Coal Tar Dissolution in Water-Miscible Solvents: Experimental ...

Environ. Sci. Technol. 1993, 27, 2831-2843

Coal Tar Dissolution in Water-Miscible Solvents: Experimental Evaluation

Catherine A. Peters' and Richard G. Luthy

Department of Civil Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

Coal tar, a dense nonaqueous phase liquid (NAPL), is a common subsurface contaminant at sites of former manufactured gas plants. A proposed remediation technology is water-miscible solvent extraction, which requires un- derstanding of the effect of water-miscible solvents on the solubility of coal tar. This study investigated this effect and the extent to which multicomponent coal tar could be represented as a pseudocomponent in thermodynamic modeling. The coal tar used in this study showed a predominance of polycyclic aromatic hydrocarbons with no single compound accounting for more than 4% (wt). The bulk solubility of coal tar in water was estimated to be 16 mg/L using composition data and Raoult's law assumption for aqueous solubility. For three solvents, n-butylamine, acetone, and 2-propanol, equilibrium phase compositions of two-phase coal tar/solvent/water mixtures were experimentally determined using radiolabeled materials and are presented as ternary phase diagrams. Results showed n-butylamine to be a good water-miscible solvent for coal tar dissolution. The validity of thermodynamic modeling of coal tar as a pseudocomponent was explored by examining the liquid-liquid solute partitioning of naphthalene, phenanthrene and pyrene and by assessing the effect of solvent extraction on coal tar phase composition. It was found that coal tar partitions as a pseudocomponent in systems with appreciable solvent, but not in systems with only coal tar and water.

Introduction

Today there is growing concern about nonaqueous phase liquids (NAPLs), a class of subsurface contaminants that are immiscible in water (I). Coal tar is a NAPL that is denser than water and often very viscous. Subsurface contamination with coal tar exists today as a result of uncontrolled disposal of process residuals a t former manufactured gas plant (MGP) sites. The manufactured gas industry ended during the 1950s due to the widespread use of natural gas and the exploitation of petroleum. Groundwater contamination at MGP sites persists decades later because of the slow, continuous dissolution of constituent compounds from subsurface coal tar (2-5). There are as many as 1000 MGP sites in the United States and likely more (6). Numerous MGP site investigations have verified the presence of coal tar and subsequent groundwater contamination, but cleanup efforts have been only sparsely applied. Conventional remediation methods, such as direct coal tar pumping or groundwater pump- and-treat, have proven to be of limited practical use to achieve low residual concentrations, as is discussed in detail elsewhere (5, 7).

The use of water-miscible solvents for the extraction of coal tar from contaminated soils is a soil treatment option

* To whom correspondence should be addressed at her present address: Environmental and Water Resources Engineering, De- partment of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125.

0013-936X/93/0927-2831$04.00/0 0 1993 American Chemical Society

that could be applied either in an in situ injection/recovery system or in an aboveground treatment operation. The primary objective of this work was to investigate the extent to which organic water-miscible solvents increase the solubility of coal tar and its constituent compounds. This work was part of a larger project, presented in Luthy et al. (7), aimed at investigating the feasibility of in situ solvent extraction for remediation of coal tar contaminated sites. Other aspects of the project included examination of the mass transfer limitations to insitu solvent extraction of contaminated soils (8) and large-scale subsurface modeling of an in situ solvent extraction process to explore deployment options and estimate cleanup times (9).

The primary challenge in studying coal tar NAPLs is that they are mixtures of hundreds of compounds, primarily polycyclic aromatic hydrocarbons (PAHs). It would be an insurmountable task to completely characterize the equilibrium-phase compositions of coal tar/solvent/water mixtures by measuring and describing the partitioning of every individual compound. The experimental data required to calibrate models describing phase equilibria for a mixture increases very sharply as the number of components in the mixture increases. Even for a ternary mixture, the experimental effort required is almost 1 order of magnitude larger than that needed for a binary mixture (IO). Furthermore, of the many constituent compounds that make up coal tar, only a portion can be identified and quantified through chromatographic methods. The challenge, then, is to adequately describe the dissolution behavior of a multicomponent mixture such as coal tar, which itself cannot be fully characterized, without the burden of enormous data requirements. The approach used in this investigation involves a simplification, referred to here as the pseudocomponent simplification, in which coal tar is treated as a single component in a system with two other components: solvent and water. Coal tar solubility was explored by studying the equilibrium phase compositions of two-phase liquid mixtures of coal tar, solvent, and water for several water-miscible solvents. The coal tar pseudocomponent simplification allows the composition of the two immiscible liquid phases to be described in terms of volume fractions of only three components. This simplification facilitates experimental analysis and data representation using ternary phase diagrams and makes thermodynamic modeling tractable (II), as is presented in a forthcoming paper (12).

This paper addresses four specific objectives. First, detailed composition analyses of the coal tar used throughout this project are presented. Second, the validity of the pseudocomponent simplification for semi-empirical thermodynamic modeling is explored using composition analyses of coal tar before and after extraction and using measurements of the partitioning behavior of three PAH compounds. Third, an estimate is made of the bulk solubility of coal tar in water, which serves as a baseline for comparison with experiments using solvents and indicates the possible extent of groundwater contamination in terms of all constituent compounds. Finally, phase

Environ. Sci. Technol., Voi. 27, No. 13, 1993 2831

equilibria of coal tar/solvent/water systems based on experimental measurements of water and solvent partitioning are presented in the form of ternary phase diagrams.

Theory

Pseudocomponent Simplification. The character- ization of phase equilibria of complex mixtures can be accomplished in a number of ways. Petroleum products are often characterized using boiling point curves in which the mixture is thought of as a continuum of infinitesimal fractions of pseudocomponents (13). Researchers studying coal tar or other mixture NAPLs have, for the most part, described dissolution in terms of individual compounds (14-19). This approach is necessary when assessing groundwater contamination because cleanup standards are specified for individual compounds, usually those on the priority pollutant list. The individual compound approach was not useful for this project since the objective was to assess the bulk solubility of coal tar based on the representation of the entire coal tar mixture as a pseudocomponent.

For most water-miscible solvents and over a large range of compositions, mixtures of coal tar, solvent, and water will separate into two immiscible liquid phases, which are referred to here as the “coal tar phase” and the “solvent/ water phase”. The coal tar phase consists primarily of undissolved coal tar and small amounts of solvent and water incorporated into this organic phase. The solvent/ water phase consists of solvent, water, and dissolved coal tar. The coal tar pseudocomponent comprises all the constituent compounds in coal tar, which can be thought of as everything in the system except the solvent and the water.

Experimental data are used to determine parameters of a semi-empirical thermodynamic model of ternary liquid- liquid equilibrium (LLE) (11,12). Fitted model parameters for the coal tar pseudocomponent can be thought of as describing the equilibrium behavior of a “coal tar molecule”, representing the collective behavior of all the constituent PAH compounds. Conceptually, the pseudocomponent representation is valid because of the chemical similarity of the PAH compounds in coal tar relative to the two other components of the system. That is, the molecular interactions between two coal tar constituents are much more similar than the molecular interactions between either of these compounds and solvent or water. Strictly speaking, for model parameters to truly represent the collective thermodynamic properties of the coal tar pseudocomponent, the coal tar component in each phase must be identical in composition, Le., comprised of the same distribution of constituent compounds. The extent to which this is true for a given coal tar/solvent/water system depends on the degree of similarity of the partitioning behavior of the individual compounds. Consider the mixing of coal tar with a solvent/ water solution with the subsequent formation of a two- phase system in equilibrium. Each coal tar constituent compound, i, partitions into the solvent/water phase to an extent described by its concentration, Ctw [mg/Ll. Nor- malizing C;w to the total concentration of dissolved coal tar constituents, CC:”, results in a term that is indicative of the abundance of i relative to the total pseudocompo-

nent. The premise that the composition of the dissolved coal tar is similar to that of the undissolved coal tar, means that

where the superscripts sw and ct denote the solvent/water and coal tar phases, respectively. Rearranging eq 1 gives the partition coefficient, Kctlawi, the ratio of the concentration of i in the coal tar phase to the concentration in the solvent/water phase:

n c t F n c t

for all i

Thus, an important implication is that for a given mixture the partition coefficients of all coal tar constituent compounds must be similar to each other and similar to the overall partitioning of the coal tar pseudocomponent. Semi-empirical thermodynamic modeling of ternary coal tar/solvent/water systems is strictly valid only for systems for which eq 2 is true, and the extent to which predictions can be made in composition regions beyond where experimental data were used for calibration is determined by the extent to which eq 2 is true for a wide range of system compositions.

