Environ. Sci. Technol. 1990, 2 4 , 175 1- 1754
(2) Title 40: Protection of Environment. Code of Federal Regulations; Part 136, Method 607. ( 3 ) Bellar, T. A.; Behymer, T. D.; Budde, W. L. J. Am. SOC. Mass Spectrom. 1990,1, 92-98. (4) Vestal, M. L.; Fergusson, G. Anal. Chem. 1985, 57, 2373-2378. ( 5 ) Bellar, T. A.; Budde, W. L. Anal. Chem. 1988, 60, 2076-2083.
bination reaction in the thermospray mass spectra of N-nitrosoamines.
Acknowledgments We thank Gerard McKenna of the US.Environmental Protection Agency, Region 2, who provided the soil sample from the hazardous waste disposal site near Buffalo, NY.
Literature Cited (1) Eichelberger,J. W.; Kerns, E. H.; Olynyk, P.; Budde, W. L. Anal. Chem. 1983,55, 1471-1479.
Received for review March 26,1990. Revised manuscript received July 13, 1990. Accepted July 16, 1990.
Henry's Law Constants for Polychlorinated Biphenyls: Experimental Determination and Structure-Property Relationships Slegfrled Brunner, Eduard Hornung, Helmut Santl, Egmont Wolff, and Otto G. Piringer"
Fraunhofer Institut fur Lebensmitteltechnoiogie und Verpackung, Schragenhofstrasse 35, D-8000 Munchen 50, Germany Joachlm Altschuh and Ralner Bruggemann
GSF-Projektgruppe
Umweltgefahrdungspotentiale von Chemikalien, Ingolstadter Landstrasse 1, D-8042 Neuherberg, Germany
Methods to experimentally determine Henry's law constants (HLCs) can be roughly divided into two parts: kinetic and static thermodynamic methods. Here a dynamic method is presented, which combines the advantages and avoids the inherent difficulties of these two methods. A technical mixture of polychlorinated biphenyls (PCBs) was analyzed and 58 PCB congeners have been reliably identified. In addition, 11 PCB congeners were found, which are less reliably identified. The measurements have shown that a systematic dependence on the chlorine number and substitution pattern can be established. This set of HLCs allows formulation of some estimation equations. 1 . Introduction Equilibrium partitioning between water and a gas phase plays an important role in the prediction of the environmental fate of chemicals such as polychlorinated biphenyls (PCBs) (1). The lack of accurate Henry's law constants (HLCs) is one of the major problems in making useful predictions. The great number of congeners and the low vapor pressures of PCBs and errors due to loss by adsorption are the reasons for analytical difficulties in the direct determination of HLCs in water. To avoid such difficulties, predictions of HLCs from other physical properties of the PCB congeners were made (2,3).The assumptions and data used in these predictions, however, are too weak for reliable HLC calculations. There are two different approaches for the direct determination of partition coefficients between gas and water. One is a kinetic method based on the rate of loss of a substance from water by stripping with a gas (gas purge method) (4-6). The other is a thermodynamic method where the air and water concentrations of a substance are determined under equilibrium conditions and the partition coefficients are calculated as their ratio (7). The advantages of the kinetic method are the relatively small vessel volumes in gas-water contact and consequently small adsorption areas and equilibration times. A disadvantage of the gas purge method lies in the need of a model for data handling, such as the two-film or penetration model. All models are approximations, and the use of mass-transfer coefficients for the evaluation of ther0013-936X190/0924-1751$02.5010
modynamic partition coefficients is a principal disadvantage. A consequence of using such approximations is the possibility of introducing systematic errors of unpredictable amounts. The advantage of the static, thermodynamic method is the direct determination of the equilibrium concentrations in the two phases. The disadvantage is the long equilibration time for a large gas volume with the great gasvessel interface and the connected adsorption phenomena, as well as the need for a very careful handling of the gas extraction for analysis such that the gas-water equilibrium is not disturbed. In order to combine the advantages and avoid the disadvantages of the two above approaches, we have employed a dynamic method using a column operating in the concurrent mode that produces a guaranteed phase equilibrium (8). With this column, the HLCs of 58 PCB congeners were determined from a technical PCB mixture. The use of a concurrent flow of air/water in a wetted wall column for the determination of HLCs for pesticides has been recently described (9). In this paper, the apparatus and the mode of operation for the determination of HLCs of PCBs is described in detail. The measured data are compared with data obtained with other techniques. In addition, the measured HLCs are used to establish some semiempirical relationships.
