Desorption of Ethyl Acetate from Adsorbent Surfaces (Organoclays) by

Effect of supercritical fluid extraction parameters and clay properties on the efficiency of phenanthrene extraction. Maria Elektorowicz , Haifa El-Sa...
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Ind. Eng. Chem. Res. 2001, 40, 364-368

Desorption of Ethyl Acetate from Adsorbent Surfaces (Organoclays) by Supercritical Carbon Dioxide Gerson L. V. Coelho,† Fa´ bio Augusto,‡ and Janusz Pawliszyn* Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Supercritical fluid extraction was used to regenerate organically modified montimorilloniteclay (hexadecyltrimethylammonium-clay and tetramethylammonium-clay), after it was used to adsorb ethyl acetate from aqueous solutions. The desorption of ethyl acetate adsorbed over the clays was performed with CO2 at temperatures ranging from 301 to 333 K and pressures ranging from 69.0 to 413.8 bar. A crossover effect was observed. The experimental data was fitted to a simple model, with the best results corresponding to desorption with CO2 in its supercritical region. Introduction The unreasonable discharge of various chemical materials such as organic pollutants as industrial waste has been a major environmental concern. Therefore, the use of various physicochemical and biological techniques to remove these contaminants from wastewater has been extensively researched. Supercritical fluid extraction (SFE) has become widely accepted as a replacement for classical extraction methods mainly for environmental control. Supercritical carbon dioxide (SC-CO2) has been used to extract heavy molecular weight organics such as polychlorinated biphenyls, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), and polycyclic aromatic hydrocarbons from soils and sediments.1-6 Chemical engineers are also tackling issues surrounding SC-CO2, a promising, environmentally friendly replacement for organic solvents. For the same reasons, research regarding new adsorbents for the removal of pollutants from water has also been extensively done. The study of the regeneration of these adsorbents is also very important. The regeneration of activated carbon by SC-CO2 after adsorption of ethyl acetate had already been studied experimentally.7 Normally, the activated carbon is used to reduce and/ or to recover organic compounds in effluent streams, and ethyl acetate is one of these compounds commonly emitted in the chemical industry. Clay minerals as adsorbents, especially smectite, are of particular interest because of their large specific surface areas.8 An alternative adsorbent is modified clay. If a clay has metal cations occupying cation-exchange sites, its surface is hydrophilic because of the water molecules in the hydration shell solvating the cations. Such a surface is not a good adsorbent for removing hydrophobic organic molecules, which have poor water solubility.9 When the metal ions are replaced by large organic cations such as those from surfactants, the nature of the clay surfaces is drastically altered and turned hydrophobic or organophilic in nature. In other words, as hydrophobicity of * To whom correspondence should be addressed. Fax: (519) 746-0435. Phone: (519) 888-4641. E-mail: [email protected]. † Department of Chemical Engineering, UFRRJ, 23890-000 Serope´dica, Rio de Janeiro, Brazil. ‡ Institute of Chemistry, Unicamp, 13083-970 Campinas, Sa˜o Paulo, Brazil.

the molecule is increased, the sorption of organics increases. The use of organoclays as adsorbents for organic contaminants (both ionic and nonionic) from aqueous solutions has been studied.10-12 However, works about desorption and regeneration using supercritical fluid conditions are scarce. Modified smectite was used to extract the phenolic compounds from aqueous solutions, and SC-CO2 with and without cosolvent was employed to regenerate the adsorbent.8 In this work, the regeneration of organoclays loaded with ethyl acetate using SC-CO2 at different pressures and temperatures was studied. The clay employed was montmorillonite, and hexadecyltrimethylammonium (HDTMA+) and tetramethylammonium (TMA+) were used as organic modifiers, which gave a different hydrophobic nature to the clay. Experimental Methods Preparation of Organoclays. The clay used (montmorillonite) was purchased from Sigma-Aldrich Canada Ltd. The cation-exchange capacity of the clay was 0.504 mequiv/g of clay. TMA+ chloride and HDTMA+ bromide, both purchased from Sigma-Aldrich and used without further purification, were employed in the clay modification. The dry powder clay was digested for 24 h with concentrated hydrogen peroxide to remove organic contaminants. The peroxide in excess was then decomposed by gentle heating in a water bath.13 The purified clay was suspended in enough deionized water to allow its settling, the water was siphoned off, and the clay was dried for further use. The adsorption of HDTMA+ and TMA+ onto montmorillonite was performed in a 2 L beaker in an amount just equal to the cation-exchange capacity of the clay. For the clay preparation with TMA+, excess salt was added to promote saturation. Excess organic salt was not added for HDTMA+ because it may be retained by the clay in excess of the exchange capacity.9 A total of 30 g of washed montmorillonite was added to this container and then agitated with a magnetic stirrer for 24 h. After agitation, the treated clay was filtered, washed twice with 500 mL of deionized water, dried in an oven at 60 °C for 24 h, and kept in a brownish bottle to use.

