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Dissolved PCB congener distribution in generator column solutions
PII: S0043-1354(97)00391-6
Wat. Res. Vol. 32, No. 5, pp. 1373±1382, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
DISSOLVED PCB CONGENER DISTRIBUTION IN GENERATOR COLUMN SOLUTIONS M UPAL GHOSH1, A. SCOTT WEBER1** , JAMES N. JENSEN1 and JOHN R. SMITH2
Department of Civil Engineering, State University of New York at Bualo, Bualo, NY 14260, U.S.A. and 2Environmental Technology Center, Alcoa Technical Center, Alcoa Center, PA 15069, U.S.A.
1
(First received April 1997; accepted in revised form September 1997) AbstractÐA generator column technique previously used to study the solubility of sparingly soluble compounds, has been employed in this study to prepare high throughput aqueous PCB solutions for use in treatability studies. In the present study, Aroclor 1242 mixture was used to load the generator column. The total PCB concentration and the PCB congener distribution pattern in the euent changed with time, with lower chlorinated congeners eluting faster than the higher chlorinated ones. Collection of over 300 bed volumes of euent from two such generator columns provided PCB solutions which showed 75±90% similarity to the original Aroclor 1242 congener distribution. In this paper, the observed distribution of PCB congeners in the generator column euent with time is presented and discussed. A reasonable prediction of euent PCB concentration from the generator column was possible using Raoult's Law for ideal solutions. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐpolychlorinated biphenyls, PCB, generator column, aqueous PCB, PCB congener distribution, Aroclor 1242, PCB aqueous solubility
INTRODUCTION
PCB aqueous solutions are needed for conducting experimental investigations to identify physical and chemical properties of PCBs, their toxicological eects, and for conducting various aqueous phase destruction studies. To generate these solutions, two primary methods have been employed historically. First, co-solvents (typically acetone) have been used to facilitate the dissolution of PCBs in water (Bedard et al., 1986; Gschwend and Wu, 1985; Potter and Pawliszyn, 1994). However, the presence of a co-solvent in the aqueous solution can alter the physical/chemical/biological properties of the matrix being studied which often detracts from the utility of this method. For example, Valentine (1996) found that the linear sorption coecient of hydrophobic organic chemicals on clays decreased log± linearly with increasing amounts of organic cosolvent in water. Dissipation of the solvent, typically by evaporation, may result in phase partitioning of PCB congeners if they are present in excess of their aqueous solubilities. In studies that require true PCB aqueous solutions, the presence of PCB micro-globules may cause problems. To circumvent these concerns, alternate PCB dissolution methods have been employed. Long term equilibrium of water with neat PCBs has been used *Author to whom all correspondence should be addressed [Tel: 1-716-6452114 ext. 2331, Fax: 1-716-6453667].
by Luthy et al. (1997), Clark et al. (1979), and Lee et al. (1979). Common to the long term dissolution methods is the careful placement of neat PCBs in water and their slow dissolution. These methods have been used successfully to produce PCB aqueous phase solutions and have been useful for determining aqueous PCB congener solubility, but typically require several months for equilibrium to be achieved. Another method for creating aqueous solutions of low solubility species is to slowly dissolve them from a stationary support into a continuously ¯owing water. This is called the generator column method. The generator column is an accepted method for the estimation of octanol± water partition coecients and water solubility of sparingly soluble organic compounds (Federal Register, 1985). May et al. (1978) used a generator column technique to determine the aqueous solubility of polynuclear aromatic hydrocarbons (PAHs). Several other researchers have used this method to measure individual PCB congener solubilities (Billington et al., 1988; Miller et al., 1984; Dickhut et al., 1986; Dunnivant and Elzerman, 1988; Stolzenburg and Andren, 1983). In these studies, individual PCB congeners were loaded on the generator column and low ¯ow-rates were maintained to obtain equilibrium aqueous concentration in the euent of the generator column. Other researchers have used the generator column with PCB congener mixtures (Sokol et al., 1992; Brunner
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et al., 1990). However, in these studies, the change in congener distribution of aqueous PCB solution generated with time were not studied. The generator column method also has been used to study the dissolution of NAPL embedded on solid media by Mackay et al. (1991). It is expected that the eective solubility of individual PCB congeners in an Aroclor mixture would be dierent from the pure congener solubility. Therefore, the congener distribution in an aqueous solution obtained from a PCB Aroclor mixture is expected to be very dierent from the distribution in the Aroclor itself. This has been demonstrated in batch equilibrium studies by Lee et al. (1979) and Luthy et al. (1997) and in a generator column by Sokol et al. (1992). The lower chlorinated PCB congeners were found to dissolve to a greater extent than the higher chlorinated congeners in these studies. In their study, Sokol et al. (1992) used a mixture of equal amounts of Aroclors 1221, 1016, 1254, and 1260 in an attempt to obtain a wider distribution of PCB congeners in solution. Despite their eorts, about half of the total PCB in solution was contributed by the monochlorinated biphenyls. The predominance of the monochlorinated biphenyl in the solution may have been because only the ®rst few hundred milliliters of generator column euent were collected. In studies involving PCB treatment, adequate concentrations of the higher chlorinated PCB congeners and large volumes are required. One way to obtain larger volumes of aqueous PCB solutions is to allow the generator column to operate for a long period of time. With the depletion of the lower chlorinated congeners, the higher chlorinated congeners may start dissolving out. The primary goal of the present work was to develop a method of generating large volumes of aqueous PCB solutions having a prescribed distribution of PCB congeners using a generator column and quantify changes in the euent PCB congener distribution over time. A secondary goal was to explain the observations in terms of organic dissolution theory and provide a framework for designing generator columns for obtaining aqueous solutions having a speci®ed distribution of PCB congeners. In this paper, the modi®ed method, aqueous phase PCB congener concentrations achieved, and the temporal changes in congener distribution are described, along with a theoretical explanation of the results. MATERIALS AND METHODS
Chemicals Water used for aqueous PCB solution generation was produced by a Nanopure1 system (Barnstead Company Division of Sybron Corporation, Boston MA). Aroclor 1242 was the source of all PCBs in this study and was obtained in neat form from Ultra Scienti®c (North Kingstown, RI). Aroclor 1242 standards and individual congener standards were procured as hexane solutions
from Ultra Scienti®c. Pesticide grade hexane was used for all extractions. Uniform spherical glass beads, approximately 0.2 mm diameter, were employed as the packing media for the glass bead generator column. Generator column To prepare the glass beads for use in the generator column, 200 g of beads were washed with hexane three times and then dried overnight at 1058C. Fifty milligrams of neat Aroclor 1242, dissolved in 30 ml of hexane, were added slowly to a 250 ml glass beaker containing the prepared glass beads. The beads were mixed manually with a small glass rod until uniform wetting of the beads with the hexane/PCB solution was observed. The beaker then was covered with a glass dish and allowed to sit overnight in a fume hood at room temperature to evaporate the hexane. After 12 h in the hood, the beads were stirred again to expose any unevaporated hexane and placed in the hood for an additional six hours. The PCB coating concentration (0.025% w/w) was lower than the 0.1% (w/w) of solute used by Sokol et al. (1992), and was thought to be low enough to prevent the entrainment of PCB particles during solution generation. The column employed for these studies was approximately 200 mm in length and had an ID of 25 mm. A glass wool plug was positioned in the bottom of the column to support the beads. On top of the glass wool, a 10 mm deep layer of clean glass beads (2 mm diameter) was placed to enhance ¯ow distribution in the column. PCB-laden glass beads (0.2 mm diameter) then were added to the column from the top. A 10 mm layer of clean beads (0.2 mm diameter) was added on top of the PCB laden beads. The composite layers of clean