Effect of Temperature on the Desorption and Decomposition of

Experimental data are reported for the desorption of bis-2-chloroethyl sulfide, (a sulfur mustard or HD) and its decomposition products from activated...
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Langmuir 1999, 15, 8645-8650

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Effect of Temperature on the Desorption and Decomposition of Mustard from Activated Carbon Christopher J. Karwacki,*,† James H. Buchanan,† John J. Mahle,† Leonard C. Buettner,† and George W. Wagner‡ Research and Technology Directorate, U.S. Army Edgewood Research, Development and Engineering Center (ERDEC), Aberdeen Proving Ground, Maryland 21010-5423, and Geo-Centers, Inc., Gunpowder Branch Box 68, Aberdeen Proving Ground, Maryland 21010-0068 Received January 12, 1999. In Final Form: July 29, 1999 Experimental data are reported for the desorption of bis-2-chloroethyl sulfide, (a sulfur mustard or HD) and its decomposition products from activated coconut shell carbon (CSC). The results show that under equilibrium conditions changes in the HD partial pressure are affected primarily by its loading and temperature of the adsorbent. The partial pressure of adsorbed HD is found to increase by about a decade for each 25 °C increase in temperature for CSC containing 0.01-0.1 g/g HD. Adsorption equilibria of HD appear to be little affected by coadsorbed water. Although complicated by its decomposition, the distribution of adsorbed HD (of known amount) appears to occupy pores of similar energy whether dry or in the presence of adsorbed water. On dry CSC adsorbed HD appears stable, while in the presence of water its decomposition is marked by hydrolysis at low temperature and thermal decomposition at elevated temperatures. The principal volatile products desorbed are 1,4-thioxane, 2-chloroethyl vinyl sulfide and 1,4-dithiane, with the latter favoring elevated temperatures.

Introduction Adsorption of HD vapor on activated carbon has been perceived for many years to be essentially an irreversible process. Recent concerns over safety in handling carbon filters containing adsorbed toxic chemicals are raising interest in understanding the effect of temperature on vapor desorption to the atmosphere at ultralow concentrations.1-3 Of similar importance is the propensity of adsorbed HD to decompose, the concern of which is that the products of these reactions may also desorb.4 One approach in understanding the effect of temperature on the vapor phase concentration of HD adsorbed on activated carbon is to measure its adsorption equilibria at various temperatures and loadings, as this will provide the maximum partial pressure (and concentration) of vapor for a given set of conditions. The difficulty in measuring adsorption equilibria of low vapor pressure chemicals such as HD (Psat ) 14.08 Pa at 25 °C) is well known.5-7 These data and more recent measurements show that adsorption * Corresponding author. Tel.: (410) 436-5704/5948. Fax: (410) 436-5810. E-mail: [email protected]. † ERDEC. ‡ Geo-Centers, Inc. (1) Wagner, G. W.; MacIver, B. K.; Yang, Y. C. Magic Angle Spinning NMR Study of Adsorbate Reactions on Activated Charcoal, Langmuir 1995, 11, 1439-1442. (2) Holgate, H. R.; Scherer, L.; Talib, A. Assessment of Carbon Filter System Performance; MTR 93E0000034; MITRE: McLean, VA, September 1993. (3) Black, R. M.; Clarke, R. J.; Cooper, D. B.; Read, R. W.; Utley, D. Application of Headspace Analysis, Solvent Extraction, Thermal Desorption and Gas Chromatography-Mass Spectrometry to Analysis of Chemical Warfare Samples Containing Sulphur Mustard and Related Compounds, J. Chromatogr. 1993, 637, 71-80. (4) Wagner, G. W.; MacIver, B. K.; Rohrbaugh, D. K.; Yang, Y. C. Thermal Degradation of Mustard; Proceedings of the 1997 ERDEC Defense Conference, 1997. (5) Tolles, E. D. Sorption Properties of Activated Carbon; Report Number 6; Defense Technical Information Center, AD838297; August, 1968. (6) Cheselske, F. J.; Counas, G.; Loven, A. W.; Tolles, E. D. Evaluation of Fundamental Parameters of Carbon Sorbents; Defense Technical Information Center, AD846567; December, 1968.

