Study of the Adsorption of Polyaromatic Hydrocarbon Binary Mixtures

Energy & Fuels 2003, 17, 669-676

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Study of the Adsorption of Polyaromatic Hydrocarbon Binary Mixtures on Carbon Materials by Gas-Phase Fluorescence Detection A. M. Mastral,*,† T. Garcıá,† R. Murillo,† M. S. Calleń,† J. M. Lo´pez,† M. V. Navarro,† and J. Galbań‡ Instituto de Carboquı´mica, CSIC, P.O. Box 589, M Luesma Castan 4, 50018-Zaragoza, Spain, and Department of Analytical Chemistry, Zaragoza University, 50007-Zaragoza, Spain Received October 1, 2002

This is the first time that the adsorption of binary mixtures of polycyclic aromatic hydrocarbons (PAHs) has been studied. This study was carried out at laboratory scale in an experimental apparatus specially designed for this aim: with gas-phase fluorescence, a new detection procedure was used. Experimental conditions, mainly in terms of temperature (150°C) and contaminant concentration (approximately 1 ppmv), close to the ones observed in energy generation systems, were applied. The PAH adsorption process interpretation was carried out by recording the experimentally obtained breakthrough curves. After the detection parameter optimization, the influence of adsorbate characteristics in the hot gas cleaning of PAH was studied. In this system, the adsorption of 10 binary mixtures of five PAH [naphthalene (Np), fluorene (Fu), phenanthrene (Phe), fluoranthene (Fl), and pyrene (Py)] on an activated carbon was investigated. It was found that the adsorbent efficiency is always determined by the breakthrough of the weakly adsorbed component during its adsorption as pure compound. This efficiency and the equilibrium adsorption capacity, the sum of the adsorption capacity of both compounds, depend on binary mixture components, and as it happened for the pure compounds, microporosity is the main factor that controls the adsorption process. Finally, it was found that the pore volumes filled by the binary mixtures was always similar or lower than the ones filled by the strongly retained binary mixture component as pure compound. This difference mainly depended on the volatility of the weakly retained component.

Introduction Polycyclic aromatic hydrocarbons (PAHs) have started to be widely studied, and a special concern is being paid to their environmental effects due to their potential carcinogenic, mutagenic, or both, properties.1,2 PAHs are mainly emitted from combustion processes, having their origin associated with anthropogenic sources, such as engine exhaust,3 industrial processes,4 natural gas consumption,5 domestic heating systems,6 incinerators7,8 and smoke;8 and natural sources, such as volcanic eruptions and forest fires. In the combustion process, PAH generation mechanisms9-12 can have their origin in the two processes: pyrolysis and pyrosynthesis. In * To whom correspondence should be addressed. Phone: 34 976 733977. Fax: 34 976 733318. E-mail: [email protected]. † Instituto de Carboquı´mica. ‡ Zaragoza University. (1) Williams, P. T. J. Inst. Energy 1990, 63, 22. (2) Lee, M. L.; Novotny, M.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Hydrocarbons; Academic Press: New York, 1981 (3) Marr, L. C.; Kirchstetter, T. W.; Harley, R. A.; Miguel, A. H.; Hering, S. V.; Hammond, S. K. Fuel 1999, 33, 3091. (4) Kirton, P. J.; Crisp, P. T. Fuel 1990, 69, 633. (5) Rogge, W. F.; Hildemann, L. M.; Mazukek, M. A.; Cass, G. R.; Simonet, B. R. T. Environ. Sci. Technol. 1993, 27, 2736. (6) Oamh, N. T.; Reutergardh, L. B.; Dung N. T. Environ. Sci. Technol. 1999, 33, 2703. (7) Lee, C. W.; Lemieux, P. M.; Gullett, B. K.; Ryan, J. V.; Kilgrue, J. D. Stud. Environ. Sci. 1998, 72, 361. (8) Zimmermann, R.; Heger, H. J.; Kettrup, A. Fresenius’ J. Anal. Chem. 1999, 363, 720.