and PCB-laden glass beads occupied the bottom three quarters of the column height to allow for a settling zone in the upper part of the column should any glass beads be entrained in the water ¯ow. Three such generator columns could be prepared from the 200 g of coated glass beads. A schematic of the experimental setup is shown in Fig. 1. Column operation and sampling After placement of the glass beads into the glass columns was completed, Nanopure1 water was introduced slowly from the bottom by a positive displacement pump via Te¯on tubing while shaking and tapping the column occasionally to prevent the formation of any air pockets in the column. Euent was discharged from the glass bead generator column by Te¯on tubing and collected in amber glass bottles or clear glass carboys wrapped in aluminum foil. The columns were operated at room temperature which varied between 24 and 278C. The ®rst liter of solution generated by the column (~15 bed volumes) was discarded to minimize the potential of capturing ®ne glass beads or entrained non-aqueous phase PCBs that might elute quickly from the column. Dunnivant and Elzerman (1988) also discarded the ®rst 500 ml of the solution generated by their generator column. To minimize PCB loss by volatilization, the PCB solution collection vessel was closed with a Te¯on-lined stopper with a small vent to allow the air to leave as the vessel gradually ®lled up. Temporal euent samples were collected by switching the euent directly to a 100 ml volumetric ¯ask containing 3 ml of hexane. The hexane in the volumetric ¯ask covered the top surface of the PCB solution to minimize any volatilization during sample collection. Two generator columns were prepared and operated for performance analysis. The ¯ow rate through each column was identical and equaled 0.7 ml/min resulting in a super®cial ¯ow velocity of 0.14 ml/cm2 min and residence time of approximately 40 min. May et al. (1978) used ¯owrates between 0.1 and 5 ml/min with a residence time of 6.8±352 min and found that within this range, the solubility of PAHs in a
PCB congener distribution in a generator column
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Fig. 1. Schematic of generator column. generator column were not aected by ¯owrate. In Column 1, 20 liters of euent was collected and composite sampled for PCBs. During operation of Column 2, discrete column euent samples (100 ml each) were collected every day (approximately every 15 bed volumes) to determine the temporal congener production patterns. A ®nal composite sample for Column 2 also was taken to allow comparison with the operation of Column 1. Sample extraction, concentration, and GC analysis In this study, 100 ml of column euent was extracted sequentially with 3 ml of hexane three times. The extraction was performed in the 100 ml volumetric ¯ask in which the sample was collected. After extraction, the total hexane volume was adjusted to 10 ml. Hexachlorocyclohexane was used as an internal standard and added to the ®nal hexane extract. PCB congener speci®c analysis was performed using a slightly modi®ed version of EPA Method 608. A Hewlett Packard gas chromatograph (5890, Series II) with a fused silica capillary column (60m 0.25 mm inner diameter, DB-5, J&W Scienti®c) and an electron capture detector was used for analysis. The gas chromatograph was operated with hydrogen as the carrier gas, and a 95% argon/5% methane mixture as the make-up gas. The column temperature was varied using the following schedule: initial
temperature of 908C for 2 min, 108C/min for 6 min (until it reached 1508C), then 38C/min to the ®nal temperature of 2908C. The injection port and detector temperatures were maintained at 210 and 3008C, respectively. Hewlett Packard Chemstation software was used for data acquisition, integration, calibration and analysis. A multi-level calibration table was prepared using PCB Aroclor 1242 standard solution in hexane obtained from Ultra Scienti®c. The calibration was used to identify and report the concentration of 50 signi®cant PCB peaks of Aroclor 1242. Congener names corresponding to the 50 PCB peaks identi®ed are presented in Table 1 along with percent mass fraction of each peak based on characterization of Aroclor 1242 by Schulz et al. (1989).
RESULTS AND DISCUSSION
Fate of PCBS added to generator column It was expected that PCB loss was likely during the hexane evaporation phase of glass bead preparation through volatilization and sorption on the preparation beaker. To quantify this potential loss, the mass of PCBs on a batch of glass beads pre-
Fig. 2. Mass balance of PCBs on generator column.
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pared for column use was compared to the mass of PCBs added. As expected, loss of PCB was observed during preparation as shown in Fig. 2. On a percentage basis, the loss of each homolog group appeared to be uniform at approximately 15%. The losses during drying could be caused by volatilization (which is expected to be higher for lower chlorinated congeners) and adsorption on the glass vessel (which is expected to be higher for the higher chlorinated congeners). While PCB loss was not desired, uniform loss across the homolog group indicated that the relative congener composition was not altered signi®cantly. The total amount of PCBs eluted from the column in solution was estimated and found to vary by chlorination level as shown in Fig. 2. About 80% of the monochloro biphenyl was recovered in aqueous solution compared to 38% of the hexachloro biphenyl. Correspondingly, the measured amount of PCBs remaining on the glass beads after solution generation showed higher residual of the higher chlorinated congeners compared to the lower chlorinated ones. As shown in Fig. 2, about 80±90% of the total PCBs in each homolog group added to the generator column could be accounted for by losses
during preparation, dissolution, and residual on the glass beads. The remaining 10±20% categorized as other losses were attributed to that lost in the ®rst 1 liter of solution that was discarded, volatilization during solution collection, and adsorption to various parts of the generator apparatus. Composite PCB solutions PCB congener concentrations and pro®les for composite euent samples taken after 20 liters had been collected (approximately 300 bed volumes) from Columns 1 and 2 are presented in Fig. 3a and 3b. In each graph, individual PCB congeners measured are presented as peak numbers (the corresponding congener names are presented in Table 1). Total PCB concentrations in the 20 liter euent samples for Columns 1 and 2 were similar and equaled 243 and 219 mg/l, respectively. It is important to note that eleven temporal discrete 100 ml samples were taken of Column 2 euent during its operation and are thus not represented in the reported composite sample. When the PCB concentrations of these discrete samples are incorporated into the composite euent for Column 2 (resulting in a total composite volume of 21.1 liters), the cal-
Fig. 3. (a) PCB congener concentration distribution in composite sample of column 1. (b) PCB congener concentration distribution in composite sample of column 2.
PCB congener distribution in a generator column Table 1. Peak numbers, names of congeners and percentage in Aroclor 1242 (Schulz et al., 1989) Pk # 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
Congener structure
% in 1242 w/w
2 26 22' 24 2,5 23' 24' 2,3 22'6 Hexachloro Cyclo Hexane (ISTD) 22'5 22'4 44' 236 23'6 22'3 24'6 245 23'5 23'4 24'5 244' 233'2'34 22'56' 22'46' 234' 22'36 22'36' 22'55' 22'45' 22'44' 22'45 244'6 22'35' 344' 233'6 22'34' 22'34 234'6 22'33' 22'44'6 23'45 234'5 244'5 23'4'5 23'44' 22'35'6 22'34'6 2344' 233'4' 22'33'6 22'34'5 22'455' 22'44'5 22'33'5 22'3'45 22'345' 2344'6 22'344' 22'33'66' 33'44' 233'4'6 22'33'4 22'355'6 22'33'56' 233'4'5 2'344'5 22'34'5'6 23'44'5 22'33'46' 22'44'55' 233'44' 233'456 22'344'5' 233'44'6 22'33'55'66' 22'33'44'6 233'44'5 22'344'55' 22'33'44'5 233'44'56
Ð 3.21 1.14 1.38 7.71 0.53 10.31 0.50 2.89 0.10 1.33 0.79 11.11 5.72 3.64 1.16 0.49 4.04 3.60 1.87 3.20 1.44 3.50 0.89 0.41 0.23 2.17 3.89 4.53 0.17 2.93 0.72 1.65 0.86 0.12 0.65 0.77 0.53 0.07 1.98 0.44 0.08 0.07 2.25 1.84 0.54 0.14 0.06 0.11
culated concentration for the Column 2 composite euent becomes 226 mg/l. Accordingly, there was good agreement between the composite samples with respect to total PCB mass. The concentration of total PCB in the 20 liter composite euents of the two generator columns operated in this study were within the range of total Aroclor 1242 solubility values reported in the literature. The solubility value of total Aroclor 1242 presented in the literature vary widely between 85 to 340 mg/l as reported by Luthy et al. (1997). On a percentage basis, composite samples from both columns contained more of mono, di and trichloro biphenyls and less of the higher chlorinated congeners than does Aroclor 1242. To compare the congener distribution between the two composite euents, a similarity coecient was calculated as in Sokol et al. (1992).