10.1021/ie000433a CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2000

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Figure 2. Adsorption capacity of the modified and unmodified clays. Figure 1. Schematic diagram of the apparatus used for adsorption and supercritical extraction of ethyl acetate from organoclays: 1, liquid CO2 tank; 2, ethyl acetate solution; 3, syringe pump; 4, water bath; 5, heating; 6, preheating coil; 7, adsorption/ desorption column; 8, effluent collector; 9, restrictor; 10, collector trap.

X-ray Diffraction Analysis. To examine the structural difference of montmorillonite before and after the modification with a quaternary amine, the basal spacings (d001) were determined using a powder X-ray diffractometer (Siemens D-500). Batch Sorption Experiment. To verify the adsorptive capacity of organically modified clays and to compare with unmodified clay, a batch experiment was designed where a certain amount of ethyl acetate was added to three bottles containing deionized water and adsorbent material. To one bottle was added unmodified clay; to the second, TMA+-modified clay; and to the last, HDTMA+-modified clay. The bottles, maintained at a temperature of 308 K, were agitated vigorously by a magnetic stirrer. Samples were taken for analysis at various intervals until the concentration of ethyl acetate remained constant. Adsorption. In the adsorption experiments, the ethyl acetate solution (2.6 g/L) was pumped (syringe pump, ISCO model 100D) through the column packed with organoclay (Figure 1). Stainless steel frits were used to contain the matrix in the column. About 800 mL of ethyl acetate solution at a temperature of 308 K passed through the packed bed, and the concentration of the effluent was determined using solid-phase microextraction-gas chromatography (SPME-GC). The column (2.7 cm3) was charged with 1.0 g of organoclay, and 1.0 cm of 20-30 mesh glass beds packing above and below the organoclay packing was used to homogenize the flow distribution and avoid possible end effects. The amount of ethyl acetate adsorbed in the packed bed agrees with the batch sorption experiment. SFE. For the SFE, the same apparatus as that used for the adsorption experiment was employed (Figure 1). The extractions were performed with an ISCO model 260D syringe pump that supplies the column with CO2 continuously. The experiments were performed at pressures of 69, 137.9, 275.9, and 413.8 bar and at temperatures of 301, 313, and 333 K. The flow rate was maintained at 1.0-2.2 mL/min. The temperature was controlled with a constant-temperature bath. The CO2 stream, before reaching the column, passed through a preheated coil (ca. 2 m of 1.6 mm o.d. stainless steel tubing) in the bath where it was brought to the bath

