Ind. Eng. Chem. Res. 2007, 46, 8193-8198
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Temperature Swing Adsorption of Polycyclic Aromatic Hydrocarbons on Activated Carbons Asuncio´ n Aranda, Marı´a V. Navarro, Toma´ s Garcı´a,* Ramo´ n Murillo, and Ana M. Mastral Instituto de Carboquı´mica, CSIC, M. Luesma Casta´ n 4, 50018 Zaragoza, Spain
Temperature swing adsorption (TSA) of polycyclic aromatic hydrocarbons (PAH) has been studied. Naphthalene (Np)stwo aromatic ringssand phenanthrene (Phe)sthree aromatic ringsshave been selected as model compounds for the PAH hot gas cleaning process. Five activated carbons (AC) were produced from pyrolytic carbon black obtained in waste tire recycling. Carbon dioxide was used as a gasifying agent during five different reaction times, bringing about adsorbents with different textural properties. The influence of these properties has been assessed in the loss of adsorption capacity with the number of cycles of both model compounds. The adsorption process was performed at 150 °C in helium atmosphere with a gas hourly space velocity of 25 000 h-1. Under these conditions, the breakthrough curve of each model compound was obtained with an inlet concentration of ca. 250 ppbv. AC regeneration was carried out in air by thermal desorption under fixed conditions (300 °C, 25 000 h-1, and 100 min) during five cycles. It was observed that the AC performance depends mainly on the model compound nature, total micropore volume, and micropore size distribution. In this work, it is shown that although the Np adsorption capacity is fairly constant with the number of cycles, Phe removal drastically decreases after the first regeneration cycle, but it is maintained in the successive adsorption/desorption cycles. This fact is likely due to a pore-blocking effect by Phe molecules retained in the molecular size pores. 1. Introduction On the last years, concern about maintaining and improving air quality with respect to polycyclic aromatic hydrocarbons (PAH) has increased.1 These compounds have widely proved their carcinogenic and/or mutagenic nature.2,3 PAH are produced in industrial processes,4 engine exhaust,5 energy generation,6 etc. Unfortunately, complete PAH abatement during these processes, by controlling the specific variables, is not always possible.6 Besides, due to their high volatility and reactivity, PAH can be released not only supported onto particulate matter (PM) but also in the gas phase.8 Therefore, eliminating these species from hot flue gas would involve both PM collection and airborne PAH elimination.9 Although PM removal can be performed by using solids abatement systems, such as cyclones or electrostatic precipitators, one of the most promising technologies for gas-phase PAH abatement is adsorption on porous materials.6,10,11 However, both the treatment of the contaminated activated carbons (ACs) and/or the cyclic operation suitable from an industrial point of view are still unresolved problems. Therefore, adsorption/regeneration cycles must be studied to improve the performance of these adsorbents and to find the best operating conditions. Temperature swing adsorption (TSA) is a generally accepted industrial process for cyclic operation in adsorption systems. However, there are no published works regarding TSA processes applied to PAH abatement in flue gases. In addition, few experimental data for thermal regeneration of ACs in the abatement of other pollutants can be found. Hwang et al.12 and Yun et al.13 studied the effect of the regeneration temperature on the adsorption of methylene chloride and benzene vapors, respectively. It was found that the higher the regeneration temperature, the lower the regeneration time during the cyclic operation. On the other side, the disposal of used automotive tires is an increasing economical and environmental problem for most * To whom correspondence should be addressed. Tel.: +34 976733977. Fax: +34 976733318. E-mail:
[email protected].
