Micellar Solubilization of Naphthalene and Phenanthrene from

Masrat Maswal , Altaf Hussain Pandith , Nasrul Islam , Aijaz Ahmad Dar ... Mohammad Amin Mir , Oyais Ahmad Chat , Muzaffar Hussain Najar , Mohammad ...
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Environ. Sci. Technol. 2002, 36, 3901-3907

Micellar Solubilization of Naphthalene and Phenanthrene from Nonaqueous-Phase Liquids ALEX J. HILL AND SUBHASIS GHOSHAL* Department of Civil Engineering, McGill University, Macdonald Engineering Building, 817 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6

The equilibrium partitioning of the polycyclic aromatic hydrocarbon (PAH) compounds naphthalene and phenanthrene, from nonaqueous-phase liquids (NAPLs) into micellar solutions of five different nonionic polyethoxylated surfactants, is evaluated in this study. A series of synthesized NAPLs, comprised of naphthalene and/or phenanthrene dissolved in hexadecane at varying concentrations, were equilibrated with surfactant solutions in well-mixed batch systems. It was observed that the extent of micellar partitioning of PAH compounds increases linearly with their relative abundance in the NAPLs. A theoretical liquid-liquid partitioning framework that describes PAH equilibrium partitioning between the NAPL, aqueous, and the liquid-like micellar phases is presented. Although the maximum solubilization capacity of micelles is generally higher for naphthalene as compared to phenanthrene, results indicate that with certain NAPLs phenanthrene may be solubilized to a similar extent as naphthalene, even when equal mole fractions of the compounds are present in the NAPLs. Selective solubilization of naphthalene over phenanthrene into micellar solutions of Brij 35 was observed in systems where naphthalene and phenanthrene were both present. The extent of micellar partitioning of phenanthrene was decreased by approximately 18% in the presence of naphthalene, while naphthalene partitioning was unaffected by the presence of phenanthrene.

Introduction The contamination of soils and groundwater by polycyclic aromatic hydrocarbon (PAH) compounds is commonly found at many locations including old manufactured gas plant sites, wood treatment facilities, coal coking sites, and petroleum processing facilities. PAHs often comprise a significant fraction of the chemically complex, multicomponent NAPLs such as coal tar, creosote, and petroleum fuels found at these sites (1-3). Many PAHs are known carcinogens, and thus NAPLs containing significant fractions of PAHs when released to the environment pose human health risks (4, 5). Soil flushing with surfactant solutions increases the detergency of the water, which results in improved solubility of NAPL components and emulsification of the NAPL and is a promising technique for increasing removal rates of PAHs contained in NAPLs. Concern about the failure to contain mobilized NAPLs has directed research mostly toward the use of surfactants for increased extent and rates of solubilization (1, 6). * Corresponding author e-mail: [email protected]; phone: (514)398-6867; fax: (514)398-7361. 10.1021/es011175r CCC: $22.00 Published on Web 08/14/2002