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The similarity coecient between the two composite euents was 80% indicating high reproducibility. Comparing the congener distribution in the two euents to the congener distribution in Aroclor 1242, it was found that Column 1 and 2 euents matched 90 and 75% respectively to 1242 which is signi®cantly higher than the 33% similarity achieved in solutions prepared by Sokol et al. (1992). The two primary reasons for the greater similarity to the original Aroclor obtained in this study were low PCB loading on the generator column and the longer period of composite sample collection (>300 bed volumes). The ratio of mass of PCB added to a generator column to the total volume of solution generated appears to aect the closeness of the congener distribution of composite solution to the original Aroclor. As the total volume of water contacted approaches in®nity, all PCBs will be transferred to water and the congener distribution in solution will approach that in the original Aroclor. However, operation of a generator column for too long may yield very dilute solutions which may not be suitable for the intended studies. Temporal congener distribution pattern To investigate the pattern of congener elution as a function of solution volume produced, discrete euent samples were collected during the operation of Column 2 as reported earlier. PCB congener concentrations for six discrete Column 2 samples, out of the eleven collected during its operation, are shown in Fig. 4A±F. The total PCB concentration in the euent shows a general decreasing trend with bed volumes of solution generated, with about 400 mg/l in the ®rst sample and about 200 mg/l in the last sample. As shown in Fig. 4, two trends were observed in the temporal PCB data. First, the lower chlorinated congeners (mono, di, and tri) were found to dissolve faster and eluted in higher concentrations in the beginning, and lower concentrations near the end of column operation. Trichlorobiphenyl concentrations appeared steady at the beginning but decreased in the later discrete samples. Second, concentrations of higher chlorinated biphenyls appeared to increase as the concentrations of the lower chlorinated congeners decreased with higher bed volumes of operation, except for the 315 bed volume sample. A similar change in euent solution composition from a generator column has been reported for hydrocarbon mixtures and petroleum products by Mackay et al. (1991) and Shiu et al. (1988). In these studies, the more soluble components in the mixture were found to elute from the generator column during the initial phases of operation and the less soluble components followed later. For example, Mackay et al. (1991) observed that when benzene, toluene, and p-xylene were loaded on a generator column during the initial phases of the column operation, the most soluble compound benzene eluted
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Fig. 4. PCB congener distribution in generator Column 2 euent with bed volumes.
at the highest concentration and decreased with time of operation. The concentration of the least water soluble compound (p-xylene) in the euent was low in the beginning and increased with time as benzene was depleted from the column.
It was hypothesized that the decrease in production of the lower chlorinated biphenyls observed with time in Column 2 was caused by a decrease in the available pool of these congeners in the glass beads. As shown in Fig. 2 earlier, nearly half of the
PCB congener distribution in a generator column
higher chlorinated congeners (4±7 chlorines) remained on the glass beads after 315 bed volumes of solution generation, compared to none of the monochlorobiphenyl, only 5% of the dichloro biphenyl, and 29% of the trichloro biphenyl. Accordingly, an exhaustion of PCBs in the column would appear to be a reasonable explanation for the reduction of more soluble PCBs observed in the later discrete samples taken during Column 2 operation. Application of Raoult's law The second trend observed in the discrete samples from Column 2 was that the concentration of higher chlorinated homolog groups appeared to increase with bed volumes as shown earlier in Fig. 4. According to Raoult's Law, the solubility of a compound present in an ideal mixture is equal to the solubility of the pure compound multiplied by its mole fraction in the mixture. In this case, the mole fractions of the lower chlorinated congeners in the PCB phase decreased with time as they dissolved faster, and the mole fractions of the higher chlorinated congeners in the PCB increased with time as they dissolved at a slower rate. As seen in Fig. 2, the percentage of PCBs remaining on glass beads after solution generation was higher for the higher chlorinated homolog groups. Therefore, the apparent solubility of mono, di, and trichloro biphenyls are expected to decrease with solution generation and the apparent solubility of the higher chlorinated congeners are expected to increase. The aqueous solubility depression for hydrophobic organic chemicals in the presence of other organic chemicals in water has been studied by Coyle et al. (1997). They found that the solubility of a tetrachloro and heptachloro biphenyl was reduced to about 15% in the presence of other organic compounds in water such as methylene chloride and chloroform. They also found that the extent of solubility depression increased with the hydrophobicity of the organic compound in solution. Two possible theoretical explanations for this phenomena have been proposed by Coyle et al. (1997). The ®rst explanation is related to changes in the mole fraction of the organic compounds in the solute phase. The second alternate explanation is solventing out (similar to the salting out phenomenon) in which the dissolved organic compounds may occupy a signi®cant portion of the water molecules, rendering them unavailable for further dissolution of other hydrophobic organic compounds. The solventing-out phenomenon probably is applicable for a system where the concentration of the organic compounds in solution is high, as in the system studied by Coyle et al. (1997) (10±20 g/l of methylene chloride). In the present study, the total PCB concentration was in the range of 200± 400 mg/l. Therefore, changes in mole fractions of the dissolving solutes is likely to be a more appro-
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priate explanation of the observed behavior of changing distribution of aqueous phase PCB congener concentrations. To test the applicability of Raoult's Law in explaining the changing aqueous concentration of PCBs in the generator column euent, a simulation was performed on a homolog group basis. Aqueous solubility values for PCB congeners reported in the literature are highly variable (Opperhuizen et al., 1988; Dunnivant and Elzerman, 1988; Miller et al., 1984), typically spanning one order of magnitude. Therefore, solubility values of PCB congeners within a homolog group reported by Opperhuizen et al. (1988) were averaged to arrive at a representative solubility of the homolog group. These average solubilities of the seven homolog groups are presented in Table 2. The depletion of a PCB homolog group on the glass bead column was modeled as an outcome of dissolution of that homolog group into solution over time as shown in equation (1). The eective solubility of the PCB homolog group was obtained by multiplying the average solubility from Table 2 by the mole fraction of that homolog in the PCB phase as shown in equation (2). The activity coecient of each homolog group was assumed to be unity, as in Mackay et al. (1991). The mole fraction of each PCB homolog group at any time was calculated using equation (3). The measured initial mole fraction of each homolog group and total initial mass of PCBs in the generator column were used as initial conditions. The same ¯ow rate of water was used for the simulation as in the experimental column. In the simulation, the aqueous concentration in the euent of the generator column was assumed to be in equilibrium with the PCB phase on the glass beads at all times. It should be noted that the model in equations (1)±(3) are truly predictive with no adjustable parameters. 1 t QCi dt 1 qi qi0 ÿ m 0 Ci C *i Xi gi
2
qi =Mi Xi X n qi =Mi
3
i1
where: qi=concentration of PCB homolog group i Table 2. Average solubility of PCB homolog groups (calculated from Opperhuizen et al., 1988) Homolog group Monochlorobiphenyl Dichlorobiphenyl Trichlorobiphenyl Tetrachlorobiphenyl Pentachlorobiphenyl Hexachlorobiphenyl Heptachlorobiphenyl
Average aqueous solubility (mg/l) 3000 1100 500 70 20 4 2.4
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Fig. 5. (a) Prediction of mono, di and tri chlorobiphenyl dissolution from generator column. (b) Prediction of tetra, penta and hexa chlorobiphenyl dissolution from generator column.
