Environ. Sci. Technol. 1997, 31, 384-389
Aqueous Solubility Depression for Hydrophobic Organic Chemicals in the Presence of Partially Miscible Organic Solvents G . T . C O Y L E , † T . C . H A R M O N , * ,‡ A N D I. H. SUFFET§ Environmental Science Center, Monsanto Company, St. Louis, Missouri 63167, Civil & Environmental Engineering Department, University of California, Los Angeles, Los Angeles, California 90095-1593, and Environmental Sciences and Engineering Program, University of California, Los Angeles, Los Angeles, California
This work examines the solubility of hydrophobic organic chemicals (HOC), including naphthalene, biphenyl, 2,2′,4,4′-tetrachlorobiphenyl (PCB-47), and 2,2′,4,4′,5,5′hexachlorobiphenyl (PCB-153), in water saturated with partially miscible organic solvents (PMOSs), including methylene chloride and chloroform. A recirculating generator column technique provided solubility measurements for the stationary-phase chemical (HOC) in the mobile phase (PMOS solution). The observed solubility of the naphthalene (log Kow ) 3.4) was not noticeably impacted by the PMOS, while that of biphenyl (log Kow ) 4.1) decreased slightly with increasing PMOS concentration. In water saturated with methylene chloride and chloroform, the observed PCB-47 (log Kow ) 6.2) aqueous concentrations were reduced to about 25% and 15%, respectively, of its aqueous solubility. HOC solubility results were further reduced for PCB-153 (log Kow ) 7.2) and the same PMOS solutions. Solventingout, a phenomenon analogous to salting-out, is introduced as an explanation for the observed behavior. The solubility depressions increased with increasing chemical hydrophobicity (judged by Kow) of both the HOC and the PMOS. Implications with respect to organic mixtures and contaminant transport in soils and groundwater include the following: (1) the association of the PMOS with the HOC phase will retard the transport of this relatively mobile solute through sediments contaminated with the HOC; (2) the presence of nearly saturated solutions of PMOS will reduce the apparent solubility and therefore the mobility of the HOC.
Introduction Hazardous sites often include landfills and disposal pits in which the wastes have become intermixed through careless dumping procedures or through failure to segregate waste streams. In general, the various components of these complex mixtures will affect the dissolution and transport of one another. The nature of the effect depends on chemical types, phase, and composition of the mixture involved. For example, it is well known that each component of a mixture of similar * Corresponding author phone: (310) 206-3735; fax: (310) 2062222; e-mail:
[email protected]. † Monsanto Company. ‡ Civil & Environmental Engineering Department, UCLA. § Environmental Sciences and Engineering Program, UCLA.
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organic liquids (e.g., benzene and toluene) will exhibit an aqueous solubility proportional to its mole fraction in the mixture (1-4). It is also well known that completely watermiscible cosolvents, such as acetone and methanol, will enhance the solubility and transport of less soluble organic contaminants (5-10). However, for dissimilar compounds or for liquid-solid and solid-solid mixtures, dissolution behavior can be complex (4, 11). Furthermore, if the solvent is also a mixture, such as in the case of partially miscible organic solvents (PMOSs) (10) or ionic species (11-14), then the nature of the solvent mixture will also play a role in determining dissolution behavior. A better understanding of the solubility of organic contaminants in mixed systems such as these would provide valuable information for scientists and engineers associated with afflicted hazardous waste sites. This work examines the dissolution behavior of extremely low solubility compounds in water containing dissolved volatile organic chemicals (VOC, e.g., methylene chloride, chloroform). Using a previously introduced nomenclature (10), the lower solubility compounds, also known as nonvolatile organic chemicals (NVOC), will be referred to here as hydrophobic organic chemicals (HOC). The more soluble compounds will be referred to as PMOSs. In this parlance, the dissolution behavior of the solid-phase HOC is studied in water laden with a dissolved PMOS. Examples of HOCs include polycyclic aromatics hydrocarbons (PAHs), chlorinated pesticides, polychlorinated biphenyls (PCBs), and other low solubility chemicals. Examples of PMOSs include common industrial solvents, such as methylene chloride, chloroform, and trichloroethylene.