 2002 American Chemical Society

The objective of this research was to develop and experimentally validate a quantitative relationship between PAH mole fraction in a NAPL and a nonionic surfactant solution’s equilibrium solubilization capacity for PAH compounds. The equilibrium solubilization capacity is the maximum amount of PAH that may be removed from a NAPL by solubilization into micellar surfactant solutions. In the surfactant-aided dissolution of multicomponent NAPLs, the equilibrium concentrations of potentially soluble individual components in micellar solutions may change as individual compound mole fractions of the compound in the NAPL decrease with dissolution. Such changing magnitudes of the maximum solubilization capacity have to be accounted for in determining the rates of dissolution under nonequilibrium conditions. In micellar surfactant solutions, the true equilibrium aqueous-phase concentration of a solute affects its molar solubilization ratio (MSR), which is a measure of the amount of solute partitioned into a micelle (7). The equilibrium aqueous concentration of a hydrophobic organic compound (HOC) in a NAPL-water system is dependent on the mole fraction of the compound in the NAPL, and when surfactant micelles are introduced in the aqueous phase of such systems, the magnitude of the compound’s MSR is also influenced by its mole fraction in the NAPL. There is substantial literature on the equilibrium partitioning of HOCs such as PAHs in micellar surfactant solutions. Most studies with PAHs, however, have studied systems containing PAHs in their pure aggregate state (solid or liquid) or as sorbed onto soils (7, 8). The micellar solubilization of constituents of multicomponent NAPLs has been systematically investigated by only a few studies. In companion papers, Nagarajan et al. (9) and Chaiko et al. (10) reported that when NAPLs comprised of benzene, hexane, and cyclohexane were contacted with solutions of ionic surfactants, the MSR of benzene was only slightly affected by the presence of cyclohexane and hexane. However, those authors witnessed a synergistic increase in the MSR of hexane from NAPLs containing small mole fractions of benzene. It was concluded that benzene solubilized in the outer micellar layers caused an increase in the micellar core volume, which in turn increased the MSR of hexane. Cheng et al. (11) have demonstrated that solvent extraction of hydrophobic organic solutes from micellar solutions of anionic surfactants is generally more efficient for solvents with higher equivalent alkyl carbon numbers than the solutes. Yeom et al. (12) related estimated PAH mole fractions in aged, coal tar-contaminated soils to the solubilization capacity of the surfactants, but it is unclear if a nonaqueous liquid phase was present in the soil and if equilibrium conditions were achieved. Thus, conclusions regarding the equilibrium partitioning patterns of PAHs from NAPLs cannot be made from that study. Partitioning of PAHs in NAPL-Surfactant Systems. At surfactant concentrations in excess of the critical micelle concentration (cmc), surfactant monomers aggregate to form micelles that exist as a pseudo-phase dispersed throughout the aqueous solution. The outer region of the micelles is comprised of the surfactants molecule’s hydrophilic headgroups while the micellar core contains its hydrophobic tails. It has been suggested that the micellar core may be modeled as a drop of liquid hydrocarbon (13-15). Thus when NAPLs are contacted with surfactant solutions, PAHs initially contained in NAPLs are solubilized into both the aqueous and the micellar pseudo-phase. When expressed with respect to the total volume of the solution, the equilibrium PAH concentration in the aqueous i ) is a sum of the PAH concentration in the bulk phase (CTOT VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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aqueous pseudo-phase (CiAQ) and the mass in the micellar pseudo-phase per unit solution volume (Ci*M):

CiTOT ) CiAQ + Ci*M

(1)

In the surfactant solutions used in this study, CiAQ matches very closely to the aqueous-phase concentration in a system containing NAPL and water only. This was inferred from the observation that CiAQ does not change over the range of sub-cmc surfactant doses, and thus surfactant monomers do not associate with PAHs in aqueous solution (16). Thus, CiAQ can be related to the PAH mole fraction in the NAPL (XiN) by (17) i CiAQ ) (XiNγiNCAQ,sat )/(fis/fiL) )

KiNXiN, when γiN is constant for all XiN (2) where γiN is the activity coefficient of PAH i in the NAPL i phase, CAQ,sat is the aqueous solubility of the PAH, KiN is the NAPL-water partitioning constant, and (fis/fiL) is the solidliquid fugacity ratio that accounts for the fact that many PAHs are solids and not pure liquids at ambient temperatures. A micelle-water partitioning coefficient (KiMC) can be defined as a ratio of CiAQ to CiM (18):

CiM ) niM/VM ) KiMCCiAQ

(3)

where CiM is the number of moles of PAH i, niM, per unit volume of micellar phase. For the case of linear partitioning, KiMC is constant over a range of solute concentrations; therefore, CiM becomes a linear function of CiAQ. Assuming that the volume of the micellar phase (VM) is dependent only on the number of moles of surfactant in the micellar phase over the range of surfactant and PAH concentrations considered (8), the following equation applies:

VM ) Bnsurf M

(4)

where nsurf M is the number of moles of surfactant comprising the micellar pseudo-phase and B is a surfactant-specific proportionality constant. The MSR is defined as the number of moles of solute incorporated into micelles per mole of surfactant added to the system, at equilibrium. By combining eqs 3 and 4, the following relationship can be used to calculate MSR values for each solute (MSRi) in systems where linear partitioning occurs between the micellar pseudo-phase, aqueous pseudophase, and NAPL: i i i MSRi ) niM/nsurf M ) KMKNXN

(5)

where KiM is a lumped constant representing the product of B and KiMC. Cheng et al. (11) have presented a detailed mathematical model of solute partitioning in water-micelle-NAPL systems. Linear partitioning of the mole fractions of solutes in the micelle and aqueous phases has been assumed in that study and leads to a nonlinear relationship between solute MSR and the mole fraction in the NAPL. The validity of the theoretical framework presented here has been discussed further in a later section.