temperature. The extracted ethyl acetate was collected through a stainless steel capillary restrictor (0.50 mm i.d.) in a cold trap containing 2-propanol. Nine samples were collected for each experiment with 5 min intervals for the first 30 min and with 30 min intervals for the next 2 h. The SFE time profiles were obtained at various pressure and temperature combinations. To find out the best desorption conditions, the experiments were also performed in gas (G) and liquid (L) states. The collected samples were analyzed by SPME-GC. Gas Chromatographic and SPME Analyses. A Varian 3400 gas chromatograph with a flame ionization detector was used for this task. The analyses were carried out using a 30 m × 0.25 mm × 1 µm DB-5 column and helium as the carrier gas. SPME14 was used for extracting the ethyl acetate from the samples. A SPME fiber (Supelco) coated with 30 µm thick poly(dimethylsiloxane) was used. The samples (300 µL) were diluted in 4 mL vials with 3 mL of deionized water. Sampling was performed by exposing the SPME fiber in the solution over stirred samples for 3 min. After sampling, the fiber was withdrawn into the needle, and the SPME device was transferred to the GC. The desorption temperature was 150 °C. Experimental Results and Discussion Figure 2 shows the results of a batch sorption experiment where it is possible to compare the adsorptive capacity of the unmodified clay (6.2 mg of ethyl acetate/g of clay) with the modified clay, i.e., TMA+-clay (15.6 mg of ethyl acetate/g of clay) and HDTMA+-clay (35.6 mg of ethyl acetate/g of clay). It can be seen that, as expected, the nature of the clay surfaces was drastically altered. Montmorillonite saturated with TMA+ cations showed weak sorption when compared to that of HDTMA+, which is more hydrophobic. In general, the more hydrophobic the group(s) associated with the quaternary ammonium entity are, the greater the sorption of the sorbate is.9 The observed different adsorption capacity led to a determination of d spacings by X-ray diffraction analysis of the modified and unmodified clays. The first peak of the diffraction result represents the d001 spacing of an intercalated layer of the specimen.12 The d001 spacing of unmodified clay is 14.79 Å and occurred at 6.30° (2θ), the d001 spacing of the intercalated layer of the organically modified TMA+-clay is 14.93 Å at 5.89°, and the d001 spacing of HDTMA+-clay is 18.84 Å at 4.61°. Mainly because of the presence of the HDTMA+ ion occupying

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Figure 3. Effect of temperature on the desorption of ethyl acetate on HDTMA-clay at 69.0 bar.

the intercalated layer, the organically modified HDTMA+clay shows a relatively large d001 spacing with respect to natural clay. The modified clay containing a small organic cation (TMA+) behaved almost like the natural clay. The adsorption of ethyl acetate was performed by montmorillonite-clay and by two different organoclays (TMA+ and HDTMA+); the desorption was carried out with carbon dioxide at physical conditions defined by T and P, which allows the experiments to be conducted in different phases to find the optimal operating conditions. A mass balance for the ethyl acetate compound was performed at the end of the adsorption/desorption experiments. The total volume of carbon dioxide used for the recovery of the adsorbed ethyl acetate from the organoclay was 266 mL. The effect of temperature was studied at pressures from 69 to 413.8 bar, whereas the effect of operating pressure on the cumulative amount of ethyl acetate desorbed was studied at temperatures from 301 to 333 K. Figures 3-6 show the results. The most efficient operation condition was found in the supercritical region (333 K and 413.8 bar), where 84% of the adsorbed ethyl acetate could be recovered from HDTMA+-clay. The worst recovery efficiency result was found in the liquid phase (301 K and 69.0 bar), where the recovery was only 8.6%. The result of both gas conditions used in this work showed that the recovery efficiency decreases with increasing temperature at 69.0 bar (Figure 3). The same behavior could be noted at 137.9 bar; desorption was greatest at the lowest employed temperature above the critical temperature (Tc ) 304 K; Figure 4). This effect cannot be seen at pressures of 275.9 and 413.8 bar as shown in Figures 5 and 6. Figure 7 indicates clearly this unconventional phenomenon that is analogous to that of the solubility in a supercritical solvent, where the solubility decreases with increasing temperature at low supercritical pressure. This behavior is known as the crossover effect.15,16 A crossover effect has also been found for supercritical fluid desorption from activated carbon.17 Besides its unique solubility characteristics, a supercritical fluid owns certain other physicochemical properties that add to its attractiveness. For example, even though it owns a liquidlike density over much of the range of industrial interest, it exhibits gaslike transport properties of diffusivity and viscosity. Additionally, the very low surface tension of the supercritical fluid allows easy penetration into microporous materials to take place.18 Such properties can cause different effects.