of HD is highly favorable on activated carbon.8 In either of these studies decomposition of adsorbed HD is not discussed. Often with this class of chemicals, low relative pressures (Pi/Psat) of approximately 10-6 are obtained at moderately high adsorbed phase loadings. This is primarily due to HD’s low vapor pressure. At present there are no reported data that describe the equilibria of HD on activated carbon near the chemical’s airborne exposure limit (AEL ) 0.003 µg/L, 4.6 × 10-5 Pa at 25 °C). In this paper we report adsorption equilibria of HD vapor on activated carbon at concentrations not previously measured. The objective is to determine the effect of temperature on the adsorption equilibrium of HD on activated carbon at vapor concentrations approaching the chemical’s toxic limits. We report experimental results of data measured on a novel desorption apparatus utilizing purge and trap chromatography, a method previously described for GB (isopropyl methylphosphonofluoridate) on activated carbon.9 Where decomposition of adsorbed HD occurs, the products of these reactions and conditions are discussed. 1H and 13C MAS NMR are used to verify the adsorbed phase loading and to determine the effect of temperature on decomposition of adsorbed HD. Experimental Section Materials. The coconut based activated carbon (CSC, type CL-13) used in these experiments was provided by Barnebey & Sutcliffe Corporation. The particle size range was originally 8 × 16 U.S. sieve and was later reduced in size by grinding and sieving to 35 × 40 U.S. sieve. The size reduction was necessary to improve packing of the sample into the desorption cell (described later) and to provide a narrower size distribution. Nitrogen adsorption (7) Wigg, E. O.; Ashley, R. W.; Thomas, C. C. Retentivity of H; Annual Report, Contract No. W-18-035-CWS-1301, University of Rochester: Rochester, NY, July, 1949. (8) Buettner, L. C.; Mahle J. J.; Buchanan J. H.; Friday D. K. U.S. Army, unpublished data. (9) Karwacki, C. J.; Tevault, D. E.; Buettner, L. C.; Mahle, J. J.; Buchanan, J. H. Adsorption Equilibria of Isopropyl Methylphosphonofluoridate (GB) on Activated Carbon at Ultralow Relative Pressures. Langmuir 1999, 15, 6343-6345.

10.1021/la9900324 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/29/1999

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Table 1. Metals Analysis on CSC

a

metal

concentrationa

K Na Ca Mg Si Al Fe Mn B Ni Cr Cu Ti Sr V

11000 860 440 760 590 11.0 45.0 8.6 2.1 17.0 1.30 8.30 1.20 16.0 0.30

ppm by wt (equivalent to 0.4 mmol/g-csc).

measurements performed on a sample of the CSC provided a BET nitrogen surface area of 1107 m2/g. T-method analysis yielded a micropore volume of 0.43 mL/g and a total pore volume of about 0.54 mL/g up to a pore diameter of 7.5 nm. The ash content was reported to be less than 1.5 wt %, consisting primarily of the metals shown in Table 1.10 HD used in this work was a Chemical Agent Standard Analytical Reference Material (CASARM). Certified analysis of the material showed a purity of 97.5% with a liquid density of 1.269 g/mL. 13C-labeled HD (HD*) was used to monitor HD* decomposition in the adsorbed phase using 13C MAS NMR. HD* was synthesized as previously described and contained 8% CH3(O)OCH2CH2S-CH2CH2Cl.11 Spectroscopic grade hexane (Aldrich) was used as a solvent in preparation of the calibration standards. Vapor Uptake Apparatus. The desorption experiments consisted of five sample preparations that involved the addition of HD vapor to CSC. Three of the samples were prepared on CSC-dry containing HD loadings of 0.01, 0.03, and 0.10 g/g. The fourth and fifth sample contained water (CSC/water,) with an initial HD loading of 0.092 g/g and 0.03 g/g, respectively (gramsHD per gram-dry-carbon). The fourth sample was used to understand the effect of temperature on the decomposition of HD. These samples were prepared with HD* and monitored with 13C MAS NMR to determine the adsorbed composition over storage time. The fifth sample (CSC/water) contained HD at a loading of 0.03 g/g and was evaluated immediately after the initial 24 h loading period. Prior to each loading preparation, the carbon was heated to 200 °C for 24 h and cooled under helium purge. Samples of CSCdry were stored at room temperature in a glass screw-cap container. For the HD/CSC/water sample, water was added first followed by the addition of HD. CSC/water was prepared by placing the dry sample into a temperature controlled humidification chamber (Blue-M). The sample was exposed to flowing air with a relative humidity of 45% (27 °C dry bulb) which produced a water loading of 0.13 g/g. The adsorption apparatus used to load HD onto samples of CSC is shown in Figure 1. The system contains an adsorption cell, chemical injection port, and circulation pump. The total system volume, consisting of the cell, tubing, fittings, and pump, is appriximately 200 mL. The unique feature of the apparatus is that the sample is gently fluidized during adsorption, thus providing uniform contact between the particles and HD vapor. The sample cell is made of tapered glass and contains a glass frit near the base for support of the carbon sample. The tapered section of the cell was needed to control the carbon sample’s fluidization height. Each batch preparation used between 200 and 1000 mg of carbon and 5.0 to 20.0 µl of HD. HD* was used in the CSC/water preparations to provide an enhanced signal when monitored with the 13C MAS NMR. The carbon sample was (10) Baer, D. Barnebey & Sutcliffe Corporation: Columbus, OH, 1997; unpublished results. (11) Reiff, L. P.; Taber, D. F. Synthesis of 13C-Labeled Mustard (HD); Proceedings of the 1996 ERDEC Defense Conference, ERDEC-SP-048; October, 1997.

