Environ. Sci. Technol. 1989, 23, 1250-1253
Vapor Pressures and Henry's Law Constants of Polychlorinated Biphenyls Darryl W. Hawker
Division of Australian Environmental Studies, Griffith University, Nathan, Queensland, 41 11 Australia The relationships between the logarithms of recent and literature values of subcooled vapor pressure (PL) Henry's law constant (H) for polychlorinated biphenyl (PCB) congeners with planar total surface area (TSA) have been investigated. While log PLforms a significant linear relationship allowing prediction of PLfor all congeners, log H does not. When a previously developed relationship between the logarithm of subcooled aqueous solubility (Sd and planar TSA is utilized, values of H for all congeners may be obtained from correlated values of PL and SL. When calculated in this manner, H values for PCBs are found to vary from 118 to 2.8 Pa m3 mol-'. The subtle influences of chlorine substitution can be identified, as shown by an increasing value of H with ortho chlorine substitution for a given chlorine number.
Introduction In modeling the fate of environmental contaminants, knowledge of physicochemical parameters such as the 1-octanol/water partition coefficient (KO,), aqueous solubility (S),vapor pressure (PI, and Henry's law constant (H) is critically important (1). For many organic chemicals of environmental relevance such as polychlorinated biphenyls (PCBs) however, extremely low aqueous solubilities and vapor pressures or large KO,values present difficulties with direct measurement (2). This had led to their estimation from molecular descriptors such as molecular volume or total surface area (TSA). Values of KO,and subcooled aqueous solubility (SL)for all PCB congeners can now be determined from their TSA in a hypothetical planar configuration (3),and by the use of linear solvation energy relationships involving molar volume and solvatochromatic parameters measuring solute-solvent interactions ( 4 , 5 ) . Relatively little experimental data exist on the vapor pressures and Henry's law constants of PCBs (2, 6, 7). These properties are used to calculate airlwater interphase exchange rates, which are of interest due to the importance of atmospheric transport in distributing PCBs on both a local and a global basis (8). The existing data indicate considerable variation in the magnitude of these physicochemical properties for isomers of a given chlorine substitution level. A similar variation was encountered in KO, values, but correlation was achieved by the use of TSA in a hypothetical planar configuration, which suggests the use of this parameter in correlating PL and H values of PCBs. The aim of this study then was to investigate whether experimentally determined vapor pressures and Henry's law constants could be correlated with planar TSA so that values for the remaining congeners could be estimated. Experimental Data The vapor pressures used have been experimentally determined by a number of techniques. Murphy et al. (9) measured values directly using a headspace sampling method. Westcott et al. used a semimicro gas saturation method (IO),while Bidleman measured PCB vapor pressures from capillary gas chromatographic retention times (11). Additionally, data from a compilation by Burkhard et al. (12)are included where they have not been drawn 1250
Environ. Sci. Technol., Vol. 23,No. 10, 1989
from the above primary sources. In total, the data base consists of 117 PCB vapor pressures (Pa), covering all levels of chlorine substitution. A similar number of Henry's law constant measurements (Pa m3 mol-l) are available. Data are taken from a review by Mackay (2),where they are calculated from the ratio together with a study of of selected values of PL and SL, Giam (13) in which a direct, aqueous-phase stripping method of determination is employed. These are supplemented by more recent data derived from equilibrium air and water concentrations (9) and by use of the stripping method (7). Data exist for congeners at all levels of chlorine substitution except for nonachloro-substituted PCBs. In general, physicochemical properties of solids are functions of molecular size and melting point, whereas for liquids and subcooled liquids, they are functions of molecular size only (2). Therefore to identify any possible relationship with planar TSA, subcooled liquid values are most appropriate for solid (mp > 25 OC) congeners. Accordingly, all solid vapor pressures are converted into subcooled liquid values (PL) by using the melting point of the congener of interest and a constant entropy of fusion (ASf) of 54.8 J K-' mol-'. This value of ASf has been found to be the average for a diverse group of PCBs (14). Where Henry's law constants were calculated, values of PL and SLwere employed. Due to the paucity of relevant data, PL and H values obtained at temperatures ranging from 20 to 25 "C are combined for the purposes of this investigation. To enable comparison with similar data for related groups of compounds that are usually measured at 25 O C , values derived from correlation are nominally referred to 25 "C. Details m2) of the calculation of planar TSA values (A2or from van der Waals radii of component atoms and appropriate bond distances and angles are found in an earlier paper (3).