Materials and Methods Chemicals. Phenanthrene (purity >98%), naphthalene (purity >99%), and hexadecane (purity >99%) were obtained from Sigma Aldrich Chemical Co. and were used to synthesize different NAPLs. Relevant physicochemical data for the NAPL components are presented in Table 1. Five nonionic sur3902

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TABLE 1. Relevant Physicochemical Properties of NAPL Components

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 18, 2002

compd

MWa (g/mol)

melting pointa (°C)

boiling pointa (°C)

solid/liq fugacity ratiob

log Kow

hexadecane naphthalene phenanthrene

226.45 128.19 178.24

18.1 80.2 99.2

286.8 217.9 340

1 0.306 0.279

6.6c 3.36a 4.57a

a Schwarzenbach et al. (19). Schmedding (21).

b

Mukherji et al. (20). c Chiou and

factants (Brij 35, Brij 30, Tergitol NP10, Tween 80, and Triton X-100; Sigma Aldrich Chemical Co.) were used in this study. These industrial surfactants are all mixtures containing molecules comprised of the same functional groups with mean molecular weights as specified in Table 2. These surfactants have been commonly employed in many experimental studies on surfactant solubilization of HOCs (7, 8). HPLC-grade methanol (Fisher Scientific Co.), and distilled-deionized water was used for all experiments. All chemicals purchased were used without further purification. Analytical Methods for PAH Samples. Measurements of PAH concentrations were made using a Hewlett-Packard HP-8453 diode array, single-beam UV spectrophotometer. The detection limits of naphthalene and phenanthrene were 0.2 and 0.1 mg/L, respectively, at their absorption maxima of 220 and 251 nm. For UV spectrophotometry analysis of PAHs from surfactant solutions, the samples were diluted to at least 80% (v/v) methanol prior to analysis. Methanol dilution ensured a sufficient increase in the cmc of the surfactants as a result of which any micelles present in the samples disintegrated as a result of the dilution. When the aqueous-phase concentration of phenanthrene was below the minimum detection limit for UV spectrophotometry, phenanthrene concentrations were analyzed by fluorescence spectrophotometry using a Shimadzu RF-540 spectrofluorophotometer. An excitement beam of 250 nm was used, and light emission at a wavelength of 364 nm was measured. Methanol solutions were used for the calibration, and all samples were diluted 25:1 (v:v) with methanol prior to fluorescence measurements. Brij 35 and Tween 80, the surfactants used for phenanthrene-containing NAPLs, did not emit any significant light at 364 nm under the fluorescence measurements conditions. Some UV spectrophotometry and spectrofluorometry measurements of PAH concentrations were confirmed by HPLC measurements. A Waters W600 HPLC fitted with fluorescence and UV detectors was used and operated at a flow rate of 1.5 mL/min with a mobile phase that was 40% acetonitrile/60% water for an initial 5 min followed by a 25-min gradient mode of acetonitrile from 40% to 100%. A 15 cm × 4.6 mm C18 column, containing 5-µm particles was used (Supelcosil LC-PAH; Supelco, Bellafonte, PA). The temperature of the column was maintained at 40 °C. Additional details on the analytical techniques are presented in the Supporting Information. NAPL Synthesis and Surfactant Solution Preparation. A series of two-component NAPLs comprised of naphthalene and hexadecane or phenanthrene and hexadecane were synthesized by completely dissolving predetermined quantities of the PAH crystals in hexadecane in sealed glass bottles with minimal headspace. PAHs were the target solutes in this study, and hexadecane served as a solvent with very low aqueous solubility and for some surfactants has very low micellar-phase solubility as well. Hexadecane is representative of the high molecular weight alkanes abundantly present in many petroleum liquids.