developed countries. It is estimated that 3 million tons per year are generated in the European Union. According to the European Legislation, they must be reused, recycled, and/or used to obtain added value products. Tire is made of rubber materials (polybutadiene, styrene-butadiene rubber, and polyisoprene or natural rubber), carbon black, and some fibrous materials.15 It has a high volatile matter content that can be easily recovered by pyrolysis processes. In addition, the resulting pyrolytic carbon black has a fixed carbon content appropriate for the production of AC.16 Physical activation using CO2 or steam as the oxidizing agent is the most common activation process in the production of tire ACs. The overall process usually consists of two steps: thermal pyrolysis at relatively low temperature (typically 400-700 °C) in the presence of an inert gas, and activation with a reactive gas at 800-1000 °C for further development of the porosity of pyrolytic carbon black.17-20 At these conditions, a medium quality but low-cost AC is produced, which has proven to be effective in the hot gas cleaning of PAHs.6 However, to the best of our knowledge, it is not known if these ACs can be used in a TSA process. It is worth pointing out that regeneration steps directly restrict the adsorbent selection since the possibility of pollutant catalytic destruction downstream,21 the long-term use of the adsorbent, the process efficiency, and, therefore, the process economics depend on the adsorbent regeneration capacity. In this context, the aim of this work is to study the adsorption process and the durability of waste-tire-based ACs using a TSA schemesin order to correlate adsorbent properties to the whole adsorption/desorption process. Pyrolytic carbon black activated with carbon dioxide and a coconut-based commercial AC have been characterized and tested as adsorbents during five cycles for phenanthrene (Phe) and naphthalene (Np) removal. The subsequent characterization makes it possible to compare fresh and used ACs textural properties and to establish relationships between those initial properties and the adsorption capacity evolution along the cycles.
10.1021/ie070849p CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007
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2. Experimental Section 2.1. Activated Carbon Production and Characterization. Three different experimental assembles were required in this work to carry out the pyrolysis, the activation, and the adsorption processes, respectively. Initially, char was produced by shredded waste tire pyrolysis in a stainless steel fixed bed reactor. Once pyrolytic carbon black was obtained, carbon dioxide activation was carried out in a swept fixed bed reactor, described elsewhere.20 Different degrees of activationsvarying from 2 to 16 h of total reaction timesled to the corresponding specific textural characteristics of the resulting solids. Samples were labeled “AXXXY”, where “A” means activated, “XXX” is related to the activation temperature (°C), and “Y” is the reaction time (h). Five different materials, A9002, A9008, A90010, A90014, and A90016, obtained at 900 °C and with different grades of porosity were selected to carry out the TSA process. Additionally, a commercial coconut AC was considered for comparative purposes. These samples were characterized by N2 and CO2 adsorption at -196 and 0 °C, respectively, using a Micromeritics ASAP 2020 apparatus. AC characterization through N2 adsorption isotherms was carried out for both fresh and used materials. The selected temperatures to perform the degasification step were 300 and 150 °C for the fresh and used adsorbents, respectively. The CO2 adsorption analysis was only carried out on the fresh samples. From these data, the following textural parameters were calculated: BET surface area, (SBET), total micropore volume (VN2) by using Dubinin-Radushkevich (DR) equation, and total mesopore volume (VBJH) by using the Barrett-Joyner-Halenda (BJH) method. Additionally, narrow micropore volume (VCO2) was obtained by applying the DR equation to CO2 isotherms, and micropore size distributions were calculated using the density functional theory (DFT). 2.2. Adsorption Experiments. After activation, the selected tire-based ACs, together with the commercial coconut AC, were tested as PAH adsorbents for hot gas cleaning. Five TSA cycles were carried outsboth for Phe and Npson those materials, and the amount of model compound adsorbed and desorbed was calculated for each cycle. The adsorption experiments were conducted in the experimental rig described in detail elsewhere.10 PAH adsorption was carried out in He atmosphere at 150 ( 1 °C with a constant gas hourly space velocity (GHSV) of ca. 25 000 h-1. Thus, 250 ( 13 ppbv of PAH was passed through an adsorbent bed of 0.10 g of AC with a 0.1-0.2 mm particle size. The corresponding AC adsorption capacities (W) for the different PAH were calculated by integrating the experimental breakthrough curves (eq 1) for each cycle.
W)
Q(CPAH,0tt -
∫0t CPAH(t) dt)
mAC
f
(1)
where W is the adsorption capacity (g PAH/kg adsorbent), Q is the gas flow through the solid (mL/s), CPAH,0 is the PAH inlet concentration (g PAH/mL), CPAH(t) is the PAH concentration at any time (g PAH/mL), mAC is the weight of adsorbent (kg), and tf is the total time (s). PAH desorption processes were performed in air at 300 ( 1 °C with a GHSV of ca. 25 000 h-1 and during 100 min. Therefore, whereas adsorption is performed with helium, desorption is conducted with air, which is likely the gas to be used in an industrial scale. At these conditions, desorption profiles were obtained for the different ACs and adsorbates. These profiles were later integrated to obtain the amount of PAH
Figure 1. N2 adsorption isotherms of some representative fresh AC samples.
