiment at supra-CMC as well as sub-CMC values of CaWfat each specific surfactant dose of interest. &,surf is related to the mass fraction of HOC in the bulk solution: Kd,surf
= (l- f l Uaq/(wsedfl
(54
= (mol of HOC sorbed/g of solid)/(mol of HOC in aqueous and micellar solution/L) (5b)
In an advection-dispersion model for surfactant-facilitated HOC transport, Kd,surfwould replace Kd in the expression Envlron. Scl. Technol., Vol. 28, No. 8 , 1994
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Table 1. Properties of Phenanthrene and Triton X-100 Surfactant Pyrene Triton X-1 00 Surfactant 45.0 mL Bulk Solution 6.25 g Morton Soil
name
-2:4
-2’2
-2:O
-1.8
-116
-114
Log (Surfactant Dose, moi/L)
for the retardation factor, which would not be constant, but rather a function of the bulk solution surfactant concentration. As a descriptor of micellar surfactant systems, Kd,surf can be shown to differ from Kd,cmc. The latter parameter describes a distribution of HOC between the solid and the aqueous pseudophase alone, i.e., the bulk solution excluding surfactant micelles. Values of are thus identical to those of &,cmc only for sub-CMC systems (Le., in which no micelles are present). An expression obtained by inserting eq 4 intoeq 5 shows that, for supra-CMC systems containing highly hydrophobic organic compounds such as phenanthrene, &,surf is a function of both Kd,cmc and the micellar surfactant concentration: = Kd,cmc/(l
+ %tVwcmic)
(6)
While &,surf can be assessed directly by measuring the bulk solution concentration of HOC and by using mass balances to determine the amount of HOC that is sorbed, the value of &,cmc for a solid/aqueous system cannot be as readily measured experimentally, since direct evaluation requires that the surfactant monomer concentration in the solution be a t the solubility limit with no micelles present. Although eqs 3 and 4 permit the value Of &,cmc to be estimated if values of K d , S,, Scmc, foc, Qswrf, and E are known, such values may not always be available, and it may be necessary to experimentally measure values of Kd,surf at surfactant doses of interest. In the following example, experimental results are compared with model predictions obtained using estimates for Kd,cmc. As an illustration of the efficacy of the solubilization model described by eqs 2-5, Figure 2 shows experimental data and model predictions for pyrene partitioning in the presence of micellar Triton X-100, a nonionic alkylphenol ethoxylate surfactant, in a system containing 45.0 mL of bulk solution and 6.25g of Morton soil with 0.96% organic carbon. The logarithm of &,surf is plotted against the logarithm of the surfactant dose. Experimental values for pyrene solubility at different Triton X-100 doses (22) are used to calculate values for &,surf. Parameter values K m , Kd,foc, sw, Scmc, &surf, and CMC employed in the model for making independent predictions are obtained from the results of separate tests (7). A value for E of 1.0 is assumed for the solid since it is a soil of moderate organic carbon content. Figure 2 shows that an E value of unity yields model predictions fitting experimental results for 1552 Environ. Scl. Technol., Vol. 28. No. 8, 1994
log
(I&)
solubilit or~~i!” (mol/L)
the soil a t supra-CMC surfactant doses. This modeling approach is later applied to the case of sorption of phenanthrene onto aquifer sediment of low organic carbon content. Materials and Methods
Flgure 2. Comparison of experimental data (22)and model predictions for pyrene solid/aqueous partitloning in a system with Morton soil and Triton X-100 nonionic surfactant.