TABLE 2. Characteristics and Properties of Surfactants Used in This Study surfactant

formula

av MW (g/mol)

HLB

reported cmc (mol/L)

reported aggregation no.

max C surf AQ used (mol/L)

Brij 30a Brij 35b Tergitol NP10a Triton X-100b Tween 80b

C12E4 C12E23 C9PE10.5 C8PE9.5 C18S6E20

363 1198 683 625 1310

9.7 16.9 13.6 13.5 15

2.0E-05 9.2E-05 5.0E-05 1.70E-04 1.2E-05

na 40 na 100-155 58

0.0040 0.0046 0.0025 0.0102 0.0024

a

Edwards et al. (7).

b

Yeom et al. (12).

Naphthalene-in-hexadecane NAPLs were created with Xnaph ranging from 0.005 to 0.19. Phenanthrene-in-hexadeN cane NAPLs were created with Xphen ranging from 0.01 to N 0.06. Three-component NAPLs comprised of naphthalene, phenanthrene, and hexadecane were synthesized in a similar manner, with naphthalene mole fractions constant in one series of NAPLs (Xnaph at 0.09 and Xphen varying from 0.01 to N N 0.05) and with phenanthrene mole fractions constant in the other series (Xphen at 0.03 and Xnaph varying from 0.01 to N N 0.17). Standard surfactant solutions were prepared by dissolving a known quantity of surfactant into an appropriate amount of water followed by continuous mixing for 24 h to allow sufficient time for all of the surfactant to dissolve into the water. Dilute surfactant solutions, ranging in concentration from well below the literature reported cmc values to the limits specified in Table 2, were created by volumetric dilution of the standard solutions. PAH Equilibrium Partitioning Experiments. Equilibrium partitioning of naphthalene and phenanthrene between NAPLs and the aqueous bulk phase was studied by contacting NAPLs with aqueous-phase or surfactant solutions in 40-mL glass vials sealed by caps fitted with Teflon-lined silicone septa. The vials containing 2 mL of NAPL and 25 mL of aqueous solution were agitated for a predetermined equilibration time of 96 h in an orbital shaker set at 175 rpm under a constant temperature environment of 25 °C. Orbital shaking allowed for adequate mixing within both the aqueous phase and the NAPL, with no visually observable NAPL emulsification. Prior to use, all glassware was acid-washed and rinsed with water. Syringes, filter holders, and septa were methanolwashed prior to use. PAH losses caused by any leakage from the headspace were determined to be negligible during contact periods up to 44 days. The initial PAH mole fractions in the NAPL were reduced as a result of surfactant dissolution by an average of 2.5% and no more than 5% in any system. Solubilization of the pure compound (crystalline) PAHs into surfactant solutions and pure aqueous phases was studied in similar systems as described above except that PAH crystals in excess of quantities that could be dissolved were present, instead of a NAPL phase. Further details on the equilibrium partitioning experiments are provided in the Supporting Information. Hexadecane Partitioning Experiments. The concentration of hexadecane partitioned in to the micellar solutions of Brij 35 and Tween 80 was measured at surfactant concentrations of 100 cmc for systems containing pure hexadecane, for two-component NAPLs (Xnaph ) 0.13 or N Xphen ) 0.04), and for a three-component NAPL (Xnaph ) 0.09 N N and Xphen ) 0.03). Prior to contact with surfactant solutions, N appropriate amounts of 14C-labeled hexadecane (Sigma Aldrich Chemical Co.) were dissolved in the two- and threecomponent NAPLs prepared using unlabeled hexadecane. The equilibrium total concentration of hexadecane in the micellar solution was analyzed by liquid scintillation counting (LS6500, Beckman-Coulter Inc.). The samples were processed prior to analysis in a manner identical to samples for PAH analysis.

FIGURE 1. Equilibrium aqueous concentrations for naphthalene (a) and phenanthrene (b) in systems containing different NAPLs comprised of hexadecane (H), naphthalene (N), and/or phenanthrene (P) and in systems containing pure compound crystalline PAH (Pure). The solid lines represent linear regression fits to data from systems containing naphthalene or phenanthrene dissolved in hexadecane. The dashed lines represent linear regression fits to data from systems containing naphthalene and phenanthrene dissolved in hexadecane.