addition, PAHs can also have their origin in the fuel structure because some small particles can be removed from the combustor as unburned material,13 but these can be controlled by improving the efficiency of the combustion process.12,13 Due to the PAHs’ high volatilities,14 PAHs can be released both supported on the particulate matter (PM) and in the gas phase.9,15 While the most volatile compounds, compounds with two or three aromatic rings, are mainly released in the gas phase, the compounds containing three or more aromatic rings in their structure are associated with the PM emission.16 The PAH gas/solid partitioning is related to the liquid vapor pressure, the ambient temperature, the size, the chemi(9) Mastral, A. M.; Calleń, M. S.; Murillo, R.; Garcıá, T. Environ. Sci. Technol. 1999, 33, 3177-3184. (10) Bonfanti, L.; DeMichelle, G.; Riccardi, J.; Lopez-Doriga, E. Combust. Sci. Technol. 1994, 101, 505. (11) Williams, P. T.; Andrews, G. E.; Bartle, K. D. Fuel 1986, 65, 1150. (12) Barbella, R.; Bertoli, C.; Ciajolo, A.; D’Anna, A. Combust. Flame 1990, 82, 191. (13) Mastral, A. M.; Calleń, M. S.; Murillo, R.; Garcıá, T. Fuel 1999, 78, 1553. (14) Sloss, L. L.; Gardner, C. A. Sampling and Analysis of Trace Emissions from Coal-Fired Power Stations. EACR/77; IEA Coal Research: London, 1995; p 48. (15) Grace Lee, W.-M.; Tong, H.-C,; Yeh, S.-Y. J. Environ. Sci. Health 1993, A28, 563-583. (16) Mastral, A. M.; Calleń, M. S.; Murillo, R.; Garcıá, T. Fuel Proc. Technol. 2000, 67, 1.

10.1021/ef020222d CCC: $25.00 © 2003 American Chemical Society Published on Web 04/24/2003

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cal composition, and the surface area of the PM.17-19 These characteristics, together with the PAHs’ volatile characteristics, will determine the way in which they will be emitted in the combustion process. The PAHs supported on the particulate matter can be trapped in cyclones, electrostatic precipitators, scrubbers, etc., whereas the most volatile compounds are released into the atmosphere through the chimney, making use of specific technologies to diminish their negative environmental impact necessary. Two different technologies are the most promising alternatives to reduce gaseous PAH emissions: PAH catalytic elimination20,21 and PAH adsorption on carbonaceous materials.22-24 Packed beds of carbonaceous materials have been used to remove organic emissions from about 100 to 10000 ppmv concentrations in industrial waste gas streams.25,26 In previous works,23,24,27 the authors accomplished the PAH adsorption as pure components onto carbonaceous materials in a laboratory-scale apparatus. To make an approximation of the complex composition of a real waste gas stream, the adsorption study of binary mixtures of gaseous PAH onto activated carbons has been carried out in this work. Experimental data on the adsorption of vapor mixtures on solids are limited because it is difficult to obtain data for complete breakthrough curves for different concentrations of solutes in vapor mixture/adsorbent systems.28 Other approaches to study the performance of adsorbent materials in the presence of binary mixtures are to develop theoretical studies29 and mathematical methods.30-32 However, these methods use the data for pure-component adsorption equilibrium only to calculate the breakthrough curves of the two components in a binary mixture. In this paper, the interpretation of the adsorption process of PAH binary mixtures was carried out by obtaining the corresponding breakthrough curves. Therefore, it was necessary to have an appropriate detector able to identify compounds in gas phase. In addition, this detector should be also able to monitor the signal throughout the experiment at the reactor exit for both PAHs. Gas chromatography has been widely used as an analytical technique for the identification of unknown (17) Yamasaki, H.; Kuwate, K.; Miyamoto, H. Environ. Sci. Technol. 1982, 16, 189-194. (18) Butler, J. D.; Crossley, P. Atmos. Environ. 1981, 15, 91-94. (19) Mastral A. M.; Calleń M. S. Environ. Sci. Techonol. 1999, 15, 3051-3057. (20) Liljelind, P.; Unsworth, J.; Maaskant, O.; Marklund, S. Chemosphere 2001, 42, 615. (21) Weber, R.; Plinnke, M.; Xu, Z.; Wilken, M. Appl. Catal. B: Environ. 2001, 31, 195. (22) Cudahy, J. J.; Helsel, R. W. Waste Manage. 2000, 20, 339. (23) Mastral, A. M.; Garcıá, T.; Calleń, M. S.; Navarro, M. V. Galbań J; Energy Fuels 2001, 15, 1. (24) Mastral, A. M.; Garcıá, T.; Calleń, M. S.; Navarro, M. V.; Galbań J. Environ. Sci. Technol. 2001, 35, 2395. (25) Mantell C. L. Chem. Metall. Eng. 1940, 47 , 305. (26) EPA-456/R-95-003; Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency: Research Triangle Park, NC, May 1995. (27) Mastral, A. M.; Garcıá, T.; Murillo, R.; Calleń, M. S.; Lopez, J. M.; Navarro, M. V. Energ. Fuel. 2002, 16, 205. (28) Brosillon, S.; Manero, M. H.; Foussard, J. N. Environ. Technol. 2000, 21, 457. (29) Vahdat, N. Carbon 1997, 35, 1545. (30) Moon, H.; Lee, W. K. Chem. Eng. Sci. 1986, 41, 1995. (31) Stoeckli, F.; Wintgens, D.; Lavanchy, A.; Sto¨ckli, M. Adsorpt. Sci. Technol. 1997, 15, 677. (32) Lavanchy, A.; Stoeckli, F. Carbon 1997, 35, 1573.