2. A p p a r a t u s and Procedure An aqueous solution of PCB congeners is produced by the purging of water through a generator column as described by Wasik et al. (10). This solution is then brought into contact with a gas stream until the partition equilibrium is achieved. An overview of the apparatus is shown in Figure 1. Water from the feed dosing funnel (A) is purged through the generator column (B) at a constant flow rate. The PCB-loaded water continues to flow into the desorption column (C), where it contacts the gas stream. At the bottom of the column, the water is separated from the gas and flows through the connecting tube (D)into the receiver dosing funnel (E). The gas is conducted into an absorption vessel (G), where the PCBs are dissolved in an organic solvent. The gas then leaves the apparatus through the reflux condenser (H).
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 11, 1990
1751
a
(2) d
C 2
D
G
E
Figure 1. Apparatus for the determination of gas-liquid partition coefficients. System components are (A) feed dosing funnel with Mariotte tube insert and temperature control jacket, (B) generator column with temperature control jacket, (C) gas-liquid desorption column with two gas fittings and temperature control jacket and thermostated inner tube, (D) connecting tube with a capillary tube insert, (E) recelver dosing funnel with gas compensation and temperature control jacket, (F) three-way stopcock, (G) absorption vessel wffh gas inlet tube and lateral outlet branch and temperature control jacket, (H) reflux condenser, temperature sensors, and s thermostat connections. Inset: Details of the desorption column are (a) temperature control jacket, (b) contactor region, (c) glass helix, and (d) thermostated inner tube. Dimensions: length 650 mm, effective glass helix length 420 mm, contactor region 12-mm i.d. and 28-mm 0.d.
0
The generator column (B) consists of a -200-mm thermostated glass tube with a 14-mm internal diameter (i.d.). The lower third of the tube is enlarged to 19-mm i.d. The upper two-thirds are filled with PCB-loaded glass beads. A stopcock a t the end of the column allows an additional regulation of the water flow. The gas-liquid desorption column (C; the column is available from Normag GmbH, Hofheim/Taunus, FRG and Greiner & Gassner, Munich, FRG) consists of a thermostated inner tube, on which a glass helix is mounted, and a thermostated outer tube (Figure 1, inset). Two gas fittings are mounted at the upper and lower ends of the column. The aqueous PCB solution flows along the glass helix as a thin film. In the very narrow contact region between the inner and outer tubes, the gas flows concurrent to the water stream, where a portion of the dissolved PCB is desorbed into the gas stream. The geometry of the helix ensures a sufficient contact time for the establishment of the partition equilibrium. The gas leaves the lower end of the column through a three-way stopcock and flows into cooled hexane (G), where the PCB congeners are absorbed. The reflux condenser (H) minimizes the loss of solvent. The liquid phase leaving the desorption column is brought into contact with cooled hexane (E). To prevent the remixing of the gas and liquid phases, a connection tube (D) is placed between (C) and (E). This tube serves 1752