Background Various systems have been included in the investigation of the aqueous solubility of organic mixtures, including multicomponent organic liquid-water systems (1-4); cosolventwater-organic liquid systems (5-10, 15); solid-liquid and solid-solid organic-water systems (4, 11), and organic liquid electrolyte solution systems (11-14). The behavior of mixtures in two-phase solid-liquid systems has received comparatively little attention in the literature. This work focuses on one such system comprising a ternary mixture in equilibrium with a solid-phase HOC. The ternary mixture is composed of water, the HOC (solubility in the range of µg/L to mg/L), and a PMOS (solubility in the range of g/L). There is assumed to be no nonaqueous phase liquid (NAPL) present. In real systems, this condition may be met when NAPL in the vicinity gives rise to aqueous concentrations approaching saturation for the dissolved PMOS. In a study related to the present one (10), the major goal was to measure the solubility of HOCs, mainly PAHs, in mixtures of water and organic cosolvents (e.g., methanol, 2-propanol, acetone) in the presence of a dissolved PMOS. Volume fractions of the cosolvents employed ranged from 0 to 0.5. Pinal et al. (10) concluded that the presence of a PMOS at a concentration of approximately 1% by volume or greater could significantly increase the solubility enhancement produced by the organic cosolvents. This was particularly true for PMOSs with strongly polar functional groups. The magnitude of this effect is similar to that induced by low levels (0.001-0.05 mole fraction) of water-miscible cosolvents (7, 16). A limited number of observations for each mixture studied by Pinal et al. (10), those corresponding to a zero volume fraction of cosolvent, are directly related to this work. From the data presented (Figures 3-5 in ref 10), HOC solubility is observed to be either unchanged, enhanced, or depressed in the presence of a PMOS. The observed depression appeared
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1997 American Chemical Society
TABLE 1. Chemical Properties for Selected Hydrophobic Organic Chemicals (HOC) at 25 °C
a
HOC
solubility [mg/L] (solid-based)
solubility [mg/L]a (subcooled liquid)
octanol-water partition constant log Kow
ref
naphthalene biphenyl PCB-47 PCB-153
30.0-34.4 6.03-7.5 0.05-0.068 0.00095-0.0056
111.6 NDb ND 0.0063-0.037
3.36 4.09 6.2 (est) 7.15
19, 24 19, 25 25-30 31, 32
Gleaned from references where available or estimated using ratio of solid to liquid phase vapor pressures (P °s/P °L).
to be roughly 50% of the solubility value for anthracene in a nearly saturated aqueous solution of nitrobenzene. Several cases of solubility depression were also observed in an earlier investigation (11) involving mixtures of water and two HOCs, both present in excess. The solubility of hexane was also observed to be depressed in aqueous solution containing methyl tert-butyl ether (MTBE) as a PMOS (15). The primary goal of this work was to measure the effect of dissolved PMOSs (methylene chloride and chloroform) on the solubility of extremely hydrophobic organic chemicals (HOCs). A secondary goal was to explain the observations in terms of currently accepted organic dissolution theory.
Materials and Methods Measurements were made using a recycling generator column to ensure equilibrium was achieved between the two phases; these measurements were verified by batch solubility experiments. This section describes the recycling generator column technique and its application in estimating the solubility of HOCs in aqueous solutions of PMOSs. After an initial overview of the apparatus, several subsections detail the materials and methods employed in the study. Generator Column Technique. The generator column technique of May et al. (17) was adopted for the solubility measurements in mixed solutions. Bellington et al. (18) provide a review of the mass transport principles underlying the technique and report that it is precise and accurate in solubility measurements for solids in aqueous solutions. In this work, the generator columns were packed with glass beads, nominally 0.25 mm in diameter, coated with the probe compound. To coat the beads, a solution of acetone and the HOC probe compound of interest was used to wet the glass beads in a glass Petri dish. The probe compound concentration was sufficient to leave approximately 0.001% (w/w) of the probe compound on the beads after the acetone had evaporated. This coating is substantially lower than the 1% employed by May et al. (17). Preliminary work indicated that entrainment of HOC particles commonly occurred when heavier coatings were employed. Furthermore, the heavier coating promoted inhomogeneous, aggregated packing. A schematic of the generator column apparatus is available as Supporting Information (Figure S1). The setup includes silanized glass contacting columns, approximately 33 cm long and 0.55 cm in diameter. All connections were Teflon or stainless steel. Three columns were operated in parallel, each equipped with a pump to drive the recirculating flow through Teflon tubing. During operation, the water or PMOS solution is introduced into each column and its associated recirculation tubing. PMOS solutions were prepared by equilibrating deionized water with excess solvent (methylene chloride or chloroform) in a sealed flask. Volumes of the saturated solution were decanted into the recirculation apparatus reservoir through glass wool to avoid entrainment of PMOS emulsions (11). An initial 10-min purging time allowed removal of any air bubbles from the apparatus through the sampling septum within a T-joint (Figure S1). When no headspace remained, liquid sampling, at the same location, commenced and continued intermittently until steady HOC and PMOS concentrations were achieved.