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compounds. As a result, different detectors have been developed which allow the identification and quantification of chemical compounds in the gas phase at the exit of the chromatographic column. Mass spectrometry detectors (MS)33-35 and gas-phase molecular spectroscopy detectors have proved suitable for the simultaneous determination of several volatile compounds. Depending on the wavelength of the light used, different molecular spectroscopy detectors have been developed: those detectors based on Fourier transform infrared spectroscopy (FTIR)36,37 are the most popular. On the other hand, and despite showing analytical advantages, ultraviolet-visible spectroscopy (UV-vis)38,39 (low-cost and high specificity) or fluorescence spectroscopy40-42 (low-cost, high sensibility, and specificity) have been only scarcely used. The specific characteristics of the experimental device used, specially taking into account the concentration range (high sensibility of the fluorescence technique), the nature of chemical compounds that show a remarkable fluorescence yield, and the experimental conditions (online detection), suggested that the best fitting detector for the monitoring and resolution of PAH binary mixtures will be the molecular spectroscopy detector in the gas-phase on the basis of fluorescence measurements, which was used in this work. Experimental Section The adsorption experiments were performed in an experimental apparatus specially designed to assess the adsorption behavior of PAH on carbonaceous materials (see Figure 1). Three different zones can be distinguished. Saturated Gas Stream Generation Zone. Two small cylindrical reactors (6.8 mm ID) connected in parallel were used to generate the gas stream saturated with PAH (see Figure 1). In these reactors, different PAHs were sublimated with a measured carrier gas stream. Two reactors were needed because there were significant differences in the PAHs’ volatilities. The generation temperatures needed to achieve the desired PAH concentrations (around 0.7 ppmv for every compound) were quite different, and so a unique reactor with a mixture of both compounds did not work. The flow through the saturation reactors was controlled using mass flow controllers. Helium was used as carrier gas, and the total flow at the inlet of the adsorption reactor was 25 mL/min. The temperature of the saturators was regulated with two independent PID controllers ((1 °C accuracy). The temperature was raised slowly until the desired PAH concentration was obtained and remained constant until the end of the experiment. (33) Lagesson, H. V.; Nilsson, A.; Tagesson, C. Chromatographia 2000, 52 (9/10), 621. (34) Zheng, M.; Fang, M. Water, Air, Soil Pollut. 2000, 117 (1-4), 175. (35) McClennen, W. H.; Vaughn, C. L.; Cole, P. A.; Sheya, S. N.; Wager, D. J.; Mott, T. J.; Dworzanski, J. P.; Arnold, N. S.; Meuzelaar, H. L. C. Field Anal. Chem. Technol. 1996, 1, 109. (36) Meyer, A.; Budzinski, H.; Powell, J. R.; Garrigues, P. Polycyclic Aromat. Compd. 1999, 13, 329. (37) Hakkarainen, T.; Mikkola, E.; Laperre, J.; Gensous, F.; Fardell, P.; Le Tallec, Y.; Baiocchi, C.; Paul, K.; Simonson, M.; Deleu, C.; Metcalfe, E. Fire Mater. 2000, 24, 101. (38) Lagesson, V.; Lagesson-Andrasko, L.; Andrasko, J.; Baco, F. J. Chromatogr., A 2000, 867, 187. (39) Sanz, I.; Cabredo, S.; Sanz, F.; Galban, J. Chromatographia 1996, 42, 435. (40) Van Engelen, D. L.; Adams, A. K.; Thomas, L. C. J. Chromatogr. 1985, 331, 77. (41) Van Engelen, D. L.; Thomas, L. C.; Piepmeier, E. H. J. Chromatogr. 1987, 405, 191. (42) Chi, Z.; Cullum, B. M.; Stokes, D. L.; Mobley, J.; Miller, G. H.; Hajaligol, M. R.; Vo-Dinh, T. Fuel 2001, 80, 1819.

