Adsorbents for Dioxins: A New Technique for Sorbent Screening for

Risk Assessment and Management at Deseret Chemical Depot and the Tooele Chemical Agent Disposal Facility; National Research Council: Washington, DC, ...
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Adsorbents for Dioxins: A New Technique for Sorbent Screening for Low-Volatile Organics Ralph T. Yang,* Richard Q. Long, Joel Padin, and Akira Takahashi Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

Tomonori Takahashi Corporate Research and Development Group, NGK Insulators, Ltd., Nagoya, Japan

Dioxins are generated in all waste incinerators, and activated carbon has been used since 1991 as the adsorbent. However, no equilibrium adsorption information is available because no experimental technique has been developed for measuring adsorption for such low-volatile organic compounds at the low concentrations that are of interest (i.e., parts per trillion to parts per billion levels). A new technique based on temperature-programmed desorption is developed for quick screening of sorbents and for determination of the activation energy for desorption. This information is used further for estimating the equilibrium isotherm. Twelve sorbents are screened for dioxin (nonchloro form) adsorption, and the equilibrium isotherms are estimated. Introduction Incineration is an efficient way for disposal of wastes such as municipal waste, medical waste, hazardous waste, and army stockpile (chemical agents). Unfortunately, trace amounts of highly toxic compounds are also generated. Among them, dioxins are the most toxic and also the most stable. Dibenzo-p-dioxins are a family of compounds consisting of two benzene rings joined by two oxygen atoms and having from zero to eight chlorine atoms attached around the rings. There are 75 chlorinated dioxins that differ in the number and positions of Cl atoms.1 The dibenzofurans are a similar family of toxic compounds differing in that only one of the bonds between the two benzene rings is bridged by oxygen. The polychlorinated dibenzo-p-dioxins and dibenzofurans (collectively known as “dioxins”) are formed downstream of the combustion zone and decompose at temperatures only above 1200 °C. The toxicity varies with the number of Cl atoms, with non- and monochloro dioxins being nontoxic while being highly toxic with more Cl atoms. The typical concentrations in the effluents of incinerators are in the range 10-500 ng/Nm3. Current regulations on dioxin emissions are complex, depending on the toxic equivalency of the actual compounds and O2 concentration, and vary in different countries. Nonetheless, removal to well below 1 ng/Nm3 is generally required (e.g., refs 2 and 3). Since 1991, activated carbon adsorption has been widely adopted for dioxin removal from municipal and other waste incinerators in Europe and Japan.2 Both powder injection and fixed-bed filters have proven to be highly effective.2-4 Catalytic oxidation has also been investigated (on catalysts such as Pt/γ-Al2O3 and vanadia5) but is apparently not as effective as adsorption. Adsorption by activated carbon is also to be used for dioxin removal in chemical stockpile disposal.6,7 * Corresponding author. Tel: (734) 936-0771. Fax: (734) 763-0459. E-mail: [email protected].

Although activated carbon is a highly effective sorbent for dioxins, no information exists on the adsorption equilibria and diffusion rates. The use of activated carbon for adsorption of dioxins, as well as many other low-volatile and semivolatile organics, is purely empirical. Because of the strong bonds between dioxins and carbon, the sorbent is not regenerable. Information on equilibrium is crucially needed for adsorber design. The lack of equilibrium information can be attributed to the extreme difficulties that are involved in experimentation; hence, no convenient experimental technique is available. Adsorption isotherm measurements for dioxins from solutions are more straightforward and have indeed been done for dioxins on clays,8,9 but the technique is not applicable to gas-phase adsorption. More recently, a technique was developed by Karwacki et al.10 for measuring gas-phase equilibrium isotherms, and it was used with success for 1- and 2-hexanol on carbon, at relative pressures as low as 10-7. This technique requires adsorption followed by constant purge with an inert carrier, when desorption occurs. The desorbed vapor is trapped and subsequently measured. Hence, with this technique one is measuring the pseudoequilibrium vapor pressure that corresponds to a given adsorbed amount. It is a novel technique, but it remains to be seen whether it is useful for measuring compounds with considerably lower volatilities and hence are much more strongly adsorbed (than hexanol) such as dioxins. The approach of Karwacki et al. was similar to that used earlier by Rordorf, 11 based on a high-sensitivity vapor pressure measurement technique. In Rordorf’s technique, the “vapor pressure” of a given amount of adsorbed chlorinated dibenzo-p-furan on a flyash was measured, to a vapor pressure as low as 2 × 10-5 Pa [or 0.2 parts per billion (ppb)]. In this work, we have developed a simple technique based on temperature-programmed desorption (TPD), and we have used the TPD technique for sorbent screening on a series of sorbents. Moreover, a method is developed for estimating the equilibrium adsorption isotherms for dioxins on different sorbents based on