Kd,surf
MW
(gimol)
phenanthrene C14HIo 178 4.57 7 X 10-8 Triton X-100 C B H ~ ? C ~ H ~ O ( C H ~ C H ~625 O ) ~ , ~ H 1.8 x 10-4
I
-3:%:6
molecular formula
Batch suspension tests to assess sorption and solubilization were conducted in 50-mE glass centrifuge tubes. The chemical compounds included both nonionic surfactant and radiolabeled and nonlabeled phenanthrene. Phenanthrenewas selected as a relatively safe, moderately nonvolatile, and fairly hydrophobic HOC. All aqueous solutions for soil tests contained 0.01 M CaC12 to facilitate solid/liquid separation and 7.4 X lo4 M of HgClzto inhibit microbial growth. Sorption and solubilization tests were performed at ambient laboratory temperatures, 22-25 OC, approximately 40 “ C below the cloud point of the surfactant. Compounds. The properties of phenanthrene are listed in Table 1. Nonlabeledphenanthrene with a purity greater than 98% was obtained from Aldrich Chemical Co. and 14C-labeledphenanthrene (13.1 pCi/pmol) was obtained from Sigma Chemical Co. Aldrich Chemical Co. was the source for nonlabeled Triton X-100 with properties given in Table 1. Triton X-100 is a heterogeneous nonionic octylphenol ethoxylate surfactant with an average of 9.5 ethoxylate groups (CH~CHPO-) per molecule. 3H-Labeled Triton X-100 was obtained from New England Nuclear. Although the selection of Triton X-100 was based primarily on its availability in radiolabeled form, it was also chosen as a representative surfactant based on its reported effectiveness in solubilizing organic compounds (e.g., ref 22). Sand. Uncontaminated Lincoln fine sand, a mixed, thermic Typic Ustifluvent from the subsurface in Oklahoma, was provided by the U.S. EPA Robert S. Kerr Environmental Research Laboratory in Ada, OK. Prior to shipment, the soil was air-dried and sieved to remove grains larger than 2 mm. The organic carbon content of the soil, as measured in duplicate samples by the WalkleyBlack method (23),was 0.053%,slightly less than the minimum value generally recommended for this procedure. The organic carbon content for the Lincoln fine sand was reported by Wilson et al. (24) to be 0.034 % . Stock Solutions. Three different stock solutions were prepared for sorption and solubilization tests. The first stock solution contained radiolabeled phenanthrene and 0.00356 g of nonlabeled phenanthrene in 10.0 mL of methanol. The resulting solution concentration and radioactivity were respectively 2.0 X M and 6.8 X lo9 dpm/mL. The concentration of phenanthrene in the methanol stock solution was selected such that it would be several orders of magnitude greater than the aqueous solubility limit of phenanthrene. This permitted aqueous concentrations of phenanthrene up to the solubility limit to exist in the batch systems following addition of the
stock solution, even accounting for the sorption of phenanthrene onto sand. The amount of methanol required for delivering sufficient phenanthrene to each system yielded methanol volumetric fractionsranging from 1 x 10-4 to 5 X 10-3. Dilution stock contained 2.0 X M CaClz and 7.4 X 10-4 M HgClz in deionized water. A surfactant stock contained 5.33 X M Triton X-100 nonionic surfactant (280 times the CMC) along with 2 X 10-2 M CaC12and 7.4 X 10-4M HgClz in deionized water. This stock solution was used to assess the effects of Triton X-100 on the sorption of phenanthrene onto sand as well as to evaluate surface tension in a separate test. Serial dilutions of the surfactant stock with 0.01 M CaClz in deionized water permitted the testing of surface tension at different bulk solution surfactant concentrations. The mercuric chloride added to the aqueous solutions served to inhibit microbial activity. The phenanthrene and surfactant stock solutions were individually sonicated in a Branson ultrasonic cleaner operating at 55 000 cps to assist in dissolution. Surfactant Sorption on Sand. Surfactant stock solution was added in different amounts to various centrifuge tubes each containing 5.00 g of Lincoln fine sand and sufficient dilution stock to make a total of 30.0 mL of solution in the tube. The centrifuge tubes were sealed with PTFE-lined septa and open-port screw caps and rotated end-over-end for 1day. At the completion of mixing, the tubes and their contents were centrifuged for 0.5 h at about 1600g on an International Equipment Co. clinical centrifuge to assist in solid/liquid separation. Aliquots of the supernatant were withdrawn by glass and stainless-steel syringe, and 4 mL of solution was passed through a Gelman 0.2-pm PTFE filter to saturate the filter both in fluid content and in sorbed solute. The necessary volume for attaining saturation was previously determined through tests in which concentrations of successive samples were measured. A total of 1.00 mL of solution was then expressed through the filter into each of two polyethylene scintillation vials containing 10.0 mL of Packard Optifluor high flash-point aqueous-sample liquid scintillation cocktail. The contents of each vial were mixed and then counted for 3Hactivity in a Beckman LS 5000 TD liquid scintillation counter (LSC) with automatic quench compensation. The bulk solution surfactant concentration was calculated using 2.22 X lo6 dpm/pCi and the predetermined nonlabeled-to-radiolabeledsurfactant mass ratio for the experiments. Phenanthrene Sorption on Sand. Phenanthrene sorption tests were conducted much like the surfactant sorption tests, using 50-mL centrifuge tubes, each containing 5.00 g of Lincoln fine sand, various amounts of phenanthrene stock, and added dilution stock to make 30.0 mL of solution. After sealing, the centrifuge tubes were rotated end-over-end for 1day, at which point they were then centrifuged for 0.5 h at about 1600g. Aliquots were withdrawn by syringe, and about 2 mL of the solution was passed through a 0.2-pm PTFE filter to condition the filter, and then an additional 1.00 mL of solution was expressed through a PTFE filter into a scintillation vial with cocktail. The contents of the vial were mixed and counted for 14C activity in the LSC. Previous tests employing similar batch techniques suggest that losses of phenanthrene due to volatilization during this type of test are negligible (7).