desorbed for each cycle. Used ACs where recovered after five cycles and characterized by N2 adsorption isotherms. 3. Results and Discussion 3.1. Activated Carbon Production and Characterization. The N2 isotherms of some representative samples used in this study are plotted in Figure 1. A clear difference is observed between the samples produced from pyrolytic carbon black and the commercial AC from coconut. On one hand, the coconutbased AC shows an isotherm that is type I in IUPAC classification, characteristic of microporous solids where the adsorption is mainly taking place at low pressures of N2. On the other hand, all the ACs produced from pyrolytic carbon black show type IV isotherms, which reflects the prevailing mesoporous structure of these solids. From the comparison of the AC isotherms obtained at different activation times, it can be observed that there is a direct relationship between activation time and adsorption capacity of N2 in the whole range of pressures, which can be related to the development of micro-, meso-, and small macroporosity. Textural parameters of fresh ACs, calculated from their N2 and CO2 isotherms, are compiled in Table 1. Regarding the ACs from pyrolytic carbon black, there is an increase in the BET surface area with activation time, which clearly shows a development of the porosity. Depending on the range of porosity studied, first, both total and narrow microporosity highly increase with activation time, reaching a value 6 times higher for the sample activated during 16 h. Second, mesoporosity also increases with activation time, but the development is less than 3 times the initial one. Therefore, the development of porosity is mainly taking place in the microporosity range. In Table 1, the different porous natures of the samples can be pointed out. The coconut sample has a microporous nature with a total microporosity of 0.57 cm3/g compared to its mesoporosity, 0.10 cm3/g. Conversely, tire-based samples have a mesoporous nature, where the contribution of mesoporosity to the total porosity always is higher than the one of the total microporosity. 3.2. Adsorption Experiments Results. Once fresh ACs were characterized, five adsorption cycles were performed on each adsorbent-adsorbate system. Results obtained from Phe and Np adsorption cycles are plotted in Figures 2 and 3, respectively (only shown for A9002, A9008, A90014, and coconut). Adsorption capacities were calculated by eq 1 for each cycle, and their corresponding amounts of PAH desorbed were obtained by integration of the desorption profiles. To throw some light on the TSA process, it is interesting to take into account how single PAH adsorption takes place.
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Figure 2. Adsorbed and desorbed quantities (g PAH/kg activated carbon) for five TSA cycles using Phe as adsorbate.
Figure 3. Adsorbed and desorbed quantities (g PAH/kg activated carbon) for five TSA cycles using Np as adsorbate.
Regarding Np,6 it has been demonstrated that its adsorption in the studied concentration range is mainly taking place in the micropores. On the other hand, it has been also demonstrated that although Phe adsorption10,20 is mainly taking place in the micropores, there also is a significant contribution of multilayer adsorption on the nonmicroporous surface. Accordingly, experimental data reported in Figures 2 and 3 show that Np and Phe adsorption capacities are directly related to the total microporosity of the ACs. Therefore, adsorption capacities increase with the degree of activation regardless the adsorbed PAH. In addition, it can be observed that Phe adsorption capacity is always higher than the amount of adsorbed Np, due to both the lower volatility of Phe and the lack of diffusional problems.22 With regards to ACs efficiency evolution during adsorptiondesorption cycles, it can be seen in Figure 2 that there is a dramatic decreasesespecially for the coconut ACsof the Phe adsorption capacity in the first TSA cycle, except for the least activated AC from pyrolytic carbon black, A9002. During the subsequent cycles, no appreciable change in the amount of adsorbed Phe is found, and each value agrees with its corresponding desorbed quantity (within experimental error of (5%). Conversely in the case of Np (Figure 3), the adsorption capacity