Polyaromatic Hydrocarbon Binary Mixtures

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Figure 1. Experimental apparatus designed to study the adsorption behavior of binary PAH mixtures on carbonaceous materials. Adsorption Zone. The adsorbing bed was composed of 25 mg of adsorbent (100-200 µm average particle size diameter). It was placed in a Teflon reactor and mixed with 1.0 g of sand as inert material, with the grains being of the same size to provide enough bed length (11 cm) ensuring uniform flow throughout and avoiding axial dispersion. Blank tests were carried out with sand to check its suitability as inert material in adsorption experiments. The Teflon reactor was placed inside a gas chromatograph oven that allowed the controlling of the adsorption temperature precisely ((1 °C accuracy). Detection Zone. The detection and resolution of the gas mixture was carried out by using a gas-phase fluorescence system.43 In previous works, a flame ionization detector (FID)23,24,27,44 was used. This system, however, is not appropriate to resolving mixtures of several compounds simultaneously passing through the detector. To avoid PAH condensation, the gas stream leaving the Teflon reactor was driven through a heated transfer line to a gas cell specially designed for this purpose.43 The gas cell was placed in the spectrofluorimeter in such a way that the fluorescence signal for a stable PAH stream was maximized. Ten binary mixtures of five PAH with different physical characteristics, see Table 1, were studied. The selected PAH were naphthalene (Np), fluorene (Fu), phenanthrene (Phe), fluoranthene (Fl), and pyrene (Py). These compounds have structures from two to four aromatic rings, which are the ones mainly emitted in the combustion process. A carbon material, reference CA-3, was used to study its behavior in the adsorption of PAH vapor mixtures: This carbon material corresponded to commercial coke from German Rhenish lignite supplied by RWE Rheinbraun. This particular coke is commercially used all over Europe for its applications in the fields of waste gas and wastewater cleaning. The apparent specific surface area SBET of this coke SBET ) 250 m2/g was obtained in an automatic volumetric sorption analyzer (model ASAP 2000, Micromeritics Instrument Co. Norcross, GA) using N2 adsorption at 77 K. Five different experiments were carried out to determine the experimental error, with the sample CA-3 having a value around 3%. Active carbon adsorption capacities of the pure compounds and PAH binary mixtures were calculated by integration of (43) Mastral, A. M.; Garcia, T.; Galban, J. “Hot gas polyaromatic compounds on-line analysis and the corresponding adjusting device.” Spain 200202902, 17th Dec, 2002. (44) Mastral, A. M.; Garcıá, T.; Calleń M. S.; Lo´pez, J. M.; Navarro, M. V. and Galbań J. Environ. Sci. Technol. 2002, 36, 1821-1826.

the obtained breakthrough curves. From these adsorption capacities, pore volumes filled by pure components and binary mixtures were calculated from the molar volumes shown in Table 1, assuming that the adsorbate state is not too different from the liquid state at the same pressure and temperature.