Results Linear regression of log PL (Pa) for the compiled data against planar TSA affords a significant linear relationship expressed by log PL= -4.88 X planar TSA + 9.40 (1) n = 117 r = 0.962 Syz = 0.327 A plot of this data, identifying data source, is presented in Figure 1, with the linear regression equation (eq 1) represented by the solid line. An analagous regression analysis with log H for the compiled data, however, results in a much poorer correlation with planar TSA. The linear regression equation is log H = -1.15 X planar TSA + 4.11 (2) n = 128 r = 0.605 SYz= 0.327 The scattered nature of these data is evident in Figure 2, in which data source is again distinguished. On considering the relationship of log H with planar TSA from each data source individually, considerable variation in significance and precision is observed. Table I contains linear regression equations and statistics for all
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Table I. Linear Regression Equations and Statistics from the Correlation of Henry's Law Constants with Planar TSA equation log H = -1.34 X plan= log H = -8.57 x 10-3pianar log H = -1.43 X lo4 plana log H = -4.43 X lo-' plan=
statistics
TSA + 4.45 TSA + 3.44 TSA + 2.26 TSA + 2.58
n = 73,S, n = 20, S, n = 10,S, n = 25, S,
ref
= 0.155,r = 0.85 = 0.303,r = 0.46 = 0.040,P = 0.35 = 0.484,r = 0.26
9 7 13 2 x
h
m
& na
-w 0
X
0 150 150
200
250
300
300
250
200
350
350
TSA TSA Figure 1. Plot of log PLversus planar TSA showing the overall linear regression line, with data from Murphy (W) (9), BMleman ( 0 )( 7 I ) , Burkhard (+) (72),and Westcotl (X) (10).
four Henry's law constant data sources. Inconsistency in regression slope and intercept is noted between all data sources, and correlation coefficients vary from 0.26 to 0.85. The most significant equation, however, predicts H to within an accuracy of a factor of 10°.'66 or 1.42.
Discussion Figure 1 and eq 1 demonstrate clearly that log PL is a linear function of planar TSA. It must be remembered that this measure of surface area is a hypothetical one for most PCB congeners, particularly those with chlorine substituents in ortho positions. The significance of this relationship is relatively high despite slightly variable experimental temperatures, different data sources, and different measurement techniques. Previously, Burkhard et al. (12)found a similar relationship, but using a more limited data base and nonplanar TSA. From the standard error of estimate (5' ) of eq I , where y is log PL and x planar TSA, PLcan estimated to within an accuracy of or 2.12. For vapor pressures of polya factor of chlorinated dibenzodioxins, estimation to within a fador of 5 was considered to provide data of sufficient reliability for most environmental assessment purposes (I). Planar TSA values can be relatively easily obtained by using computer algorithms, and a complete tabulation for all PCB congeners may be found in an earlier paper (3). Therefore, by use of the above criterion, reliable subcooled vapor pressures for all PCBs regardless of chlorine substitution pattern or level can be derived with eq 1 and then converted in solid vapor pressures by using melting point and ASf = 54.8 J K-' mol-' (3, 14). To give some indication of the range of vapor pressures for PCBs when eq 1 is used, PL for biphenyl and deca-
{e"
Flgure 2. Plot of log H versus planar TSA showing the overall linear regessbn he, with data from Murphy C)(9),Dunnlvant (D) (7),Giam (+) (73),and Mackay (X) (2).