and the amount of desorbed Np almost coincide for each cycle. That means that the AC regeneration is complete under the working conditions and that the AC efficiency remains almost constant during, at least, the first five cycles of the TSA process. Thus, the behavior of the ACs during the TSA process is depending on the nature of the PAH. While Np is almost completely removed from the AC during the desorption step, some Phe molecules are blocked inside the AC porosity at these experimental conditions, leading to a loss of adsorption efficiency during the TSA process. N2 isotherms of the used ACs were obtained in order to completely understand the role of the adsorbent characteristics on the TSA process. A9002, A90014, and coconut isotherms are plotted in Figures 4 and 5, together with the isotherms of the fresh samples for comparative purposes. A similar behavior was observed for the other ACs. When fresh to Phe used samples are compared, see Figure 4, used ACs isotherms show lower adsorption values for the whole range of partial pressures, this effect being especially remarkable on the coconut-based AC. Therefore, high values of microporosity seem to be negative for the TSA process, due to the more deficient regeneration of the ACs with higher microporosity development. Phe molecules are likely retained in the molecular size micropores (0.7 nm calculated by using Materials Studio software) as can be observed in Figure 6, where the micropore size distribution (calculated by the DFT method) of Phe used A90014 and coconut samples are shown. After Phe adsorption, it can be observed that there is a remarkable decrease in the molecular size micropore volume of both ACs. Np used adsorption isotherms are reported in Figure 5. It is observed for the least activated samples, A9002 (similar trends were observed for A9008 and A90010), that ACs isotherms after five Np adsorption cycles agree with the corresponding fresh AC isotherms. However, N2 adsorption capacity slightly decreases at higher activation times, A90014, A90016 (not shown), which would mean that those ACs do not regenerate completely. It can be observed in Figure 5 that there is a slight decrease in the amount of N2 adsorbed for the whole range of the relative pressures, which means that a small part of the molecular size microporosity seems to be blocked during the TSA process, according to the Np molecular size (0.5 nm calculated by using Materials Studio software). This fact can also be observed in Figure 6, where the micropore size distribution of the fresh and Np used ACs is shown. Nevertheless, this blocking effect is negligible, as the Np adsorption capacity is kept constant within the experimental error along the five TSA cycles, see Figure 3. A similar behavior can be observed for the coconut-based AC. In this case, the Np used adsorption isotherm shows a rounder knee, which can be clearly related to the blocking of the narrow micropores. Textural parameters of used ACs are also shown in Table 1, which includes SBET, VN2, and VBJH values. It can be observed that surface area and mesopore volume are generally lower after the TSA cycles, this effect being more remarkable in the case
Table 1. Adsorbents Textural Parameters: Specific Surface Area (SBET), Micropore Volume (VN2), Mesopore Volume (VBJH), and Narrow Micropore Volume (VCO2) SBET (m2/g)
VN2 (cm3/g)
VBJH (cm3/g)
VCO2 (cm3/g)
sample
fresh
Phe
Np
fresh
Phe
Np
fresh
Phe
Np
fresh
A9002 A9008 A90010 A90014 A90016 coconut
67 153 194 290 373 1118
36 92 149 188 278 603
66 155 185 216 325 946
0.034 0.084 0.10 0.14 0.17 0.57
0.024 0.045 0.07 0.09 0.13 0.37
0.033 0.082 0.097 0.11 0.13 0.53
0.12 0.16 0.17 0.21 0.26 0.10
0.081 0.13 0.12 0.18 0.26 0.09
0.12 0.16 0.16 0.19 0.25 0.11
0.010 0.071 0.082 0.084 0.095 0.31
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Figure 4. N2 adsorption isotherms of fresh and used ACs after five TSA cycles with Phe as the model compound: (A) A9002, (B) A90014, and (C) coconut, the line being for fresh sample and the filled dots for Phe used samples.
of Phe. Regarding the SBET and VN2, it is observed that this decrease is due to the presence of molecular size pores, where the adsorption potential is maximum and the energy provided is not enough to perform the desorption process. On the other hand, it can be tentatively proposed that the decrease in the VBJH values of the least activated adsorbents is due to the presence of necked-bottle-type mesopores, where the Phe molecules are retained, by blocking the entrance into that pores. As can be observed in Table 1, higher activation times favor the AC efficiency during the TSA process because of the opening of pore entrances. Thus, the TSA process seems to be a feasible technology to concentrate PAH from contaminated streams and to regenerate the ACs in industrial applications. Unfortunately, it has been observed that there is a decrease in the performance of the ACs from the first to the second cycle, depending on the PAH nature and adsorbent characteristics. While Np removal efficiency is
Figure 5. N2 adsorption isotherms of fresh and used ACs after five TSA cycles with Np as the model compound: (A) A9002, (B) A90014, and (C) coconut, the line being for fresh sample and the filled dots for Np used samples.