Results and Discussion In industrial hot gas cleaning systems, the inlet gas stream consists of a complex mixture of different chemical compounds. In this case, the work is focused on the different PAH molecules that should be retained in the adsorption system because of their harmful effects on human health and the environment. Therefore, the aim of this paper has been to investigate the behavior of a carbon material in the adsorption of different binary PAH mixtures to assess how the binary mixture components interfere with each other. This work should be considered as a first step toward a deeper understanding of the complex process of PAH adsorption in industrial hot gas cleaning systems. The PAH adsorption process interpretation was carried out by recording experimentally the obtained breakthrough curves. Therefore, it was necessary to use a detector that could analyze and monitor different compounds in a gas stream throughout an experiment. After the breakthrough curve is recorded, it must be interpreted so that the solid material adsorption capacity can be calculated. On-line fluorescence spectroscopy was chosen as analytical technique because of the special characteristics of the studied molecules (remarkable fluorescence yield). This technique enables a precise quantification and resolution of the mixture at the working concentration range and under the experimental conditions used. The gas-phase spectra were obtained in the specially designed gas cell43 at 200 °C. Figure 2 shows the excitation and emission spectra obtained for the PAHs at the gas phase. The selection of the appropriate wavelengths for the monitoring of binary breakthrough curves was carried out by comparing the excitation and emission spectra in such a way that they comply with the following two

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Table 1. Physical Properties and Single Adsorption Capacities for PAHs

a

CPAH ) 1.0 ( 0.1 ppm PAH.

requirements. First, the inner filter quenching produced by the compounds must be avoided. Therefore, those wavelengths where the absorption spectrum of one molecule does not coincide with the fluorescence spectrum of another molecule must be chosen. In this way, the binary breakthrough curves can be obtained using the calibration constants found for the pure compounds. Second, the fluorescence signal of every PAH should show the maximum sensitivity in order to avoid possible problems for fluorescence signal quantification. In all the studied binary mixtures, the final fluorescence signal corresponding to a 0.8 ( 0.1 ppm concentration was 10 times higher than the standard deviation of the blank in those working conditions. This fact makes possible the correct determination of the breakthrough time and the initial concentration for every PAH. The selected conditions (wavelengths and slit widths) for each binary mixture resolution are compiled in Table 2. Once the wavelengths were selected, the calibration constants that relate the obtained signal in the spectrofluorimeter with the PAH concentration were calculated. The calibration was performed for every pure compound, and it was obtained as the average of three different measurements with a RSD lower than 10% in all the cases. The linearity of the obtained signal in the spectrofluorimeter for the studied concentration range used for every PAH was assured from the linear relationship between the weighed PAH mass and the integrated area in the fluorescence register. In all the cases, the linear correlation coefficients were higher than 0.98. These calibration constants are a function of the studied PAH, the working wavelength selected, and the helium flow in the gas cell. In these operation conditions, it was observed the following relationship between

binary mixture fluorescence intensity and each PAH concentration:

Iλ1 ) KPAH1λ1CPAH1 + KPAH2λ1CPAH2

(1)

