Envlron. Sci. Technol. 1982, 16, 75-78
Isosteric Heats of Adsorption of Selected Compounds on Diesel Particulate Matter Mark M. Ross and Terence H. Rlsby”
Division of Environmental Chemistry, Department of Environmental Health Sciences, The Johns Hopkins University, School of Hygiene and Public Health, Baltimore, Maryland 21205 Samuel S. Lestzt and Ronald E. Yasbid
Department of Mechanical Engineering and Department of Microbiology, Cell Biology, Biophysics, and Biochemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Isosteric heats of adsorption were determined for 18 compounds with low injection volumes. The isosteric heats of adsorption of benzene and hexane on the pretreated Diesel samples were compared with those of the same compounds on a graphitized carbon black. The peak shapes of the Diesel-sample chromatograms and the relationship between the injection volume and the retention volume are evidence of a heterogeneous surface and a type I1 adsorption isotherm. For two compounds, benzene and hexane, the effect of varying the surface coverage, by changing the injection volume, on the adsorption heats was determined. The differences between the heats of adsorption measured for the two Diesel samples is attributed to the difference in the quantities and identities of the extractable compounds (or presorbed materials) on the particulate samples. The adsorption energies reported are significant with respect to the bioavailability of the sorbed species.
Introduction In 1977 the US. Environmental Protection Agency (1) projected that by 1985 25% of new automobiles sold in the United States of America would be equipped with Diesel engines. This change from gasoline engines to Diesel engines may be environmentally significant since it is known that combustion in a compression-ignitionengine produces 70 times more particulate matter than in a conventional spark-ignition engine. Therefore, it is important to investigate the Diesel engine and its emissions for its potential effect on public health as a result of the different future air pollution. From the time in the Diesel combustion process where the particles are formed in a heterogeneous turbulent flame until they are exhausted into the atmosphere, the Diesel particles are in contact with numerous organic and inorganic compounds in an environment of rapidly decreasing temperature. A number of studies have estimated that more than 10000 different compounds can be found in Diesel exhaust ( 2 ) ,and many of these are known to be potentially carcinogenic or mutagenic. The majority of these compounds have been shown to be associated with the particulate matter, and, depending upon the engine operating parameters, as much as 50% by mass of the particulate matter can be extracted with dichloromethane. The particles alone are potential health hazards since 80-90% by mass are less than 1 hm in diameter (3) and are, consequently, easily respired into the deep alveolar regions of the lung (4). Since these particles have significant lifetimes in the lung and could possibly have toxic species sorbed on their surfaces, there is a probability that Department of Mechanical Engineering. *Departmentof Microbiology,Cell Biology, Biophysics, and Biochemistry. 0013-936X/82/0916-0075$01.25/0
adverse health effects could arise from their inhalation. In this context, a number of studies have reported on the elution of polycyclic aromatic hydrocarbons from carbon soot particles by biological fluids (5, 6). Also, there have been many investigations into the chemistry that can take place at the gas-solid interface between adsorbed compounds, pollutants present in the atmosphere, and sunlight (7-11). The bioavailability of the adsorbed species, the probability of certain substances adsorbing onto the particle surfaces, and the likelihood of atmospheric reactions will to a great extent be determined by the energy of adsorption and the nature of the adsorbate-adsorbent forces involved in the sorption. Although much work has been done in the area of chemical analysis of the particles emitted by a variety of sources using ESCA, SIMS, AES, and GC/MS (12-15,3), fewer reports have dealt with their physical properties. Diesel particulate matter was determined to have a high surface area, implying a high adsorptive capacity (16,17), but, since these reports, there have been relatively few additional studies on the particulate porosity, surface area, and adsorption characteristics of particles. These fundamental physical properties of particulate matter should enable a better understanding of the chemistry of the particles and of their resultant environmental health implications. This paper represents the initial step in the determination of the physical characteristics of Diesel particulate matter. In this work, gas-solid chromatography was used to