Results and Discussion NAPL-Water Equilibrium Partitioning. NAPL-water equilibrium partitioning experiments were conducted using a series of two- and three-component NAPLs containing different naphthalene and phenanthrene mole fractions. The equilibrium aqueous concentrations for each of the NAPLs are shown in Figure 1. The error bars in these figures represent the precision of the CiAQ measurements as quantified by the standard error of measurements from a series of systems. The results demonstrate that for both naphthalene and phenanthrene the equilibrium aqueous-phase concentration VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Comparison of NAPL-Water Partitioning Coefficients (KiN) and NAPL Activity Coefficients (γiN) for Two- and Three-Component NAPLsa target PAH naphthalene phenanthrene a

other solute

i Csat AQ, (µmol/L)

K iN (µmol/L)

γNi

phenanthrene none naphthalene none

228.2 ( 2.3 228.7 ( 9.7 6.32 ( 0.16 5.75 ( 0.22

1172 ( 36.9 1240 ( 43.4 97.7 ( 5.4 85.5 ( 2.9

1.64 ( 0.07 1.65 ( 0.13 4.32 ( 0.22 4.12 ( 0.24

Error values indicate 95% confidence intervals.

is a linearly increasing function of the NAPL mole fraction. A series of three-component NAPLs with Xphen ) 0.03 and N Xnaph varying from 0.01 to 0.17 were contacted with water, N and the equilibrium aqueous-phase PAH concentrations naph (CiAQ) were measured. Cnaph and AQ increased linearly with XN phen naph CAQ did not vary with XN , suggesting that the range of naphthalene mole fractions in the NAPL employed did not affect the activity of phenanthrene in the NAPL. Similarly, a phen linear relationship exists between Cphen , and Cnaph AQ and XN AQ phen i does not vary with XN . KN values were calculated as the slope of the linear regression line fitted to CiAQ versus XiN data in Figure 1 and are listed in Table 3. In all systems, the γiN was reasonably constant at all mole fractions of the PAH in the NAPL, as shown in Table 3. γiN was calculated from the KiN values, the aqueous solubilities of PAHs, and their solidliquid fugacity ratios listed in Table 1. i The aqueous solubility (CAQ,sat ) of naphthalene and phenanthrene was measured in systems where the aqueous phase was contacted with pure compound (crystals) of naphthalene and phenanthrene and with a mixture of both i values fall right onto the corresponding compounds. CAQ,sat CiAQ versus XiN lines at the saturation XiN values, confirming that a linear relationship is observed at all feasible PAH mole fractions in the NAPL. The saturation XiN represents the maximum dissolvable mole fraction of the PAH compounds in hexadecane, which were experimentally determined to be 0.19 for naphthalene and 0.065 for phenanthrene. Comparisons of the KiN values for each PAH in the twoand three-component NAPLs show a difference only for phenanthrene in the three-component NAPL. A small but significant increase in KiN was observed. This suggests that equilibrium phenanthrene aqueous concentrations in the three-component NAPL systems are greater than in the twocomponent NAPLs. The increase in the aqueous phenanthrene concentrations in three-component NAPL systems is phen attributable to an increased CAQ,sat when both PAHs are present. The slightly greater value of solubility suggests that the activity of phenanthrene in water is lowered by the presence of naphthalene in the aqueous phase. This may be caused by a minor alteration of the water structure caused by the presence of naphthalene (22). Overall, the aqueous solubility of naphthalene and phenanthrene measured in the single-PAH systems matched reported values closely (8, 23). Linear partitioning of PAHs from complex real-world NAPLs has been demonstrated in a number of studies (2, 3, 17, 24), and thus the NAPLs used in this study serve as a useful surrogate for studying the partitioning of PAHs from real-world NAPLs. The range of PAH mole fractions used in this work is similar to that commonly found in environmentally significant NAPLs. Micellar Solubilization of PAHs from Two-Component NAPLs. MSR values of naphthalene and phenanthrene were i determined from lines fit to CTOT versus Csurf AQ data at surfactant concentrations above the cmc according to the method of Zimmerman et al. (25). In all cases a minimum of two sets of vials, each of which included at least three 3904