Iλ2 ) KPAH1λ2CPAH1 + KPAH2λ2CPAH2

(2)

where Iλi is the intensity measured with the fluorescence spectrometer at every wavelength λ1 and λ2, KPAHiλi is the proportionality constant obtained from the pure compounds calibration for every wavelength and every carrier gas flow, and CPAHi is the concentration of every PAH along the experiment. Data acquisition was performed using the “ratio data collection” option in the computer software package Flwinlab 1.0, which allows us to simultaneously follow the signal evolution at two different wavelengths. The application of eqs 1 and 2 to the obtained signal in the spectrofluorimeter made possible the acquisition of the breakthrough curves of different active carbons when a binary mixture of PAH passed through the adsorbent bed. These breakthrough curves allow the PAH adsorption process to be interpreted. Adsorption process was studied for binary mixtures obtained from the combination of five PAH. Figure 3 shows breakthrough curves for the simultaneous adsorption of binary mixtures formed by Phe and other PAH on the CA-3 active carbon. It can be observed that these binary curves are different and more complicated than the ones obtained in the adsorption of a single compound because of the different adsorption capacity of the same active carbon bed for every single compound,44 which depends mainly on a compound’s volatility. During the binary adsorption process, the molecules



Figure 2. Excitation and emission gas-phase spectra of Np, Fu, Phe, Fl, and Py.

of component 1, which is the weakly adsorbed compound and the most volatile, are replaced by the strongly adsorbed molecules, those of component 2, and therefore, component 1 moves faster throughout the bed. In this Figure 3, it is shown that component 1’s breakthrough determines the active carbon efficiency during a hot gas cleaning process and that component 2’s breakthrough shows an adsorption equilibrium of the PAH binary mixtures. It can be observed that the mass transfer zone height (MTZH) in the bed for component 1 is not a function of volatility. Similar MTZHs (ca 1.0 cm) are obtained for Np, Fu, and Phe when they behave as weakly adsorbed components (see Figure 3A,B). It seems to be that component 1’s MTZH is not affected by the presence of component 2 or by its nature. However, component 2’s MTZH is clearly affected by the nature of component 1 when the difference of volatilities between binary mixture components is small. Figure 3A shows how the breakthrough curves are almost identical or parallel because both components 2 (Fl and Py) are compounds with four rings, while component 1 (Phe) is a three-ring

compound. However, when two molecules with the same number of rings (Phe and Fu) are present in the feed mixture; that is, when two similar molecules with similar volatilities are present in the feed mixture, the MTZH of component 2 increases compared to the MTZH observed when Np is present in the feed mixture (see Figure 3B). Fu molecules are more difficult to be moved out from the surface than Np molecules and therefore the MTZH increases from 1.5 to 3.0 cm. Although compounds with similar volatilities produce higher MTZHs in component 2’s breakthrough, it is observed that the column utilization degree increases because the time between component 1’s and component 2’s breakthroughs is shorter. Regarding the adsorbent efficiency, Figure 3 shows how this parameter is always a function of the breakthrough of the compound with the lower adsorption capacity as a pure compound and the numerical value depends on the compounds which constitute the binary mixture. In this way, Table 3 shows that the lower component 2’s volatility in the binary mixture, the higher the carbonaceous material efficiency. The same

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Figure 3. Adsorbate influence on PAH breakthrough curves of a binary mixture. (A) Phe and strongly adsorbed component as pure compound (Fl and Py). (B) Phe and weakly adsorbed component as pure compound (Np and Fu). T ) 150 °C, Q ) 25 mL/ min, and CA-3. Table 2. Wavelengths λ and Slit Widths L (in Parentheses) Selected for PAH Binary Mixtures Resolution binary mixtures 1 λexc L λem L

2

3

4

5

6

7

8

9

10

Np

Fu

Np

Phe

Np

Fl

Np

Py

Fu

Phe

Fu

Fl

Fu

Py

Phe

Fl

Phe

Py

Fl

Py

260 (10) 338 (10)

254 (10) 325 (10)

260 (15) 352 (20)

236 (15) 352 (20)

260 (15) 370 (20)

275 (15) 425 (20)

260 (15) 352 (20)

236 (15) 380 (20)

254 (15) 347 (20)

236 (15) 347 (20)

254 (15) 360 (20)

277 (15) 437 (20)

254 (15) 360 (20)