measure the heats of adsorption of some selected compounds on a standard graphitized carbon black and particles collected from two different Diesel engines. This dynamic adsorption measurement technique was employed so that the results obtained could be compared with those obtained with the standard static method, which are currently in progress in this laboratory. It is important to make both measurements in order to characterize the particle surface. One aspect of this study was treatment of particulate samples by heating and flushing with helium in the column and heating under vacuum prior to adsorption measurement. The effects of the variation of the injection volume on the measured adsorption heats was also determined. The heats of adsorption for some basic hydrocarbons on different carbon particle samples at varying pretreatments are expected to allow insight into the nature of the particulate surfaces and into the gas-solid interactions involved in the adsorption of vapors onto Diesel particulate matter. Theory The specific method used in this study has been reported previously by Ross et al. (18)to measure the heats of adsorption for molecules on graphitized carbon blacks. These workers showed that, if adsorbate surface concen-
0 1982 American Chemical Society
Environ. Sci. Technol., Vol. 16, No. 2, 1982
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tration tends to zero, then the following expression can be derived: d(ln t,')/d(l/TJ = qst/R (1) where t,' is the corrected retention time and T , is the column temperature. Therefore, if a plot of log t,' vs. 1/T, is made, then the resulting straight line has a slope equal to qst/2.3O3R (where qst is the limiting isosteric heat of adsorption). This value of the heat of adsorption was determined to be appropriate for comparison with those measured by the static method. Since the flow rate changed from sample to sample as a result of differences in the packing densities of the adsorbents, the specific retention volume was used instead of the retention time. The specific retention volume was calculated by using the following standard expression:
where t,' is the corrected retention time, F is the flow rate, m is the mass of the adsorbent in the column, T, is the column temperature, Tf is the temperature at the bubble flowmeter, and Pi and Poare the pressure at the inlet and the outlet of the column, respectively. Experimental Section A gas chromatograph with a thermal conductivity detector (Varian 920) was used for all of these studies with a glass column (2-mm i.d., 25 cm long) packed with the adsorbents (Spheron, 0.3 g; DPM-PSU, 0.1 g; and DPMEPA, 0.06 g). Helium (HP, Matheson) was used as the carrier gas with a flow rate in the range of 1.5-3 mL/min. The retention times of the various compounds were measured over a range of column temperatures (50-300 "C) with the various adsorbents. Three adsorbents were investigated in this study. The first was graphitized Spheron 6 (Graphon), with an approximate surface area of 80 m2/g (19) with no pretreatment. The second was Diesel particulate matter collected from an Avco-Lycoming Bernard W-51 industrial engine at The Pennsylvania State University with a fuel consisting of a 1:l by volume mixture of n-tetradecane and 2,2,4trimethylpentane with an approximate surface area of 104 m2/g (19). The third adsorbent waa Diesel particulate matter collected from an Oldsmobile 350 engine by the U.S. Environmental Protection Agency (run nos. 8311-8319) with an approximate surface area of 112 m2/g (19). In addition to measuring retention volumes of various compounds and columns packed with raw Diesel particles, we also obtained chromatographic data after each of the following pretreatments: (1)300 "C with helium flow, in the column, for 15 h and (2) 500 "C under vacuum torr) for 15 h followed by the first treatment. The following compounds were used as the adsorbates: benzene, cyclohexene, cyclohexane, hexene, hexane, methanol, octane, ethylbenzene, benzaldehyde, acetophenone, naphthalene, phenol, phenanthrene, fluorenone, fluoranthene, and anthracene (all 99+% Aldrich reagents). The liquid compounds were introduced separately onto the column in a pulse size of 0.01-10.0 pL. The solid compounds were introduced in solution (25% in methylene chloride) in a pulse size of 1.0 pL. The column void volume was obtained from the air peak. Results and Discussion The specific retention volumes of each adsorbate on the various adsorbents were calculated from corrected retention times by the application of eq 2. These data were then 76
Environ. Scl. Technol., Vol. 16, No. 2, 1982
OIDPN-EPA A DPM-PSU
, ,
0.05 U L , 0 05 AIL ,
8 8 KCAL/MOLE 8 Q KCAL/MOLE
2.504 w
2
1
1
2 304
1
70'
2 . h
2.b0
2.80
2.50
2.60
2.Q0
3,00
3.;0
I000/T
Flgure 1. Gas-chromatographic log V , vs. 1/T, plots for benzene.