b
ND ) not determined.
Fully packed, the column porosity was approximately 0.40, providing a total pore volume of about 3.1 mL. Preliminary experiments focused on confirming that equilibrium was achieved in the generator columns. Flow rates of 0.1-5.0 mL/min, depending on the HOC, were employed to allow for the most rapid approach to equilibrium while minimizing the potential for entrainment of a HOC phase in the flowing aqueous phase. The duration of experiments ranged from 30 to 120 min. Results indicated that 120 min was more than adequate to ensure equilibrium for even the most insoluble HOC. The measured methylene chloride concentrations for one such recirculation experiment are plotted in Figure S2 in the Supporting Information. Partially Miscible and Hydrophobic Organic Chemicals. The HOCs employed in the study included naphthalene, biphenyl and two polychlorinated biphenyls, or PCBs (2,2′,4,4′tetrachloro- and 2,2′,4,4′,5,5′-hexachlorobiphenyl, referred to here as PCB-47 and PCB-153, respectively). Table 1 contains a summary of aqueous solubility values and octanol-water partitioning coefficients (Kow) reported for these chemicals. The compounds are extremely hydrophobic and span a range of solubility and Kow values that encompasses 3-4 orders of magnitude. It is also noteworthy that the uncertainty of the reported values is substantial for the solubility values less than about 0.1 mg/L. In order to achieve the sensitivity required for quantification of these extremely low solubility compounds, radiolabeled PCB-47 and PCB-153 were obtained (Sigma Chemicals, St. Louis, MO) and diluted in solutions of their unlabeled counterparts. The purity of the radiolabeled compounds was greater than 99%. Nonlabeled naphthalene and biphenyl were also obtained (Sigma Chemicals) and used as received (98% purity). The PMOSs employed were methylene chloride (CH2Cl2) and chloroform (CHCl3) dissolved in water. Methylene chloride has an aqueous solubility of approximately 20 g/L (19) and is a commonly used industrial solvent (20). Chloroform exhibits behavior similar to methylene chloride and has a solubility of approximately 8 g/L (19). It is also commonly identified at hazardous waste sites. Analytical Methods. PMOS concentrations were sampled periodically at the T-joint. Concentrations were quantified using a gas chromatograph (Shimadzu Mini-2C) equipped with a flame ionization detector (FID). Samples withdrawn for PMOS analysis were diluted with equal volumes of methanol and injected directly onto the GC in duplicate. A 6-ft glass column packed with 0.1% SP-2100 on 60/80 mesh Supelcoport (Supelco Co.) was used for the analyses. The gas chromatograph was operated isothermally at 100 ˚C with helium as the carrier gas (40 mL/min). The radiolabeled HOCs (PCB-47 and PCB-153) were analyzed using standard liquid scintillation counting (LSC) techniques. Aqueous samples of 0.5 mL were delivered to 10 mL of scintillation cocktail and counted using a Packard Model 4530 LSC (Packard Instruments). Automatic quench correction using an external standard and fluorescence correction were employed on all samples. The limited number of experiments with nonlabeled HOCs (naphthalene and biphenyl) were quantified using standard HPLC techniques with a UV detector (Hewlett Packard Model 1050).