on the glass beads, qi0=initial concentration of PCB homolog group i on the glass beads, Ci=concentration of homolog group i in the aqueous phase, C*i =aqueous solubility of homolog group i, m = mass of glass beads, Xi=mole fraction of homolog group i in the PCB phase, gi=activity coecient of homolog group i in the PCB phase (assumed = 1), Q = ¯ow rate of water through the generator column, n = number of homolog groups, Mi=molecular weight of PCB homolog group i. The simulated and measured euent concentrations are plotted against bed volumes generated for mono through hexachloro biphenyl in Fig. 5a±b. As in the experimental data, the predicted mono, di, and trichloro biphenyl concentrations decreased with
time and that of the higher chlorinated congeners increased with time. Predictions of mono, di, and trichloro biphenyls match well with the experimental data. An average percentage dierence between the data and the model was estimated for each homolog group using the following equation: n C ÿ C m 100 X ij ij average percentage difference Cij n j1 4 where: n = number of observed data points and Cm i =model prediction of PCB homolog concentration. The average percentage dierence for mono, di and tri chlorobiphenyls were 23, 22 and 13% respectively. Predictions of tetra and pentachloro
PCB congener distribution in a generator column
biphenyl are reasonable in trends but smaller in magnitude than the experimental data. The prediction of hexachloro biphenyl is much smaller that the observed values. The average percentage dierence for tetra, penta and hexa chlorobiphenyls were 52, 71 and 95% respectively. Deviations between observed and predicted performance are likely related to the uncertainty in the reported values of PCB congener solubility obtained from the literature and simplifying assumptions made in the model. These assumptions include: (1) a homogeneous distribution of PCBs throughout the column; and (2) the eective solubility of each homolog group is a function of the average mole fraction of that homolog group. The homogeneous distribution assumption may not be valid, since rapid depletion of more soluble PCBs in the initial portion of the column could result in higher localized mole fractions of higher chlorinated homolog groups than would be calculated assuming homogeneous distribution. Accordingly, it is not unreasonable that concentrations of pentachloro and hexachloro biphenyls in the column euent were higher than the model predictions. The calculated and observed concentration of pentachloro and hexachloro biphenyls are within the reported solubility limits (4±23 mg/l for pentachloro and 1±10 mg/l for hexachloro biphenyls; Opperhuizen et al., 1988). The total PCB concentration in the euent was slightly underpredicted by Raoult's Law, but the trends matched very well. The observed total PCB values in the euent decreased from 418 mg/l to 203 mg/l whereas the predicted values decreased from 400 mg/l to 160 mg/l. Therefore, it does appear possible to predict with some accuracy the distribution of PCB congeners in the generator column euent with time. Accordingly, if aqueous solutions of speci®c homolog distribution were desired for experimental studies, their generation appears possible. The concentration of any homolog group in the euent of the generator column at any time is given by equation (2). The cumulative concentration of euent collected from the generator column over a period of time t can be expressed as: t QCi dt Ci 0 5 Qt The cumulative concentration which can be estimated by solving equations (1)±(3) and (5) simultaneously is a function of two variables: initial PCB homolog concentration on the glass beads (qi0), and duration of solution generated (time t or bed volumes). Therefore the choice of these two variables can be determined by performing the simulation and ®nding the ones that result in the closest match to the required homolog distribution.
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CONCLUSIONS
Speci®c conclusions from this work are: . The generator column method is capable of producing reproducible concentrations of soluble PCBs under the same set of experimental conditions. Operation of two similar generator columns yielded PCB solutions with a total PCB concentration of 243 and 226 mg/l after about 300 bed volumes. Similarity analysis con®rmed the reproducibility of congener concentrations. . The generator column method is capable of producing equivalent volumes of PCB solution much faster than the quiescent dissolution method. Twenty liters of PCB solution was obtained in twenty days compared to 4±6 months typically required by quiescent dissolution method. . PCB congener concentrations in the generator column euent change with solution volume generated. Initial volumes are richer in lower chlorinated PCBs, while later volumes are richer in higher chlorinated PCBs. Accumulation of column euent for a large number of bed volumes can provide an aqueous solution that approaches the congener distribution of the parent Aroclor. . Time dependent changes in PCB homolog distribution in the euent of a generator column can be explained and predicted using Raoult's Law. . It is possible to design an experimental protocol to produce aqueous solutions of a desired PCB homolog distribution. The two important operational variables that determine the PCB composition of solution generated in a generator column are: starting Aroclor type and the ratio of PCB mass added to the total volume of water eluted. REFERENCES
Bedard D. L., Unterman R., Bopp L. H., Brennan M. J., Haberl M. L. and Johnson C. (1986) Rapid assay for screening and characterizing microorganisms for the ability to degrade polychlorinated biphenyls. Appl. Environ. Microbiol. 51, 761±768. Billington J. W., Huang G. -L., Szeto F., Shiu W. Y. and Mackay D. (1988) Preparation of aqueous solutions of sparingly soluble organic substances: I Single component systems . Environ. Toxicol. Chem. 7, 117±124. Brunner S., Hornung E., Santi H., Wol E. and Piringer O. G. (1990) Henry's law constants for polychlorinated biphenyls: experimental determination and structure± property relationships. Environ. Sci. Technol. 24, 1751± 1754. Clark R. R., Chian E. S.K. and Grin R. A. (1979) Degradation of polychlorinated biphenyls by mixed microbial cultures. Appl. Environ. Microbiol. 36, 680±685. Coyle G. T., Harmon T. C. and Suet I. H. (1997) Aqueous solubility depression for hydrophobic organic chemicals in the presence of partially miscible organic solvents. Environ. Sci. Technol. 31, 334±389. Dickhut R. M., Andren A. W. and Armstrong D. E. (1986) Aqueous solubilities of six polychlorinated biphenyl congeners at four temperatures. Environ. Sci. Technol. 20, 807±810. Dunnivant F. M. and Elzerman A. W. (1988) Aqueous solubility and Henry's law constant data for PCB con-
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geners for evaluation of quantitative structure±property relationships (QSPRs). Chemosphere 17, 525±541. Federal Register (1985) Vol. 50, No. 188, September 27. Gschwend P. H. and Wu S. (1985) On the constancy of sediment±water partition coecients of hydrophobic organic pollutants. Environ. Sci. Technol. 19, 90±96. Lee M. C., Chian E. S.K. and Grin R. A. (1979) Solubility of polychlorinated biphenyls and capacitor ¯uid in water. Wat. Res. 13, 1249±1258. Luthy R. G., Dzombak D. A., Shannon M. J.R., Unterman R. and Smith J. R. (1997) Dissolution of PCB congeners from an aroclor and an aroclor/hydraulic oil mixture. Wat. Res. 31, 561±573. Mackay D., Shiu W. Y., Maijanen A. and Feenstra S. (1991) Dissolution of non-aqueous phase liquids in groundwater. J. Contaminant Hydrol. 8, 23±42. May W. E., Wasik S. P. and Freeman D. H. (1978) Determination of the aqueous solubility of polynuclear aromatic hydrocarbons by a coupled column liquid chromatographic technique. Anal. Chem. 50, 175±179. Miller M. M., Ghodbane S., Wasik S. P., Tewari Y. B. and Martire D. E. (1984) Aqueous solubilities, octanol/ water partition coecients, and entropies of melting of chlorinated benzenes and biphenyls. J. Chem. Eng. Data 29, 184±190. Opperhuizen A., Gobas F. A.P. C., Van den Steen J. M.D. and Hutzinger O. (1988) Aqueous solubility of