Environ. Sci. Technol., Vol. 24, No. 11, 1990
to separate the gas flow from the gas present in (E). This is accomplished by a capillary tube, which always contains a drop of the liquid phase. It is important that when the room temperature lies below the experimental temperature the thermostating liquid must first flow through the outer jacket of (C) and then into the inner tube. This prevents the condensation of water vapor on the walls within the contact region. The regulation of the experimental temperature is accomplished by the external sensor T2. Additional temperature control occurs at T1 and T3 (Figure 1). A constant gas flow is maintained by a flow regulator and is controlled at the entrance of the desorption column (C) and a t the exit of the condenser (H). The preparation of the PCB loaded glass beads (100/120 mesh) was performed by applying a solution of PCB technical mixtures (Clophen-A30 and Clophen-AGO,1:2, from Bayer AG) of different chlorine contents in dichloromethane. After the removal of the solvent, the loaded beads were placed into the generator column, which was previously filled with water. The enlarged lower section of the column contained quartz wool. Before the connection of the generator column to the apparatus, it was conditioned with a water flow of -24 h. Demineralized, distilled water and nitrogen were used as the liquid and gas phases, respectively. The feed dosing funnel (A), the generator column (B), and the desorption column (C) were thermostated at 25O C. To avoid errors by adsorption of PBCs on the desorption column walls, the column was equilibrated for several hours. This was done by applying the experimental flow rates of water (80-90 mL/h) and gas (35 L/h). During this conditioning period, the gas flowed through the stopcock (F) into the atmosphere and the receiver dosing funnel (E) was not connected to the apparatus. After the conditioning period, the receiver flasks (E) and (G) were each filled with 50 mL of hexane and cooled together with the reflux condenser (H) to +2 "C. The experiment was begun by switching the stopcock (F)to the absorption vessel (G). An experimental run lasted from 3 to 5 h. For the control of mass balance, samples from the initial aqueous PCB solution were taken at the end of the generator column (B) before and after the experiment and compared with the solutions in the receiver flasks from the water (E) and the gas phase (G). This is important to estimate uncontrolled losses of PCBs, for example, by adsorption. The analysis of the PCB congeners was performed by gas chromatography using an ECD detector and a 60-m fused-silica capillary column with SE-30 as the stationary phase. The determination of the number of chlorine atoms in the separated congeners has been made by mass spectrometry. The identification of isomers with the same chlorine content was possible on the basis of retention indexes (11). 3. Results