Figure 1. Schematic of batch uptake apparatus.

Figure 2. Schematic of thermal desorption apparatus. loaded from the top of the cell then sealed to minimize water exchange to and from the adsorbent. Once loaded into the cell the CSC-dry samples were purged with nitrogen at ambient temperature (22 °C). With the 4-way valve placed in the recycle position, liquid HD was injected into the system while the pump was off. After sealing the injection port, the pump was turned on and the nitrogen/HD mixture was circulated in an upflow manner for 24 h at ambient temperature. Following the uptake period, the sample was removed and placed into a tared screwcap container. HD loadings were determined by mass difference for both the CSC-dry and CSC/water samples. Samples of HD*/ CSC/water were further analyzed by 1H MAS NMR to verify the actual ratio of HD and water present. This procedure was necessary since the gravimetric determinations indicated a net reduction in mass of sample following the adsorption period. The apparent decrease in sample mass was due to either loss of adsorbed water or HD due to the initial uptake of HD vapor. Desorption Apparatus. A diagram of the desorption apparatus is shown in Figure 2. The system consists of a flowthrough desorption cell, a concentrator trap that collects desorbed vapor eluted from the adsorbent sample, and a gas chromatograph for determining the amount of vapor (HD and reaction product) collected over a known purge volume. Three conditions were

Desorption of Mustard from Activated Carbon

Figure 3. Desorption of HD from CSC-dry at 50, 100, and 150 °C. Initial loading 0.10 g/g. necessary to establish equilibrium between the vapor and adsorbed phases: (1) a residence time (bed volume divided by purge flow rate) long enough to allow adsorbed vapor to diffuse from the porous carbon and mix with the inert carrier, (2) negligible change in adsorbed phase loading during the desorption period, and (3) a uniform bed temperature. The desorption cell consisted of a glass tube with an inside diameter of 0.7 cm and an overall length of 11.5 cm (Dynatherm AO-06-2707) and contained a glass frit at one end to support the adsorbent sample. Approximately 100 mg of sample was used in each desorption experiment, which produced a bed length of about 0.5 cm. The temperature of the adsorbent sample was maintained by surrounding the cell within a heated core, which was controlled to within 0.5 °C of the desired set-point. Equilibrium between the HD vapor and adsorbed phases was established by flowing nitrogen through the packed bed at a flow rate of 1.0-5.0 sccm. Over this flow range the mass of chemical desorbed remained proportional with flow volume. In all experiments the flowrate was 1.0 sccm, which was the minimum flow rate that could be accurately controlled. During each desorption cycle eluted vapor was sent to an adsorbent trap (TENAX-TA) which concentrated the volatile chemicals above the detection threshold of the flame ionization detector (0.1 ng HD). The desorption period of each cycle varied from 90 to 4000 min depending on the mass of vapor desorbed. Higher adsorbed phase loadings and temperature experiments were run for shorter times. During any desorption cycle less than 0.1% of the adsorbed HD was removed from the sample. The average concentration (equilibrium vapor pressure) was determined by dividing the detected mass by the purge volume (corrected to temperature of sample) for the designated desorption period. NMR Apparatus. 1H and 13C solution and MAS NMR spectra were obtained using Varian INOVA 200 and Unityplus 300 NMR spectrometers equipped with Doty Scientific 7 mm high-speed VT-MAS NMR probes. MAS spinning speeds of 3000 Hz were used. Spectra were referenced to external TMS (0 ppm). HD* (0.10 g/g) was loaded onto CSC/water (0.132 g/g) using the batch uptake apparatus (Figure 1). Three 150 mg portions were packed into three 7 mm, double O-ring sealed, macor MAS NMR rotors (Doty Scientific). The 0.10 g/g HD* loading was verified by 1H MAS NMR prior to heating. The three samples were stored in either 30, 60, or 90 °C ovens, and periodically analyzed at room temperature by 13C MAS NMR. At the end of the study, CDCl3 extractions were performed. The extracts were analyzed by 13C NMR to verify the amount of remaining HD* determined by 13C MAS NMR and to aid in the identification of the decomposition products.