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FIGURE 2. MSR values of naphthalene (a) and phenanthrene (b) for micellar surfactant solutions contacted with different twocomponent NAPLs and pure compounds. Dashed lines represent linear regression fits to the data. The solid line (a) represents fit to a MSR vs Xnaph equation based on an alternate model (refs 7 and 11). i -Csurf CTOT AQ combinations, were used to calculate each MSR value. The MSRs of PAHs in systems where NAPLs with various PAH mole fractions were contacted with surfactant solutions are presented in Figure 2. The error bars on the pure compound (crystals) data points represent the standard i error of the mean MSR values calculated from CTOT versus surf CAQ data. Factors such as partitioning of the surfactant into the NAPL and the altering of the cmc as a result of solubilization

TABLE 4. NAPL-Surfactant Micelle Partitioning Coefficients (KiM)a surfactant

target PAH

other solute

K iMb (L/mol)

naphthalene 861 ( 48 naphthalene 861 ( 40.4 naphthalene phenanthrene 907 ( 47.6 phenanthrene 18300 ( 280 phenanthrene naphthalene 13100 ( 460 Triton X-100 naphthalene 936 ( 56.8 Tween 80 naphthalene 1840 ( 174 phenanthrene 27300 ( 426 Tergitol NP10 naphthalene 1080 ( 76.8 Brij 30 Brij 35

a Error values indicate 95% confidence intervals. b Calculated from i KN and the slopes of the lines fit to the data in Figures 2 and 3.

of PAHs may cause errors in determining the true MSR in surfactant solutions contacted with NAPLs (25). Significant changes in the cmc caused by solubilization are also indicative of changes in the free energy of micellization (13), which may invalidate the assumption that the micelle volume and structure remain essentially constant over the range of PAH concentrations studied. The apparent cmc was measured by solubilization, in systems containing crystalline PAH or NAPLs with various PAH mole fractions, according to the method of Edwards et al. (7). From comparison of the apparent cmc in systems containing NAPLs with varied PAH concentrations or PAH crystals, it was determined that solubilization of PAHs did not affect the cmc of the surfactants except for Brij 30. Surfactant partitioning into the NAPL was ascertained to be the reason for i a 28-fold increase in cmc Brij 30, and Csurf AQ and XN values were adjusted to account for this (16). For each surfactant-PAH combination, a linear relationship is apparent between MSRi and XiN (R2 > 0.93). Moreover, the MSRi values in systems containing PAH crystals, represented by the open symbols in Figure 2, agree closely with the expected MSR for a PAH-saturated NAPL. This is analogous to the observation that the PAH concentration in an aqueous phase in contact with a PAH-saturated NAPL will be equal to its aqueous solubility. Considering that the aqueous pseudo-phase PAH concentrations do not change significantly in the presence of surfactant monomers, the linear relationships between MSRi and XiN observed in Figure 2 and predicted by eq 5 support the solute partitioning model for NAPL-surfactant solution systems presented. Linear relationships between solute aqueous concentrations and their micellar-phase concentrations have been observed in prior studies (26-28). For condensed aromatic compounds, the solubilization capacity has been observed to decrease with increases in molecular size (13). Naphthalene has a higher MSR than phenanthrene when present in excess of saturation in solutions of either Brij 35 or Tween 80, as is seen in Figure 2. The KiM values for two-component NAPLs presented in Table 4 and, when used in eq 5, suggest that at the same NAPL mole fraction phenanthrene may have a higher MSR than naphthalene in solutions of Brij 35 and Tween 80. Considering this, it is possible that, when a real-world NAPL is being solubilized, with some surfactants the micellar concentration of phenanthrene may exceed that of naphthalene, even if the mole fraction of the two PAHs are similar in the NAPL. KiM is a measure of the compound’s relative solubility in water and the micelle, and thus higher KiM values i do not necessarily indicate higher CM,sat or maximum MSRi. Some amount of hexadecane was solubilized in the surfactant micelles. The MSR of hexadecane was determined i from a single data point for CTOT at a Csurf AQ of 100 cmc and was

