104a
Anal. Chem. 1993, 65, 1048-1053
Langmuir-Derived Equations for the Prediction of Solid Adsorbent Breakthrough Volumes of Volatile Organic Compounds in Atmospheric Emission Effluents Pau Comes, Norbert Gonzalez-Flesca, and Tamara M6nard INERIS, Groupe Air, Parc Technologique Alata, BP 2, 60550- Verneuil-en-Halatte, France
Joan 0. Grimalt' Department of Environmental Chemistry, CID-CSIC, Jordi Girona 18, 08034-Barcelona, Catalonia, Spain
The analyrir of dynamlc atmospheres contalnlng known concentrath of vdatlk organk compounds(VOC) hasshown the Langmuirlan behavlor of these chemlcai specles when adsorbed on Tenax GC. Equatlons for the predlction of breakthroughvolumes In mlxtureowlth hlghVOC concentration and Important competltlve adsorption effects have been developed. These equations are deflned from lndlvldual compound parameterswch as adsorptioncapacity, and dlstrlbutlon coefflclent, b. They can also be used to evaluate the correct condlons for the appllcatlon of other more conventional methods for breakthrough predlctlon such as the retentlon volume approach. The lack of dependence of and b on the gas-phase concentratlon of the organlc specles suggests that there are the parameters to take as reference data for the characterization of the adsorption propertks of VOC.
INTRODUCTION Many volatile organic compounds (VOC) are proven bacterial mutagens and suspected carcinogens182 and major efforts are being made to assess their environmental occurrence and health hazards.374 These goals require the study of VOC in both ambient atmospheres and emission sources. Diverse analytical methods have been proposed for the analysis of the former, but the analytical methodology for the study of the latter is scarce. Adsorption on solid surfaces, the current technique for these studies,f-9 has essentiallybeen developed in ambient atmospheric analysis. In consequence, the operational conditions already known can only be extrapolated to emission measurements with caution. Two main aspects must be considered (a) the high concentrations of organic compounds in the emission effluents and (b) the crossed-interferenceeffects between different organic species. An important criterion for a solid adsorbent is the correspondencebetween collectionbreakthrough volumes and (1) Guicherit, R.; Schulting, F. L. Sci. Total Enuiron. 1985,43, 193219. (2) Tanmede,M.;Wilson,R.;Zeise,L.;Crouch,E. A. C. Atmos.Enuiron. 1987,21, 2187-2205. (3) Shah, J . J.; Singh, H. B. Enuiron. Sci. Technol. 1988, 22, 13811388. (4) Wallace, L. A.; Pellizzari, E. D.; Hartwell, T. D.; Whitmore, R.; Zelon, H.; Perritt, R.; Sheldon, L. Atmos. Enuiron. 1988,22,2141-2163. (5) Butler. L. D.: Burke. M. F. J. Chromaton. Sci. 1976.14.117-122. (6) Vidal-Madjar, Cl.; Gonnord, M.-F.; Benchah, F.; Guiochon, G. J. Chromatogr. Sci. 1978, 16, 190-196. (7) Brown, R. H.; Purnell, C. J. J. Chromatogr. 1979, 178, 79-90. (8) Namiesnik, J.; Torres, L.; Kozlowski,E.; Mathieu,J. J.Chromatogr. 1981.208. 239-252. ,--(9) Figge, K.; Rabel, W.; Wieck, A. Fresenius Z . Anal. Chem. 1987, 327, 261-278. ~~~~
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0003-2700/93/0365-1048$04.00/0
retention volumes in a chromatographic system using the same adsorbent as stationary phase. This relationship between frontal and elution chromatography was reported in the first applications of these adsorbents.5-6J0-12 Extensive accounts of retention volumes have subsequently appeared as guidelinesfor the adequate use of the adsorption systems.13 An implicit assumption of this model is the lack of dependence between retentionlbreakthrough volumes and VOC concentrations. However, the influence of VOC concentration on breakthrough volumes has repeatedly been o b s e ~ e d . ~ f j J ~ J ~ l ~ VOC levels of l o O , 7 20,12 or 1ppm15 have been indicated as the upper limit beyond which the breakthrough volumes are concentration dependent. In emission effluents, VOC levels higher than these limits are often encountered. Therefore, important differences in sorption behavior are to be found relative to the performance of the adsorbents in ambient atmosphere measurements. However, the above-mentioned upper limits are also of concern for regular atmospheres. Values higher than these may also be found in ambient air, and several reference concentrationsfor occupational safety'8are above these limits. The obvious alternative to the lack of correspondence between breakthrough and retention volumes is to mimic the sampling procedure by passing a continuous flow of the vapor mixture of interest through the adsorbent bed and recording the effluent concentrations vs time.14J7 Unfortunately, this empirical approach gives rise to results that are very dependent on the specific properties of the sample analyzed. Mathematical expressions allowing the prediction of breakthrough volumes in conditions of high concentration are needed. Another problem that cannot be avoided in the VOC studies of either ambient atmospheres or emission effluents is the competitiveeffect between different compounds. These crossinteractions modify the breakthrough volumes.12J4J7 They are, however, difficult to model from elution chromatography retention volumes because these parameters are intrinsically independent of the other compounds present in the mixture. This problem has also been empirically approached. Again, (10) Cropper, F. R.; Kaminsky, S. Anal. Chem. 1963,35, 735-743. (11) Raymond, A.; Guiochon, G. J. Chromatogr. Sci. 1975, 13, 173177. (12) Russell, J. Enuiron. Sci. Technol. 1975, 9, 1175-1178. (13) Pankow, J. F. Anal. Chem. 1988,60, 950-958. (14) Bertoni, G.; Bruner, F.; Liberti, A.; Perrino, C. J. Chromatogr. 1981,203, 263-270. (15)Van der Straeten, D.; Van Langenhove, H.; Schamp, N. J. Chromatogr. 331, 207-218. (16) Coppi, S.; Betti, A.; Ascanelli, M. J.Chromatogr. 1987,390,349355. ~ .