10.1021/ie990170o CCC: $18.00 © 1999 American Chemical Society Published on Web 06/02/1999

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the TPD technique. The first estimated isotherm for dioxin on activated carbon is reported. The TPD technique can also be used for determining the regenerabilities of sorbents as well as the conditions for regeneration. New Technique for Measuring Adsorption Energies and Estimating Isotherms for Low-Volatile and Semivolatile Organics The main difficulty for measuring adsorption isotherms for low-volatile and semivolatile organics such as dioxins lies in their high melting and boiling points. Hence, it is difficult to generate the vapor and control its concentration. Moreover, the adsorption bonds for these compounds are generally very strong and the isotherms are very steep. Information in the very low concentration range, in the parts per trillion (ppt) to ppb range, is extremely difficult to obtain. It is the information in this low concentration range that is needed for practical applications. For dioxins, information in the ppt range is important because removal to the order of ng/m3 is required. To overcome these difficulties, a technique based on TPD is developed. The technique involves TPD at different heating rates. From the desorption temperature peaks, the activation energy for desorption (or the bond energy) can be calculated. In the experiment, a small amount of adsorbate, either in its solid form or in solution, is dosed at the inlet of the column of the sorbent. The TPD experiment ensues with an inert gas flowing through the column. As the temperature is increased, the adsorbate at the inlet is vaporized and is subsequently adsorbed in the bed. As the temperature is further increased, the adsorbate is eventually desorbed. In case the amount of the dosed adsorbate exceeds the saturated amount for the sorbent in the column, two peaks will appear: the first one is at or slightly above the boiling point, and the second is the true desorption peak temperature. From the area of the second peak, the saturated amount (or monolayer amount) at the peak temperature can be determined. In case the amount of sorbate that is dosed is less that the saturated amount, all sorbate will be adsorbed and only the second peak will be observed. Because the desorption peak temperature is related to the adsorption bond strength, a stronger bond gives rise to a higher TPD peak. This technique provides a convenient and quick way for screening sorbents. It can also be used for estimating the adsorption isotherm. To this end, the activation energy for desorption is calculated by12

2 ln Tm - ln b ) E/(RTm) + ln ZA

(1)

where Tm is the peak desorption temperature, b is the heating rate, E is the activation energy for desorption, R is gas constant, and ZA is a constant that depends on the desorption kinetics. The desorption is assumed to follow first-order kinetics, and the heating rate is constant. Although the Dubinin-Radushkevich isotherm is widely used for adsorption of hydrocarbons on mesoporous activated carbon, it is not appropriate for the very low concentration range.13 For adsorption of low-volatile and semivolatile organic compounds, such as dioxins, the sorbate-surface bond is much stronger than the sorbate-sorbate bond. Therefore, the Langmuir iso-

therm is more meaningful to be applied.14 This is particularly the case for the low concentration range. The Langmuir isotherm is given by

θ)

q BP ) qm 1 + BP

(2)

where θ is fractional surface coverage, q is the amount adsorbed at absolute temperature T and vapor pressure P, qm is the monolayer amount, and B is the Langmuir constant. Once the activation energy for desorption (E) is known, the Langmuir constant can be calculated by15

B)

σN eE/RT βx2πMRT

(3)