Results HD/CSC-Dry. Figures 3-6 show the effect of temperature on desorption of HD for the 0.10, 0.03, and 0.01 g/g samples. For the 0.10 and 0.03 g/g samples (Figures 3 and 4) there is a steady release of HD vapor for each temperature experiment (50-150 °C) that covers a pressure range of about 0.000 08 to 0.15 Pa. As expected, there

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Figure 4. Desorption of HD from CSC-dry at 50, 75, 100, and 150 °C. Initial loading 0.03 g/g.

Figure 5. Effect of temperature on the desorption concentration of HD and principal reaction products from CSC (dry). Initial HD loading 0.1 g/g.

Figure 6. Desorption of HD and principal reaction products from CSC-dry at 100 °C. Initial loading 0.01 g/g.

is a change in pressure that is proportional, although nonlinear, with loading and temperature. HD desorption is also accompanied by the release of 2-chloroethyl vinyl sulfide, 1,4-thioxane and 1,4-dithiane, which appears to increase with mustard loading and temperature (Figure 5). Note that the onset of 1,4-dithiane favors higher temperatures, a condition previously observed in HD contaminated solutions and solid materials.3,4 The reaction product data shown are estimates based on the relative detector response compared to the calibrated response for HD. Comparison of these results and those obtained during calibration show that no measurable decomposition of HD occurs in the desorption apparatus. For the 0.01 g/g sample (Figure 6), detectable amounts of HD are present only during the 100 °C experiment, which shows a significant decrease in partial pressure over the course of the desorption period. The decrease in HD, however, is accompanied by a steady release of 1,4-

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Figure 7. 1H MAS NMR spectra (200 Mz) of CSC/water (0.132 g/g H2O) obtained before (right) and after (left) loading with 0.10 g/g HD*.

dithiane and 1,4-thioxane. The formation of 1,4-thioxane appears to be the result of HD reaction with the adsorbent’s surface, possibly involving the metal oxides contained on the carbon (Table 1) and trace amounts of water. Analysis of the adsorbed phase by 13C MAS NMR shows that HD appears to remain stable on CSC as long as water is not present. HD/CSC/Water-Adsorbed Phase. Sample 4, prepared with 0.092 g/g HD* and 0.08 g/g water on CSC, was used to determine the effect of temperature and storage time on the decomposition of HD. The aged sample was divided equally and placed in sealed containers and stored in a temperature-controlled oven at 30, 60, and 90 °C. Periodically the samples were evaluated with 13C MAS NMR to determine their adsorbed phase composition. Following 70 days of aging at these temperatures the samples were desorbed at 50 and 100 °C in a manner similar to the CSC-dry samples. 1H MAS NMR spectra of the CSC/water sample before and after the addition of HD* are shown in Figure 7. In both spectra, the broad peak for adsorbed water is near -1 ppm, and in the left spectrum, the broad peak for the overlapping proton resonances of HD* is at -3.6 ppm. The peaks are shifted by about -6 ppm from normal solution values due to micropore adsorption1 and indicate an initial water loading of 0.132 g/g. Gravimetric determination of the combined HD*/CSC/water sample showed a net weight loss of 0.05 g/g. The weight reduction was attributed to the loss of water due to the displacement of HD* as confirmed by 1H MAS NMR immediately following the uptake period (24 h). The loss of water in favor of HD* is typical of a displacement process where two chemicals with significantly different adsorptive strengths and low miscibilities compete for pore space of similar energy.12 A comparison of the 1H peak intensities before and after the addition of HD* showed the water loading to be 0.08 g/g. The 13C MAS NMR spectra obtained for the HD*/CSC samples stored at 30, 60 and 90 °C at various time intervals are shown in Figure 8. As in the 1H spectra, the peaks for HD* and various products are shifted by about -6 ppm from normal solution values. 13C MAS NMR spectra showed 24 h after the initial exposure that the HD* loading is 0.092 g/g and about 0.008 g/g H3C(O)OCH2CH2S CH2CH2Cl which is not resolved. Decomposition profiles for HD* obtained from the spectra are shown in Figure 9 and show a rapid loss of HD* in the first several days followed by a gradual loss (12) Rudisill, E. N.; Hacskaylo, J. J.; LeVan, M. D. Coadsorption of Hydrocarbons and Water on BPL Activated Carbon. I&EC Res. 1992, 31, 1122.