chlorobiphenyl are 2.5 Pa and 3.5 X lo4 Pa, respectively. These translate into solid vapor pressures of 0.94P a and 6.9 X 10-9 Pa, which may be compared with some literature values of 1.15 and 1.01 P a (12,15)and 2.8 X lo* and 2.8 X 10-OPa (derived from subcooled liquid values) (1I,16). The apparent lower precision with decachlorobiphenyl may reflect in part the larger experimental error associated with measurement of such small vapor pressures. By way of comparison with analagous dibenzodioxins, PL for the unsubstituted parent compound is 0.5 Pa, while for octachlorodibenzodioxin, a value of 1.2 X Pa has been reported (I). The relatively poor correlation between log H and planar TSA shown in Figure 2 and represented by eq 2 is consistent with previous observations of little overall relationship or systematic trend for Henry's law constants of PCBs with descriptors such as molecular weight and chlorine number (6,7).The Henry's law constant may be regarded as an airlwater partition coefficient and is often derived from the ratio of PL/SL. Figure 3 represents a plot of 167 PCB subcooled aqueous solubilities of congeners containing all levels of chlorine substitution versus planar TSA. The solubilities were obtained from use of either a generator column, a circulating water saturation system, or an enclosed equilibrated airlwater system and can be considered reasonably reliable (7, 9, 14, 17,18). Linear regression of subcooled values against planar TSA results in log SL = -3.52 X
n = 167
planar TSA
r = 0.922
+ 4.82
(3)
SYz= 0.370
which allows estimation to within a factor of 2.3,which is sufficient for investigations of environmental fate ( I ) . Environ. Sci. Technol., Vol. 23, No. 10, 1989
1251
I
01
-1
h1 \:.
-2
1 C . m
9 E
..
\ 150
200
250
300
350
TSA planar TSA showing the overall linear regression line, with data from Murphy (S),Opperhuizen ( 78), Miller ( 7 4 ) , Dickhut (77), and Dunnivant ( 7 ) . Flgure 3.
Plot of
SL versus
Both log PL and log SLare therefore linearly related to planar TSA for all congeners, and by simple subtraction of eq 3 from eq 1 log H = log P L - log S L = -1.36 X planar TSA + 4.58 (4) The slope and intercept of this expreasion differ from those of eq 2, which was derived from experimental values. It is unlikely that the approximation of H from PL/SLis invalid for PCBs. It is generally accepted that Henry’s law constants derived in this manner break down when vapor pressures exceed (0.5-1) X lo6 Pa, aqueous solubilities exceed 1.6 X lo3 mol m-3, or both (6). Of the PCB congeners, biphenyl possesses among the highest vapor pressures (approximately 1 Pa) and aqueous solubilities (4.5X mol m-3) (2). These values are clearly within the limits of applicability stated above. In addition, correlation of log H calculated from the ratio of P L / S L with molar volume has been achieved for similar molecules such as polychlorinated dibenzodioxins (1). The observed poor correlation of experimental data and their inconsistency with values derived from eq 4 may be due to a number of factors. It may reflect a propagation of errors involved in the determination of vapor pressure and solubility both experimentally and from planar TSA. As illustrated in Figures 1 and 2, planar TSA is not a perfect description of either process. The calculation of planar TSA itself may involve some error. While the assumption of a constant AS, for all congeners introduces error into the derivation of SLand PLindividually, if H is given by PL/SL, this error will cancel for H. To assess the possible influence of experimental method the data are subdivided and considered on the basis of source and method (Table I). Murphy obtained H for PCBs possessing up to seven chlorines from direct measurement of vapor pressure and aqueous concentration within an enclosed, equilibrated air/water system (9). When regressed against planar TSA, log H i s found to be highly correlated, and the regression equation log H = -1.34 X
planar TSA
+ 4.45
(5)
is very similar to eq 4. This is perhaps not surprising since both expressions involve determination of H from vapor 1252
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10, 1989