where σ is the molecular area, N is Avogadro’s number, β is the vibration frequency of the adsorbate against the surface, and M is the molecular weight. Experimental Section Experimental Details. The TPD experiments were performed from room temperature to 500-800 °C at different heating rates ranging from 2 to 10 °C/min. In each experiment, 0.1 g of sorbent sample, with a particle size range of 60-100 U.S. mesh, was loaded in a stainless steel tubing (1/8-in.) with an inner diameter of 2 mm. The length of the column was approximately 5 cm, depending on the sample. Either solid dioxin (0.5 mg) or its solution in N,N-dimethylformamide (DMF; 5 µL) was used as the adsorbate. The solution consisted of 50 mg of dibenzo-p-dioxin dissolved in 1 mL of DMF. The solid sample or the solution was then loaded at the inlet of the sorbent column. The column was then purged thoroughly with helium (50 mL/min) at room temperature. Subsequently, the sample was heated in the He flow at a constant heating rate. The effluent or the desorption products were analyzed continuously with a gas chromatograph (GC, Shimadzu 14A). Both a thermal conductivity detector (TCD) and a flame ionization detector (FID) were used is each experiment. The total area under the GC peak should be equal to the total amount of the dioxin that was adsorbed. Hence, the GC signal could be quantified. However, this was not done in this work. The He gas (high-purity grade from Matheson) was pretreated with a 5A molecular sieve column and a model 1000 oxygen trap column before entering the sorbent column. The tubings between the sample column and the TCD/FID detectors were heated with heating tapes to 300 °C to prevent deposition of dioxin (bp 266 °C and mp 122 °C). Sorbents Studied and Materials. The following sorbents were subjected to the TPD experiments: three activated carbons, CuCl2-impregnated activated carbon, γ-Al2O3, CuCl2-impregnated γ-Al2O3, two zeolites, Ag+-exchanged zeolite, two clays, and Al2O3-pillared clay. The three activated carbons were BPL carbon from Calgon (Pittsburgh, PA), ZX-4 carbon from Mitsubishi (Tokyo, Japan), and TURUMI-Coal 7GM from TURUMI (Yokohama, Japan). All three were gas-phase carbons. The 20 wt % CuCl2/carbon (BPL) was prepared by the method of incipient wetness impregnation. γ-Al2O3 was PSD-450 grade from Aluminum Company of America. The 20 wt % CuCl2/Al2O3 was also prepared by the method of incipient wetness impregnation. A

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Figure 1. Temperature-programmed desorption (TPD) profiles of dioxin (nonchloro) at a heating rate of 5 °C/min (in helium) on (a) γ-Al2O3, (b) 20 wt % CuCl2/γ-Al2O3, (c) Y zeolite, (d) Ag+exchanged Y zeolite, (e) clinoptilolite, (f) laponite, (g) bentonite, and (h) Al2O3-pillared bentonite.

Y-type zeolite with Si/Al ) 195 was obtained from Tosoh Corp. (Japan). Ag ion-exchanged Y-zeolite (Si/Al ) 195) was prepared using the conventional ion-exchange procedure. A total of 4 g of Y-zeolite was added to 200 mL of a 0.05 M AgNO3 solution with constant stirring. The exchange process was carried out at room temperature for 24 h. Subsequently, the mixture was filtered and washed five times with deionized water. The obtained sample was first dried at 120 °C for 12 h and then calcined at 500 °C for 4 h in air. A natural zeolite, clinoptilolite, was also included in the work. The sample was obtained from Steelhead Co. (Seattle, WA), with details given elsewhere.16 Two clays, bentonite (a natural clay from Wyoming) and laponite, were obtained from Fisher Co. and Southern Clay Products (of Laporte), respectively. The laponite is a synthetic hectorite. Details of these two clays are available elsewhere.17 The Al2O3-pillared bentonite was prepared according to the procedure described elsewhere.17 Nonchloro dioxin was used in this study because it is nontoxic. The nonchloro dioxin was supplied by Chem Service Co., and had a purity of 99%. DMF was from Aldrich, at 99.8% purity. Results and Discussion TPD Results: Rank Order of Sorbents. As described in the foregoing, all TPD experiments were conducted with a pure-helium purge. TPD peak temperatures were compared for 12 sorbents at the same heating rate of 5 °C/min. The results are shown in Figures 1 and 2. Figure 1 shows the TPD spectra of dioxin for sorbents other than activated carbon. One desorption band was observed for γ-Al2O3, with a maximum desorption temperature at 353 °C. When 20 wt % CuCl2 was doped on γ-Al2O3, the desorption band shifted to 360 °C. The slight increase in desorption temperature suggests that

Figure 2. TPD profiles of dioxin (nonchloro) at a heating rate of 5 °C/min on (a) TURUMI activated carbon, (b) Mitsubishi carbon, (c) BPL carbon, and (d) 20 wt % CuCl2/C(BPL).