polychlorinated biphenyl related to molecular structure. Environ. Sci. Technol. 22, 638±646. Potter D. W. and Pawliszyn J. (1994) Rapid determination of polyaromatic hydrocarbons and polychlorinated biphenyls in water using solid-phase micro-extraction and GC/MS. Environ. Sci. Technol. 28, 298±305. Schulz D. E., Petrick G. and Duinker J. C. (1989) Complete characterization of polychlorinated biphenyl congeners in commercial aroclor and cyclophen mixtures by multidimentional gas chromatography Ð electron capture detection. Environ. Sci. Technol. 23, 852± 859. Shiu W. Y., Maijanen A., Ng A. L.Y. and Mackay D. (1988) Preparation of aqueous solutions of sparingly soluble organic substances L II. Multicomponent systems Ð hydrocarbon mixtures and petroleum products. Environ. Toxicol. Chem. 7, 125±137. Sokol R. C., Bush R., Wood L. W. and Jahan-Parwar B. (1992) Production of aqueous PCB solutions for environmental toxicology. Chemosphere 24, 483±495. Stolzenburg T. R. and Andren A. W. (1983) Determination of the aqueous solubility of 4-chlorobiphenyl. Anal. Chem. Acta 151, 271±274. Valentine A. N. (1996) Organic cosolvent eects on sorption equilibrium of hydrophobic organic chemicals by organoclays. Environ. Sci. Technol. 30, 89±96.
Wat. Res. Vol. 32, No. 5, pp. 1373±1382, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
DISSOLVED PCB CONGENER DISTRIBUTION IN GENERATOR COLUMN SOLUTIONS M UPAL GHOSH1, A. SCOTT WEBER1** , JAMES N. JENSEN1 and JOHN R. SMITH2
Department of Civil Engineering, State University of New York at Bualo, Bualo, NY 14260, U.S.A. and 2Environmental Technology Center, Alcoa Technical Center, Alcoa Center, PA 15069, U.S.A.
1
(First received April 1997; accepted in revised form September 1997) AbstractÐA generator column technique previously used to study the solubility of sparingly soluble compounds, has been employed in this study to prepare high throughput aqueous PCB solutions for use in treatability studies. In the present study, Aroclor 1242 mixture was used to load the generator column. The total PCB concentration and the PCB congener distribution pattern in the euent changed with time, with lower chlorinated congeners eluting faster than the higher chlorinated ones. Collection of over 300 bed volumes of euent from two such generator columns provided PCB solutions which showed 75±90% similarity to the original Aroclor 1242 congener distribution. In this paper, the observed distribution of PCB congeners in the generator column euent with time is presented and discussed. A reasonable prediction of euent PCB concentration from the generator column was possible using Raoult's Law for ideal solutions. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐpolychlorinated biphenyls, PCB, generator column, aqueous PCB, PCB congener distribution, Aroclor 1242, PCB aqueous solubility
INTRODUCTION
PCB aqueous solutions are needed for conducting experimental investigations to identify physical and chemical properties of PCBs, their toxicological eects, and for conducting various aqueous phase destruction studies. To generate these solutions, two primary methods have been employed historically. First, co-solvents (typically acetone) have been used to facilitate the dissolution of PCBs in water (Bedard et al., 1986; Gschwend and Wu, 1985; Potter and Pawliszyn, 1994). However, the presence of a co-solvent in the aqueous solution can alter the physical/chemical/biological properties of the matrix being studied which often detracts from the utility of this method. For example, Valentine (1996) found that the linear sorption coecient of hydrophobic organic chemicals on clays decreased log± linearly with increasing amounts of organic cosolvent in water. Dissipation of the solvent, typically by evaporation, may result in phase partitioning of PCB congeners if they are present in excess of their aqueous solubilities. In studies that require true PCB aqueous solutions, the presence of PCB micro-globules may cause problems. To circumvent these concerns, alternate PCB dissolution methods have been employed. Long term equilibrium of water with neat PCBs has been used *Author to whom all correspondence should be addressed [Tel: 1-716-6452114 ext. 2331, Fax: 1-716-6453667].
by Luthy et al. (1997), Clark et al. (1979), and Lee et al. (1979). Common to the long term dissolution methods is the careful placement of neat PCBs in water and their slow dissolution. These methods have been used successfully to produce PCB aqueous phase solutions and have been useful for determining aqueous PCB congener solubility, but typically require several months for equilibrium to be achieved. Another method for creating aqueous solutions of low solubility species is to slowly dissolve them from a stationary support into a continuously ¯owing water. This is called the generator column method. The generator column is an accepted method for the estimation of octanol± water partition coecients and water solubility of sparingly soluble organic compounds (Federal Register, 1985). May et al. (1978) used a generator column technique to determine the aqueous solubility of polynuclear aromatic hydrocarbons (PAHs). Several other researchers have used this method to measure individual PCB congener solubilities (Billington et al., 1988; Miller et al., 1984; Dickhut et al., 1986; Dunnivant and Elzerman, 1988; Stolzenburg and Andren, 1983). In these studies, individual PCB congeners were loaded on the generator column and low ¯ow-rates were maintained to obtain equilibrium aqueous concentration in the euent of the generator column. Other researchers have used the generator column with PCB congener mixtures (Sokol et al., 1992; Brunner
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et al., 1990). However, in these studies, the change in congener distribution of aqueous PCB solution generated with time were not studied. The generator column method also has been used to study the dissolution of NAPL embedded on solid media by Mackay et al. (1991). It is expected that the eective solubility of individual PCB congeners in an Aroclor mixture would be dierent from the pure congener solubility. Therefore, the congener distribution in an aqueous solution obtained from a PCB Aroclor mixture is expected to be very dierent from the distribution in the Aroclor itself. This has been demonstrated in batch equilibrium studies by Lee et al. (1979) and Luthy et al. (1997) and in a generator column by Sokol et al. (1992). The lower chlorinated PCB congeners were found to dissolve to a greater extent than the higher chlorinated congeners in these studies. In their study, Sokol et al. (1992) used a mixture of equal amounts of Aroclors 1221, 1016, 1254, and 1260 in an attempt to obtain a wider distribution of PCB congeners in solution. Despite their eorts, about half of the total PCB in solution was contributed by the monochlorinated biphenyls. The predominance of the monochlorinated biphenyl in the solution may have been because only the ®rst few hundred milliliters of generator column euent were collected. In studies involving PCB treatment, adequate concentrations of the higher chlorinated PCB congeners and large volumes are required. One way to obtain larger volumes of aqueous PCB solutions is to allow the generator column to operate for a long period of time. With the depletion of the lower chlorinated congeners, the higher chlorinated congeners may start dissolving out. The primary goal of the present work was to develop a method of generating large volumes of aqueous PCB solutions having a prescribed distribution of PCB congeners using a generator column and quantify changes in the euent PCB congener distribution over time. A secondary goal was to explain the observations in terms of organic dissolution theory and provide a framework for designing generator columns for obtaining aqueous solutions having a speci®ed distribution of PCB congeners. In this paper, the modi®ed method, aqueous phase PCB congener concentrations achieved, and the temporal changes in congener distribution are described, along with a theoretical explanation of the results. MATERIALS AND METHODS
Chemicals Water used for aqueous PCB solution generation was produced by a Nanopure1 system (Barnstead Company Division of Sybron Corporation, Boston MA). Aroclor 1242 was the source of all PCBs in this study and was obtained in neat form from Ultra Scienti®c (North Kingstown, RI). Aroclor 1242 standards and individual congener standards were procured as hexane solutions