From the ratios of the concentrations of the PCB congeners in the gas phase, C,, to the concentrations in the liquid phase, CI, the dimensionless partition coefficients K = C,/CI are calculated. In Table I, the K values and the corresponding HLC values (atm m3/mol) are shown. The mean values of six experimental runs are given together with the corresponding standard deviations. For statistical evaluation, standard methods were used (12). From all congeners in the technical mixtures only 55 could be separated by gas chromatography into pure substances. Three additional compounds with a purity of more than 95% were considered, giving a total of 58 HLCs.
Table I. Experimentally Determined K and HLC Values of Individual PCB Congeners K x 103 SD
no.
IUPAC no.
substitution pattern
mean
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
6 12 18 19 24 28 29 31 36 37 40 47 52 54 62 67 70 74 85 97 99 102 120 128 129 130 132 134 135 136 138 141 146 147 151 153 159 160 163 165 170 172
2,3’ 394 2,2’,5 2,2’,6 2,3,6 2,4,4’ 2,4,5 2,4’,5 3,3‘,5 3,4,4’ 2,2’,3,3‘ 2,2’,4,4’ 2,2’,5,5’ 2,2’,6,6’ 2,3,4,6 2,3’,4,5 2,3’,4’,5 2,4,4’,5 2,2’,3,4,4‘ 2,2’3’,4,5 2,2’,4,4’,5 2,2’,4,5,6’ 2,3’,4,5,5’ 2,2‘,3,3‘,4,4’ 2,2‘,3,3‘,4,5 2,2’,3,3’,4,5’ 2,2’,3,3’,4,6’ 2,2‘,3,3‘,5,6 2,2’,3,3’,5,6’ 2,2’,3,3’,6,6’ 2,2’,3,4,4’,5’ 2,2’,3,4,5,5’ 2,2’,3,4’5,5’ 2,2’,3,4‘,5,6 2,2’,3,5,5’,6 2,2‘,4,4‘,5,5‘ 2,3,3’,4,5,5’ 2,3,3’,4,5,6 2,3,3’,4’,5,6 2,3,3’,5,5’,6 2,2’,3,3’,4,4’,5 2,2‘,3,3‘,4,5,5‘
10 5.9 10 9.2 8.8 7.9 7.9 7.6 7.0 4.2 4.1 7.7 8.0 8.1 8.6 3.9 3.9 4.2 2.7 3.0 3.2 3.7 2.3 0.53 1.2 1.5 1.8 2.0 2.3 3.6 0.84 0.94 1.0 2.1 2.4 0.93 0.82 0.78 0.60 1.2 0.38 0.52
0.49 2.0 2.1 0.70 0.73 1.2 0.70 1.0 1.6 1.1 1.0 1.9 2.1 1.6 1.8 1.4 1.2 1.2 1.0 1.1 1.1 1.4 1.0 0.24 0.55 1.1 0.88 0.85 1.0 1.4 0.27 0.26 0.30 1.0 1.1 0.24 0.47 0.22 0.16 0.79 0.18 0.23
HLC x 104, atm m3/mol 2.5 1.4 2.5 2.3 2.2 2.0 2.0 1.9 1.7 1.0 1.0 1.9 2.0 2.0 2.1 1.0 1.0 1.0 0.66 0.74 0.78 0.90 0.56 0.13 0.29 0.37 0.44 0.49 0.56 0.88 0.21 0.23 0.25 0.51 0.59 0.23 0.20 0.20 0.15 0.29 0.09 0.13
no. 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
IUPAC no.
substitution pattern
173 174 178 179 180 185 194 195 196 198 199 201 202 22“ 51 95” 66 91
2,2‘,3,3‘,4,5,6 2,2‘,3,3‘,4,5,6‘ 2,2’,3,3’,5,5’,6 2,2’,3,3’,5,6,6’ 2,2’,3,4,4’,5,5’ 2,2’,3,4,5,5’,6 2,2’,3,3’,4,4’,5,5’ 2,2’,3,3’,4,4’,5,6 2,2‘,3,3‘,4,4‘,5,6‘ 2,2‘,3,3’,4,5,5’,6 2,2’,3,3’,4,5,5’,6’ 2,2’,3,3’4,5’,6,6’ 2,2’,3,3’,5,5’,6,6’ 2,3,4’ 2,2’,4,6’ 2,2’,3,5’,6 2,3’,4,4’ 2,2‘,3,4‘,6 2,2’,4,5,5’ 3,3’,4,5’ 2,2’ 2,6 2,3 2,4’ 2,4 2,s 2,2‘,3 2,4‘,6 2,3,3’ 2’,3,4 2,3’,5 2’,3,5 2,2’,3,4 2,3,4’,6 2,2’,3,4’ 2,2’,3,5’ 2,2’,4,5’ 2,3’,4,6 2,2’,3,4,5’ 2,3’,4,4’,6 2,2’,3,3’,4,6 2,2‘,3,4,5,6‘
l O l C
79 4 10 5 8 7 9 16 32 20 33 26 34 41 64 42 44 49 69 87 119 131 143
K x 103 SD
mean
HLC x 104, atm m3/mol
0.56 0.56 0.95 0.96 0.40 0.65 0.40 0.44 0.41 0.59 0.40 0.70 0.74 5.6
0.17 0.17 0.52 0.21 0.19 0.18 0.12 0.21 0.11 0.43 0.21 0.36 0.35 0.48
0.14 0.14 0.23 0.24 0.10 0.16 0.10 0.11 0.10 0.14 0.10 0.17 0.18 1.4
5.0
1.5
1.2
3.7
1.3
0.90
9.5
4.0
2.3
9.2
1.6
2.3
1.2
2.8
7.9
1.2
2.0
6.5
0.60
1.6
8.3
0.89
2.0
5.6
1.6
1.4
5.9
1.6
1.4
8.4
2.2
2.1
3.0
1.0
0.74
1.6
0.80
0.39
12
Main congener with more than 95% Duritv.