pressure and aqueous solubility. While Mackay (2) also derived H in this manner, little relationship is found with planar TSA, possibly because of inaccuracy associated with the selected experimental measurements. For example, calculated H values for 2,2’,4,4/,6,6/-hexachlorobiphenyl and 2,2’,3,3‘,4,4’,6-heptachlorobiphenyl of 818 and 5.4 Pa m3 mol-’ arise from subcooled aqueous solubilities of 1.5 X and 4.6 X mol m-3 and subcooled vapor pressures of 1.2 X and 2.5 X lo4 Pa, respectively. As a comparison, on the basis of eq 3,correlated solubilities are 8.5 X and 1.6 X mol m-3, and from eq 1, vapor pressures are 1.2 X and 1.2 X lo4 Pa, respectively, affording H values of 14 and 7.3 P a m3 mol-’. The remaining two Henry’s law constant data sources employed the aqueous-phase stripping method, but for both, poor correlation with planar TSA is observed, and the regression equations are dissimilar to eq 4. The accuracy of this method is dependent upon the sensible attainment of equilibrium between the solute in the purge gas leaving the system and that in aqueous solution, which may not be achieved in all cases (6). Furthermore, the method was developed with relatively hydrophilic chemicals (naphthalene, chlorobenzene,ethylbenzene),and its applicability for very hydrophobic PCBs is uncertain. Recently, Valsaraj suggested that such compounds have a greater affinity for the air/water interface than the air phase itself (19)) indicating that consideration of both phases in Henry’s law constant determination may be necessary. When eq 4 is utilized to determine H for PCB congeners, values range from 118 Pa m3 mol-’ for biphenyl to 2.8 Pa m3 mol-’ for decachlorobiphenyl. As a comparison, Henry’s law constants for polychlorinated dibenzodioxins range from 15 to 0.13 Pa m3 mol-’. Use of eq 4 in deriving H also enables the subtle influence of chlorine-substitution patterns to be revealed. For a given chlorine number, subcooled vapor pressures and aqueous solubilities of isomers may vary by an order of magnitude or more, with increasing ortho chlorine substitution resulting in larger values. This trend is also apparent with Henry’s law constants, though the variation is much less than that observed with PLand SLand confirms earlier findings based on more limited data sets (6, 11). These results indicate that the more toxic non-ortho chlorine substituted PCBs possess lower subcooled vapor pressures and partition into the air to a lesser extent than other isomers (7, 20).
Conclusions A significant linear relationship exists between the logarithm of the subcooled vapor pressure and planar TSA (A2 or m2) for PCBs, similar to one previously found with log KO,,,. Planar TSA calculations are relatively easily made so that reliable vapor pressures are now available for all congeners of sufficient reliability for use in environmental fate modeling. When combined with a significant linear relationship between the logarithm of subcooled aqueous solubility and planar TSA based on recent experimental data, a new relationship may be developed, in which log H is a linear function of planar TSA, enabling estimation of H for all congeners. Available experimental data are unable to satisfactorily confirm this relationship because of propagation of errors involved in determining vapor pressure and aqueous solubility individually, particularly for very hydrophobic compounds such as PCBs, and additionally because the commonly used aqueousphase stripping method for Henry’s law constant determination may be inappropriate. Based on the linear relationship developed in this investigation, H values of PCBs are found to vary between
Environ. Sci. Technol. 1989,23, 1253-1258
Murphy, T. J.; Mullin, M. D.; Meyer, J. A. Environ. Sci. Technol. 1987, 21, 155. Westcott, J. W.; Simon, C. G.; Bidleman, T. F. Environ. Sci. Technol. 1981, 15, 1375. Bidleman, T. F. Anal. Chem. 1984, 56, 2490. Burkhard, L.P.;Andren, A. W.; Armstrong, D. E. Environ. Sei. Technol. 1985, 19, 500. Atlas, E.; Foster, R.; Giam, C. S. Environ. Sei. Technol. 1982, 16, 283. Miller, M. M.; Ghodbane, S.;Wasik, S. P.; Tewari, Y. B.; Martire, D. E. J . Chem. Eng. Data 1984,29, 184. Burkhard, L.P.;Armstrong, D. E.; Andren, A. W. J. Chem. Eng. Data 1985,29, 248. Dobbs, A. J.; Cull, M. R. Environ. Pollut., Ser. B 1982,3, 289. Dickhut, R.M.; Andren, A. W.; Armstrong, D. E. Environ. Sci. Technol. 1986, 20, 807. Opperhuizen,A.; Gobas, F. A. P. C.; Van der Steen, J. M. D.; Hutzinger, 0. Environ. Sei. Technol. 1988, 22, 638. Valsaraj, K.T. Chemosphere 1988, 17, 875. McKinney, J. D.; Singh, P. Chem.-Biol. Interact. 1981,33, 271.
118 and 2.8 Pa m3 mol-’. Increasing ortho chlorine substitution results in larger values of both PLand H, so that non-ortho chlorine substituted isomers have the lowest values for a given chlorine number. Such PL and H data should enable congener-specific environmental behavior for PCBs to be more accurately determined.