Figure 3. TPD profiles of dioxin on γ-Al2O3 at heating rates of (a) 10, (b) 5, and (c) 2 °C/min.

copper dichloride enhanced the interaction between dioxin and the sample. On Y zeolite (Si/Al ) 195), the desorption peak of dioxin appeared at 241 °C. By comparison, the desorption band of dioxin increased to 261 °C on Ag ion-exchanged Y zeolite. This result indicates that a weak π complexation existed between the benzene rings and Ag+. On the clay and pillared clay samples, the desorption temperature of dioxin decreased in the order of Al2O3-pillared bentonite (282 °C) > bentonite (275 °C) > laponite (202 °C) ≈ clinoptilolite (200 °C), as shown in Figure 1. For bentonite, laponite, and clinoptilolite, the adsorption probably took place on the external surfaces of the particles because of the lack of pores (in clays) or the pores being too small (in

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Figure 4. TPD profiles of dioxin on TURUMI carbon at heating rates of (a) 10, (b) 5, and (c) 2 °C/min.

Figure 6. Relationship between the maximum desorption temperature (Tm) and the heating rate (b) on TURUMI carbon. Table 1. Peak Desorption Temperatures of (Nonchloro) Dioxin at Different Heating Rates and Activation Energies for Desorption on Different Sorbents peak desorption temperature (°C) at different desorption heating rates activation 2 5 10 energy °C/min °C/min °C/min (kcal/mol)

sorbent BPL carbon (Calgon) ZX-4 carbon (Mitsubishi) 7GM carbon (TURUMI) γ-Al2O3

497 481 468 306

522 517 492 353

563 543 517 394

27.1 28.3 34.7 11.4

Table 2. Equilibrium Adsorption of 2-Hexanol on BPL Carbon at 75 °C

Figure 5. Relationship between the maximum desorption temperature (Tm) and the heating rate (b) for dioxin on γ-Al2O3.

clinoptilolite with pores < 0.4 nm). For all of these samples, the desorption temperature of dioxin was below 360 °C. Hence, these sorbents can be regenerated at temperatures below 360 °C. The TPD spectra of dioxin on activated carbons and 20 wt % CuCl2/carbon (BPL) are shown in Figure 2. The peak desorption temperatures of dioxin from the carbons were considerably higher than that on the non-carbon samples, suggesting stronger interactions between dioxin and carbons. The desorption band on BPL carbon appeared at 523 °C, which was 31 °C higher than that on TURUMI carbon. After 20 wt % CuCl2 was doped on the BPL carbon, the desorption temperature of dioxin was increased to 693 °C. This indicates that CuCl2

pressure, Pa

exptl amt ads,10 mmol/g

amt ads estd from E, mmol/g

0.01 0.1 1 10

0.22 0.51 1.3 1.9

0.28 1.3 2.0 2.2

significantly enhanced the adsorption of dioxins on carbon. The increased interaction by CuCl2 was, possibly, also due to weak π complexation.18 Following the TPD results, the sorbents can be ranked according to their strengths of interactions with dioxin as follows (with peak desorption temperaure): CuCl2/ BPL carbon (693 °C) > BPL carbon (523 °C) > Mitsubishi ZX-4 carbon (517 °C) > TURUMI carbon (492 °C) > CuCl2/γ-Al2O3 (360 °C) > γ-Al2O3 (353 °C) > Al2O3pillared bentonite (282 °C) > bentonite (275 °C) > Ag-Y zeolite (261 °C) > Y zeolite (241 °C) > laponite (202 °C) ≈ clinoptilolite (200 °C). Heats of Adsorption from TPD. As discussed above, TPD yields information on the activation energy for desorption, via eq 1. For physical adsorption, the heat of adsorption is equal to the activation energy for desorption. This is not true for strong chemisorption when breakage and formation of chemical bonds are

2730 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 3. Norit Incinerator Data for Dioxins Removal by Carbon4 conc of dioxins in gas phase (ng/dsm3 a) inlet outlet 128 411 416 94 a

4.66 16 6 2

amt of dioxins adsorbed (g/kg of carbon)

amt of Hg adsorbed (g/kg of carbon)

incinerator type

temp (°C)

0.34 × 10-3 1.2 × 10-3 3.5 × 10-3 0.32 × 10-3

1.7 2.1 21.1 31.3

municipal waste municipal waste medical waste medical waste

132 176 unknown unknown

equilibrium amt ads calcd (g/kg of carbon) 125 5.2

Dry m3 at STP.