In 11 further cases, the values refer to mixtures of two isomers that could not be separated. Nevertheless, these values were included in Table I (no. 59-69) because of their similar molecular structure and the same number of chlorine atoms in ortho positions. In order to prove the reliability of the results it was demonstrated that (a) the phase equilibrium was established in the concurrent operation mode of the column and (b) the mass balance is fulfilled. Preliminary studies were performed with the column operating in countercurrent mode. By use of the same gas and liquid flows as in the concurrent mode, the partition of a selected mixture of substances with known partition coefficients between gas and water was measured (8). From the concentrations of these substances in the gas and the liquid phases, respectively, the number of theoretical plates of the column has been determined (8). Within the experimentally used flows (-15-35 L/h for gas and 40-150 mL/h for water) about five theoretical plates were obtained. This guarantees that an equilibrium is obtained by operating in the concurrent mode under the same hydrodynamic conditions. For each experimental run the mass balance was determined as mentioned before. For the calculation of the partition coefficients only the best six runs were used. The
mass balances for these runs ranged between 80 and 115%, with most of them between 90 and 107%. 4. Discussion
4.1. Comparison with Previous Measurements. Henry’s law constants of PCBs have been published by a number of authors. Dunnivant et al. gave an overview of the results (4). The published values (dimensionless)range from 0.6 X to 40 X including PCBs with two to seven chlorine atoms. These measured HLCs are found to be unrelated to molecular weight (i.e., number of chlorine atoms) except for the results of Murphy et al. (7). The values presented here range from 0.4 X to 1 2 X agreeing with the previously measured range. They also exhibit a distinct dependence on molecular weight, as shown in Figure 2. In general, there is good agreement with Murphy et al. (7), though their values are slightly higher. The verification of the mass balance and the guaranteed equilibrium as discussed before confirm the high reliability of our values despite of the large standard deviation range. Small standard deviations, as for example those given by Dunnivant et al. (4), do not necessarily exclude systematic errors. Especially there is some potential of systematic error if dynamic models are used that may not be approEnviron. Sci. Technol., Vol. 24, No. 11, 1990
1753
log K
,
-3,5
'
PCBs based on literature data determined by the gas-purge method. Two connectivity indexes, 4xpand 4x turned out to be appropriate descriptors in their m o c i . Therefore, in this paper molecular connectivity indexes will additionally be used to describe the HLCs for the PCB congeners, although eqs 1 and 2 give two satisfactory structure-property relations. In fact, the HLCs are highly correlated with various molecular connectivity indexes. If only one index is used, the 4xpcindex shows the best result: log K = -1.15 - 0.42 4 ~ p c (3)
I
N = 58
I
2
3
L 5 6 7 Number of CI-atoms
8
Figure 2. Dependence of log K values on the number of chlorines. The number of ortho chlorines: zero (o), one (+), two (0),three (O), and four (X).
