Environ. Sci. Technol. 1997, 31, 3736-3737
Comment on “Partide/Gas Concentrations and Distributions of PAHs in the Atmosphere of Southern Chesapeake Bay” SIR: The recent contribution of Gustafson and Dickhut (1) presents experimental data for the particle/gas partitioning of PAHs. I would like to make several comments concerning these data and their interpretation. (i) As is common practice, the authors use the following equation for analysis of their experimental data:
log Kp ) -mr log p°L + br
(1)
where Kp represents the measured sorption coefficient of the studied compounds and p°L is the corresponding saturated vapor pressure over the pure liquid. I have two comments concerning the interpretation of the slope mr in this equation. (a) The authors state that the slope mr should theoretically equal unity and that significant deviations from unity point to non-equilibrium sorption or non-exchangeable PAHs. However, I would like to argue that this conclusion is not compelling since the slope mr can easily deviate from unity for equilibrium sorption processes without any artifacts. For partitioning processes of a compound between two phases, the following relationship is known:
∆Ge ) -RT × 2.303 × log × K
(2)
where ∆Ge is the excess free energy of the transfer of the considered compound between the two phases and K is the thermodynamic partition coefficient. This equation can be applied to the partitioning of a compound between air and its pure liquid phase (evaporation/condensation) where ∆Ge is the excess free energy of evaporation and log K ) log p°L + constant: e ∆Gevap ) -RT × 2.303 × log × p°L + constant
(3)
Similarly, it can be applied to the partitioning between the gas phase and any condensed phase (sorption/desorption) in which case ∆Ge is the excess free energy of the sorption process and log K ) log Kp + constant: e ∆Gsorp ) -RT × 2.303 log Kp + constant
(4)
The latter is valid no matter whether the sorption process is of the absorption or the adsorption type. Dividing eq 4 by eq 3 and rearranging yields an e equation with the principal form of eq 1 (note: ∆Gsorp ) e -∆Gdesorp):
log Kp ) -
e ∆Gdesorp e ∆Gevap
log p°L + br
(5)
This equation demonstrates that the slope of a plot of log Kp vs log p°L depends on the gain in excess free energy in the sorption process as compared to the process of condensation. Only if both processes are energetically similar will the slope be close to unity. However, it is easily conceivable that the slope differs significantly from unity depending on the sorption medium. Experimental evidence for slopes that are significantly different from unity in case of adsorption as well as absorption come from gas chromatographic data (2-5). Therefore, I do not agree to the authors statement that a slope which deviates from unity points to non-equilibrium
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or to a change in the type of sorption process within the set of compounds (p 144). A slope that differs from unity may simply indicate that the sorption medium is significantly different from the pure liquid phase of the studied compounds with respect to intermolecular interactions. (b) Likewise, I do not comply with the authors statement (p 144) that potential affects of relative humidity (RH) do not influence the slope in eq 1. Adsorption on hydrophilic surfaces (such as many minerals) is influenced by RH in such a way that water preferentially sorbs to the surface so that organic pollutants can only adsorb onto the adsorbed water layer in cases where complete water coverage is reached (usually above 30% RH (6)). The interaction free energy between organic compounds and adsorbed water is smaller than between the molecules and the original surface (7), and thus the presence of water leads to a reduction in the adsorption coefficient of the organic compound. As a consequence, this must have an impact on the slope in the above eq 5 and eq 1 since the influence of water is connected to its influence on the interaction free energy of the sorption e process ∆Gdesorp . Both points (a and b) are corroborated by the experimental data from Dorris and Gray (8) for the sorption of alkanes on silica at different relative humidities where the slopes are as follows:
25.7% RH:
mr ) -0.82 ( 0.00
r 2 ) 1.00
n)6
62.3% RH:
mr ) -0.74 ( 0.00
r 2 ) 1.00
n)6
88.0% RH:
mr ) -0.68 ( 0.00
r 2 ) 1.00
n)6
(ii) Throughout the text, the authors frequently use the term ‘condensation’. However, under ambient conditions, the relative gas phase concentrations of PAHs are always smaller than p/p° ∼ 1, and the transfer never occurs from the gas phase to a pure PAH phase. Thus, the removal of PAHs from the gas phase under ambient conditions falls into the category of sorption (be it adsorption or absorption) not condensation. (iii) Comments on the enthalpies ∆H of the sorption process: (a) On page 145, the authors state that enthalpies of desorption and vaporization are expected to vary little for a particular compound class. However, the intermolecular interactions of PAHs and other non-polar compounds that determine their sorption and vaporization behavior are dominated by van der Waals forces, which are known to depend on the size of the molecule. The molecular size may vary greatly within one compound class and so do the enthalpies of partitioning. The alkanes may serve as an illustration: the enthalpy of evaporation of pentane is 26.4 kJ/mol whereas the enthalpy of evaporation of decane is almost twice as high (51.4 kJ/mol) (9). Similar differences can be found for the enthalpies of sorption (3, 4). These differences have an great impact on the sorption coefficients Kp that depend exponentially on thermodynamic entities like ∆H. Thus, the averaging of enthalpies of sorption for whole compound classes as it is done in Table 2 is questionable. (b) The enthalpies of desorption ∆Hdesorp that can be calculated from the data in Table 2 for PAH sorption on the Haven Beach site particles (and to a lesser extent on the Hampton site particles) are extremely high. Usually enthalpies of desorption lie in the range of the enthalpies of evaporation ∆Hevap not only for PAHs but for all kinds of non-polar organic compounds. For example, experimental ∆Hdesorp from the literature for phenantrene lie around 76 kJ/mol (10-12), which compares to ∆Hevap of 75 kJ/mol (9).
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1997 American Chemical Society
In Table 2, the authors report a value for phenanthrene that corresponds to ∆Hdesorp ) 193 kJ/mol. This difference in enthalpy is huge and would indicate that the sorption characteristics of the Haven Beach particles are completely different from all atmospheric particles for which experimental sorption data have been measured till now. They can only be compared with enthalpies for the sorption on pure microporous carbon black (13). However, carbon black-like particles would be expected to be found in urban areas rather than in rural areas. Furthermore, under ambient conditions, carbon black surfaces would oxidize and adsorb water, which would greatly reduce the sorption enthalpy of the PAHs. Thus, I wonder whether experimental artifacts should not be considered as a possible explanation.
Literature Cited (1) Gustafson, K. E.; Dickhut, R. M. Environ. Sci. Technol. 1997, 31, 140-147. (2) Pecsok, R. L.; de Yllana, A.; Abdul-Karim, A. Anal. Chem. 1964, 36, 452-457. (3) Arancibia, E. L.; Catoggio, J. A. J. Chromatogr. 1982, 238, 281290. (4) Arancibia, E. L.; Catoggio, J. A. J. Chromatogr. 1980, 197, 135145.
(5) Castells, R. C.; Arancibia, E. L.; Nardillo, A. M. J. Phys. Chem. 1982, 86, 4456-4460. (6) Goss, K.-U.; Eisenreich, S. J. Environ. Sci. Technol. 1996, 30, 2135-2142. (7) Bernett, K. M.; Zisman, W. A. J. Colloid Interface. Sci. 1968, 29, 413-423. (8) Dorris, G. M.; Gray, D. G. J. Phys. Chem. 1981, 85, 3628-3635. (9) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1995. (10) Pankow, J. F.; Storey, J. M. E.; Yamasaki, H. Environ. Sci. Technol. 1993, 27, 2220-2226. (11) Baek, S. O.; Godstone, M. E.; Kirk, P. W. W.; Lester, J. N.; Perry, R. Chemosphere 1991, 22, 503-520. (12) Subramanyam, V.; Valsaraj, K. T.; Thibodeaux, L. J.; D. D., R. Atmos. Environ. 1994, 28, 3083-3091. (13) Ludwig, S.; Schmidt, H.-D. J. Chromatogr. 1990, 520, 69-74.
Kai-Uwe Goss Swiss Federal Institute for Environmental Science and Technology (EAWAG) CH-8600 Du ¨ bendorf, Switzerland ES970508A
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