254 (15) 380 (20)

250 (15) 380 (20)

277 (15) 437 (20)

250 (15) 390 (20)

304 (15) 390 (20)

277 (15) 420 (20)

317 (15) 420 (20)

Table 3. Efficiency and Total Adsorption Capacity on CA-3 Carbon Material for Np Binary Mixtures Calculated from Breakthrough Curves binary mixtures

efficiency [min]

total adsorption capacity [mgPAH/mgCA-3]

Np/Fu Np/Phe Np/Fl Np/Py

40 48 52 54

0.094 0.11 0.13 0.14

Table 4. Efficiency and Total Adsorption Capacity on CA-3 Carbon Material for Py Binary Mixtures Calculated from Breakthrough Curves binary mixtures

efficiency [min]

total adsorption capacity [mgPAH/mgCA-3]

Np/Py Fu/Py Phe/Py Fl/Py

54 58 64 66

0.14 0.083 0.093 0.094

behavior is observed when component 1’s volatility is changed in the binary mixture (see Table 4). Regarding the total adsorption capacity of the binary mixture, see Tables 3 and 4, a trend similar to the one observed for the efficiency is observed; that is, the numerical value depends on the nature of the compounds implied in the process. The binary mixtures that drove to a higher total adsorption capacity were those in which Np was present. This is probably due to the contribution of two different effects. First, a molecular

sieve effect is observed: the low size of Np molecules implies that they can enter into pores where other greater PAH molecules cannot penetrate. Second, the contribution of the multilayer interactions is observed: it has been shown24 that Np is mainly adsorbed into molecular size pores and no multilayer formation is observed. Therefore, it could be assumed that Np molecules are not going to hinder the multilayer adsorption processes of the other PAHs as is observed for the other studied binary mixtures. The different adsorption mechanisms shown by the binary mixtures and their differences in comparison to the observed for the adsorption of pure compounds can be analyzed from the comparison of the pore volumes V0 filled by the pure compounds and by the binary mixtures in both breakthroughs. The V0 values found for every PAH as a pure compound are shown in Figures 4 and 5. It can be observed that these values increase as the volatilities of the compounds decrease.. In addition, the studied PAH have different adsorption mechanism in the ppmv concentration range.24,44 Therefore, it was observed that, while Np adsorption happens only in those pores with adsorbate molecular size, in the case of Fu, its adsorption is going to be also favored by the presence of pores with a higher diameter, where cooperative effects between adsorbate molecules can take place. Finally, in the cases of Phe, Fl, and Py, their adsorption capacities increases in the carbonaceous



Figure 4. Filled pore volume by five pure compounds and 10 binary mixtures in the breakthrough point of weakly adsorbed component.

Figure 5. Filled pore volume by five pure compounds and 10 binary mixtures in the breakthrough point of strongly adsorbed component.

material with mesopores where multilayer adsorption can also take place. Once the adsorption mechanism of the pure compounds is understood, it is possible to study the behavior of the binary mixtures during the adsorption process from the V0 values in both breakthroughs. Figure 4 shows the V0 value in the weakly adsorbed component breakthrough. It can be deduced that component 1’s breakthrough happens once the pores with the molecular size diameter of molecules constituting the binary mixture have been filled. This fact seems to agree with the observation of quasi-identical pore volumes for all the mixtures and with the fact that this value is similar to the observed for Fu adsorption. It has been shown that the Fu adsorption in this concentration range is mainly due to the micropores filling mechanism,44 first in those pores with a diameter close to the molecule size (1σ-2σ) and later in pores with a greater diameter (2σ-5σ) through cooperative effects. Figure 4 also shows that the V0 values for the binary mixtures containing Np is slightly higher than the values observed for the rest of mixtures and this value increases at decreasing volatilities of the second compound. This could be due to the contribution of two effects described earlier: lower Np molecular size that allows a deeper penetration into small pores and weak adsorption as pure compound that would allow multilayer interactions between the second compound’s molecules before the Np breakthrough in the binary mixture. This is corroborated by the increase observed in the V0 value in the binary mixture when component 2’s volatility is decreased. In this way, microporosity is the main factor that controls the adsorption of PAH binary mixtures, and the higher the microporosity development, the higher the adsorbent efficiency. Although the mesoporosity influence should be thoroughly studied, it is worth commenting that its presence in the carbonaceous material is always positive because it not only drives the adsorbate molecules to the micropores but also promotes the multilayer interactions increasing the adsorbent efficiency. Figure 5 shows the adsorption equilibrium V0 values. As mentioned above and as shown in the breakthrough curves in Figure 3, once component 1’s breakthrough has occurred, it is observed that component 1’s molecules are moved out of the carbonaceous material surface by component 2’s molecules that remain adsorbed. In this way, while the V0 value for component 1