Phenanthrene sorption onto Lincoln fine sand in the presence of micellar surfactant solution was assessed by adding 4.0 mL of 4.99 X 10-2 M Triton X-100 solution to the remaining solution of each of the batch system centrifuge tubes just described and re-equilibrating over 1day. The resulting solution volume in each sample was 31.0 mL. A mass balance on each system before and after the initial phenanthrene sampling allowed total initial phenanthrene mass to be calculated prior to the start of tests for sorption in the presence of surfactant. Aside from the use of surfactant,the latter tests were conducted similarly to the tests for phenanthrene sorption in the absence of surfactant, resulting in multiple data points for phenanthrene sorption at a single surfactant dose. Surfactant Solubilization of Phenanthrene. Tests for Triton X-100 micellar surfactant solubilization of phenanthrene in the presence of sediment were conducted like the sorption tests described above except with varied concentrations of Triton X-100 and a constant mass of phenanthrene. Duplicate samples were obtained for each surfactant dose. The mass of phenanthrene added was such that the aqueous phenanthrene concentration in the absence of surfactant would be 4.0 X 10-6 M, slightly more than half the solubility limit ofphenanthrene in pure water. Surfactant was added to the different tubes such that the bulk solution surfactant concentrations in the tubes varied from slightly less than the CMC to about 150 times the CMC. The tubes were rotated for 1 day, centrifuged, filtered, sampled, and evaluated for [l4C1-phenanthrene radioactivity. Chemical Oxygen Demand. A closed reflux, titrimetric procedure for measuring chemical oxygen demand, Method 5220 C (251, was used to validate radiolabeled tests for Triton X-100 sorption for higher bulk solution surfactant concentrations. Prior to analysis, a chemical oxygen demand calibration curve was created by adding known amounts of surfactant to supernatant fluid that had been equilibrated with sediment in the absence of surfactant. Blank samples were also tested. Samples for both calibration and analysis were filtered through a 0.2pmPTFE filter. For each test, a2.00-mL sample of filtered supernatant from an equilibrated batch system was pipeted into a vial containing prescribed amounts of acid, catalyst, and mercuric chloride. A measured excess of potassium dichromate was added, and each sample was refluxed at 150 "C for 2.0 h in a Hach Model 16500 aluminum-block micro-COD apparatus. After cooling, the samples were titrated against a standard solution of ferrous ammonium sulfate. Surface TensionTests. Surface tension was measured with a Central Scientific Co. Model 70545 DuNuoy tensiometer. Samples were prepared for surface tension measurement by diluting a stock solution consisting of deionized water, 5.0 X lo4 M Triton X-100, and 1.0 X 10-2 M CaC12 to create solutions of varying surfactant concentration. The lowest concentration obtained as a result of serial dilutions was 2.5 X M. Each aqueous sample was tested with the tensiometer at controlled temperature until at least three consistent surfacetension readings were obtained, Le., to within 1% precision. Between each reading, the ring was cleaned with acetone and heated to redness in a gas flame. The three values were averaged, and dial and ring corrections were applied. Test procedures and an assessment of the critical micelle concentration are described in ref 26. Environ. Scl. Technol., Vol. 28, No. 8, 1994
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