Figure 8. 13C MAS NMR spectra obtained for HD*/CSC at 50 and 75 MHz. Left column, 30 °C (top to bottom): day 0 (50 MHz), day 13 (50 MHz), day 70 (75 MHz), day 115 (75 MHz). Middle column, 60 °C: day 0 (50 MHz), day 8 (50 MHz), day 70 (75 MHz), day 115 (75 MHz). Right column, 90 °C: day 0 (50 MHz), day 2 (50 MHz), day 13 (50 MHz), day 111 (75 MHz).

Figure 9. Decomposition of HD adsorbed on CSC/water determined by 13C MAS NMR at 30, 60, and 90 °C. Initial HD loading 0.092 g/g.

until 70 days. The adsorbed phase loading of HD after the 70-day aging period is 0.074, 0.045, and 0.035 g/g for the 30, 60, and 90 °C samples, respectively. The 13C MAS NMR spectra reveal products consistent with hydrolysis, predominantly thiodiglycol and chlorohydrin, while thermal decomposition yields 1,2-dichloroethane, 1,4-dithiane, and polyethylene sulfides.4 Additional reactions of HD* and various products yield 2-chloroethanol, 1,4-thioxane, and polyethylene sulfides/ethers. For the 60 and 90 °C samples there is an abrupt slowing of HD* decomposition with about 45% of the HD* hydrolyzed (marked by the knee of the curve in Figure 9). 13C MAS NMR spectra reveal that hydrolysis stops at this point and products consistent with thermal decomposition begin to appear. Post analysis of the 60 °C parent sample after 112 days shows the adsorbed phase composition to consist of approximately 22% HD, 12% 1,4-thioxane, 2% 1,4-dithiane, and the remaining as higher molecular weight ethers. Analysis of the 90 °C sample shows that the adsorbed phase composition consists of approximately 27% HD, 43% 1,4-thioxane, 11% 1,4-dithiane, and the

Desorption of Mustard from Activated Carbon

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remaining as hemi-sulfur, dichloroethane, and higher molecular weight ethers. The reaction kinetics of HD on activated carbon suggests two competing reaction mechanisms involving (1) the hydrolysis of HD at lower temperatures and (2) thermal decomposition at elevated temperatures. The proposed reaction rate mechanism is kh

HD + CSC/water 98 thiodiglycol + chlorohydrin (1) heat, kd

HD + CSC/products 98 1,2-dichloroethane + 2-chloroethanol + 1,4-thioxane + ,4-dithiane + sulfides/ethers (2) The rate of HD loss is therefore the sum of both rate mechanisms,

-d[HD] ) kh[HD][H2O] + kd[HD] dt

(3)