Table I. Isosteric Heat of Adsorption (kcal/mol) for Various Graphitized Carbon Blacks adsorbent graphitized carbon blacks adsorbate Spheron 6 ref 20 ref 21 benzene 8.7 9.4 9.8 n-hexane 9.4 10.1 10.4
plotted against column temperature by using the relationship expressed in eq 1 to obtain the apparent heats of sorption or the isosteric heats of adsorption. Examples of these plots are shown in Figure 1. The major source of error in the determinations was the variation of the retention time. Consequently, the error associated with the heats is *5%. Isosteric Heat of Adsorption on Spheron 6. Preliminary experimenb were performed with Spheron 6 to compare the values obtained for the isosteric heats of adsorption for benzene and n-hexane with the heats obtained with gas chromatography by other workers (20,21) for similar adsorbents. Injection volumes (1.0 pL) of adsorbates, which correspond to -1% coverage of the available surface area of the adsorbent, were used since this coverage falls within the Henry's law region of the adsorption isotherm (22). The chromatographic peaks obtained with Spheron 6 were sharp and symmetrical with very little tailing which is characteristic of a homogeneous and relatively nonporous surface. The values of the heats obtainedin this study as compared to the previous studies are contained in Table I. The heats for the Spheron 6 are low compared with the heats on completely graphitized carbon blacks since graphitized Spheron 6 is known to have residual heterogeneity (Le., it is not 100% C H) in its surface. Pretreatment of Diesel Particulate Matter. The samples of Diesel particulate matter which were used in this and subsequent studies have significant quantities of sorbed materials on their surfaces (PSU 5% extractable (22)and EPA 18% extractable into dichloromethane (2)).These sorbed species will play significant roles in the surface properties of the particles. Therefore, the effects of various pretreatments were studied, and the apparent heats of sorption of selected hydrocarbons were used to monitor these pretreatments. Unfortunately, the injection volume used (1.0 pL) yielded heats of adsorption less than the adsorbate heat of vaporization. The adsorbate surface coverages were calculated (on the basis of BET surface areas of the adsorbents (19)) to be quite high (-40%), which must exceed the Henry's law region of the isotherm. The heats measured, therefore, have very little significance and are not reported here. Yet, since the DPM-EPA
+
-
-
particles do have a larger quantity of presorbed material than the DPM-PSU particles, high-temperature pretreatment should affect the adsorption heats on the former to a greater extent. This has been found to be the case with the static measurements which have been reported elsewhere (19). The peak shapes for the adsorbates after all pretreatments were characteristic of heterogeneous, porous surfaces and type I1 isotherm adsorbents (24). The presence of type I1 isotherms was confirmed by the observation that as the injection volume was increased the retention volume decreased (23). The equilibrium constant decreases as the amount of the adsorbate increases, and the isotherm is concave with respect to the pressure axis. Effect of Surface Coverage on the Apparent Heats of Sorption. A Diesel particle is formed in the cylinder as a result of the combustion conditions present in the turbulent, heterogeneous flame. The particles once formed are transported from the cylinder to the ambient air in the exhaust gas which contains high concentrations of species in the gas phase. Therefore, during the process of emission, the particles will be undergoing numerous collisions with the gas-phase molecules and as a result the molecules will condense or adsorb onto the surfaces of the particles. The extent of condensation will be dependent upon the vapor pressure of the adsorbate and the temperature of the exhaust gases. Typical emission rates of particulate matter of 0.622 g/mi have been observed by other workers (25) for light-duty automobile Diesel engines of which 0.0715 g/mi is extractable into organic solvents. Similarly, total hydrocarbon emission rates of 0.50 g/mi have been observed with a hot flame ionization detector. If this hydrocarbon fraction is separated by gas chromatography, 50% of the total hydrocarbons have sufficient molecular weight to adsorb or condense on the particles. These figures suggest that approximately 28% of the available gas phase adsorbs or condenses on the surfaces of the particles. The extent of the adsorption of the molecules will be related to the surface properties of the absorbent and the heat of adsorption of the adsorbate. However, once the particles have adsorbed molecules on their surfaces, the heat of sorption of multilayers is lower than the heat of adsorption of the monolayer. The particles, which are collected on the filter, contain at least 18% by weight of organic extractable compounds, which means that the particles have multilayers of molecules sorbed on their surfaces. It is reasonable to expect that, once a particle enters the ambient air, it has sufficient sorbed molecules on its surface that its properties are solely dependent upon the nature of the sorbed species. Since the bioavailability of sorbed molecules is dependent upon the nature and the energy of sorption of the active species, a study was performed in which the apparent heat of sorption was measured as a function of concentrations of the adsorbate. The following adsorbates were investigated: acetophenone, benzaldehyde, benzene, cyclohexane, cyclohexene, ethylbenzene, n-hexane, l-hexene, and n-octane. The adsorbents used in this study have been subjected to the highest-temperature activation pretreatment. Table I1 lists some examples of the results of this study. These apparent heats of sorption suggests that the adsorbate first sorbs on the most energetic adsorption sites and then sorbs on sites with lower energies. The heats continue to decrease until there is a sufficient quantity of sorbed molecules on the surface that the energy of sorption becomes the differential heat of evaporation from solution. Therefore, this study suggests that the history of collisions between the exhaust gases and the particles will have a major effect on the energy of sorption.