Figure 4. Effect of temperature on the desorption of ethyl acetate on HDTMA-clay at 137.9 bar.

Figure 5. Effect of temperature on the desorption of ethyl acetate on HDTMA-clay at 275.9 bar.

Figure 6. Effect of temperature on the desorption of ethyl acetate on HDTMA-clay at 413.8 bar.

Higher density may enhance the solubility, and higher viscosity may be unfavorable to the rate of diffusion. The isothermal curves (Figure 7), at a temperature of 301 K in the liquid phase and 313 K at the beginning of the gas phase going into the supercritical region, show analogous behavior where the recovery efficiency increases at low pressures and decreases at high pressures. In these cases, the viscosity effect seems to be dominant. The relative ethyl acetate concentration on organoclay decreases with isothermal increases in pressure as shown in Figure 8. The result could be explained by the dependency existing between desorption and carbon dioxide density. A similar case was reported concerning the supercritical desorption of DDT on activated carbon.19 The increase in desorption with isothermal

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modified and unmodified montmorillonite. These qualities make regeneration of all montmorillonite easier. For modeling the desorption profiles, a simple preliminary approach has been considered.1,8 It was considered that the packed bed is perfectly mixed with carbon dioxide. The mass-transfer resistances are negligible in this case, with equilibrium being the only contributing factor in the extraction process. A mass balance in the extraction column is expressed by

W(dθ/dt) ) -QC

Figure 7. Isothermal desorption of ethyl acetate from HDTMAclay with SC-CO2 versus pressure.

(1)

where W is the clay weight, θ is the ethyl acetate concentration in the clay phase in g/g of clay, Q is the flow rate of carbon dioxide in cm3/min, and C is the ethyl acetate concentration in the carbon dioxide phase in g/cm3. By using a linear partition relationship among the phases

θ ) KC

(2)

where K is the distribution coefficient in cm3/g of clay, the solution of eq 1 is given by

ln(θ/θ0) ) (-Q/KW)t

(3)

where θ0 is the initial concentration in clay. A linear regression of the extraction data, ln(θ/θ0) vs t, allows the determination of the distribution coefficient and the prediction of the extraction profiles.1 Figure 8 is plotted using these values. Figure 8. Predicted extraction profiles of ethyl acetate at 333 K with SC-CO2. The solid curve is the data by using the simple equilibrium model (eq 3).

Figure 9. Regeneration of modified and unmodified clays loaded with ethyl acetate by SC-CO2 at 413.8 bar and 333 K.

increases in the carbon dioxide density is due to the greater solvating power of the carbon dioxide, which results in more rapid desorption. The result in Figure 9 does not indicate a difference in regeneration efficiency between the two modified and the unmodified montmorillonite, despite the different adsorptive capacities as previously shown in Figure 2. Another thing to consider is that CO2, having desirable physicochemical properties, could induce considerable swelling in the clay at moderate pressure. This is analogous to the swelling already seen in the polymer matrix.20 The result confirms that ethyl acetate is both soluble in SC-CO2 and not strongly adsorbed on the

Conclusion The supercritical regeneration with carbon dioxide of modified and unmodified clays was experimentally studied. Two quaternary amine modifiers (HDTMA+ and TMA+) were used. The regeneration capacity was almost coincidental, and high pressure was more favorable to regeneration. The effect of pressure and temperature was characterized under different conditions (gas, liquid, and supercritical), and the supercritical condensation was shown to be the best. A crossover effect for desorption was observed; i.e., desorption decreases with increasing temperature. The roles of the density and viscosity were observed to be determinants of the optimal conditions. The most active form of montmorillonite for ethyl acetate adsorption was the clay that had been modified with HDTMA+. Acknowledgment G.L.V.C. and F.A. acknowledge fellowships received from CAPES (Ministry of Education of Brazil) and FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo), respectively. The authors express their appreciation to Denise Souza for her assistance in X-ray analysis and gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC). Literature Cited (1) Brady, B. O.; Kao, C.-P. C.; Dooley, K. M.; Knopf, F. C.; Gambrell, R. P. Supercritical Extraction of Toxic Organics from Soils. Ind. Eng. Chem. Res. 1987, 26, 261. (2) Alexandrou, N.; Lawrence, M. J.; Pawliszyn, J. Cleanup of Complex Organic Mixtures Using Supercritical Fluids and Selective Adsorbents. Anal. Chem. 1992, 64, 301.