involved. For adsorption of low-volatile organics such as dioxin on carbon, the process is reversible. Hence, the activation energy for desorption is a good approximation for the heat of adsorption. The TPD spectra of dioxin on γ-Al2O3 and TURUMI carbon at different heating rates are shown respectively in Figures 3 and 4. On γ-Al2O3, the maximum desorption temperature increased from 306 to 394 °C as the heating rate was increased from 2 to 10 °C/min (Figure 3). The plot following eq 1 is given in Figure 5. From this plot, the activation energy for desorption of dioxin on γ-Al2O3 was 11.4 kcal/mol. A similar plot for dioxin on the TURUMI carbon is shown in Figure 6, which yielded an activation energy of 34.7 kcal/mol. TPD experiments at different heating rates were also made for the other two activated carbons. The results are summarized and compared with the TURUMI carbon and γ-Al2O3, as shown in Table 1. The comparison among the three carbons is interesting. The BPL carbon showed the highest TPD peak temperatures at all heating rates, yet it had the lowest activation energy for desorption. This inverse trend is seen clearly among the three activated carbons. The reason for this inverse trend is not understood; however, it might be caused by different functional groups on the surface19 that bonded differently with the dioxin molecule. Estimated Isotherms. As discussed in the foregoing, for adsorption of low-volatile organics on carbon, the sorbate-surface bond is usually much stronger than the sorbate-sorbate bond. Hence, the adsorption is limited to the monolayer, and this is particularly the case for low pressures or concentrations. Consequently, the Langmuir isotherm is a meaningful representation for adsorption equilibrium. The Langmuir isotherm is a two-constant isotherm, with constants qm and B as shown by eq 2. The Langmuir constant B can be calculated from eq 3, given the information on E, the activation energy for desorption. For the value of the monolayer amount, qm, an estimate can be made based on two-dimensional molecular packing (p 44 of ref 13), which is similar to the “Gurvitsch rule”.20 The monolayer amount is hence estimated by dividing the surface area by the molecular area. Estimates made in this manner are in excellent agreement with experimental data for strongly adsorbed systems such as benzene on silica gel and hydrocarbons (higher than methane) on carbon.21 The molecular area for dioxin1 at room temperature is 186 × 10-20 m2. Hence, dividing the surface areas of the different sorbents by this value yields estimates for the monolayer amounts. The surface areas for the activated carbons were as follows: BPL (891 m2/g), Mitsubishi (876 m2/g), and TURUMI (887 m2/g). From eq 3, the Langmuir constant for dioxin on BPL carbon is

B ) 3.2 × 10-2(1/T1/2) exp(E/RT) (1/atm)

(4)

Adsorption equilibria data are available for the system 2-hexanol on BPL carbon at low pressures.10 A test of the proposed methodology for estimating isotherms can be made using this system. The equilibrium data for the above system were measured at three temperatures, with the most complete data set obtained at 75 °C, which covered the pressure range from 0.01 to 10 Pa.10 From the temperature dependence, the heat of adsorption is 15 kcal/mol. The molecular area is estimated at 65 × 10-20 m2. Using the proposed methodology, the predicted amounts adsorbed for 75 °C at four pressures (0.01, 0.1, 1, and 10 Pa, i.e., covering the whole pressure range) are compared with the experimental data in Table 2. Considering the simplicity of the proposed methodology, the comparison is fairly satisfactory. No experimental data have been published on the equilibrium adsorption of dioxins on carbon. However, Norit4 has released data from several commercial incinerators where a powdered activated carbon was injected in the effluents at 130-176 °C. The carbon removed mercury, polychlorinated dioxins, and dibenzofurans as well as many other compounds. The data are listed in Table 3. On the basis of the data on BPL carbon for dioxin obtained in this work, an estimate was also made on the equilibrium amounts, which are also listed in Table 3. From the comparison in Table 3, it is clear that the amounts adsorbed in the commercial operation were far from equilibrium. The short contact time (in seconds) was likely one of the reasons. Obviously, the coadsorption of large amounts of Hg (104105 times that of the dioxins) and other compounds does not make the comparison meaningful. Nomenclature A ) constant b ) heating rate B ) Langmuir constant E ) activation energy for desorption, or bond energy M ) molecular weight N ) Avogadro’s number P ) pressure q ) equilibrium amount adsorbed qm ) monolayer amount adsorbed R ) gas constant T ) absolute temperature Tm ) peak desorption temperature Z ) constant Greek Letters β ) vibrational frequency σ ) molecular area per site θ ) fractional surface coverage