regenerated during the desorption process, Phe is strongly affected by the AC textural properties as can be seen in Table 2, where the loss of Phe adsorption capacites from the first to the second cycle of the TSA process, together with the VCO2/ VN2 ratio for the different ACs, are compiled. It has been previously reported by this research group20 that the evolution of pyrolytic carbon black porosity during a CO2 activation process can be described as a double phenomenon. In the first step, narrow micropores are created once the reaction begins. Later, the destruction of narrow microporosity to generate wider pores is taking place in a second step. Thus, the porosity evolution in the AC as the reaction progresses is producing an increase in Phe adsorption capacity. In addition, the destruction of molecular size micropores should bring about a decrease in the adsorption strength, which is the most important factor during the AC regeneration step. According to this assumption,
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Thus, the desorption step would not be hindered by steric effects and energy requirements.23 Nevertheless, the net adsorption capacity of this AC is extremely low, which would restrict its feasible applicability in spite of its suitable performance during the TSA process. Finally, the TSA performance of the coconutbased AC confirms the above assumptions. This commercial AC shows a similar loss in Phe adsorption capacity from the first to the second adsorption cycle (in percentage) than the waste tire adsorbents activated for 14 and 16 h, according to the similar values found for the VCO2/VN2 ratio. 4. Conclusions
Figure 6. Micropore size distribution of fresh (black line) and used activated carbons after Np (dashed line) and Phe (gray line) adsorption: (A) coconut and (B) A90014. Table 2. Narrow Micropore Volume/Total Micropore Volume Ratio (VCO2/VN2) and Phe Adsorption Capacity Decrease from the First to the Second TSA Cycle (in Percentage) sample
VCO2/VN2 (fresh sample)
Phe adsorption capacity decrease (%)
A9002 A9008 A90010 A90014 A90016 coconut
0.29 0.85 0.82 0.60 0.56 0.54
5.6 38 36 30 29 30
it can be observed in Table 2 that there is a decrease in the VCO2/VN2 ratio with the activation time for medium and highly activated adsorbents (from 8 to 16 h), which favors the efficiency of the TSA process. This fact shows that the presence of molecular size micropores, where the adsorption potential is maximum, are not effective adsorption sites for the Phe molecules during the TSA process. Thus, although these adsorption sites are accounting for the total adsorption capacity of the ACs in the first cycle, Phe molecules cannot be desorbed at the experimental conditions used in this work, due to energetic constraints during the desorption step. On the other hand, it can be observed in Table 2 that the Phe adsorption process on the least activated carbon (A9002) shows a remarkably low value for the loss of Phe adsorption capacity in the TSA process. This fact could be explained taking into account the Phe adsorption mechanism on ACs. As commented above, Phe adsorption is not only taking place in the micropores but also there is a significant contribution of multilayer adsorption on the nonmicroporous surface. It can be seen in Tables 1 and 2 that the microporosity contribution in the A9002 sample is very scarce, and therefore, Phe molecules are supposed to be mainly adsorbed on the external surface and the mesopores.