This concept was explored experimentally using the solvent n-butylamine, which has been identified as a good water-miscible solvent for coal tar dissolution (7). First, solute partitioning tests were done for three coal tar constituent compounds: naphthalene, phenanthrene, and pyrene, which represent compounds with a range of aqueous solubilities. Second, the effect of extraction with n-butylaminelwater solutions on the composition of the coal tar phase was studied using quantitative chromatographic analyses of extracted coal tar samples.

Coal Tar Solubility in Water. The aqueous solubility of constituent compounds from coal tar into water has been discussed in recent years (14-19) in an effort to understand groundwater contamination at coal tar sites. I t has been found (e.g., ref 15) that predicting aqueous solubilities of coal tar compounds using the Raoult’s law assumption of ideality and an approximation for pure liquid aqueous solubilities based on heats of fusion offers reasonable agreement with experimental measurements. This approach was adopted for this work for the purpose of estimating the bulk solubility of coal tar as a pseudocomponent.

Equilibrium solute partitioning in liquid-liquid systems is characterized by equal fugacities of the compound in each liquid phase (20). If it is assumed that the solute behaves ideally in both the aqueous and coal tar phases, the equilibrium relation for solute i becomes (21)

(3)

where x p is the mole fraction of solute i in the water phase, xpt is the mole fraction of i in the coal tar phase, and xy‘ is the mole fraction equivalent of the aqueous solubility of pure liquid i. Assuming that the aqueous phase is sufficiently dilute such that the volume of the solution is approximately equal to that of pure water (ZZ), the aqueous concentration expressed as a mole fraction is

xp = ct WL x i x i

2832 Envlron. Sci. Technol., Vol. 27, No. 13, 1993

proportional to mass concentration. An equation similar to eq 3 can be written

cp = "FtS? (4)

where Cy is the mass concentration of i in the water phase [mg/Ll, and S? is the aqueous solubility of pure liquid i [mg/Ll. Coal tar constituent mole fractions were computed by x p = ( w t % j / l O O ) (MWdMWi), where wt% j is the weight percent of i in the coal tar, MWCt is the average molecular weight of the coal tar, and MWi is the molecular weight of compound i.

For many coal tar compounds, SF is a hypothetical quantity since these compounds are solids in the pure state at ambient temperatures. An expression relating SF to Sy, the pure solid aqueous solubility a t the system temperature, is derived from the thermodynamics of solid- liquid equilibrium where the standard state in the liquid phase is defined as the pure subcooled liquid at the temperature of the solution (20). Applying this relation to eq 4 results in

(5)

where the term on the right is the ratio of the pure component fugacities in the subcooled liquid and the solid states. Fugacity ratios are often available in the literature (e.g., ref 23) or can be approximated by an expression that accounts for the free-energy change between the liquid and the solid state (20), using a constant entropy of fusion for organic compounds (24). The bulk coal tar dissolved concentration is the sum of the concentrations of all the dissolved species, i.e., C: = Cin4_1Cy, where m is the total number of compounds in coal tar. Substituting for CT from eq 5

m

an expression is derived to compute the solubility of bulk coal tar in terms of individual component properties and their relative abundances.

Methods Coal Tar, The coal tar used for all laboratory exper-

iments was a sample of the free-flowing liquid tar residing in the subsurface at the former manufactured gas plant site in Stroudsburg, PA. Several published reports describe the chemical and physical characteristics of this coal tar (25-28). A t this site, it is possible to collect liquid coal tar by pumping from a NAPL pool in a stratigraphic depression in the confining layer. Coal tar was collected (7, 11) from one of several existing wells that had been installed for remediation of the site through coal tar pumping. This coal tar sample was well-suited for laboratory investigations because it is a thin liquid and is virtually free of dirt and water bubbles.

Materials. The n-butylamine, acetone, and 2-propanol solvents were ACS grade from Fisher Scientific Co. Deionized water was used for solvent/water solutions. Solute partitioning and coal tar/solvent/water phase equilibria were determined using radiolabeled techniques. This provided a means of observing the behavior of individual compounds or system components without the need for chromatographic methods, which are difficult

and time-consuming for coal tar because of the large number of similar hydrocarbon compounds. 14C-Labeled naphthalene, phenanthrene, and pyrene were obtained from Amersham Corp., with specific activities ranging from 30 to 60 pCi/mg and chemical purities of >98%. The 14C- labeled solutes were stored in stock solutions by rinsing the glass ampules with methanol. Stock solution concentrations ranged from 20 000 to 60 000 dpm/pL (dpm refers to disintegrations per minute in which 2.22 x lo6 dpm = 1 pCi). This was sufficiently concentrated so that when used to prepare coal tar radioactive stock solutions, methanol accounted for less than 1 % of the total system volume. With this small amount, there was little concern for cosslvency effects on constituent solubilities (29).

14C-Labeled solvents, n-butylamine, acetone, and 2-propanol, were purchased from the Sigma Chemical Co. in specific activities ranging from 2.8 to 8.2 mCi/mmol with chemical purities of >98%. Stock solutions were prepared by diluting the radioactive materials in pure, unlabeled solvents, resulting in concentrations ranging from 90 000 to 200 000 dpm/pL. Tritiated (3H) water was purchased from NEN Research Products of DuPont Co. with a specific activity of 25 mCi/g (5.5 X 1O1o dpm/g). A tritiated water stock solution was prepared by diluting by a factor of 100.

Coal Tar Composition Analysis. Composition analyses that were performed on the Stroudsburg coal tar sample included analysis of volatile aromatic compounds, chromatographic analyses of polycyclic aromatic hydrocarbons (PAH), and a molecular weight determination. Number-average molecular weight determinations were done by Galbraith Laboratories, Knoxville, TN, on two replicate coal tar samples using vapor pressure osmometry. This procedure (30) is based on the relationship between the vapor pressure of a solution relative to that of pure solvent and the molar concentration of solute in the solution. A Knauer-Dampfdruck osmometer was used with toluene as the solvent.

Volatile aromatic compounds were extracted with methanol according to EPA method 8020. The methanol extract was analyzed according to EPA method 5030, using helium for purging and as the carrier gas in the gas chromatograph (GC). The GC had a 105-m VOCOL capillary column by Supelco Co. The injection temperature was 240 "C, and the carrier gas flow rate was 6 mL/ min. The temperature program was 10 min at 45 "C, to 200 "C at 4 "C per min, and held for 8 min. Detection was achieved by photoionization, a t a temperature of 250 "C. Quantification was done using fluorobenzene as a surrogate standard compound. A five-point calibration was done over the 10-100 ppb range.

The primary chromatographic analysis of PAH compounds was done in collaboration with the Coal Science Division, Pittsburgh Energy Technology Center (PETC), Pittsburgh, PA. A 10-mg sample of coal tar was dissolved in methylene chloride to make a l-mL solution. A sample of this was injected into a 5988A Hewlett-Packard gas chromatograph-mass spectrophotometer (GCIMS) with a 35 m X 0.2 mm SB-Phenyl-5 column with 0.3 pm film thickness. The injection temperature was 300 "C, the helium carrier gas had a linear velocity of 32 cm/s a t 30 "C, the ionization potential was 70 eV, and the voltage was 2551 V. The program was 3 min at 30 "C, to 320 "C at 4 "C per min, and 5 min at 320 "C. Thirty-five compounds were identified by mass spectra and retention

Environ. Sci. Technol., Vol. 27, No. 13, 1993 2833

indices from the literature. Fourteen compounds were quantitatively determined using individually run calibra- tions. Benzo[blthiophene was added to the coal tar as an internal standard to check instrument response. For the remaining 21 identified compounds, an average response factor was calculated from the 14 calibrated runs and used to convert peak area into mass injected to estimate the weight percents.