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TABLE 2. Observed HOC SolubIlities and Experimental Precision of Generator Column Technique (All Values for 22 ( 1 °C) chemical
measured solubility [mg/L]
no. of observations, n
relative SD [%]
naphthalene biphenyl PCB-47 PCB-153
33.9 6.99 0.066 0.0028
3 3 3 5
4.9 5.4 3.2 3.6
Experimental Approach. The mixed systems were examined using the generator column technique. In all systems, the stationary phase was composed of single-component HOC coating. The first set of experiments employed water as the mobile phase, providing measurements of the HOC aqueous solubility values. The purpose of this system was to evaluate the accuracy of the generator column technique. The second set of experiments employed single-component aqueous solutions of the PMOSs as the mobile phase. In these experiments, HOC concentrations were probed at the following PMOS levels: 50% and 100% methylene chloride saturation for naphthalene and biphenyl; 25%, 50%, 75%, and 100% methylene chloride saturation for PCB-47; 25% and 100% chloroform saturation for PCB-47; and 100% only for both methylene chloride and chloroform with PCB-153. Here, 100% refers to solutions prepared as approximately 97% of saturation in order to avoid the formation of PMOS emulsions in the recirculating system. A final experiment employed aqueous mixtures of the two PMOSs as the mobile phase and PCB-153 as the HOC. Batch Experiments. A validation study was performed for the generator column method by comparing its results with batch solubility results obtained using an independent method (21). In this procedure, PCB-47-coated glass beads were placed in batch systems with Milli-Q water saturated or nearly saturated with methylene chloride. Batch samples were equilibrated in headspace-free vials for 16 h, after which the PCB-47 concentrations were quantified.
Results and Discussion At the outset of the experiments, two types of behavior were hypothesized: (1) the observed solubility of the HOC would increase, that is, the systems would exhibit behavior analogous to that induced by low levels of miscible cosolvents (in this case, the PMOS); (2) the observed solubility of the HOC compound would decrease as the PMOS adsorbed onto, or partitioned into, the HOC phase. In this section, the experimental results are presented and discussed in the framework of these hypotheses. Aqueous Systems. The preliminary experiments aimed at validating the generator column measurements yielded solubility values for naphthalene, biphenyl, PCB-47, and PCB153. The results from triplicate measurements are given in Table 2. The measured solubility values for all of the compounds fall within the range of values reported in the literature (see Table 1). In addition to being reasonably accurate, the generator column technique yielded precise results for the compounds tested. The coefficient of variation was approximately 5% or less for all of the measured solubility values. It is worthwhile to note the substantially wide range of literature values cited for PCB-153. Given the accuracy of the generator column technique for the other compounds, it is suggested that the measured value (0.0028 mg/L) is more appropriate than the previously published aqueous solubility values. PMOS Systems. Following the aqueous solubility measurements, the influent reservoir (Figure S1) was amended to deliver constant concentrations of the PMOSs. Results for
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FIGURE 1. Effect of methylene chloride (PMOS) on the aqueous solubility of solid-phase naphthalene and biphenyl (HOCs). Linear regression for naphthalene results in a statistically insignificant slope. For biphenyl, best fit based on a linear expression is shown with the solid line; best fit based on log-linear expression (eq 4) is shown with the dashed line. Constants derived from the regression analyses are summarized in Table 4.
FIGURE 2. Effects of methylene chloride (PMOS) on the aqueous solubility of solid-phase PCB-47 (HOC) using recirculating column and batch techniques. The best fits based on a linear expression are shown with the solid lines; best fits based on log-linear expression (eq 4) are shown with the dashed lines. Constants derived from the regression analyses are summarized in Table 4. the more soluble of the HOCs (naphthalene and biphenyl) with methylene chloride are shown in Figure 1. Although the precision is fairly poor for the naphthalene case, the data suggest that there is no observable effect of the dissolved PMOS. The biphenyl solubility appears to decrease slightly with increasing methylene chloride concentration. In a PMOS solution saturated with methylene chloride, the solubility of the biphenyl decreased to approximately 50% of its measured solubility. For PCB-47, which is more hydrophobic than biphenyl, the effects of the dissolved methylene chloride and chloroform are more evident, as shown in Figures 2 and 3, respectively. As the methylene chloride level approached saturation, the observed PCB-47 solubility decreased to about 25% of its clean water solubility. The apparent PCB-47 solubility was observed to decrease to about 15% of its clean water solubility in the chloroform-saturated solution. Results from the batch equilibration method for PCB-47 as the HOC and methylene chloride as the PMOS are also included in Figure 2. The batch method reproduced the solubility depression observed in the generator column results, although the impact of the PMOS was slightly less according to the batch results. In a saturated methylene