Early in the reaction period kh appears to be pseudo first order involving the hydrolysis of HD, but rapidly diminishes, possibly due to the formation of Cl- and sulfonium ion intermediates.1,13-15 Assuming the Clproduced remains in the water phase and that complete conversion to thiodiglycol occurs, the resulting Cl- concentration at 45% HD decomposition is 7.9 M (0.551 mmol Cl -/g-csc in 3.9 mmol/g-csc water). The first-order rates are calculated as k30 °C ) 3.2 × 10-7, k60 °C, ) 1.22 × 10-6 and k90 °C ) 6.58 × 10-6 s-1, which are similar to values reported for HD in solution.14,15 The calculated half-lives are 2530 °C, 9.560 °C, and 1.290 °C days and are reasonably close to the measured values shown in Figure 9, except for the 30 °C sample where only 28.6% of the HD decomposed over the storage period. An Arrhenius plot of the first-order rates provided an activation energy of 10.94 kcal/mol, which is about half the reported value for the thermal decomposition of HD in solution.4 HD/CSC/Water-Vapor Phase. Desorption measurements on the unaged and aged samples show that adsorbed water has only a marginal effect on the partial pressure of HD compared to CSC-dry. Sample 5 contained 0.03 g/g HD on CSC/water (0.10 g/g water) and was evaluated immediately following the initial loading period (24 h). This sample produced partial pressures of HD of approximately 3.7 × 10-6, 2.7 × 10-5, and 3.6 × 10-4 Pa at 50, 75, and 100 °C, respectively, and are within a decade of the CSC-dry samples of comparable loading and temperature. The results of the CSC/water sample indicate that the adsorbed HD had not fully equilibrated with the highest energy sites and is likely due to the presence of water that was loaded onto the sample prior to the addition of HD. For the aged samples, complications due to the increasing number of decomposition products in the adsorbed phase appears to have only a marginal effect on the equilibrium of HD when compared to the dry samples. Table 2 shows the average partial pressure of HD and product vapors measured for each desorption experiment conducted at 50 and 100 °C (q ) 0.074, 0.045, and 0.035 (13) Yang, Y. C.; Szafraniec, L. L.; Beaudry, W. T.; Ward, J. R. Kinetics and mechanism of the hydrolysis of 2-Chloroethyl Sulfides. J. Org. Chem. 1988, 53, 3293-3297. (14) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Decontamination of Chemical Warfare Agents. Chem. Rev. 1992, 92, 1729-1743. (15) Bartlett, P. D.; Swain, C. G. Kinetics of Hydrolysis and Displacement Reactions of β,β′-Dichlorodiethyl Sulfide (Mustard Gas) and of β-Chloro-β′-hydroxydiethyl Sulfide (Mustard Chlorohydrin). J. Am. Chem. Soc. 1949, 71, 1406-15.

Table 2. Partial Pressure of HD and 1,4-Thioxane at 50 and 100 °Ca partial pressure at equilibrium (Pa) storage temperature 70 days 30 °C 60 °C 90 °C

desorption temperature 50 °C

desorption temperature 100 °C

HD

1,4-thioxane

HD

1,4-thioxane

0.0002 7.98 × 10-5 4.98 × 10-5

0.000 17 0.002 44 0.011 08

0.004 213 0.001 426 0.000 656

0.0034 0.0129 0.007 28

a Samples aged for 70 Days at 30 °C (q ) 0.074 g/g HD), 60 °C (q ) 0.045 g/g HD), and 90 °C (q ) 0.035 g/g HD).

g/g HD, 0.08 g/g water on CSC). For the 50 °C samples the partial pressures ranged from 5.0 × 10-5 Pa (0.035 g/g) to 2.0 × 10-4 Pa (0.074 g/g) and is comparable to the partial pressures obtained on the CSC-dry samples over a similar loading range (8.0 × 10-5 Pa at 0.03 g/g to 4.0 × 10-4 Pa at 0.10 g/g). A similar trend also occurs for the 100 °C samples. In each of the six samples 1,4-thioxane is the only product vapor identified and exists about a decade higher than the concentration of HD for the 60 and 90 °C aged samples. Discussion The relationship between vapor phase concentration, adsorbed phase loading, and temperature for a particular adsorbent can be conveniently expressed in the form of an equilibrium potential plot. Dubinin proposed a form of the potential plot for microporous materials known as the Dubinin-Radushkevich (D-R) equation and relates the potential energy of adsorption to the filling of pores with adsorbed vapor16,17