Supplementary analysis was done in collaboration with the Analytical Section of the Research and Development Division of Texaco, Inc., Beacon, NY. Prior to GC/MS analysis, the coal tar was fractionated using ASTM method D2007, in which characteristic groups were quantified in weight percent using clay-gel adsorption chromatography. This step was done to make a crude separation of the PAH compounds residing in the aromatic fraction from the other coal tar compounds, such as heterocyclics, oxygenated compounds, and very high molecular weight substances. This facilitated the GC/MS analysis, however, the lack of specificity in the operational definitions prescribed by this procedure limits the usefulness of the analysis, and the crude separation procedure may have caused some inaccuracies in the quantification step. The aromatic fraction of the coal tar was diluted 1 0 0 ~ in methylene chloride and injected into a GC/MS with a 30 m X 0.25 mm SPB5 capillary column. The program was 5 min at 100 "C and then up to 300 "C at 5 "C per min. Mass spectral analysis was by positive electron ionization, between 3 and 250 amu. Approximate weight percentages were estimated for the groups of compounds identified using a single internal standard, assuming identical responses and using relative peak areas.

GUMS analyses of coal tar samples that had been extracted with n-butylaminelwater solutions were also performed by the PETC laboratory. Coal tar samples were equilibrated with n-butylaminelwater solutions in 500- mL separatory funnels using a coal tar-to-solvent/water solution volume ratio of 1:4. For two of the samples, the coal tar was extracted once. The solvent/water solutions used were 20% (vol) n-butylamine/80% water and 40% (vol) n-butylamine/60% water. For the third sample, the coal tar was extracted twice sequentially, each time with fresh 40 5% n-butylamine/water solution, to observe a trend in composition change upon sequential extraction. Over a 24-h period, the mixtures were gently agitated inter- mittently to prevent emulsion formation and then settled for 24 h to allow phase separation. Samples of the extracted coal tar phases were collected from the bottom of the separatory funnels. GC/MS analyses were performed in the manner described above for the PETC analysis of the original coal tar. The amounts of dissolved n-butylamine, estimated using thermodynamic LLE model predictions, were found for all three extracted samples to be approximately 5% of the weight (11). The GC/MS results were corrected to represent weight percent relative to the coal tar portion only, allowing an assessment of the change in mass distribution of PAH compounds.

Solute Partitioning in Coal Tar/Water Systems. For each of the three solutes (naphthalene, phenanthrene, and pyrene), coal tar was spiked with several microliters of radioactive solute stock solution giving 100 000-900 000 dpm/mL. Common procedures for LLE experiments involve mixing the two liquids in a vial and then agitating, settling, and analyzing each phase. Variations on this approach were tried for coal tadwater systems, but these

were found to produce erroneously high solute concentration measurements in the aqueous phase, possibly due to erroneous sampling of either a floating organic film or a microemulsion of coal tar micelles in the water (11). A more successful experimental method was designed to eliminate the free-flowing coal tar phase. IMPAQ RG20 porous silica gel beads with pore diameters of 200 A, supplied by the PQ Corp., were used. The beads were dehydrated by heating at 800 "C for 1 h and then cooled. A 1-mL sample of the coal tar radioactive stock solution was imbibed into 5 g of beads in a glass vial with a Teflon septum and mixed on an orbital rotator for 24 h. This amount of coal tar was sufficient to discolor the beads, but not enough to saturate them. This provided ample interfacial surface area for mass transfer and eliminated the formation of coal tar phase emulsions. The coal tar imbibed beads were mixed with water in 50-mL centrifuge tubes, with calcium chloride to aid in the settling of any particulate matter. Equilibration times ranging from 24 h to 1 month were used to test for kinetic hindrances to solute dissolution from bead pore spaces, but no significant differences in aqueous concentrations were observed. In all, four replicate experiments were performed for naphthalene and phenanthrene, and seven were performed for pyrene. The vials were centrifuged. A several milliliter sample, taken from the top of the vial through the septum using a syringe, was passed through a 0.2-pm Teflon filter and, discarding the first milliliter to precondition the filter, 1-mL volumes were expressed into 20-mL scintillation vials containing 15 mL of Ultima Gold, Packard Instrument co.

The concentration of 14C-labeled solute in the vial was measured using a Beckman 5000 TD liquid scintillation counter. The channel window was set to record events with pulse heights from 0 to 670. Quenching was corrected automatically with the H# method and an internally stored quench curve generated from 14C standard solutions, obtained from Amersham Corp. Each test vial was counted twice and for a sufficient time such that the 2a error in dpm was less than 1 % . Measurements were corrected for background radiation which averaged 40 dpm in the laboratory. The random coincidence monitoring (RCM) option was used to indicate samples with high numbers of light-producing events other than radiation, which was problematic especially for coal tar containing samples. To reduce the measurement error caused by these added counts, the scintillation vials were stored in the dark for at least 24 h, until the RCM %, the percentage of nonradiation events relative to total light-producing events, decreased to less than 1 % .

For each experiment, the radioactivity concentration in the water phase, (dpm/mL)w was measured directly. The radioactivity concentration in the coal tar phase, (dpm/mL)Ct, was estimated from knowledge of the total radioactivity in the system, dpmT, and the volume of coal tar, VCt, since the amount of dissolved solute is negligible relative to the undissolved solute. The coal tar/water partition coefficient, Kctlw, was computed by

( 7 ) Cct (dpm/mL)ct (dpmT)/VCt

(dpm/mL)" (dpm/mL)" N - - Kct,w = -

The solute's aqueous solubility, Cw, was computed from the aqueous concentration of the radiolabeled solute

2834 Environ. Scl. Technol., Vol. 27, No. 13, 1993

relative to the total radioactivity, and an estimate of the total amount of the solute in the system, mT:

(8)

where C W is in milligrams per liter with mT in milligrams. mT was computed from the wt % of the compound in the coal tar, the volume of coal tar, and the density of the coal tar.

Solute Partitioning in Coal Tar/n-Butylamine/ Water Systems, In 50-mL centrifuge tubes, coal tar spiked with 14C-labeled solute was added to n-butylamine/ water solution. A range of solvent concentrations were used. In all cases, the coal tar-to-solvent/water solution volume ratio was 1:4. For each of the three solutes, a t each solvent concentration, replicate experiments were performed. The mixture was brought to equilibrium at 25 "C over a period of 24 h. Since vigorous agitation led to emulsion formation, the vials were agitated intermit- tently (I I). After centrifugation, the solvent/water phase was sampled with a syringe and expressed through a 0.2- pm Teflon filter, discarding the first milliliter to precondition the filter. The filtering step removed any suspended coal tar, giving a clear filtrate. Coal tar phases were sampled by decanting the solvent/water phase from the vial and injecting a syringe into the coal tar. The sampled coal tar was checked for emulsification by expressing through a fine needle; emulsions were evidenced by expulsion of intermittent slugs of solvent/water phase. Observations from such systems were not used. The coal tar phase sample was diluted by a factor of 30-40 in n-butylamine before adding to the scintillation counting cocktail to lighten its color and reduce quenching for more accurate liquid scintillation counting. Dilution was also necessary for solvent/water phase samples that were very dark due to significant coal tar dissolution. The solute partition coefficient, Kdlsw, was computed similarly to Kdlw (eq 7), except that for these experiments (dpm/mL)ct was measured directly.

CoalTar/Solvent/Water Phase Equilibria. For the three solvents, n-butylamine, acetone, and 2-propanol, LLE of coal tar/solvent/water systems was experimentally determined for a range of compositions that resulted in two-phase systems. To determine a single tie line of the ternary phase diagram, an overall composition of coal tar/ solvent/water was chosen. Parallel experimental systems were set up: one using 14C-labeled solvent and one using tritiated water. For the solvent partitioning experiments, the solvent/water solution and a spike of 14C-labeled solvent stock solution were mixed in a 35- or 50-mL centrifuge tube such that the total level of radioactive solvent, dpm:, was in the range of (5 X 106)-(9 x 107) dpm. For the water partitioning experiments, 0.5-1 mL of tritiated water stock solution was added to the solvent/ water solution resulting in a total water radioactivity, dpm:, on the order of 5 X lo8 dpm. Coal tar was added, and the vials were equilibrated, centrifuged, and sampled in the manner described above. The solvent/water phase samples for the vials containing 3H had to be diluted because of the high dpm: needed for these experiments, For liquid scintillation counting of these samples, the channel window was set for pulse heights of 0-400. The H# quench curve was generated from 3H standard solutions from NEN Research Products of DuPont Co.

c w = (dpm/mL)w 1ooomT dpmT

The volume fraction of component i in the cy phase, vE, is computed by

where (dpm/mL); is the measured concentration of radioactive i in the cy phase, dpm' is the total radioactivity in the system, and Vi is the total volume of i in the system in milliliters. The subscript i denotes the component, either water (w) or solvent (s), and the superscript cy denotes either the solvent/water phase (sw) or the coal tar phase (ct). The volume fraction of the coal tar pseudocomponent in each phase is calculated by the difference from unity:

v f c t = 1 - vfs* - VfW*

Throughout, when the symbol ct is used as a superscript it refers to the coal tar phase, and when used as a subscript it refers to the coal tar pseudocomponent. Since both phases were sampled, these calculations do not require the relative phase volumes, which changed significantly for high solvent systems.