found to be 0.03 for Brij 35 and 0.17 for Tween 80. The amounts of hexadecane solubilized into micelles vary with the chain length of the hydrophobic tail of the surfactant monomers (29). It has been reported that the partitioning of one solute into a micelle may be influenced by the presence of other solutes (9, 30). However, given that MSR for pure compound PAHs agree closely with the expected MSR for a PAH-saturated NAPL for all surfactant solutions examined, the hexadecane in the micelles likely did not compete with PAHs for solubilization space and did not affect PAH solubilization. Micellar Solubilization of PAHs From Three-Component NAPLs. Comparison of the MSR of a PAH in single and multiple solute systems was used to determine the effect of the presence of another PAH has on its concentration within the micelle. The three-component NAPLs were contacted with Brij 35 solutions, and the MSR values for each PAH were determined. Brij 35 was chosen for these experiments since micellar solubilization of hexadecane is negligible as compared to the PAH solutes, and thus systems containing Brij 35 allow evaluation of micellar solubilization of PAHs from NAPLs primarily composed of PAHs, such as coal tars. Moreover, systems containing Brij 35 provided the most reproducible results from the two-component NAPL experiments with naphthalene and phenanthrene. The MSRi values varied linearly with the XiN of the corresponding compound as shown in Figure 3. Phenanthrene MSR at Xphen ) 0.3 was unchanged over a range of N naphthalene mole fractions in the three-component NAPLs, and the same is observed for the naphthalene MSR with NAPLs of various phenanthrene content and Xnaph ) 0.09. N The results in Table 4 suggests that Knaph values in threeM component NAPLs are only very slightly different from those obtained using the two-component NAPLs. However, the phenanthrene MSR values and the Kphen values from the M three-component NAPLs are substantially lower than those estimated from two-component NAPL systems. The product of the partitioning constants, KiMKiN, is 18% lower for phenanthrene in three-component NAPLs in comparison to that in the two-component NAPLs. For naphthalene the difference is only 0.4%. At any given solute mole fraction in the NAPL, the MSRs are altered proportionately, and a comparison of the results of the two- and three-component NAPL partitioning experiments indicate that the micellar partitioning of phenanthrene is reduced in systems containing both phenanthrene and naphthalene. Naphthalene solubilization does not appear to be affected by the presence of phenanthrene in the micelles. The multicomponent relative solubilization ratio (Si) can be used to quantify the degree to which one compound is solubilized relative to the others. The multicomponent relative solubilization ratio is defined as the ratio of the MSR values of the solutes in the multiple solute system divided by the ratio of the MSR values of the two components of interest in single solute systems. The multicomponent relative solubilization of naphthalene over phenanthrene in a multiple PAH NAPL is calculated by

Snaph )

(MSRnaph/MSRphen)multiple PAH system

(6a)

(MSRnaph/MSRphen)single PAH system

By dividing each of the MSRi values by XiN and replacing single solute system (MSRi/XiN) terms with (KiMKiN), the Si of a given solute can be expressed as a function of the respective NAPL mole fractions. Thus VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Micellar-phase activity coefficient of PAHs and the multicomponent relative solubilization ratio for naphthalene in different three-component NAPLs of varying total micellar PAH mole fraction contacted with Brij 35 solutions.

FIGURE 3. MSR values of naphthalene (a) and phenanthrene (b) in Brij 35 solutions contacted with three-component NAPLs with different PAH mole fractions. Snaph ) (MSRnaph/Xnaph )multi-PAH system/(MSRphen/Xphen )multi-PAH system N N naph phen (Knaph /Kphen )single PAH system M KN M KN

(6b) Snaph values greater than unity indicate situations where naphthalene is selectively solubilized over phenanthrene, and conversely, Snaph values less than one represent situations where other solutes are selectively solubilized over naphthalene. Figure 4 shows Snaph values for different XPAH M (the sum of XiM values), where XiM can be estimated as

XiM ) ni/(ni + nj + nk + nsurf) ) MSRi/(MSRi + MSRj + MSRk + 1) (7) where ni, nj, nk, and nsurf are respectively the number of moles of the solutes (PAHs, i and j; hexadecane, k) and surfactant in the micellar pseudo-phase. Figure 4 shows that the highest values of Snaph are at the lowest XPAH M , which indicates that at low micellar PAH concentrations naphthalene is preferentially solubilized over phenanthrene to a larger degree. At higher XPAH the Snaph approaches unity, suggesting that M 3906