( ) ( ) [ ( )]

ln

q TR )qsat Eβ

2

ln

psat pi

2

(4)

where q(cm3/kg) is the measured adsorbed phase capacity, qsat is the adsorption capacity at saturation, pi (Pa) is the partial pressure of adsorbate and psat is the vapor pressure at saturation. The constant E is the characteristic free energy of adsorption, R is the ideal gas constant and T (K) is the absolute temperature. In this work β is taken as the adsorbate molar volume Vm (cm3/mol). Adsorption equilibria measured in this investigation are correlated with higher concentration data obtained in previous work on HD/CSC-dry using a gravimetric technique.8 These data are compared to toluene which is used as a reference adsorbate for CSC. Figure 10 shows the D-R plot of the combined data qi on the ordinate and energy of adsorption as (T/Vm)2 ln(Ps/Pi)2 on the abscissa with slope of -(R/E)2. A least-squares fit of the data is provided (solid line) and shows a good correlation with the high concentration HD data and the toluene reference. Note that values to the left of the potential fit (or below) indicate higher vapor-phase concentrations or adsorbed phase loadings that are lower than estimated. Values to the right of the potential fit (or above) indicate lower vaporphase concentrations or adsorbed-phase loadings higher than estimated. Close analysis of Figure 10 shows that for the samples that lie below the potential fit, there is a trend toward increasing adsorption energy (lower vapor concentration) with increasing temperature. The lower temperature samples that produce higher vapor concen(16) Yang, R. T. Gas Separation By Adsorption Processes; Butterworth: Stoneham, MA, 1987; Chapter 3, pp 49-100. (17) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982.

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Figure 10. Adsorption equilibrium potential plot of HD adsorbed on CSC. Toluene is shown as a reference adsorbate.

trations suggest that HD is not fully equilibrated and may reside in lower energy sites. With increasing temperature the measured values approach the estimated potential fit, indicating that the sample is more completely equilibrated. Interestingly, HD equilibrium is little affected by coadsorbed water. The data shown in Figure 10 (square symbols) indicates that comparable vapor phase concentrations are obtained for carbons of similar loading whether dry or in the presence of adsorbed water. Although complicated by HD decomposition, these data suggest that HD is preferentially adsorbed compared to water and is supported by the earlier observation of the loss of water from the sample resulting from the addition of HD. The order in which the adsorbate pair (HD/water) is loaded onto the adsorbent as well as its miscibility may also be determining factors that affect equilibrium of either component.12 By applying the potential fit of the combined data shown in Figure 10 a simulation of estimated adsorption equilibria is obtained. Figure 11 shows isotherms of HD on CSC at 25, 50, 75, 100, and 150 °C. As evident in the measured data there is about a decade change in partial pressure (vapor concentration) with each 25 °C increase in temperature. For a given temperature a factor of 10 increase in HD loading produces about five decade increase in partial pressure at lower temperatures. The change in partial pressure over this loading increment decreases with increasing temperature as is evident in the slope of the curves.

Figure 11. Isotherm simulation of HD on CSC at 25, 50, 75, 100, and 150 °C

Conclusions A novel desorption apparatus is used to measure equilibria of HD on activated coconut shell carbon at vapor concentrations approaching the chemical’s toxic airborne exposure limits. The vapor phase equilibrium concentration of HD is strongly influenced by its adsorbed phase loading and the temperature of the adsorbent. The results show that the ratio of vapor and adsorbed HD over a range of temperatures follows normal equilibrium behavior yet is complicated by decomposition in the adsorbed phase. On dry CSC the partial pressure of HD is found to increase by about a decade for each 25 °C increase in temperature on CSC containing 0.01-0.1 g/g HD. Under these conditions HD adsorbed on CSC appears relatively stable, producing only low level reactions that may be confined to the adsorbent’s surface which contain metal oxides that are likely hydrated. The principal volatile products desorbed are 1,4-thioxane, 2-chloroethyl vinyl sulfide, and 1,4-dithiane. In the presence of coadsorbed water HD is found to initially undergo rapid decomposition. The extent of the reaction was found to be temperature dependent and ranged from about 20 to 70% over a temperature range of 30 to 90 °C, respectively. Interestingly, HD equilibria are little affected by coadsorbed water. Although complicated by its decomposition, the distribution of adsorbed HD (of known amount) appears to occupy pores of similar energy whether dry or in the presence of adsorbed water. LA9900324