Each experimentally determined tie line is the result of a single or duplicate measurements of solvent and water partitioning. The experiments were designed so that the relative standard deviations (a,/x) from random error associated with vc and vr*, measurements were less than 2% (see ref 11). Because eq 10 is additive, the relative standard deviation for random error in vet is not approximated by a constant value; the absolute standard deviation for each vet is approximated by uu2 = ( c ~ , , ~ + a2 )112, assuming Gaussian distributions for error terms. For very small values of $2, the random errors likely have skewed distributions, so this analysis is only an approximation.

Quantitative tie line data were difficult to obtain near the plait points of the ternary phase diagrams due to sampling difficulties which resulted because the two liquid phases were very similar in composition and thus similar in appearance, and often one of the phases was present in a very small quantity. For overall compositions thought to be near the plait point, an additional series of experimental systems was visually examined for heterogeneity to identify compositions that are conclusively within the two-phase composition region. Heterogeneity was observed by slight color differences between phases, by light reflected from the interphase meniscus, and by overturning the vial to see a thicker coal tar phase clinging to the glassware. Tests were continued, moving toward the expected direction of the plait point, until a composition was found beyond which two phases could not be visually discerned. This point is referred to here as a two-phase check point.

Water Solubility in Coal Tar. In experiments where coal tar is mixed with water containing a spike of tritiated water, the concentration of radioactivity in the coal tar phase can, in principle, be used to compute the volume fraction of water using eq 9. While this method was successfully applied to systems containing solvents, it was not practically applied to coal tar/water systems since the amount of tritiated water in coal tar was at or below the analytical detection limit due to the low solubility and to the large dilution necessary to count coal tar phase samples.

UG


Naphthalene 2-Methyl Naphthalene 1 -Methyl Naphthalene Biphenyl 2-Ethyl Naphthalene 1 -Ethyl Naphthalene 2,6-Dirnethylnaphthalene Acenaphthylene Acenaphthene Dibenzothiophene Phenanthrene Anthracene

1

1 -Phenylnaphthalene 3-Methylphenathrene 2-Methylphenanthrene 2-Methylarithracene 4 H-Cyclopenta [def] phenanthrene 9-Methylphenanthrene 1 -Methylphenanthrene 2-Phenylnaphthalene 9-Ethylphenathrene 2-Ethylphenathrene Fluoranthene Aceanthrylene

Pyrene Benzo (c) Phenanthrene Benzo (a) Anthracene Chrysene + Triphenylene Benzo (j) Fluoranthene

Benzo (k) Fluoranthene Benzo (e) Pyrene Benzo (a) Pyrene Perylene lndeno (1, 2 , 3-04 Pyrene Picei-te

+ Benzo (b) Fluoranthene

27

I I l l l / I / - 1 I I I I I I I I I I I I I I I I I I I I 12 18 24 30 36 42 48 54 60 66 72

Time (min.) Figure 1. Chromatogram of Stroudsburg coal tar with 35 peaks identified.

Table I. Classification of Stroudsburg Coal Tar into Characteristic Groups Using ASTM Method D2007

w t % classification

34 17

41

8

asphaltenes, the n-pentane insoluble fraction polar compounds, material retained on adsorbent clay after

aromatics, material that passes through a column of adsor-

saturates, material in an n-pentane eluent that is not ad-

percolation of the samples in an n-pentane eluent

bent clay in an n-pentane eluent but adsorbs on silica gel

sorbed on either the clay or silica gel

The amount of tritiated water required for precise measurement would have made these experiments ex- pensive and would have involved much higher than normal radioactivity levels employed for routine laboratory work. I t was possible, however, to use information about the precision of the experimental method to estimate an upper bound for the water concentration in the coal tar phase.

Results and Discussion

Stroudsburg Coal Tar Composition. Results of the characteristic group analysis of the Stroudsburg coal tar are shown in Table I, giving weight fractions and operational definitions. The PAH compounds comprise the aromatics fraction and are likely also to be present as very high molecular weight compounds in the asphaltenes fraction. The results from the PETC chromatographic analysis of the Stroudsburg coal tar are shown in Figure 1. A total of 280 peaks were found, indicating the complexity of the mixture. The results of the quantification of compound weight percents from all the chromatographic analyses are shown in Table 11. The compounds that were quantified using calibration standards

are identified with CS; the others having been estimated using average response factors. Summing up the weight percents in Table 11, this composition analysis accounts for just under half (46% ) of the total coal tar weight, for constituent compounds in the molecular weight range from 78 to 278 that produced peak areas sufficiently large and distinct. This analysis cannot account for very high molecular weight material not quantifiable by GUMS techniques, for trace compounds such as between peaks 9 and 10 in Figure 1, or for compounds that co-elute resulting in peaks that cannot be separated.

The predominance of PAHs is consistent with analyses of other coal tars (3, 311, the primary group here being naphthalenes accounting for 18% of the total weight of this coal tar. It is noteworthy that no single compound accounts for more than 4% of the coal tar weight. The low concentrations of volatile aromatic compounds such as benzene, xylene, and toluene (BXT), together accounting for just under 1 wt %. This is consistent with analyses of certain other coal tars (3) where the BXT fraction accounts for about 1 wt 5% , and is usually closer to 0.5 wt %. Volatile aromatics were indeed produced at MGP facilities, from the volatile fraction of the raw coal and from the aromatic fraction of certain carburettor oils (32- 34). However, these compounds may not be present in significant concentrations in subsurface coal tar because light fractions may have been recovered as byproducts at MGP facilities and because the relative dissolution and migration in groundwater of these compounds is higher than for other coal tar constituents. The absence of acid- extractable organics, such as phenols, is consistent with what is expected given the method of production that was used at the Stroudsburg MGP. The process a t this plant employed anthracite coal in a carburetted water gas process, which did not produce large quantities of oxygenated

2836 Environ. Scl. Technol., Vol. 27, No. 13, 1993

Table 11. Stroudsburg Coal Tar Composition from GC/MS Analyses

benzene toluene m-, p-xyleneb o-xylene o-xylene naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthylene biphenyl acenaphthalene 2-ethylnaphthalene 1-ethylnaphthalene 2,6-dimethylnaphthalene 9H-fluorene 1H-phenalene trialkylated naphthalenes phenanthrene anthracene methyl-9H-fluorenes 4-methyl-l,l’-biphenyl dibenzothiophene 3-methylphenanthrene 2-methylphenanthrene 2-methylanthracene 4H-cyclopenta[deflphenanthrene

and 9-methylphenanthreneb 1-methylphenanthrene methyldibenzothiophene fluoranthene aceanthr ylene pyrene 1-phenylnaphthalene 2-phenylnaphthalene 9-ethylphenanthrene 2-ethylphenanthrene dimethylphenanthrene methylpyrene benzo[a]anthracene acepyrene chrysene and triphenylene b methylchrysenes benzopyrenes picene total

molecular weight

78 92

106 106 106 128 142 142 152 154 154 156 156 156 166 166 170 178 178 180 184 184 192 192 192 192

192 198 202 202 202 204 204 206 206 206 216 228 228 228 242 252 278

w t %

0.050 0.094 0.32 0.32 0.41 2.16 (CS)II 3.75 3.80 (CS) 0.68 0.50 (CS) 1.52 (CS) 1.84 (CS) 0.45 (CS) 1.99 (CS) 1.4 0.4 4.3 2.12 (CS) 0.59 (CS) 1.7 0.2 0.22 (CS) 0.55 0.43 0.31 0.57

0.33 (CS) 0.3 0.30 0.29 0.50 0.29 0.22 0.17 0.20 2.3 3.9 0.31 (CS) 0.4 0.27 (CS) 4.4 1.8 0.13 (CS) 46.46

a CS, quantification was by individually run calibration standards. * Elute together.

wastes as was often true with the coal carbonization processes with bituminous coal (3).