in the aqueous phase for HOC i, a component of the nonaqueous mixture. Equation 1 assumes that the HOCPMOS coating behaves as a well-mixed liquid. Clearly this assumption is flawed if the HOC remains in a solid state. However, if we continue with the assumption intact, then according to eq 1, partitioning of the PMOS into the HOC would reduce the observed concentration of HOC in the aqueous phase. A common simplifying assumption is that p of the equivalence of γi,aq and γi,aq because of the low solubility of nonpolar organics in water (1-4). Given this assumption, eq 1 simplifies to the following expression (e.g., 4):
Ci ) xi,orgγi,org Si FIGURE 3. Effect of chloroform (PMOS) on the aqueous solubility of solid-phase PCB-47 (HOC). Best fit based on a linear expression is shown with the solid line; best fit based on log-linear expression (eq 4) is shown with the dashed line. Constants derived from the regression analyses are summarized in Table 4. chloride solution, the apparent PCB-47 solubility was observed to decrease to about 40% of its clean water solubility. The dissolved PMOS experiments were repeated for PCB153 as the HOC. The observed PMOS effects were similar to those observed with PCB-47. Over the same range of methylene chloride PMOS concentrations, the observed PCB153 solubility also decreased to about 25% of its clean water solubility. Chloroform again impacted the solubility behavior more strongly than methylene chloride, decreasing the apparent solubility of PCB-153 to 15% of the observed clean water value. Regarding the working hypotheses presented, none of the experimental results indicated concentrations elevated above a compounds’ pure water solubility. Thus, the first hypothesis, that asserting a minor cosolvent effect with a PMOS, was not considered further. An increasing activity of the PMOS in the HOC phase is a possible explanation for the apparent depression in the HOC solubility. Such behavior might occur if the PMOS is partitioning into a liquefied HOC phase or if the PMOS is somehow entering the solid state of the HOC. This explanation was investigated further here in a set of calculations based on the following equation (e.g., 4): p Ci xi,orgγi,orgγi,aq ) Si γi,aq
(1)
where Ci is the observed aqueous concentration of HOC i [mg/L], Si is the aqueous solubility of HOC i [mg/L], xi,org is the mole fraction of HOC i in the nonaqueous phase, γi,org is the activity coefficient for HOC i in the nonaqueous mixture p (mole fraction basis), γi,aq is the activity coefficient in the aqueous phase of pure HOC i, and γi,aq is the activity coefficient
(2)
To predict the observed concentration in the mixed system, it is first necessary to estimate xi,org using the partitioning coefficient for the water-HOC system. The following equation was used to calculate the distribution coefficients (22):
Kjd )
( )
∞ γj,aq Vw ∞ V γ org j,org
(3)
where Kjd is the aqueous-organic phase distribution coef∞ ficient for PMOS j [-], γj,aq is the infinite dilution aqueous∞ phase activity coefficient for PMOS j in water, γj,org is the infinite dilution aqueous-phase activity coefficient for PMOS j in the organic phase (HOC), Vw is the molar volume of water [L/mol], and Vorg is the molar volume of the organic phase (HOC) [L/mol]. Estimates of the activity coefficient values in eq 3 were calculated using the UNIFAC approach (fifth revision, 23). The estimated partitioning coefficients and predicted aqueous concentrations are summarized in Table 3. The calculations indicate that a significant portion of methylene chloride and chloroform (0.14 mole fraction) will partition into the HOC phase, reducing the HOC mole fraction to about 0.86. Despite the presence of this level of methylene chloride and chloroform, UNIFAC estimates of γi,org values are greater than 0.99 for the four chemical combinations, suggesting that liquid-phase HOC would form an ideal mixture with the PMOS. Calculations employing the subcooled liquid solubility for the HOCs in eq 2 produce concentrations well in excess of those observed in the PMOS-free experiments (Table 3). If the solubility of the HOC in a solid state is used in the calculations, then the estimates become more comparable to the observed values (Table 3). However, it is important to note that this correction is inconsistent with the assumption of a well-mixed liquid HOC phase (on which eqs 1-3 are based). Given the apparent incompatability of the underlying assumptions and the experimental system at hand, the poor predictions are not surprising. The HOC phase in the