from Ultra Scienti®c. Pesticide grade hexane was used for all extractions. Uniform spherical glass beads, approximately 0.2 mm diameter, were employed as the packing media for the glass bead generator column. Generator column To prepare the glass beads for use in the generator column, 200 g of beads were washed with hexane three times and then dried overnight at 1058C. Fifty milligrams of neat Aroclor 1242, dissolved in 30 ml of hexane, were added slowly to a 250 ml glass beaker containing the prepared glass beads. The beads were mixed manually with a small glass rod until uniform wetting of the beads with the hexane/PCB solution was observed. The beaker then was covered with a glass dish and allowed to sit overnight in a fume hood at room temperature to evaporate the hexane. After 12 h in the hood, the beads were stirred again to expose any unevaporated hexane and placed in the hood for an additional six hours. The PCB coating concentration (0.025% w/w) was lower than the 0.1% (w/w) of solute used by Sokol et al. (1992), and was thought to be low enough to prevent the entrainment of PCB particles during solution generation. The column employed for these studies was approximately 200 mm in length and had an ID of 25 mm. A glass wool plug was positioned in the bottom of the column to support the beads. On top of the glass wool, a 10 mm deep layer of clean glass beads (2 mm diameter) was placed to enhance ¯ow distribution in the column. PCB-laden glass beads (0.2 mm diameter) then were added to the column from the top. A 10 mm layer of clean beads (0.2 mm diameter) was added on top of the PCB laden beads. The composite layers of clean and PCB-laden glass beads occupied the bottom three quarters of the column height to allow for a settling zone in the upper part of the column should any glass beads be entrained in the water ¯ow. Three such generator columns could be prepared from the 200 g of coated glass beads. A schematic of the experimental setup is shown in Fig. 1. Column operation and sampling After placement of the glass beads into the glass columns was completed, Nanopure1 water was introduced slowly from the bottom by a positive displacement pump via Te¯on tubing while shaking and tapping the column occasionally to prevent the formation of any air pockets in the column. Euent was discharged from the glass bead generator column by Te¯on tubing and collected in amber glass bottles or clear glass carboys wrapped in aluminum foil. The columns were operated at room temperature which varied between 24 and 278C. The ®rst liter of solution generated by the column (~15 bed volumes) was discarded to minimize the potential of capturing ®ne glass beads or entrained non-aqueous phase PCBs that might elute quickly from the column. Dunnivant and Elzerman (1988) also discarded the ®rst 500 ml of the solution generated by their generator column. To minimize PCB loss by volatilization, the PCB solution collection vessel was closed with a Te¯on-lined stopper with a small vent to allow the air to leave as the vessel gradually ®lled up. Temporal euent samples were collected by switching the euent directly to a 100 ml volumetric ¯ask containing 3 ml of hexane. The hexane in the volumetric ¯ask covered the top surface of the PCB solution to minimize any volatilization during sample collection. Two generator columns were prepared and operated for performance analysis. The ¯ow rate through each column was identical and equaled 0.7 ml/min resulting in a super®cial ¯ow velocity of 0.14 ml/cm2 min and residence time of approximately 40 min. May et al. (1978) used ¯owrates between 0.1 and 5 ml/min with a residence time of 6.8±352 min and found that within this range, the solubility of PAHs in a
PCB congener distribution in a generator column
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Fig. 1. Schematic of generator column. generator column were not aected by ¯owrate. In Column 1, 20 liters of euent was collected and composite sampled for PCBs. During operation of Column 2, discrete column euent samples (100 ml each) were collected every day (approximately every 15 bed volumes) to determine the temporal congener production patterns. A ®nal composite sample for Column 2 also was taken to allow comparison with the operation of Column 1. Sample extraction, concentration, and GC analysis In this study, 100 ml of column euent was extracted sequentially with 3 ml of hexane three times. The extraction was performed in the 100 ml volumetric ¯ask in which the sample was collected. After extraction, the total hexane volume was adjusted to 10 ml. Hexachlorocyclohexane was used as an internal standard and added to the ®nal hexane extract. PCB congener speci®c analysis was performed using a slightly modi®ed version of EPA Method 608. A Hewlett Packard gas chromatograph (5890, Series II) with a fused silica capillary column (60m 0.25 mm inner diameter, DB-5, J&W Scienti®c) and an electron capture detector was used for analysis. The gas chromatograph was operated with hydrogen as the carrier gas, and a 95% argon/5% methane mixture as the make-up gas. The column temperature was varied using the following schedule: initial
temperature of 908C for 2 min, 108C/min for 6 min (until it reached 1508C), then 38C/min to the ®nal temperature of 2908C. The injection port and detector temperatures were maintained at 210 and 3008C, respectively. Hewlett Packard Chemstation software was used for data acquisition, integration, calibration and analysis. A multi-level calibration table was prepared using PCB Aroclor 1242 standard solution in hexane obtained from Ultra Scienti®c. The calibration was used to identify and report the concentration of 50 signi®cant PCB peaks of Aroclor 1242. Congener names corresponding to the 50 PCB peaks identi®ed are presented in Table 1 along with percent mass fraction of each peak based on characterization of Aroclor 1242 by Schulz et al. (1989).
RESULTS AND DISCUSSION
Fate of PCBS added to generator column It was expected that PCB loss was likely during the hexane evaporation phase of glass bead preparation through volatilization and sorption on the preparation beaker. To quantify this potential loss, the mass of PCBs on a batch of glass beads pre-
Fig. 2. Mass balance of PCBs on generator column.
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pared for column use was compared to the mass of PCBs added. As expected, loss of PCB was observed during preparation as shown in Fig. 2. On a percentage basis, the loss of each homolog group appeared to be uniform at approximately 15%. The losses during drying could be caused by volatilization (which is expected to be higher for lower chlorinated congeners) and adsorption on the glass vessel (which is expected to be higher for the higher chlorinated congeners). While PCB loss was not desired, uniform loss across the homolog group indicated that the relative congener composition was not altered signi®cantly. The total amount of PCBs eluted from the column in solution was estimated and found to vary by chlorination level as shown in Fig. 2. About 80% of the monochloro biphenyl was recovered in aqueous solution compared to 38% of the hexachloro biphenyl. Correspondingly, the measured amount of PCBs remaining on the glass beads after solution generation showed higher residual of the higher chlorinated congeners compared to the lower chlorinated ones. As shown in Fig. 2, about 80±90% of the total PCBs in each homolog group added to the generator column could be accounted for by losses
during preparation, dissolution, and residual on the glass beads. The remaining 10±20% categorized as other losses were attributed to that lost in the ®rst 1 liter of solution that was discarded, volatilization during solution collection, and adsorption to various parts of the generator apparatus. Composite PCB solutions PCB congener concentrations and pro®les for composite euent samples taken after 20 liters had been collected (approximately 300 bed volumes) from Columns 1 and 2 are presented in Fig. 3a and 3b. In each graph, individual PCB congeners measured are presented as peak numbers (the corresponding congener names are presented in Table 1). Total PCB concentrations in the 20 liter euent samples for Columns 1 and 2 were similar and equaled 243 and 219 mg/l, respectively. It is important to note that eleven temporal discrete 100 ml samples were taken of Column 2 euent during its operation and are thus not represented in the reported composite sample. When the PCB concentrations of these discrete samples are incorporated into the composite euent for Column 2 (resulting in a total composite volume of 21.1 liters), the cal-
Fig. 3. (a) PCB congener concentration distribution in composite sample of column 1. (b) PCB congener concentration distribution in composite sample of column 2.