The results of average molecular weight determinations of two replicate coal tar samples were 209 and 211, giving an average molecular weight for the Stroudsburg coal tar of 210. This is relatively high compared to the compounds in Table I1 that have been quantified by GC/MS, indicating that the majority of the mass that has not been accounted for in the GC/MS analysis lies in the high molecular weight range. The average molecular weight of the Stroudsburg coal tar is low relative to the range of average coal tar molecular weights of 230-1600 from a study of MGP site tar residues (311, but that study was not limited to free- flowing, liquid coal tars as is present a t Stroudsburg. Additional data that characterize the Stroudsburg coal tar are a viscosity of 9.93 CP (30 “C), determined by capillary viscometry, and a density of 0.994 g/mL (30 “C), determined using hydrometers (11).

Coal tars vary from site to site in terms of composition and physical properties, resulting from production dif-

Table 111. Solubilities in Water-Phase (Experimental and Raoult’s Law Prediction), Coal Tar/Water Partition Coefficients (Experimental and Raoult’s Law Prediction), and Octanol/Water Partition Coefficients for Three Selected Solutes

log C; (mg/L) 1% (KD) log

pre- (Kctlw) y e - Wow) exptlo dicted exptP dicted ref 23

naphthalene 3.3f 0.2 3.9 3.81 f 0.03 3.7 3.37 phenanthrene 0.68 f 0.06 0.16 4.49 f 0.04 5.1 4.57 PFene 0.08f 0.03 0.014 4.8f 0.2 5.6 5.18

error estimates. 0 Experimental values are shown with 3u (99 % confidence) random

ferences as well as environmental factors, as is discussed elsewhere (3, 11). Nevertheless, the composition of the Stroudsburg coal tar is typical of liquid coal tars because the constituent compounds represented are commonly found in all coal tars and because the PAH portion is predominantly composed of naphthalenes. This suggests that the experimental and modeling results from this study may be modestly extended for making predictions about other coal tar sites having pumpable liquid coal tar.

Coal Tar Solubility in Water. Results of solute solubility and partitioning in coal tar/water systems are shown in Table 111. The experimental values for the aqueous solubilities are shown with 3a (99% confidence) estimates of the random errors based on repeated measurements, as well as the predicted aqueous solubilities based on the Raoult’s law approximation (eq 5 ) . Also reported in Table I11 are the measured coal tadwater partition coefficients with their estimated 3arandom errors estimates (log units). Just as the Raoult’s law assumption was used to derive an expression for Cy, an expression for the partition coefficient can be derived (see, e.g., ref 15) in terms of SF and the molar volume of the organic phase. The Raoult’s law estimate of the partition coefficient KD was computed for each solute using the average molecular weight of 210 and a density of 0.994 g/mL to compute the molar volume of the coal tar phase. These values and, for the sake of comparison, literature values of the octanol/ water partition coefficients are included in Table 111. High correlation between Kcnw and KO, has been demonstrated by others (14, 17, 19) and is to be expected given the correlation between partitioning of solutes in different immiscible organic/water systems as is commonly described as the linear free-energy relationship (35).

Lee et al. (15) found that for several coal tar samples, and for several PAH compounds, experimental Kdw values were generally within a factor of 2 of predicted KD’s. In logarithmic units, this corresponds to zt0.30. This agreement implies that even though the coal tar phase is a complex mixture it can be modeled as ideal in the Raoult’s law sense which assumes that the molecular interactions are equivalent to those in a pure organic liquid. For this investigation, the agreement between experimental and Raoult’s law predictions are good for naphthalene, but the measured Ciw and Kctlw of phenanthrene and pyrene indicate higher aqueous solubilities tha predicted by Raoult’s law. The difference between log KD and log Kctlw for pyrene corresponds to a Raoult’s law estimate of the partition coefficient five times greater than the measured value. The discrepancies for phenanthrene and pyrene are not likely due to an inaccurate coal tar molar volume estimate since this would have resulted in a constant bias


Table IV. Calculation of the Bulk Solubility of Coal Tar in Water (25 “C) from Estimates of Constituent Solubilities Using eq 5.

benzene toleuene m-xylene p-xylene o-xylene naphthalene 2-methylnaphthalene 1-methylnaphthalene acenap hthylene biphenyl acenaphthalene 2-ethylnaphthalene 1-ethylnaphthalene 2,6-dimethylnaphthalene 9H-fluorene trialkylated naphthalene phenanthrene anthracene methyl-9H-fluorenes 4-methyl-l,l’-biphenyl 2-methylanthracene 1-methylphenanthrene fluoranthene pyrene benzo[a]anthracene chrysene triphenylene benzopyrenes total

X?

1.35 X 103 2.15 X 10-9 3.17 X 10-9 3.17 X 103 8.12 f 103 3.54 x 10-2 5.55 x 10-2 5.62 X 9.39 x 103 6.82 X 103 2.07 X 10-2 2.48 X 10-2 6.06 X 10-9 2.68 X 10-2 1.77 X 5.31 X le2 2.50 X 10-2 6.96 X 103 1.98 X 2.28 X 10-9 3.39 f 103 3.61 X 103 3.12 X 10-9 5.20 X 103 2.86 X 103 1.24 X 1.24 X 10-3 1.50 X

0.42

S: (mg/L) 1780 515 160 215 220 31 25 28 16.1 7 3.80 8

10.1 1.7 1.9 2.1 1.10 0.045 1.09 4.05 0.03 0.27 0.26 0.132 0.011 0.002 0.043 0.004

P/tS)pwe i

1 1 1 1 1 3.53 1.24 1 4.61 2.85 5.05 1 1 6.62 7.94 2.43 5.65

3.92 1.59

9.35 7.09

11.5

66.2

19.8 21.6

52.6 32.3

189

(mg/L) 2.4 1.1 0.51 0.68 1.8 3.9 1.7 1.6 0.70 0.14 0.40 0.20 0.061 0.30 0.27 0.27 0.16 0.024 0.085 0.015 0.0067 0.0091 0.0058 0.014 0.00068 0.00047 0.0028 0.0019

16.3 Pure compound aqueous solubilities and fugacity ratios are from ref 23.

in log& for all three solutes. The discrepancies are likely due to measurement error resulting from aqueous-phase sampling difficulties in systems which may have an oily floating phase and tendencies to form microemulsions. K,, is used in this work solely for comparison with KdlSw, the solute partition coefficient in coal tar/solvent/water systems.

The coal tadwater partition coefficients vary over orders of magnitude, which is indicative of the variation of aqueous solubilities of the coal tar constituent compounds. Thus, it is immediately obvious that the constituent compounds do not equally partition to the aqueous phase, and eq 2 does not hold for a system with only coal tar and water. This implies that the application of a thermodynamic model to describe the binary LLE of a coal tar/ water system is not strictly valid since the coal tar component of the equilibrated water phase does not have the same composition as the coal tar component in the coal tar phase. A following section discusses the validity of the pseudocomponent simplification for ternary systems which include solvent.

Estimation of the bulk solubility of coal tar in water, Cz, was accomplished using the predicted aqueous solubilities of constituent compounds based on Raoult’s law (eq 6). Data for this calculation are shown in Table IV, for compounds which had been quantified (Table 11) and for which solubility data and fugacity ratios were available from the literature (23). For co-eluting compounds, e.g. m- and p-xylene, the weight fractions were taken to be half of the total for both, effectively using an equally- weighted average aqueous solubility for these compounds. For quantified groups with limited data available, such as trialkylated naphthalenes, solubility and fugacity ratio data for a reported compound in the group were used,

such as 1,4,54rimethylnaphthalene. These approxima- tions do not contribute significantly to uncertainty in the estimation of qv The aqueous solubilities of these compounds sum to 16.3 mg/L, providing an estimate of CW,. Converting to volume fraction, this corresponds to uEt = 1.6 X 10-5. Only a portion of the coal tar has been quantitatively characterized due to analytical limitations; the sum of the mole fractions used in this calculation is 0.42, and the corresponding sum of the weight fractions is 0.32. However, the majority of the compounds that are not analyzable by GC/MS methods are high molecular weight compounds which have very small aqueous solubilities. Their contribution to the sum in eq 6 is negligible. For example, pyrene’s aqueous solubility is more than 2 orders of magnitude less than that of naphthalene. Also, this calculation is limited by the availability of solubility data for constituent compounds. Again, the more soluble compounds are the ones for which data are available.