Literature Cited Shiu, W. Y.; Doucette, W.; Gobas, F. A. P. C.; Andren, A.; Mackay, D. Environ. Sci. Technol. 1988, 22, 651. Shiu, W. Y.;Mackay, D. J. Phys. Chem. Ref. Data 1986, 15, 911. Hawker, D. W.; Connell, D. W. Environ. Sei. Technol. 1988, 22, 382. Doucette, W. J.;Andren, A. W. Environ. Sci. Technol. 1987, 21, 82. Kamlet, M. J.; Doherty, R. M.; Carr, P. W.; Mackay, D.; Abraham, M. H.; Taft, R. W. Environ. Sci. Technol. 1988, 22, 503. Burkhard, L.P.;Armstrong, D. E.; Andren, A. W. Environ. Sei. Technol. 1985, 19, 590. Dunnivant, F. M.; Elzerman, A. W. Chemosphere 1988,17, 525. Doskey, P.V.; Andren, A. W. Environ. Sci. Technol. 1981, 15, 705.
Received for review October 3,1988. Revised manuscript received April 7, 1989. Accepted May 22, 1989.
Long-Term Measurements of Atmospheric Polychlorinated Biphenyls in the Vicinity of Superfund Dumps Mark H. Hermanson+ and Ronald A. Hites”
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 The concentrations of polychlorinated biphenyls (PCBs) in the atmosphere of Bloomington, IN, were analyzed over a period of 22 months. The sampling sites were within 14 km of three landfills contaminated with PCBs. The atmospheric PCB concentrations varied with the atmospheric temperature; thus, there was a large seasonal component to the data. The vapor-phase PCB concentrations averaged 1.7-3.8 ng m-3 in the summer and 0.27-0.58 ng m-3 in the winter. Particulate-phase PCBs did not exhibit consistent chan es with season; the concentrations averaged 0.04 ng m- The logarithm of the ratio of the PCB concentration in the vapor phase to that in the particulate phase was a linear function of reciprocal absolute temperature. Atmospheric PCB concentrations in Bloomington differed only by a factor of 2-3 compared to other areas in the Great Lakes region, indicating that the atmosphere may effectively disperse PCBs within short distances from sources.
f.
Introduction Polychlorinated biphenyls (PCBs) are ubiquitous contaminants in the Earth’s atmosphere (1-16). Previous studies have given us some information on typical atmospheric concentrations, geographical variability, and vapor-particulate partitioning. However, there are two problems with these data. First, because data reporting schemes have evolved over time, there is a considerable variation in the published data. For example, some studies have identified the commercial PCB mixture (6-8,10,12); some studies have quantitated “total PCB” as the sum of ‘Present address: Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53204. 0013-936X/89/0923-1253$01.50/0
two (9,13,12)or three ( 6 , I O ) commercial mixtures; and some studies have identified representative PCB congeners (I,3,II). Other studies have reported PCB concentrations in both vapor and particulate phases (I, IO, I3), while others have reported only a “PCB” value (2-5, 7-9). The second problem relates to seasonal variability. In our earlier studies on Isle Royale in Lake Superior ( I ) , we observed a large seasonal variation in atmospheric PCB levels. For example, the PCB concentrations in the summer were 5 times higher than the concentrations in the winter. If there is a large seasonal variation, this further complicates the comparison of the data cited above. We, therefore, thought it was important to determine the seasonal variability of PCB concentrations at one location. If it were possible to understand the functional relationship between atmospheric concentration and temperature, it would then be possible to correct future atmospheric measurements to a common temperature in order to achieve comparability. Our study used long-term sampling at multiple sites within a local area, employed quantitation of a large number of PCB congeners in both vapor and particulate phases, and included temperature observations. Specifically, we sampled airborne vapor and particulate-adsorbed PCBs at three sites in the Bloomington, IN, area from October 1986 to August 1988. Long-term sampling enabled us to observe seasonal changes in individual congener and total vapor and particulate PCB concentrations, changes in vapor-to-particulate partitioning, and variability of PCB concentrations among the three sites. In addition, this study provides base-line atmospheric PCB concentrations for the Bloomington area, where a trash-fueled incinerator is proposed for thermal destruction of 650000 m3 of
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 10, 1989 1253