PCB congener distribution in a generator column Table 1. Peak numbers, names of congeners and percentage in Aroclor 1242 (Schulz et al., 1989) Pk # 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
Congener structure
% in 1242 w/w
2 26 22' 24 2,5 23' 24' 2,3 22'6 Hexachloro Cyclo Hexane (ISTD) 22'5 22'4 44' 236 23'6 22'3 24'6 245 23'5 23'4 24'5 244' 233'2'34 22'56' 22'46' 234' 22'36 22'36' 22'55' 22'45' 22'44' 22'45 244'6 22'35' 344' 233'6 22'34' 22'34 234'6 22'33' 22'44'6 23'45 234'5 244'5 23'4'5 23'44' 22'35'6 22'34'6 2344' 233'4' 22'33'6 22'34'5 22'455' 22'44'5 22'33'5 22'3'45 22'345' 2344'6 22'344' 22'33'66' 33'44' 233'4'6 22'33'4 22'355'6 22'33'56' 233'4'5 2'344'5 22'34'5'6 23'44'5 22'33'46' 22'44'55' 233'44' 233'456 22'344'5' 233'44'6 22'33'55'66' 22'33'44'6 233'44'5 22'344'55' 22'33'44'5 233'44'56
Ð 3.21 1.14 1.38 7.71 0.53 10.31 0.50 2.89 0.10 1.33 0.79 11.11 5.72 3.64 1.16 0.49 4.04 3.60 1.87 3.20 1.44 3.50 0.89 0.41 0.23 2.17 3.89 4.53 0.17 2.93 0.72 1.65 0.86 0.12 0.65 0.77 0.53 0.07 1.98 0.44 0.08 0.07 2.25 1.84 0.54 0.14 0.06 0.11
culated concentration for the Column 2 composite euent becomes 226 mg/l. Accordingly, there was good agreement between the composite samples with respect to total PCB mass. The concentration of total PCB in the 20 liter composite euents of the two generator columns operated in this study were within the range of total Aroclor 1242 solubility values reported in the literature. The solubility value of total Aroclor 1242 presented in the literature vary widely between 85 to 340 mg/l as reported by Luthy et al. (1997). On a percentage basis, composite samples from both columns contained more of mono, di and trichloro biphenyls and less of the higher chlorinated congeners than does Aroclor 1242. To compare the congener distribution between the two composite euents, a similarity coecient was calculated as in Sokol et al. (1992).
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The similarity coecient between the two composite euents was 80% indicating high reproducibility. Comparing the congener distribution in the two euents to the congener distribution in Aroclor 1242, it was found that Column 1 and 2 euents matched 90 and 75% respectively to 1242 which is signi®cantly higher than the 33% similarity achieved in solutions prepared by Sokol et al. (1992). The two primary reasons for the greater similarity to the original Aroclor obtained in this study were low PCB loading on the generator column and the longer period of composite sample collection (>300 bed volumes). The ratio of mass of PCB added to a generator column to the total volume of solution generated appears to aect the closeness of the congener distribution of composite solution to the original Aroclor. As the total volume of water contacted approaches in®nity, all PCBs will be transferred to water and the congener distribution in solution will approach that in the original Aroclor. However, operation of a generator column for too long may yield very dilute solutions which may not be suitable for the intended studies. Temporal congener distribution pattern To investigate the pattern of congener elution as a function of solution volume produced, discrete euent samples were collected during the operation of Column 2 as reported earlier. PCB congener concentrations for six discrete Column 2 samples, out of the eleven collected during its operation, are shown in Fig. 4A±F. The total PCB concentration in the euent shows a general decreasing trend with bed volumes of solution generated, with about 400 mg/l in the ®rst sample and about 200 mg/l in the last sample. As shown in Fig. 4, two trends were observed in the temporal PCB data. First, the lower chlorinated congeners (mono, di, and tri) were found to dissolve faster and eluted in higher concentrations in the beginning, and lower concentrations near the end of column operation. Trichlorobiphenyl concentrations appeared steady at the beginning but decreased in the later discrete samples. Second, concentrations of higher chlorinated biphenyls appeared to increase as the concentrations of the lower chlorinated congeners decreased with higher bed volumes of operation, except for the 315 bed volume sample. A similar change in euent solution composition from a generator column has been reported for hydrocarbon mixtures and petroleum products by Mackay et al. (1991) and Shiu et al. (1988). In these studies, the more soluble components in the mixture were found to elute from the generator column during the initial phases of operation and the less soluble components followed later. For example, Mackay et al. (1991) observed that when benzene, toluene, and p-xylene were loaded on a generator column during the initial phases of the column operation, the most soluble compound benzene eluted
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Fig. 4. PCB congener distribution in generator Column 2 euent with bed volumes.
at the highest concentration and decreased with time of operation. The concentration of the least water soluble compound (p-xylene) in the euent was low in the beginning and increased with time as benzene was depleted from the column.