When estimating properties of mixtures, the approach of selecting a representative compound is often used. The premise of this method is that there is a single constituent compound whose property is representative of the bulk property of the mixture as a whole. While this method is tempting because of its simplicity, the above calculation indicates that it is not immediately obvious how a representative compound would be chosen. Since all the coal tar constituents are present in small quantities, a representative compound cannot be chosen on the basis of predominance. Alternatively, the selection of a representative compound based on a close match with the number-average molecular weight of coal tar leads to a low bias for CW,. The compound with available aqueous solubility data, whose molecular weight most closely matches 210, is 9,lO-dimethylanthracene with a molecular

2838 Envlron. Scl. Technol., VoI. 27, No. 13, 1993

weight of 206 and subcooled liquid solubility of 2 mg/L at 25 "C (23), which is significantly less than 16 mg/L, the estimated CW, based on eq 6. For a mixture with relatively evenly distributed weight, the selection of a representative compound based on molecular weight is inherently biased low because there is an approximate logarithmic relation between aqueous solubilities and molecular weight for nonpolar hydrocarbons, and thus the averages do not correspond with each other. For purposes of predicting aqueous solubility, fluorene is more representative of the Stroudsburg coal tar mixture, with a subcooled liquid aqueous solubility of 15.1 mg/L and a molecular weight of 166 (23).

Water Solubility in Coal Tar. The solubility of water in coal tar is of interest because together with coal tar solubility in water this completely characterizes the mutual solubility of these two components. This is useful for calibration of the binary molecular interaction parameters in a thermodynamic model describing coal tar/solvent/ water phase equilibria ( I I , 2 2 ) . An upper bound estimate of 0.001 was obtained for uc (11). Conversion to mole fraction gives an upper bound estimate for x$ of 0.01, which is comparable to water solubilities in other organic liquids of 0.0030 for water in benzene and 0.0034 in 1-methylnaphthalene (36).

Solvent Selection. A literature survey of organic compounds used as solvents was conducted to identify an initial list of 13 water-miscible solvents with suitable properties for use in a solvent extraction site remediation system. As described elsewhere (3, criteria for initial selection included the following: suitable chemical properties for separation from coal tar and water by distillation, relatively low volatility and flammability for industrial safety and handling, commercial availability, and biode- gradability. The 13 solvents were evaluated in laboratory screening tests to assess their capacity to dissolve coal tar and their effect on the physicochemical properties of the coal tar phase. As a result of this work, three solvents were identified for evaluation for potential use in asolvent extraction coal tar remediation process: n-butylamine, acetone, and 2-propanol.

Pseudocomponent Simplification. As discussed ear- lier, the validity of thermodynamic modeling of coal tar/ solvent/water phase equilibria as ternary LLE depends on the extent to which the composition of the dissolved coal tar component is the same as that of the undissolved coal tar. Experimental solute partitioning observations in coal tarln-butylaminelwater systems are presented in Figure 2. The data points shown on the ordinate, for 0% solvent, are the coal tar/water partition coefficient data (Table 111). With increasing amounts of n-butylamine in the solvent/water solution, the partition coefficients for all three solutes approach a comparable value. This observation indicates that the effect of n-butylamine on each compound is different. Specifically, with increasing concentration of solvent the enhancement of the solute partitioning to the solvent/water phase is greater for compounds such as pyrene than for compounds such as naphthalene. This is consistent with what has been observed in studies of cosolvents on PAH solubilities in which the cosolvency power, represented by u, the slope of the log-linear solubility curve, is theoretically predicted and experimentally verified (29,37) to be proportional to the logarithm of the solute's KO,. In other words, the more hydrophobic compounds experience a larger en-

8 0 L """"Y

10000

lo!

A pyrene

0 phenanthrene

0 naphthalene

! 4

I 0 20 40 60 80 0

volume % n-butylamine in initial solvent/water solution

Flgure 2. Solute partitioning in coal tar/n-butylamlne/water systems with 20% (vol) coal tar overall.

hancement of solubility in a given cosolvent solution. In terms of partition coefficients, this corresponds to a more significant decrease in K values with increasing solvent concentration for more hydrophobic compounds. This is, in fact, what is shown in Figure 2 for K,t/,,. Thus, for systems with appreciable n-butylamine (>lo% with respect to the solvent/water solution), the condition for similar compositions of dissolved and undissolved coal tar, i.e., eq 2, is satisfied. Since the validation of the pseudocomponent simplification depends on similarity of K&,,, values, these results may be extended to other coal tars with similar constituencies of PAH compounds. Coal tars that contain acidic compounds, such as phenols, and basic compounds such as anilines may not be suitable for this simplification since the solubilities and hydrogen- bonding characteristics of these compounds are very different from those of the neutral fraction (17, 35).

The underlying premise of the pseudocomponent simplification was further tested by studying the effect of n-butylaminelwater extractions on the composition of the coal tar phase. The weight percents of 27 compounds were quantified for this analysis. The compositions of the coal tar phases, corrected for fractions of dissolved n-butylamine, are presented as weight percent distributions over molecular weight in Figure 3. In Figure 3a, the compositions of coal tar samples that were singly extracted with either 20% or 40% n-butylaminelwater solutions are shown relative to the original coal tar composition as reported in Table 11. In Figure 3b, the compositions of a coal tar sample that had been sequentially extracted once, then again with a solvent/water solution of 40% (vol) n-butylamine are shown relative to the original coal tar. The total mass accounted for in each of the extracted coal tar samples was approximately 16%. Note that true histograms of the coal tar mass distribution over molecular weight would likely have long tails extending beyond molecular weight 280.

The changes in compound weight percentages from the original coal tar and extracted coal tar samples depicted in Figure 3 range from reductions of 15 % to 92 % , but the majority of the reductions are about 50%. Given that the initial weight percentages were already small (<4%), the reductions are not significant in an absolute sense. The effect of extracting with a 20% n-butylamine solution or with a 40% n-butylamine solution is practically the same

Environ. Sci. Technol., Vol. 27, No. 13, I993 2899

Table V. Experimental Measurements of Component Volume Fractions in Solvent/Water and Coal Tar Phases.

Solvent = n-Butylamine 0.048 (0.014) 0.052 (0.001) 0.900 (0.014) 0.959 (0.001) 0.029 (0.001) 0.012 (0.0002)

0.022 (0.0004) 0.080 (0.013) 0.113 (0.001) 0.807 (0.013) 0.920 (0.001) 0.058 (0.001) 0.065 (0.009) 0.165 (0.002) 0.770 (0.009) 0.899 (0.002) 0.073 (0.001) 0.028 (0.0004) 0.106 (0.011) 0.300 (0.005) 0.594 (0.009) 0.910 (0.001) 0.070 (0.001) 0.020 (0.0004) 0.106 (0.008) 0.398 (0.006) 0.496 (0.006) 0.893 (0.002) 0.087 (0.002) 0.020 (0.0003) 0.162 (0.007) 0.456 (0.005) 0.382 (0.004) 0.891 (0.001) 0.090 (0.001) 0.019 (0.0003) 0.196 (0.007) 0.500 (0.006) 0.304 (0.003) 0.887 (0.001) 0.095 (0.001) 0.018 (0.0003) 0.352 (0.008) 0.465 (0.007) 0.183 (0.003) 0.841 (0.003) 0.133 (0.003) 0.026 (0.0005)

Solvent. = Acet.nna

Solvent = 2-Propanol 0.001 (0.014) 0.160 (0.002) 0.839 (0.014) 0.928 (0.001i 0.070 (0.001) 0.002 (0.0004) 0.041 (0.008) 0.297 (0.003) 0.662 (0.007) 0.888 (0.002) 0.107 (0.002) 0.005 (0.0007) 0.048 (0.007) 0.485 (0.005) 0.467 (0.005) 0.850 (0.002) 0.144 (0.002) 0.006 (0.0009) 0.041 (0.007) 0.620 (0.006) 0.339 (0.004) 0.814 (0.003) 0.179 (0.003) 0.007 (0.0009) 0.075 (0.010) 0.699 (0.009) 0.226 (0.004) 0.743 (0.005) 0.251 (0.005) 0.006 (0.0011)

L1 10 error estimates are indicated in parentheses

molecular weight

molecular weight Flgure 3. Weight percent distributions of coal tar samples that have been extracted with Rbutylaminelwater solutions.

with regard to weight percent distribution. Furthermore, no significant difference in composition is observed for the coal tar sample that has been extracted once and then again. Another important observation is that, in the molecular weight range of 128-278, the effect of extraction on weight percent does not vary with molecular weight. That is, the compounds in this range are being extracted to the same extent, supporting the solute partitioning observations shown in Figure 2 for three solutes. The decrease in weight percentages in the 128-278 molecular weight range signifies an increase in weight percentages of compounds beyond this range. The cumulative increase in the unquantified portion of the coal tar is estimated to be 9% based on a summation of the decreases in mass of the quantified compounds. Spread over a large number of compounds, this represents only a slight increase in relative abundance of each compound.