Table 11. Variation of Apparent Heats of Sorption with Concentration of Adsorbate apparent heat of injection sorption, adsorbate adsorbent vol, M L kcal/mol benzene DPM-EPA 0.01 12.8 0.05 0.10 1.0 10.0 0.01 0.05 0.10
DPM-PSU
1.0 n-hexane
DPM-EPA
10.0 0.01 0.05 0.1
1.0 10.0
DPM-PSU
0.01 0.05 0.1 1.0 10.0
9.8 9.1 5.9 8.7 12.0 9.9 10.4 7.6 10.5 15.0 10.6 10.4 5.4 12.8 11.4 11.4 11.9 8.4 10.7
Table 111. Isosteric Heats of Adsorption heat of adsorption, kcal/mol heat of vaporgraphitized ization, DPM- DPMcarbon adsorbate kcal/mol PSU EPA blacks (27) water methanol dichloromethane n-hexane 1-hexene benzene cy clohexene cyclohexane ethyl benzene acetophenone benzaldehyde n-octane phenol naphthalene anthracene phenanthrene
9.72 8.98 7.63 7.79 8.15 7.83 9.3 11.73 11.66 9.2 11.89 12.31 16.82 14.18
10.3 9.0 8.6 11.4 10.6 12.0 10.2 8.7 15.2 16.1 17.4 15.2 16.4 16.4
6.9 4.8 5.0 15.0 10.2 12.8 9.7 9.3 8.3
15.1 12.2 8.4 12.2 12.3 11.1 19.0
5.6 5.3 10.4 9.8 9.1 8.7 12.7 13.0 13.4 13.0 17.3
Isosteric Heats of Adsorption for Diesel Particulate Matter. The small injection volumes (0.01-0.05 pL) which were used in the previous study correspond to coverage in the Henry's law region of the adsorption isotherm and can therefore be used to quantify the isosteric heats of adsorption. Table I11 lists these heats for various adsorbates on the two samples of Diesel particulate matter activated at the highest-temperature pretreatment. There are significant increases in the heats of adsorption for the activated DPM-PSU as compared to the activated DPMEPA for every compound investigated, and as a result the higher molecular weight adsorbates phenanthrene, fluorenone, fluoranthene, and anthracene could not be eluted from the column containing DPM-PSU. These results can be rationalized by the fact that the DPM-EPA sample is expected to have sorbed materials on its surface which cannot easily be desorbed. These materials will probably result from partially combusted lubricating oils (25). Summary and Conclusions The isosteric heats of adsorption determined for benzene and hexane on Spheron 6 Bpe close to those reported in the Environ. Sci. Technol., Vol. 16, No. 2, 1982
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literature for graphitized carbon blacks. The heats for the same two compounds on both Diesel samples are somewhat greater, and this is probably due to the presence of higher-energy adsorption sites on the heterogeneous porous Diesel particles. This heterogeniety is confirmed by the peak shapes and increasing retention volumes with decreasing injection volumes, indicative of a type I1 adsorption isotherm. The difference between the isosteric heats of adsorption may be explained by the increased quantity of sorbed species on the DPM-EPA particle surfaces. Therefore, with the DPM-EPA samples, there is a higher probability of the adsorbate interacting with substances condensed on the particles than with the carbon particles themselves. The variation of adsorption energies with injection volumes indicates a similar phenomenon. With increasing surface coverage, adsorbate molecules will be encountering fewer unoccupied adsorption sites and more adsorbed molecules. Multilayer adsorption will then occur and the corresponding interaction energies should be less. This has significance when considering the bioavailability of the sorbed species on inhaled particles. The molecules adsorbed on the high-energy carbon sites will be tightly held as indicated by the isosteric heats. But, additional layers of sorbed substances may be more easily liberated. This is currently being investigated by measurement of multilayer adsorption isotherms in this laboratory.