decreases, the corresponding V0 value for component 2 increases. This fact can be observed in Figures 4 and 5. Regarding component 1, first it can be observed that, when Np is present in the binary mixture, the V0 value remains practically constant, indicating that Np molecules are moved from all active sites, except from those having a diameter close to Np molecular size where the other PAHs cannot penetrate, because of the molecular sieve effect. For the other binary mixtures it is observed that, when the adsorption equilibrium is reached, the pore-filled volume of component 1 decreases at increasing adsorption capacities of component 2 as pure compound, according to the adsorption equilibrium theory.45 Regarding component 2, it is generally observed that its V0 value increases when its volatility decreases in binary mixtures with the same component 1 because the multilayer interactions between component 2’s molecules are promoted. It is also observed that, when similar molecules are forming the binary mixture, these multilayer interactions are hindered. For example, it has been observed that for the Fl-Py binary mixture the breakthroughs of both components practically happen simultaneously. According to component 1’s and component 2’s adsorption mechanisms described above, the total binary mixture V0 value depends on component nature. So, it is observed that, in those binary mixtures where Np is present, total V0 values higher than the ones for the pure compounds are obtained, because Np allows for the multilayer adsorption of the less volatile component. When Np is not present in the binary mixture, the obtained V0 values are lower than the adsorption capacity of component 2 as pure compound. Therefore, once again microporosity seems to be a determinant factor in the equilibrium because adsorption takes place mainly in those pores due to their higher adsorption potential. However, mesoporosity is also important because, as it was previously commented, not only mesopores drive the adsorbate molecules to the micropores but they also promote the multilayer interactions increasing the equilibrium adsorption capacity. Acknowledgment. The authors would like to thank ECSC their partial financial support (Project 7220/EC/ (45) Tien C. Adsorption Calculations and Modeling; ButterworthHeinemann: Boston, 1994; pp 43-70.

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089), the Autonomic Government of Aragoń, DGA, Spain (T. Garcıá and J. M. Lopez fellowships), and Spanish Ministry of Science and Technology, “Ramoń y Cajal” Program (M. S. Calleń and R. Murillo contracts). Nomenclature C: outlet PAH concentration at any time, ppm C0: inlet PAH concentration, ppm CA-3: carbonaceous material reference Fl: fluoranthene Fu: fluorene I: fluorescence intensity, relative units Iλi: intensity measured with the fluorescence spectrometer at every wavelength λi, relative units ID: internal diameter, mm KPAHiλi: calibration constant obtained from pure compounds at every wavelength

Mastral et al. L: slit width in the spectrofluorimeter, nm MTZH: mass transfer zone height, cm Np: naphthalene PAH: polycyclic aromatic hydrocarbon Phe: phenanthrene PID: proportional integral derivative controller PM: particulate matter Pv150°C: PAH vapor pressure at 150 °C Py: pyrene RSD: relative standard deviation Tb: boiling point, °C Vm: molecular volume, cm3/mol V0: Filled pore volume by the PAH molecules, cm3/g Greek Symbols λ: wavelength, nm σ: molecular diameter, nm EF020222D

Study of the Adsorption of Polyaromatic Hydrocarbon Binary Mixtures

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