2840 Environ. SCI. Technol.. Vol. 27. NO. 13. 1993

Flgure 4. Experimental coal tarlRbutyhminelwater ternary phase diagram. Error bars on solventlwater phase tie line end points are 30 random error estimates; the errors on the coal tar phase end polnts are insignificant. The shaded circle is the two-phase check point.

The primary conclusion to be drawn from this analysis is that there is not a large change in coal tar composition uponextractionwith n-butylaminelwater solutions. These studies indicate that theassumptionof coaltar partitioning as a single component in coal tarlsolventlwater systems is plausible.

Coal Tar/Solvent/Water Ternary LLE. For each of the three solvents, experimental results of coal tar/ solvent/water phase equilibria are presented in Table V as volume fractions of the three components. The data are presented as tie lines on ternary phase diagrams in Figures 4-6, for n-butylamine, acetone, and 2-propanol, respectively. Inaternarydiagram, theaxisforaparticular component is a line drawn from the apex (100% of that component) perpendicular to the opposite triangular face (0% of that component). The triangular space is divided intotwo regions representingmixturesthat are completely miscible and mixtures that separate into two phases. Tie lines within the immiscible region connect points that indicate the equilibrium compositions of the two immiscible phases resulting for any overall composition represented by a point on the tie line.

The error bars that are shown in Figures 4-6 are 30 (99 % confidence) random error estimates calculated from

acetone I

eo 60

FlgunS. Experlmntal coaitar/aCB1one/watBrternary phasediagram. Error bars on soiventlwater phase tie line end points are 30 random error estimates: the errors on the coal tar phase end point are insignificant. The shaded circle is the two-phase check point.

pro pa no^ fie 80

Figure 6. Experimental coal tarl2-propanoilwater ternary phase diagram. Error bars on soiventlwater phase tie line end points are 3c random enw estimates: the errors on the coal tar phase end points are lnsigniflcant. The shaded circle is the two-phase check point.

the standard deviations in Table V. Only the error bars for the solventlwater end points of each tie line are displayed since the error bars for the coal tar phase end points are insignificant. Since the relative errors for u t w and ucw are roughly constant a t 1-2 % , the size of the absolute error increases with the magnitude of the ucw or urww measurement value. Three-dimensional error dis- play on a ternary phase diagram must be viewed carefully to accurately visualize the size of the error space. The 3u error space can be approximated on the ternary phase diagram by a hexagon bounded by the ends of the three error bars as shown in Figure 7a. For points with large error bars in two dimensions but with a small error bar in the third dimension, the error space is more like a thick bar (Figure 7b). For example, the solvent/water endpoint of the bottom tie line on the n-butylamine ternary phase diagram has a small error in the ucw dimension. The resulting error space is a horizontal bar roughly parallel with the tie line. This suggests that the slope of the tie line has been precisely determined, and the uncertainty due to random error lies in the position of the end point along this line.

The representation of coal tar as a single component in a ternary system allowed the bulk dissolution behavior of coal tar to be indirectly observed by measuring the partitioning behavior of the other two components. Since the estimated error in uf": will always be greater than the larger of the error in u t w or .Ew. the measurement

(b) Figure 7. Three-dimensional error space as determined by error bars for vf, vis. and vf,

precision of ut": diminishes when it is much smaller than uc" or .rew. As a result, relative errors greater than 100% result for very small values of $7. In general, this method provides a means of determining LLE of coal tar/ solvent/water systems with measurements on the order of volume percents. Below ut": estimates of about 0.05, order of magnitude precision can be expected. Even with this level of precision, the data in Table V show that, even with small amounts of solvent, coal tar solubility is appreciable relative to its bulk solubility in water.

QualitativeTests for Heterogeneity. The plait point is the point on a ternary phase diagram where the tie line end points converge, and the two phases have identical composition. Qualitative tests performed on compositions near the plait point were done to gain additional information about the boundaries of the two-phase regions. The two-phasecheckpoints, shown asgray dots in Figures 4-6, were found to be the same for the acetone and 2-propanolsystems: 10% coal tar, 80% solvent, and 10% water. It is expected that there are compositions slightly above this point that are also heterogeneous, but visual inspection of these systems were not conclusive because of the small overall volume of coal tar. For n-butylamine, the region thought to contain the plait point was checked for heterogeneity, but determination of the two phases becameimpossiblewithsimplevisualtests. Apoint (shown in Figure 4) with 80% n-butylamine in the solvent/water solution, with overall composition of 30% coal tar, 56% n-butylamine, and 14% water, was found to be two phases. Although this point is not expected to be near the plait point, it provides further information about the upper boundary of the curve. Note that the uppermost tie line in Figure 4 is not consistent with this observation, since the solventlwater end point does not extend high enough to be in line with the two-phase observation. The error bars calculated based on random experimental error do not account for this discrepancy, suggesting a systematic error likely attributable to errors in solvent/water phase sampling, given the difficulty in distinguishing between the two phases.

Plait points exist for ternary systems that have only one partially miscible pair (type I ternary system). In this

Envlron. Sci. Technol.. VoI. 27. NO. 13. 1993 2841

case, the coal tadwater pair is known to be immiscible, all three solvent/water pairs are miscible, and the coal tar/ solvent pairs are assumed to be miscible based on visual observations that fail to identify two phases. For n-butylamine, an attempt was made to verify this assumption without reliance on visual observations (11). Coal tar/ n-butylamine mixtures were prepared with spikes of radiolabeled solvent. These tests were done in separatory funnels to facilitate sampling from the bottom of the vial, which would contain the coal tar phase if indeed two phases were present. The measured concentrations of radiolabeled solvent in samples from the top and the bottom of the vial and the overall concentration were found to be within experimental error of each other, suggesting single- phase systems. The conclusion can be drawn that coal tarln-butylamine is a completely miscible pair and that some water must be present to result in phase separation.

Solvent Effectiveness. The information in a ternary phase diagram provides a useful metric for assessment of the effectiveness of a solvent, and it provides necessary quantitative data for larger scale process modeling (8,9). The simplest piece of information from the ternary phase diagram is the vertical height of the two-phase region, delineated by the end points of the tie lines. The two- phase region for n-butylamine is smaller than for acetone and 2-propanol, indicating that less solvent is required to completely dissolve coal tar. For example, on the n-butylamine diagram, it is shown that if the overall composition of a mixture is 20 % coal tar, 56% solvent, and 24% water (i.e., 70% solvent-to-water ratio), then the mixture is completely miscible. Another indicator of the effectiveness of a solvent is the amount of coal tar dissolved in the solvent/water phase, as indicated by the position of the right-side tie line end points, i.e., the solvent/water rich region. The farther the end points from the solvent/ water edge, the greater the amount of coal tar dissolved in this phase. For example, with a mixture of 30% coal tar, 40% n-butylamine, and 30% water, the solvent/water phase will contain just under 20% coal tar. With asimilar mixture using 2-propanol, the amount of coal tar in the solvent/water phase is less than 5 % , and with acetone, coal tar dissolution is less than 1 % . The ternary phase diagram can also be used to quantify the extent of solvent dissolution into the coal tar phase, as is indicated by the slopes of the tie lines. A horizontal tie line, such as is approximated by the very lowest tie line in the acetone ternary phase diagram (Figure 5), depicts approximate equal partitioning of solvent between coal tar and water. The case of limited solvent dissolution in coal tar is depicted by a tie line steeply sloped down from right to left, such as is observed for n-butylamine with increasing solvent content. The amount of solvent in the coal tar phase determines the extent to which the physical properties of this phase are altered, i.e., the change in volume, density, viscosity, and surface-wetting properties. The amount of solvent that remains in the solvent/water phase determines the solvent-to-water ratio in this phase and, thus, affects the extent of coal tar dissolution.