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neither compound is selected over the other relative to the single solute situation. This result is in contradiction with the observations of Chaiko et al. (10) of an increase in the MSRs of more hydrophobic solutes caused by the solubilization of aromatic compounds at the micelle’s core-water interface (i.e., palisade layer) leading to an increase in the micelle’s core volume. If this were the case in the experiments reported in this study, the MSR of phenanthrene would have increased in the presence of naphthalene. The contradiction between the results of Chaiko et al. (10) and this study may be attributable to the use of ionic surfactants in that study while this study employed nonionic surfactants and that errors that may arise from MSR determination using results for only one surfactant concentration have not been accounted for in that study. Surfactant solubilization patterns for multiple PAH solutes have been reported by Guha et al. (30). In experiments where the partitioning of aqueous-phase naphthalene, phenanthrene, and pyrene to micelles of the nonionic surfactant Triton X-100 was evaluated, it was found that in some cases an increase in the solubilization capacity of Triton X-100 for phenanthrene occurred in the presence of naphthalene. Similar to Chaiko et al. (10), they concluded that naphthalene solubilized at the micelle core-water interface likely increased the solubilization of the other more hydrophobic PAHs. However, this trend is not consistently observed from the results of Guha et al. For instance, the solubilization of pyrene was generally not increased by the presence of naphthalene. Because pyrene, like phenanthrene, is more hydrophobic than naphthalene and is also solubilized predominantly within the micelle core, an increase in the pyrene solubilization should have been observed if naphthalene solubilization had increased the micelle core volume. The micellar-phase activity coefficients (γiM) of naphthalene and phenanthrene in the three-component NAPLs are shown in Figure 4 and were calculated as δiM ) XiNδiN/XiM. The γnaph values increased by a factor of 1.7, and γphen M M increased by a factor of 1.5 over the range of XPAH studied. M Comparable increases in γiM are observed for the twocomponent NAPL systems (results not shown). Because XiM/ XiAQ ) γiAQ/γiM at equilibrium and it is observed that γiM increases with XiM (and XiN), linear partitioning between the mole fractions of the solute in the micellar and aqueous phases (7, 11) is not applicable to the results in this study, as confirmed by the poor fit of the solid line to the naphthalene MSR data for Tween 80 in Figure 2. The ratio

CiM/CiAQ ) (XiM/V′M)/(XiAQ/V′AQ) ) γiAQV′AQ/γiMV′M is however observed to be constant possibly because with increasing XiM the molar volume of the micellar pseudo-phase, V′M, decreases in magnitude (negligible change in the volume of the micellar pseudo-phase with increasing amounts of PAH solutes solubilized) and compensates for increases in γiM. Assuming that the micellar pseudo-phase volume does not change, calculations of the change in molar volume in fact indicate an inversely proportional decrease as compared to the increase in γiM. The observed linear relationship between XiN and MSR, as stated in eq 5, may be used to predict the total PAH concentration in micellar solutions during surfactant flushing operations. The coefficient KiN can be determined from the aqueous solubility of the compound in its pure subcooled liquid state, and calculated activity coefficients of the PAHs in the NAPL (for example, using the UNIFAC method) and the coefficient KiM can be determined from the MSR of the compound in systems containing solutions of the surfactant of interest and an excess of the compound in its pure aggregate phase. However, deviations from this relationship can occur when competitive solubilization between solutes occur, and further research is required to determine the important phenomena involved in the dissolution of multiple PAH mixtures.

Acknowledgments Funding for this project was provided in part by the National Science and Engineering Research Council, Canada. We thank Dr. J. Hawari, National Research Council Biotechnology Research Institute, for his assistance in providing PAH analysis of several samples. The insightful comments of an anonymous reviewer are greatly appreciated.

Supporting Information Available Additional details on the analytical techniques employed and on the methods for the PAH equilibrium partitioning experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review July 30, 2001. Revised manuscript received February 18, 2002. Accepted June 18, 2002. ES011175R

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