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Literature Cited (1) Mackay, D.; Shu, W. Y.; Ma, K. C. Polynuclear Aromatic Hydrocarbons, Polychlorinated Dioxins and Dibenzofurans. Handbook of Physical Chemical Properties and Enviromental Fate for Organic Chemicals; Lewis Publishers: Ann Arbor, MI, 1992; Vol. II. (2) Harstenstein, H. U. Activated Carbon for Flue-Gas Polishing of MWIs (Municipal Waste Incinerators). Proceedings of International Specialty Conference, VIP-32; Municipal Waste Combustion Conference, Williamsburg, VA, March 1993; Air and Waste Management Association: Pittsburgh, PA, 1993; pp 87-105. (3) U.S. Environmental Protection Agency, Proposed Rule: Revised Standards for Hazardous Waste Combustors Maximum Achievable Emissions Control. Proposal. EPA Office of Solid Waste. GI Federal Register 17358-536; Washington DC, U.S. Government Printing Office: April 19, 1996. (4) Norit Americas Inc. Norit Acitvated Carbons for Removal of Mercury and Dioxin for Flue Gas. Report NA 74-3; Norit Americas Inc.: Atlanta, GA, 1994. (5) Ide, Y.; Kashiwabara, K.; Okada, S.; Mori, T.; Hara, M. Catalytic Decomposition of Dioxin from MSW Incinerator Flue Gas. Chemosphere 1996, 32, 189.5. (6) National Research Council. Risk Assessment and Management at Deseret Chemical Depot and the Tooele Chemical Agent Disposal Facility; National Research Council: Washington, DC, 1997. (7) Goldgarb, A.; Anderson, G.; Mahle, J.; Croft, D. Pollution Abatement System. Carbon Filter Simulation Model Development. Mitretek Report to U.S. Army; Mitretek Systems: McLean, VA, 1997. (8) Srinivasan, K. R.; Fogler, H. S.; Gulari, E.; Nolan, T.; Schultz, J. S. The Removal of Trace Levels of Dioxins from Water by Sorption on Modified Clay. Environ. Prog. 1985, 4 (4), 239. (9) Srinivasan, K. R.; Fogler, H. S. Use of Modified Clays for the Removal and Disposal of Chlorinated Dioxins and Other Priority Pollutants from Industrial Wastewaters. Chemosphere 1989, 18, 333. (10) Karwacki, C. J.; Buettner, L. C.; Buchanan, J. H.; Mahle, J. J.; Tevault, D. E. Low-Concentration Adsorption Studies for Low-Volatility Vapors. Fundamentals of Adsorption; Meunier, F., Ed.; Elsevier: Amsterdam, The Netherlands, 1998; p 315.

(11) Rordorf, B. F. Thermodynamic and Thermal Properties of Polychlorinated Compounds: The Vapor Pressures and Flow Tube Kinetics of Ten Bibenzo-p-Dioxins. Chemosphere 1985, 14, 885. (12) Cvetanovic, R. J.; Amenomiya, Y. A Temperature Programmed Desorption Technique for Investigation of Practical Catalysts. Catal. Rev. 1972, 6, 21. (13) Kapoor, A.; Ritter, J. A.; Yang, R. T. On the DubininRadushkevich Equation for Adsorption in Microporous Solids in the Henry’s Law Region. Langmuir 1989, 5, 1118. (14) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Boston, 1987. Reprinted in paperback, Imperial College Press: River Edge, NJ, 1997. (15) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1976; p 554. (16) Ackley, M. W.; Yang, R. T. Diffusion in High-Exchange Clinoptilolite. AIChE J. 1991, 37, 1645. (17) Yang, R. T.; Cheng, L. S. Pillared Clays and Ion Exchanged Pillared Clays as Gas Adsorbents and Catalysts for Selective Catalytic Reduction of NO. In Access in Nanoporous Materials; Pinnavaia, T. J., Thorpe, M., Eds.; Plenum: New York, 1995; p 73. (18) Tamon, H.; Kitamura, K.; Okazaki, M. Adsorption of Carbon Monoxide in Activated Carbon Impregnated with Metal Halides. AIChE J. 1996, 42, 422. (19) Puri, B. R. Surface Complexes on Carbons. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1970; Vol. 6, p 191. (20) Myers, A. L. Adsorption of Pure gases and Their Mixtures on Heterogeneous Surfaces. In Fundamentals of Adsorption; Myers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1983. (21) Valenzuela, D. P.; Myers, A. L. Adsorption Equilibrium Data Handbook; Prentice Hall: Englewood Cliffs, NJ, 1989.

Received for review March 5, 1999 Revised manuscript received April 29, 1999 Accepted April 30, 1999 IE990170O