TABLE 3. Comparison of Observed and Predicted Aqueous Concentrations of HOCs (PCB-47 and PCB-153) Assuming PMOSs (Methylene Chloride and Chloroform) Partition from Aqueous Phase into Nonaqueous HOC Phase HOC
PMOSa
Vorg [cm3/mol]
γ∞j,aqb
γj∞,orgc
Kd [-]d
xj,orge
Cipred [µg/L]f
Cipred [µg/L]g
Ciobs [µg/L]
PCB-47 PCB-153 PCB-47 PCB-153
methylene chloride methylene chloride chloroform chloroform
268 310 268 310
248 248 864 864
0.50 0.44 0.50 0.44
33 33 107 100
0.14 0.14 0.13 0.12
NDh 31 ND 32
57 2.4 58 4.5
18 0.7 9 0.4
a PMOS is assumed to be in a saturated aqueous solution and equilibrated with nonaqueous phase HOC. b Infinite dilution activity coefficient for PMOS in water; calculated using UNIFAC. c Infinite dilution activity coefficient for PMOS in organic phase (HOC); calculated using UNIFAC. d Distribution coefficient for PMOS; calculated using eq 3. e Mole fraction of PMOS in HOC phase ) K x s (where x s is the aqueous solubility of the d j j PMOS expressed as mole fraction). f Calculated using eq 2 using subcooled liquid HOC solubility values (Table 1) without temperature correction. g Calculated using eq 2 using observed solid HOC solubilities (Table 2). h ND, not determined, solubility data for PCB-47 as subcooled liquid not available.
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TABLE 4. Estimated Solventing-Out Constants Based on Observed Linear and Log-Linear Regressions on Solubility Depression Data for Dissolved PMOS-HOC Systems solution components (HOC-PMOS)a
min obsd rel HOC concn [Cobs/S]
estd solventing-out constants, Ks and Ks′ [L/mol]b
correl coeff, r 2
biphenyl-CH2Cl2
0.51
PCB-47-CH2Cl2
0.27
PCB-47-CH2Cl2c
0.43
PCB-47-CHCl3
0.13
PCB-153-CH2Cl2
0.24
PCB-153-CHCl3
0.14
1.10 1.93 2.17 2.84 1.30 2.18 13.8 13.0 2.74 3.35 13.5 13.5
0.84 0.89 0.88 0.93 0.82 0.89 0.97 0.99 ND ND ND ND
FIGURE 4. Correlation of the estimated solventing-out constants (Ks*) with the HOC (biphenyl, PCB-47, and PCB-153) octanol-water partitioning coefficients, log Kow.
a Mixture comprises ternary mixture of water with dissolved PMOS and HOC in equilibirum with solid-phase HOC. b First number based on log-linear relation (eq 5); second number based on linear relation. c From batch data.
experimental system appears to be composed of either a solid solution or a solid-liquid mixture of the HOC and PMOS or may involve adsorption of the PMOS at the water-HOC interface. Aqueous solubility depression of organic chemicals in these types of systems has been observed previously (4, 11) but has not been explained. An alternative explanation addresses the problem from a new perspective, analogous to the salting-out phenomenon. According to this analogy, the observed solubility depression is a product of a solventing-out phenomenon in which dissolved PMOS may occupy a significant portion of the water molecules, rendering them unavailable for HOC dissolution. The solventing-out phenomenon has not been reported previously, although it is consistent with past experimental observations with MTBE (PMOS) and benzene (15) and with nitrobenzene (PMOS) and anthracene (10). A common expression for describing the salting-out phenomenon is referred to as the Setchenow equation (e.g, 11):
Si log ) KsCs Ci
(4)
where Si is the solubility of organic species i in pure water [mol/L], Ci is the observed concentration of organic species i in a salt-water solution [mol/L], Ks is the Setchenow or salting-out (or solventing-out) constant [L/mol], and Cs is the salt concentration in water [mol/L]. Given the relatively narrow range of PMOS concentrations observed in this work, a linear relationship was also used to summarize the observed HOC concentration reduction with increasing PMOS concentration. Solventing-out constants characterizing the observed HOC solubility depressions for the range PMOS solutions in this work were estimated using the log-linear (eq 4) and the linear expression. The regression analyses, summarized in Table 4, suggest that the linear solventing-out expression describes the data significantly better than the log-linear expression. The observed solubility depression results in solventingout constants that are approximately 1-2 orders of magnitude greater than analogous salting-out constants (11, 12). However, because of the low solubility of the VOCs relative to inorganic salts, the magnitude of the effect is approximately the same: a maximum of roughly 90% reduction in concentration of the HOC. For the HOCs tested, with methylene chloride as the PMOS, there was no observable depression in the naphthalene solubility, and depressions to roughly 50%, 25-45%, and 25% of respective solubility values for