Acknowledgments We thank William A. Steele for his advice and help in interpreting these data.
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B.; Hynds, P. M. Science 1978,202, 515. (8) Falk, H. L.; Markul, I.; Kotin, P. Am. Med. Assoc. Arch. Ind. Health 1971, 13, 365. (9) Tebbens, B. D.; Jukai, M.; Thomas, J. F. Am. Ind. Hyg. Assoc. J . 1971, 32, 365. (10) Thomas, J. F.; Mukai, M.; Tebbens, B. D. Environ. Sci. Technol. 1968, 2, 33. (11) Korfmacher, W. A.; Wehry, E. L.; Mamantov, G.; Natusch, D. F. S. Environ. Sci. Technol. 1980, 14, 1094. (12) Keyser, T. R.; Natusch, D. F. S.; Evans, C. A., Jr.; Linton, R. W. Environ. Sci. Technol. 1978, 12, 768. (13) Boyer, K. W.; Laitenen, H. A. Environ. Sci. Technol. 1975, 9, 457. (14) Gardella, J. A., Jr.; Hercules, D. H., “Surface Spectroscopic Examination of Diesel Particulates-A Preliminary Study”, presented at the 9th Annual Symposium on the Analytical Chemistry of Pollutants, Jekyll Island, GA, May 1979. (15) Linton, R. W.; Williams, P.; Evans, C. A,, Jr.; Natusch, D. F. S. Anal. Chem. 1977,49, 1515. (16) Frey, J. W.; Corn, M. Nature (London) 1967, 216, 615. (17) Frey, J. W.; Corn, M. Am. Ind. Hyg. Assoc. J . 1967,28,468. (18) Ross, S.; Saelens, J. K.; Olivier, J. P. J . Phys. Chem. 1962, 66, 696. (19) Ross, M. M. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1981. (20) Elkington, P. A.; Gurthoys, G. J . Phys. Chem. 1969, 73, 2321. (21) Kiselev, A. V.; Migunova, L. A.; Yashin, Y. I. Russ. J . Phys. Chem. (Engl. Transl.) 1968,42, 644. (22) Gale, R. L.; Beebe, R. A. Russ. J . Phys. Chem. (Engl. Transl.) 1964, 68,555. (23) Risby, T. H.; Yasbin, R. E.; Lestz, S. S. Proc. Int. Symp. Health E f f .Diesel Eng. Emiss. 1980, 1 , 359. (24) Brumaner, S.; Deming, L. S.; Deming, W. E.; Teller, E. J . Am. Chem. SOC.1940,62,1723. (25) Bradow, R., U.S. Environmental Protection Agency, personal communication, 1979. (26) Mayer, W. J.; Lechman, D. C.; Hilden, D. L. “The Contribution of Engine Oil to Diesel Exhaust Particulate Emission”; SAE Paper No. 800256, 1980. (27) Kiselev, A. V.; Yashin, Y. I. “Gas-Adsorption Chromatography”; Plenum Press: New York, 1969. Received for review January 16,1981. Revised manuscript received June 29,1981. Accepted October5,1981. This work was supported by a grant from the U S . Environmental Protection Agency (R-806558) to T.H.R. The computational facility was purchased with funds from the U.S. Environmental Protection Agency, the National Institute of Allergy and Infectious Diseases, and the Division of Research Resources at the National Institutes of Health.