It was hypothesized that the decrease in production of the lower chlorinated biphenyls observed with time in Column 2 was caused by a decrease in the available pool of these congeners in the glass beads. As shown in Fig. 2 earlier, nearly half of the
PCB congener distribution in a generator column
higher chlorinated congeners (4±7 chlorines) remained on the glass beads after 315 bed volumes of solution generation, compared to none of the monochlorobiphenyl, only 5% of the dichloro biphenyl, and 29% of the trichloro biphenyl. Accordingly, an exhaustion of PCBs in the column would appear to be a reasonable explanation for the reduction of more soluble PCBs observed in the later discrete samples taken during Column 2 operation. Application of Raoult's law The second trend observed in the discrete samples from Column 2 was that the concentration of higher chlorinated homolog groups appeared to increase with bed volumes as shown earlier in Fig. 4. According to Raoult's Law, the solubility of a compound present in an ideal mixture is equal to the solubility of the pure compound multiplied by its mole fraction in the mixture. In this case, the mole fractions of the lower chlorinated congeners in the PCB phase decreased with time as they dissolved faster, and the mole fractions of the higher chlorinated congeners in the PCB increased with time as they dissolved at a slower rate. As seen in Fig. 2, the percentage of PCBs remaining on glass beads after solution generation was higher for the higher chlorinated homolog groups. Therefore, the apparent solubility of mono, di, and trichloro biphenyls are expected to decrease with solution generation and the apparent solubility of the higher chlorinated congeners are expected to increase. The aqueous solubility depression for hydrophobic organic chemicals in the presence of other organic chemicals in water has been studied by Coyle et al. (1997). They found that the solubility of a tetrachloro and heptachloro biphenyl was reduced to about 15% in the presence of other organic compounds in water such as methylene chloride and chloroform. They also found that the extent of solubility depression increased with the hydrophobicity of the organic compound in solution. Two possible theoretical explanations for this phenomena have been proposed by Coyle et al. (1997). The ®rst explanation is related to changes in the mole fraction of the organic compounds in the solute phase. The second alternate explanation is solventing out (similar to the salting out phenomenon) in which the dissolved organic compounds may occupy a signi®cant portion of the water molecules, rendering them unavailable for further dissolution of other hydrophobic organic compounds. The solventing-out phenomenon probably is applicable for a system where the concentration of the organic compounds in solution is high, as in the system studied by Coyle et al. (1997) (10±20 g/l of methylene chloride). In the present study, the total PCB concentration was in the range of 200± 400 mg/l. Therefore, changes in mole fractions of the dissolving solutes is likely to be a more appro-
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priate explanation of the observed behavior of changing distribution of aqueous phase PCB congener concentrations. To test the applicability of Raoult's Law in explaining the changing aqueous concentration of PCBs in the generator column euent, a simulation was performed on a homolog group basis. Aqueous solubility values for PCB congeners reported in the literature are highly variable (Opperhuizen et al., 1988; Dunnivant and Elzerman, 1988; Miller et al., 1984), typically spanning one order of magnitude. Therefore, solubility values of PCB congeners within a homolog group reported by Opperhuizen et al. (1988) were averaged to arrive at a representative solubility of the homolog group. These average solubilities of the seven homolog groups are presented in Table 2. The depletion of a PCB homolog group on the glass bead column was modeled as an outcome of dissolution of that homolog group into solution over time as shown in equation (1). The eective solubility of the PCB homolog group was obtained by multiplying the average solubility from Table 2 by the mole fraction of that homolog in the PCB phase as shown in equation (2). The activity coecient of each homolog group was assumed to be unity, as in Mackay et al. (1991). The mole fraction of each PCB homolog group at any time was calculated using equation (3). The measured initial mole fraction of each homolog group and total initial mass of PCBs in the generator column were used as initial conditions. The same ¯ow rate of water was used for the simulation as in the experimental column. In the simulation, the aqueous concentration in the euent of the generator column was assumed to be in equilibrium with the PCB phase on the glass beads at all times. It should be noted that the model in equations (1)±(3) are truly predictive with no adjustable parameters. 1 t QCi dt 1 qi qi0 ÿ m 0 Ci C *i Xi gi
2
qi =Mi Xi X n qi =Mi
3
i1
where: qi=concentration of PCB homolog group i Table 2. Average solubility of PCB homolog groups (calculated from Opperhuizen et al., 1988) Homolog group Monochlorobiphenyl Dichlorobiphenyl Trichlorobiphenyl Tetrachlorobiphenyl Pentachlorobiphenyl Hexachlorobiphenyl Heptachlorobiphenyl
Average aqueous solubility (mg/l) 3000 1100 500 70 20 4 2.4
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Fig. 5. (a) Prediction of mono, di and tri chlorobiphenyl dissolution from generator column. (b) Prediction of tetra, penta and hexa chlorobiphenyl dissolution from generator column.
on the glass beads, qi0=initial concentration of PCB homolog group i on the glass beads, Ci=concentration of homolog group i in the aqueous phase, C*i =aqueous solubility of homolog group i, m = mass of glass beads, Xi=mole fraction of homolog group i in the PCB phase, gi=activity coecient of homolog group i in the PCB phase (assumed = 1), Q = ¯ow rate of water through the generator column, n = number of homolog groups, Mi=molecular weight of PCB homolog group i. The simulated and measured euent concentrations are plotted against bed volumes generated for mono through hexachloro biphenyl in Fig. 5a±b. As in the experimental data, the predicted mono, di, and trichloro biphenyl concentrations decreased with
time and that of the higher chlorinated congeners increased with time. Predictions of mono, di, and trichloro biphenyls match well with the experimental data. An average percentage dierence between the data and the model was estimated for each homolog group using the following equation: n C ÿ C m 100 X ij ij average percentage difference Cij n j1 4 where: n = number of observed data points and Cm i =model prediction of PCB homolog concentration. The average percentage dierence for mono, di and tri chlorobiphenyls were 23, 22 and 13% respectively. Predictions of tetra and pentachloro
PCB congener distribution in a generator column
biphenyl are reasonable in trends but smaller in magnitude than the experimental data. The prediction of hexachloro biphenyl is much smaller that the observed values. The average percentage dierence for tetra, penta and hexa chlorobiphenyls were 52, 71 and 95% respectively. Deviations between observed and predicted performance are likely related to the uncertainty in the reported values of PCB congener solubility obtained from the literature and simplifying assumptions made in the model. These assumptions include: (1) a homogeneous distribution of PCBs throughout the column; and (2) the eective solubility of each homolog group is a function of the average mole fraction of that homolog group. The homogeneous distribution assumption may not be valid, since rapid depletion of more soluble PCBs in the initial portion of the column could result in higher localized mole fractions of higher chlorinated homolog groups than would be calculated assuming homogeneous distribution. Accordingly, it is not unreasonable that concentrations of pentachloro and hexachloro biphenyls in the column euent were higher than the model predictions. The calculated and observed concentration of pentachloro and hexachloro biphenyls are within the reported solubility limits (4±23 mg/l for pentachloro and 1±10 mg/l for hexachloro biphenyls; Opperhuizen et al., 1988). The total PCB concentration in the euent was slightly underpredicted by Raoult's Law, but the trends matched very well. The observed total PCB values in the euent decreased from 418 mg/l to 203 mg/l whereas the predicted values decreased from 400 mg/l to 160 mg/l. Therefore, it does appear possible to predict with some accuracy the distribution of PCB congeners in the generator column euent with time. Accordingly, if aqueous solutions of speci®c homolog distribution were desired for experimental studies, their generation appears possible. The concentration of any homolog group in the euent of the generator column at any time is given by equation (2). The cumulative concentration of euent collected from the generator column over a period of time t can be expressed as: t QCi dt Ci 0 5 Qt The cumulative concentration which can be estimated by solving equations (1)±(3) and (5) simultaneously is a function of two variables: initial PCB homolog concentration on the glass beads (qi0), and duration of solution generated (time t or bed volumes). Therefore the choice of these two variables can be determined by performing the simulation and ®nding the ones that result in the closest match to the required homolog distribution.
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CONCLUSIONS
Speci®c conclusions from this work are: . The generator column method is capable of producing reproducible concentrations of soluble PCBs under the same set of experimental conditions. Operation of two similar generator columns yielded PCB solutions with a total PCB concentration of 243 and 226 mg/l after about 300 bed volumes. Similarity analysis con®rmed the reproducibility of congener concentrations. . The generator column method is capable of producing equivalent volumes of PCB solution much faster than the quiescent dissolution method. Twenty liters of PCB solution was obtained in twenty days compared to 4±6 months typically required by quiescent dissolution method. . PCB congener concentrations in the generator column euent change with solution volume generated. Initial volumes are richer in lower chlorinated PCBs, while later volumes are richer in higher chlorinated PCBs. Accumulation of column euent for a large number of bed volumes can provide an aqueous solution that approaches the congener distribution of the parent Aroclor. . Time dependent changes in PCB homolog distribution in the euent of a generator column can be explained and predicted using Raoult's Law. . It is possible to design an experimental protocol to produce aqueous solutions of a desired PCB homolog distribution. The two important operational variables that determine the PCB composition of solution generated in a generator column are: starting Aroclor type and the ratio of PCB mass added to the total volume of water eluted. REFERENCES
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