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(3) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Effects of Temperature and Pressure on Supercritical Fluid Extraction Efficiencies of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls. Anal. Chem. 1993, 65, 338. (4) Bjo¨rklund, E.; Bowadt, S.; Mathiasson, L.; Hawthorne, S. B. Determining PCB Sorption/Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 1. Desorption from Historically Contaminated Samples. Environ. Sci. Technol. 1999, 33, 2193. (5) Pilorz, K.; Bjo¨rklund, E.; Bowadt, S.; Mathiasson, L.; Hawthorne, S. B. Determining PCB Sorption/Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 2. Describing PCB Extraction with Simple Diffusion Models. Environ. Sci. Technol. 1999, 33, 2204. (6) Hawthorne, S. B.; Bjo¨rklund, E.; Bowadt, S.; Mathiasson, L. Determining PCB Sorption/Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 3. Sorption from Water. Environ. Sci. Technol. 1999, 33, 3152. (7) Tan, C.-S.; Liou, D.-C. Desorption of Ethyl Acetate from Activated Carbon by Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1988, 27, 988. (8) Park, S.-J.; Yeo, S.-D. Supercritical Extraction of Phenols from Organically Modified Smectite. Sep. Sci. Technol. 1999, 34, 101. (9) Boyd, S. A.; Shaobai, S.; Lee, J.-F.; Mortland, M. M. Pentachlorophenol Sorption by Organo-Clays. Clays Clay Miner. 1988, 36, 125. (10) Zhang, Z. Z.; Sparks, D. L.; Scrivner, N. C. Sorption and Desorption of Quaternary Amine Cations on Clays. Environ. Sci. Technol. 1993, 27, 1625. (11) Nzengung, V. A.; Voudrias, E. A.; Nkedi-Kizza, P.; Wampler, J. M.; Weaver, C. E. Organic Cosolvent Effects on Sorption Equilibrium of Hydrophobic Organic Chemicals by Organoclays. Environ. Sci. Technol. 1996, 30, 89.

(12) Lo, I. M. C.; Mak, R. K. M.; Lee, C. H. Modified Clays for Waste Containment and Pollutant Attenuation. J. Environ. Eng. (Reston, Va.) 1997, 123, 25. (13) van Olphen, H. An Introduction to Clay Colloid Chemistry; John Wiley and Sons: New York, 1977. (14) Pawliszyn, J. Solid-Phase MicroextractionsTheory and Practice; Wiley-VCH: New York, 1997. (15) Chimowitz, E. H.; Pennisi, K. J. Process Synthesis Concepts for Supercritical Gas Extraction in the Crossover Region. AIChE J. 1986, 32, 1665. (16) Schaeffer, S. T.; Zalkow, L. H.; Teja, A. S. Supercritical Extraction of Crotalaria spectabilis in the Cross-Over Region. AIChE J. 1988, 34, 1740. (17) Srinivasan, M. P.; Smith, J. M.; McCoy, B. J. Supercritical Fluid Desorption from Activated Carbon. Chem. Eng. Sci. 1990, 45, 1885. (18) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid ExtractionsPrinciples and Practice; Butterworth-Heinemann: Boston, 1993. (19) Macnaughton, S. J.; Foster, N. R. Supercritical Adsorption and Desorption Behavior of DDT on Activated Carbon Using Carbon Dioxide. Ind. Eng. Chem. Res. 1995, 34, 275. (20) Wissinger, R. G.; Paulaitis, M. E. Swelling and Sorption in Polymer-CO2 Mixtures at Elevated Pressures. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 2497.

Received for review April 25, 2000 Revised manuscript received September 14, 2000 Accepted September 21, 2000 IE000433A