Conclusions The challenge of characterizing the phase equilibria of

a complex mixture was met by representing coal tar as a pseudocomponent. Phase equilibria of coal tar/solvent/ water systems were experimentally determined and presented as tie lines on ternary phase diagrams. Of the three

solvents studied, n-butylamine was shown to have the smallest two-phase region and enhance the solubility of coal tar to the largest extent. While it is not directly apparent from the ternary phase diagrams, coal tar dissolution in solvent/water solutions containing acetone or 2-propanol is appreciable in terms of being orders of magnitude higher than the bulk coal tar solubility in water.

The coal tar/solvent/water phase equilibria provide the necessary equilibrium chemistry for predicting mass transfer limitations in porous media (8) and in solvent extraction process modeling (9). Furthermore, these data are used to determine parameters for a semi-empirical thermodynamic model describing ternary LLE of highly nonideal liquid mixtures (11,12).

Acknowledgments

Mr. James Villaume of Pennsylvania Power and Light, Mr. Curt Kramer of Atlantic Environmental Services, and Dr. David Nakles and Mr. Robert Weightman of Reme- diation Technologies Inc. assisted in enabling coal tar sample collection. Dr. Curt White and Ms. Louise Douglas of the U S . DOE Pittsburgh Energy Technology Center, Coal Science Division, and Dr. Edward C. Nelson and Dr. Ingeborg D. Bossert of Texaco Research Center, Beacon, NY, arranged for chromatographic analyses. Mr. Zhong- Bao Liu assisted with solute partitioning measurements. The authors thank Dr. David Dzombak and Dr. Babu Nott for their review of this manuscript. The Electric Power Research Institute was the primary sponsor for this research project through contract RP 3072-2. Dr. Babu Nott was the project manager. Additional fellowship support was provided by the Patricia Harris Government Opportunities Program.

Literature Cited

Mackay, D. M.; Cherry, J. A. Environ. Sei. Technol. 1989, 23,630-636. Harkins, S. M.; Truesdale, R. S.; Hill, R.; Hoffman, P.; Winters, S. US. Production of Manufactured Gases: Assessment of Past Disposal Practices; Research Triangle Institute: Research Triangle Park, NC, 1987. Management of Manufactured Gas Plant Sites; Gas

Research Institute: Chicago, IL, 1987; Vol I: Wastes and Chemicals of Interest. Moore, T. EPRI J. 1989, 14, 22-31. Luthy, R. G.; Dzombak, D. A.; Peters, C. A.; Roy, S. B.; Nakles, D. V.; Nott, B. Technological Directions for Remediation of Tar Contaminated Soils a t MGP Sites. To be submitted for publication. Salvensen, R. H. In Proceedings of the 5th National Conference on Management of Uncontrolled Hazardous Waste Sites; Hazardous Materials Control Research Institute: Silver Spring, MD, 1984; pp 1-15. Luthy, R. G.; Dzombak, D. A,; Peters, C. A.; Ali, M. A.; Roy, S. B. Solvent Extraction forRemediationof Manufactured Gas Plant Sites; Final report, EPRI TR-101845, Project 3072-2; Carnegie Mellon University: Pittsburgh, PA, 1992. Roy, S. B.; Dzombak, D. A.; Ali, M. A. Assessment of in situ solvent extraction for remediation of coal tar sites: Column studies. Submitted to Water Enuiron. Res. Ali, M. A.; Dzombak, D. A.; Roy, S. B. Assessment of in situ solvent extraction for remediation of coal tar sites: Process modeling. Submitted to Water Environ. Res. Orye, R. V.; Prausnitz, J. M. Ind. Eng. Chem. 1965, 57,

Peters, C. A. Ph.D. Dissertation, Carnegie Mellon University, Pittsburgh, PA, 1992.

18-26.

2842 Environ. Sci. Technol., Vol. 27, No. 13, 1993

(12) Peters, C. A.; Luthy, R. G. Coal tar dissolution in water- miscible solvents: Semiempirical thermodynamic modeling. To be submitted for publication.

(13) Edmister, W. C. Applied Hydrocarbon Thermodynamics, 2nd ed.; Gulf Publishing Co.: Houston, TX, 1988; Vol. 2.

(14) Lane, W. F.; Loehr, R. C. Environ. Sci. Technol. 1992,26, 983-990.

(15) Lee, L. S.; Rao, P. S. C.; Okuda, I. Environ. Sci. Technol. 1992,26,2110-2115.

(16) Mackay, D.; Shiu, W. Y.; Maijanen, A.; Feenstra, S. J. Contam. Hydrol. 1991,8, 23-42.

(17) Picel, K. C.; Stamoudis, V. C.; Simmons, M. S. Water Res. 1988,22, 1189-1199.

(18) Groher, D. M. Master's Thesis, Department of Civil Engineering, Massachusetts Institute of Technology, 1990.

(19) Rostad, C. E.; Pereira, W. E.; Hult, M. F. Chemosphere 1985, 14, 1023-1036.

(20) Prausnitz, J. M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria, 2nd ed.; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1986; Chapter 9.

(21) Banerjee, S. Environ. Sci. Technol. 1984, 18, 587-591. (22) Chiou, C. T.; Schmedding, D. W.; Manes, M. Environ. Sci.

Technol. 1982,16,4-10. (23) Mackay, D.; Shiu, W. Y.; Ma, K. C. Illustrated Handbook

of Physical-Chemical Properties and Environmental Fate for Organic Chemicals;Lewis Publishers: Boca Raton, FL, 1992; Vole. 1 and 2.

(24) Yalkowsky, S. H. Ind. Eng. Chem. Fundam. 1979,18,108- 111.

(25) Ferry, J. P.; Dougherty, P. J.; Moser, J. B.; Schuller, R. M. In Petroleum Hydrocarbons and Organic Chemicals in Ground Water-Prevention, Detection, and Restoration; NWWAIAPI Houston, TX, 1986, pp 722-733.

(26) Lafornara, J. P.; Nadeau, R. J.; Allen, H. L. In Proceedings of the 1982 Hazardous Material Spills Conference; Bur. Explo.: Washington, DC, 1982, pp 37-42.

(27) Unites, D. F.; Housman, J. J. In Proceedings of the 5th Annual Madison Waste Conference of Applied Research and Practice on Municipal and Industrial Waste; Uni- versity of Wisconsin: Madison, WI, 1982, pp 344-355.

(28) Villaume, J. F. Ground Water Monit. Rev. 1985,5, 60-74. (29) Fu, J. K.; Luthy, R. G. J. Environ. Eng. 1986,112,328-345. (30) Bonnar, R. U.; Dimbat, M.; Stross, F. H. Number-Average

Molecular Weights; Interscience Publishers: New York, 1958.

(31) Chemical andphysical Characteristicsof Tar Samples from Selected Manufactured Gas Plant (MGP) Sites; Final report for EPRI Contract RP 2879-12; META Environ- mental, Inc.: Watertown, MA, 1993.

(32) Rhodes, E. 0. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; John Wiley & Sons: New York, 1945; Vol. 11, Chapter 31.

(33) Rhodes, E. 0. In Bituminous Materials: Asphalts, Tars, and Pitches; Hoiberg, A. J., Ed.; Kreiger Publishing Co.: Huntington, NY, 1979; Vol. 111, Chapters 1 and 2.

(34) Morgan, J. J. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; John Wiley & Sons: New York, 1945; Vol. 11, Chapter 37.

(35) Campbell, J. R.; Luthy, R. G.; Carrondo, M. J. T. Environ. Sci. Technol. 1983, 17, 582-590.

(36) Sorensen, J. M.; Arlt, W. Liquid-Liquid Equilibrium Data Collection; DECHEMA: Frankfort, Germany, 1979; Vol. 5, Part 1.

(37) Morris, K. R.; Abramowitz, R.; Pinal, R.; Davis, P.; Yalkowsky, S. H. Chemosphere 1988, 17, 285-298.

Received for review March 18, 1993. Revised manuscript received July 23, 1993. Accepted July 28, 1993.'

@ Abstract published in Advance ACS Abstracts, October 1,1993.


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