priate for evaluations of thermodynamically defined quantities. Burkhard et al. (2) have estimated HLCs for all 209 PCB congeners from vapor pressure and water solubility. They found no systematic variation with molecular weight but a large variability within one molecular weight group. The results presented here and by Murphy et al. suggest that the prediction of HLCs from vapor pressure and water solubility may be doubtful for PCBs. 4.2. Structure-Property Relationships. The comprehensive set of experimentally determined HLCs for PCBs presented here opens the possibility of studying some quantitative structure-property relationships. First of all, the HLCs strongly depend on the number of chlorine atoms, i.e., they decrease with increasing molecular weight. Nevertheless there exists a noticeable variability within isomers of constant chlorine number. The variability can be explained to some extent if the number of chlorine atoms in ortho positions is taken into account. Increasing substitution of the ortho positions results in increasing values for the HLC. This effect has been discussed before by Burkhard et al. (2) and Dunnivant et al. (4). From the experimental results a predictive equation for K can be derived. log K = -1.38 - 0.32(no. of C1) + 0.18(no. of 0-C1) N = 58 rDF = 0.957 s = 0.136
(1)
Recently, Hawker (13) studied relationships of vapo; pressure and HLCs of PCBs taken from the literature with the total surface area (TSA). While the vapor pressure formed a linear relationship with TSA, log HLC did not. With our experimentally determined K values and the TSA values of Hawker and Connell(14), which have been calculated for a coplanar conformation, a fairly good correlation results. log K = 2.41 - 0.020 TSA (2) N = 58
rDF = 0.939
s = 0.161
Molecular connectivity indexes have been used frequently as structural descriptors in QSAR studies (for an overview, see ref 15). One main advantage in using such indexes is the fact that they may be calculated exclusively from molecular structure. Recently, Nirmalakhandan and Speece proposed a QSAR model to predict HLCs that is based on molecular connectivity indexes (16). Such topological indexes have been applied by Sabljic and Giisten (17),who developed a predictive model for the HLCs of 1-
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Environ. Sci. Technol., Vol. 24, No. 11, 1990
rDF = 0.929
s = 0.173
Introducing two connectivity indexes, the best descriptors are 4xpcand 6xpcand rDF is now 0.953. log K = -1.08 - 0 . 7 5 4 ~ p+c 0 . 1 3 6 ~ p c (4)
N
= 58
rDF = 0.953
s = 0.142
Equation 4 corresponds to eq 1 because the 4xpcindex is correlated with the molecular weight (correlation coefficient 0.9841, and the 6xpcindex has the highest correlation with the number of o-chlorine atoms (correlation coefficient 0.783). However, it should be noted that these two indexes are correlated with a correlation coefficient of 0.960. By use of the connectivity indexes of Sabljic and Giisten and our experimental K values, a regression equation with rDF = 0.934 is derived, which is worse than that for eq 1 or 4. This demonstrates how crucial the quality of data affects the corresponding structure-property relationship.
Literature Cited Mackay, D. In Handbook of Environmental Chemistry; Hutzinger, O., Ed.; Springer-Verlag: Berlin, New York, 1980; Vol. 2, Part A, pp 31-46. Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. Environ. Sci. Technol. 1985, 19, 590-596. Shiu, W. Y.; Mackay, D. J. Phys. Chem. Ref. Datu 1986, 15, 911-929.
Dunnivant, F. M.; Coates, J. T.; Elzerman, A. W. Environ. Sci. Technol. 1988, 22, 448-453.
Atlas, E.; Foster, R.; Giam, C. S. Environ. Sci. Technol. 1982, 16, 283-286.
Oliver, B. G. Chemosphere 1985, 14, 1087-1106. Murphy, T. J.; Mullin, M. D.; Meyer, J. A. Environ. Sci. Technol. 1987,21, 155-162.
Piringer, 0.; Skories, H. In Analysis of Volatiles;Schreier, P., Ed.; Walter de Gruyter & Co.: Berlin, New York, 1984; pp 49-60.
Fendinger, N. F.; Glotfelty, D. W. Environ. Sci. Technol. 1988,22, 1289-1293. Wasik, S. P.; DeVoe, H.; Miller, M. M. J. Res. Nutl. Bur. Stand. (U.S.) 1981, 86, 361-366. Ballschmiter,K.; Zell, M. Fresenius Z. Anal. Chem. 1980, 302, 20-31. Sachs, L. Angewandte Statistik; Springer-Verlag: Berlin,
1978. Hawker, D. W. Environ. Sci. Technol. 1989,23, 1250-1253. Hawker, D. W.; Connell, D. W. Environ. Sci. Technol. 1988, 22, 382-387. Kier, L. B.; Hall, L. H. Molecular Connectivity in Struc-
ture-Activity Analysis; Research Studies Press: Letchworth, England, 1986. Nirmalakhandan, N. N.; Speece, R. E. Environ. Sci. Technol. 1988,22, 1349-1357. Sablijc, A.; Gusten, H. Chemosphere 1989, 19, 1503-1511.
Received for review April 10, 1990. Revised manuscript received July 2, 1990. Accepted July 24, 1990.