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(17) Schoene, K.; Steinhanses, J.; Konig, A. Freseniw J.Anal. Chem. 1990,336, 114-119. (18) Pocket guide t o chemical hazards; DHHS Publ. 90.117; DHHS.PHS.C.D.C. NIOSH: Cincinnati, OH, 1990.
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 8, APRIL 15, 1993
mathematical expressions affording a description of the breakthrough volumes for the VOC present in the mixture are desired. In the present study we have derived several equations from the Langmuir isotherm.19 A gadsolid equilibrium can be described by a Langmuir isotherm when the following conditions are met: (a) only a monolayer of gas molecules is adsorbed on the solid surface, (b) all active sites in the adsorbent are equivalent, and (c) no interactions occur between adjacent adsorbate molecules.20 These conditions place some restrictions in the applicability of these isotherms to gadsolid adsorption, but as it will be shown, they can be used to describe the adsorption behavior of VOC present in emission effluents in concentrations up to 300 ppm, an order of concentration in which linear equations are not adequate. The equationsdeveloped in the present study are evaluated by reference to experimental results obtained in the measurement of emission effluents of known VOC concentrations, either pure compounds or mixtures. These effluents have been generated with airstream equipment that affords the production of dynamic atmospheres with controlled flow, temperature, humidity, and VOC concentration.
THEORY
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Equation 7 can be transformed into a linear expression,
= l/(bmma) + c/m,, (8) Equation 8 provides an easy way for the calculation of the b and mmaxparameters of the individual compounds in the adsorption system. The linear regression coefficient of 1/ v b vs c gives an indication of the Langmuirian behavior of the compounds in the system. Multicomponent VOC Mixtures. For a mixture of compounds,kinetic considerations21*22 allow the generalization of eq 1 to a competitive Langmuir adsorption equation, l/vb
where KJ, KB,cj, and 8i are defined for the j compound as in eq 1and Cei is summed over all the compounds present in the mixture. Equation 9 can be rearranged as
Cei)
CY = & j ( -i (10) in which b' = K,j/Kd. The summation of all 8i in eq 10 provides an expression for Cei,
Ed = E(b'c')/(l + C(bici))
(11)
From eqs 10 and 11,
Individual VOC. A gadsolid equilibrium governed by a site-limited Langmuir sorption process19can be described by
(12) Similarly to eqs 3 and 6,
K&(1- 8 ) N = Kd8N
(1) where K, and Kd are the adsorption and desorption constants, respectively, c is the gas-phase concentration,8 is the fraction of occupied sites, and N is the total number of sites in the surface layer of the adsorbent in the tube. This equation can be transformed to 8 = ((Ka/&)C)/(l
+ (K,/&)C)
(2)
This equation is equivalent to other forms of the Langmuir isotherm previously described in elution chromatography.20 The fraction of occupied sites can also be expressed as 8 = m/mm,
(3) where m is the mass of the adsorbed compound and mma, adsorption capacity, is the maximum mass that can be retained to form a monolayer on the adsorbent surface. From eqs 2 and 3, mlm,,
= bc/(l
+ bc)
(4) where b = KJKb is the distribution coefficient between the gas and solid phases. The parameters b and mma depend on the physicochemical properties of the analyte, the temperature, and the type and amount of adsorbent within the tube. While no analyte is detected at the tube outlet, m can be expressed as m = CV (5) where V is the total air volume that has passed through the tube. When detectable amounts of compound start to be observed, this parameter corresponds to the breakthrough volume, Vb. vb
= m/c
(15) This equation allows the calculation of breakthrough volumes of any organic compound in a mixture provided that its mmax and b parameters and the b parameter of the remaining organic compounds are known. These parameters are related to the breakthrough volumes of the individual compounds in the adsorption system (eq 8). The combination of eqs 8 and 15 provides a simple link between single-component and multicomponent adsorption paralleling the reasoning of the ideal adsorbed solution theory.23 In this respect, it has to be indicated that eq 15 is only in agreement with the GibbsDuhem relationship when m m dare equal for all j.24,26 This equation is therefore a simplified expression. It does not give rise to major discrepancies between experimental and calculated data, but it has a lower capacity to predict VOC breakthrough volumes than in the case of individual compounds. EXPERIMENTAL SECTION
Atmospheres with Known VOC Concentrations. A schematic of the equipment designed for the generation of dynamic atmospheres with known concentrations of VOC is shown in Figure 1. A preliminary description of this equipment has been reported elsewhere.%
(6)
From eqs 4 and 6, V, = bmm,/(l
where w',mmd,and v b j are defined for the j compound as in eqs 3 and 6. From eqs 12-14,
+ bc)
(7)
(19) Langmuir, I. J. Am. Chem. SOC. 1916,38, 2221-2295. (20) Snyder, L1. R. Principles of Adsorption Chromatography; Arnold London, U.K., 1968.