It can be concluded that the PAH desorption stage is critical when determining the adsorbent optimum properties for hot gas cleaning. It has been found that, for Np, carbon regeneration is easily accomplished over the ACs studied in this paper. Although the narrowest pores seem to be blocked, the adsorbent efficiency is hardly affected during the TSA process. Therefore, the initial adsorption capacities can be maintained during adsorption/desorption cycles for ACs with high meso- and micropore volumes and low VCO2/VN2 ratio. Conversely, it has been found for Phe that its removal efficiency drastically decreased after the first regeneration cycle, but it was maintained in the successive adsorption/desorption cycles. This fact was likely due to a pore-blocking effect by Phe molecules retained in the molecular size pores, where the adsorption potential is maximum. Thus, not only the total micropore volume, but also their micropore size distribution and their accessibility through mesopores, are important variables in the desorption stage, and these parameters should also be taken into account when an AC for PAH abatement is selected. It is worth pointing out that real exhaust gases are composed of a complex mixture of PAH, from two to four aromatic rings. Therefore, strongly activated ACs with a high porosity development, micropores with a size bigger than PAH molecular sizes, and pores without neckedbottle-type form seem to be the most suitable adsorbents to be implemented in the TSA processes for PAH hot gas cleaning. Notation AC ) activated carbon CPAH,0 ) polycyclic aromatic hydrocarbon initial concentration (g PAH‚mL) CPAH(t) ) polycyclic aromatic hydrocarbon concentration at any time (g PAH‚mL) DFT ) density functional theory GHSV ) gas hourly space velocity (h-1) mAC ) activated carbon mass (kg) Np ) naphthalene PAH ) polycyclic aromatic hydrocarbon Phe ) phenanthrene PM ) particulate matter Q ) gas flow (mL‚s-1) SBET ) surface area obtained by applying the BrunauerEmmett-Teller equation to the N2 adsorption isotherm (m2‚g-1) t ) time (s) TSA ) temperature swing adsorption VN2 ) total micropore volume (cm3‚g-1) obtained from N2 adsorption isotherm applying the Dubinin-Radushkevich equation VCO2 ) narrow micropore volume (cm3‚g-1) obtained from CO2 adsorption isotherm applying the Dubinin-Radushkevich equation
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VBJH ) mesopore volume (cm3‚g-1) obtained from N2 adsorption isotherm obtained by using the Barrett-Joyner-Halenda method W ) adsorption capacity (g PAH‚kg activated carbon-1) Literature Cited (1) Directive 2004/107/EC of the European Parliament and of the Council of 15th December 2004 relating to arsenic, cadmium, mercury and polycyclic aromatic hydrocarbons in ambient air. Official Journal of the European Union, L: Legislation 23/3. (2) International Agency for Research on Cancer (IARC), Chemical Environmental and Experimental Data. Polynuclear Aromatic Compounds, Part 1; Lyon, France, 1983. (3) EValuation and Estimation of Potential Carcinogenic Risks of PAH: Carcinogen Assessment; Office of Health and Environmental Assessment, Office of Research and Development, U.S. EPA, U.S. Government Printing Office: Washington, DC, 1985. (4) Kirton, P. J.; Crisp, P. T. The sampling of coke oven emissions for polycyclic aromatic hydrocarbons: a critical review. Fuel 1990, 69 (5), 633-638. (5) Marr, L. C.; Kirchstetter, T. W.; Harley, R. A.; Miguel, A. H; Hering, S. V.; Hammond, S. K. On-road measurement of fine particle and nitrogen oxide emissions from light- and heavy-duty motor vehicles. Atmos. EnViron. 1999, 33 (18), 2955-2968. (6) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lo´pez, J. M.; Navarro, M. V. Measurements of polycyclic aromatic hydrocarbon adsorption on activated carbons at very low concentrations. Ind. Eng. Chem. Res. 2003, 42, 155-161. (7) Mastral, A. M.; Calle´n, M. S. A review on polycyclic aromatic hydrocarbon (PAH) emissions from energy generation. EnViron. Sci. Technol. 2000, 34, 3051-3057. (8) Lee, W. M. G.; Tong, H. C.; Yeh, S. Y. Partitioning model of PAHs between gaseous and particulate phases with consideration of reactivity of PAH in an urban atmosphere. J. EnViron. Sci. Health, Part A 1993, 28 (3), 563-583. (9) Cudahy, J. J.; Helsel, R. W. Removal of products of incomplete combustion with carbon. Waste Manage. 2000, 20 (5-6), 339-345. (10) Mastral, A. M.; Garcı´a, T.; Calle´n, M. S.; Navarro, M. V.; Galba´n, J. Removal of naphthalene, phenanthrene and pyrene by sorbents from hot gas. EnViron. Sci. Technol. 2001, 35 (11), 2395-2400. (11) Murillo, R.; Garcı´a, T.; Aylo´n, E.; Calle´n, M. S.; Navarro, M. V.; Lo´pez, J. M. Adsorption of phenanthrene on activated carbons: breakthrough curve modelling. Carbon 2004, 42, 2009-2017.
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ReceiVed for reView June 21, 2007 ReVised manuscript receiVed August 30, 2007 Accepted August 30, 2007 IE070849P