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FIGURE 5. Effect of mixed aqueous PMOS solutions (chloroform and methylene chloride) on the apparent aqueous solubility of PCB-153 (HOC). The predicted solubility depressions based on log-linear (solid line) and linear (dashed line) solventing-out constants (Table 4) weighted by mole fraction. biphenyl, PCB-47, and PCB-153. Chloroform imparted about a five times greater effect than methylene chloride, causing depressions to roughly 15% of the solubility of PCB-47 and PCB-153, respectively. The estimated solventing-out constants (based on a linear relationship) were compared with HOC solubility and Kow values. Although the data are sparse, they do suggest a correlation between estimated constants and the logarithm of the thermodynamic properties. This point is illustrated in Figure 4 in which the estimated solventing-out constants (i.e., solubility depression) increase with increasing HOC and PMOS hydrophobicity (as judged by Kow). Alternatively, the estimated solventing-out constants (i.e., solubility depression) decrease with increasing HOC and PMOS solubility (see Figure S3 in Supporting Information). Extrapolation of either correlation suggests that a solventing-out constant of about 1.5 L/mol for the naphthalene-methylene chloride system. This would imply that a solubility depression for naphthalene (about 25%) should have been observed. Thus, the lack of observable depression in this system merits further investigation. Mixed Systems. The solubility experiments were also performed with methylene chloride and chloroform as cosolutes. The HOC solubility (PCB-153) results, which are limited to one or two points per incremental PMOS increase, are plotted in Figure 5 as a function of total PMOS molarity. The predicted behavior plotted in Figure 5 is based on loglinear (eq 4) and linear solventing-out constants from Table 4. The composite solventing-out constant is assumed to be the mass-average of the two ternary system constants, in accord with salting-out behavior. The experimental observations agree reasonably well with the predicted results, suggesting a simple additive effect by the two PMOSs. The
apparent change in slope at a total molarity of about 0.13 (approximately 0.065 mol/L for both methylene chloride and chloroform) corresponds to the point at which the water is saturated with respect to chloroform. At this point, increases in the total molarity are due solely to the addition of methylene chloride. Environmental Implications. Finally, the results reported here have several implications with respect to organic mixtures and contaminant transport in soils and groundwater. First, the association of the PMOS with the HOC phase will retard the transport of this relatively mobile solute through sediments contaminated with the HOC. It is important to recall, however, that elevated concentrations of PMOS are usually indicative of proximate DNAPL and that the issue of DNAPL removal may supercede the retardation issue. Second, assuming that nonaqueous PMOSs and nonaqueous HOCs are separate in space, the presence of saturated solutions of PMOSs will reduce the solubility and therefore the mobility of the HOC. In some cases, this may reduce the risk of dangerous HOCs, such as PCBs, being transported toward drinking water supplies. In other cases in which remediation is the primary goal, the presence of PMOSs may hinder HOC removal.
Acknowledgments This work was funded by the U.S. Army Corps of Engineers Construction Engineering Research Center (CERL), through an award to I.H.S. (CERL Project DACA 88-88-D-0019), Project Officers R. Scholze and S. Maloney, and by the U.S. Environmental Protection Agency Office of Exploratory Research (Award R-823579-01-0). Indirect support was provided by the National Science Foundation through an award to T.C.H. (Award BES-9502170). Portions of this work were presented at the ACS Environmental Chemistry Section Meeting, September 1989, Miami Beach, FL, and at the Society of Environmental Toxicology and Chemistry Meeting, 1988, Alexandria, VA. This work has not been subject to agency review, and endorsement should not be inferred.
Supporting Information Available Three figures showing the headspace-free generator column apparatus, the observed methylene chloride concentration, and the correlation of the estimated solventing-out constants with HOC solubility (3 pp) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the Supporting Information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title, of article, names of authors, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $12.00 for photocopy ($14.00 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7%
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Received for review February 28, 1996. Revised manuscript received September 17, 1996. Accepted September 24, 1996.X ES960184A X
Abstract published in Advance ACS Abstracts, December 1, 1996.
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