(21) Butler, J. A. V.; Ockrent, C. J. Phys. Chem. 1930,34,2841-2859. (22) Markham, E. C.; Benton, A. F. J.Am. Chem. SOC. 1931,53,497501. (23) Myers, A. L.; Prausnitz, J. M. AZChE J . 1965,11, 121-127. (24) Kemball, C.; Rideal, E. K.; Guggenheim, E. A. Tram. Faraday SOC.1948,44,948-954. (25) Broughton, D. B. Ind. Eng. Chem. 1948,40,1506-1508. (26) Comes, P.; Gonzalez, N.; Grimalt, J.; Gomez, R. Pollut. Atmos. 1991, 211-218.
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ANALYTICAL CHEMISIRY, VOL. 65, NO. 6, APRIL 15, 1993
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Flguro 1. Scheme of the generator of dynamlc atmospheres with known concentratlons of VOC: (1) hot air generator, (2) dilution alr inlet, (3) puhrerlzed water inlet, (4) heat exchanger, (5) VOC pump, (6) VOC pulverlrer, (7) homogenizationplpe (5 m X 0.29 m i.d.), (8) stream parameters monitoring controllers, (9) sampilng probe, (10) nltrogen inlet, (1 1) dilution device, (12) pump (Impeller mode), (13) three-way valves, (14) adsorbent tube, (15) to detector, (16) flowmeters, (17) totalizer, (18) extraction fan.
A circular oven equipped with a natural gas burner and a primary air entrance generates a current of hot air (ca. lo00 "C) that is diluted with ambient air and mixed with sprayed water. The resulting gas mixture is cooled in a heat exchanger (ca. 200 OC) and receives known amountsof VOC (individualor mixtures) that are introduced with a Chromatem 380 high-performance liquid chromatographic pump. The VOC are pumped through a 1-mrn4.d.Teflon tubing into a pulverizer that is situated at the base of a vertical 5 m X 29 cm i.d. pipe. This pipe homogenizes and channels the spiked airstream (average flow rate 300 m3/h), which is subsequently released to the outside with an extraction fan. The fan keeps all the system below atmospheric pressure (600Pa). The pipe is equipped with three type K thermocouples for the control of the temperature at the top and at the base. The stream flow is measured with a Pitot tube connected to a capacitive electronic micromanometer (Furness Controls FCO14). This micromanometer is used in a range of 0-10 Pa with time constant fitting. A sampling probe situated at 50 cm below the top of the pipe introduces a portion of the spiked air into a glass dilution chamber where it is mixed with nitrogen (1:lO). The dilution ratio is controlledwith two oxygenanalyzerssituated before and after the air/nitrogen mixing device. Condensation is avoided by external heating at 150 OC. Two types of detectors are used to monitor the stability of the VOC concentrations in the resulting air effluent. A flame ionization detector (FID) Cosma RS-55 (working range between 1and lo00 ppm methane equivalents) is often used when only one compound is introduced into the airstream. In some of these cases, or when the airstream contains mixtures of organic compounds, the monitoring device is a mass spectrometer (MS) Balzers 420 equipped with a heated capillary for atmospheric sampling. Repeated measurements of the concentrations of the spiked compounds have shown a range of variation of 3-4 % with respect to the mean. This dispersion is essentially introduced by the pulses of the chromatographic injection pump. Once the system is in a steady-state condition, the average VOC concentration do not exhibit any significant drift (
