Supercritical Fluid Extraction and Temperature-Programmed

The desorbed components were analyzed by GC−MS (HP 5890 Series II, HP 5989A mass engine, HP 9000 CHEM Station computer) equipped with a ...
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Ind. Eng. Chem. Res. 1998, 37, 3089-3097

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Supercritical Fluid Extraction and Temperature-Programmed Desorption of Phenol and Its Oxidative Coupling Products from Activated Carbon Raashina Humayun, Gurkan Karakas, Philip R. Dahlstrom, Umit S. Ozkan, and David L. Tomasko* Department of Chemical Engineering, The Ohio State University, 140 W. 19th Avenue, Columbus, Ohio 43210-1110

Activated carbon remains one of the most economical adsorbents for the removal of contaminants from water. In particular, activated carbon is known to have an extremely high affinity for phenol and its derivatives. This has been shown to be the result of a catalytic process wherein activated carbon catalyzes the oxidative coupling reactions of phenol in aqueous solution when molecular oxygen is present. These reactions are believed to be the source of the difficulty of regenerating activated carbon loaded with phenol. This paper reports on our efforts toward using supercritical fluids to regenerate activated carbon combined with a concurrent temperatureprogrammed desorption study to identify reaction products and their binding strength to the carbon surface. The results show unequivocally that part of the phenol is chemisorbed on the surface and part of it undergoes polymerization. Dihydroxybiphenyls and phenoxyphenols are the major reaction products present on the surface. Isotope studies showed that surface carbon atoms do not directly participate in these reactions. Supercritical extraction was found to perform as well as solvent extraction for the regeneration of activated carbon loaded with phenol. However, due to the chemisorbed nature of these oxidative coupling products, the reduced masstransfer limitations afforded by supercritical extraction cannot improve the overall extent of extraction even though the rate is improved with the addition of cosolvents. Introduction Activated carbons are unique and versatile adsorbents because of their extended surface area, microporous structure, high adsorption capacity, and high degree of surface reactivity. Many studies have shown them to be the best broad-spectrum control strategy available for the removal of organics from water. In addition to being good adsorbents, activated carbons have been known to catalyze a variety of surface reactions including oxidation, combination, decomposition, and oxidative coupling reactions.1 The efficient use of activated carbon depends on a basic knowledge of the adsorption mechanism. Phenolic compounds exist widely in industrial effluents such as those from oil refineries, coal tar, plastics, leather, paint, pharmaceutical, and steel industries.2 Activated carbon has been commonly used for their removal from aqueous streams and has been shown to have an unusually high affinity for phenols. However, different researchers have reported different isotherms and loading capacities of phenolic compounds on activated carbon.3 There have also been conflicting reports regarding the regenerability of carbons loaded with phenols. While some researchers have reported complete desorption of phenols from activated carbon, several others have encountered varying degrees of difficulty during regeneration.4-6 Recently, the role of oxygen in the irreversible adsorption of phenols on activated carbons has been elucidated.7 These findings * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 614-292-4249. Fax: 614-2923769.

point to the oxidative coupling of phenol catalyzed by activated carbon surfaces as a major cause of the irreversible adsorption.8 Earlier researchers focused on the high affinity of carbon for phenolic compounds and were not aware of the irreversible adsorption in the presence of oxygen. Since the ease of regeneration is an important economic factor in the use of activated carbon, it is important to reevaluate regeneration processes with this new knowledge in mind. Supercritical fluid extraction (SFE) is an attractive option for contaminant removal from activated carbon and presents several advantages over conventional regeneration methods.9 There is little loss of surface area compared to thermal regeneration where harsh temperatures and attrition result in a reduction of capacity. Supercritical fluids (SCFs) have enhanced mass-transfer properties over liquid solvent extraction, and issues of solvent disposal and solvent removal from the solid matrix that are a major concern in solvent regeneration do not pose a problem for SFE. Previous researchers have reported that while the regeneration of activated carbon with SCFs is feasible, complete extraction of phenol from the carbon surface may not be achieved.10,11 They have acknowledged that phenol may be irreversibly adsorbed on the surface of carbons but have not considered the effect of oxygen-induced coupling reactions. In this paper we have examined the potential of supercritical fluid extraction of phenols from activated carbon in light of the phenomenon of carbon-catalyzed oxidative coupling reactions. The dependence of the SFE results on the aqueous phase adsorption procedure has been analyzed, and the results are further interpreted through an analysis of the carbon surface via

S0888-5885(97)00936-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/21/1998

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temperature-programmed desorptionsdifferent aspects of this complex problem that were previously studied in an isolated manner. With this combination of fluid phase and surface studies and drawing on recent literature results on phenol oxidative coupling catalyzed by activated carbon, we hope to provide a more complete evaluation of the SFE process for the regeneration of activated carbon loaded with strongly bound contaminants. Background The effect of oxygen on the adsorption of phenol by activated carbon and the catalytic role of the carbon surface are important but often ignored aspects of adsorption studies. Vidic et al.3 found that the presence of molecular oxygen during adsorption increased the capacity of granular activated carbon (GAC) for phenols. The aerobic isotherm was found to depict 42-85% higher capacities than the anaerobic isotherm. Grant and King7 pointed out that the increase in adsorptive capacity in the presence of molecular oxygen was accompanied by irreversible adsorption of phenol on the carbon surface. They identified oxidative coupling as the mechanism responsible for the increased capacity and irreversible adsorption. Several subsequent studies have confirmed that activated carbon catalyzes the oxidative coupling of phenols in the presence of molecular oxygen.8,12-17 Phenolic polymers of the form C6nH4n+2On, specifically 2,2′-dihydroxybiphenyl, 4phenoxyphenol, and dibenzofuran have been identified in concentrated acetone extracts of the phenol-loaded carbon.7,16 These oxidative coupling products were found in significant quantities in the extracts from the “oxic” sample, whereas traces were present in the “anoxic” extract. Phenol recovery efficiencies are significantly affected by the presence of oxygen during adsorption; e.g., phenol recoveries by Soxhlet extraction were around 70% and 23% for anoxic and oxic isotherms, respectively.16 The extent of irreversibility increases with the amount of oxygen available during the adsorption, though excluding oxygen does not preclude oxidative coupling.7 Adsorption equilibrium is achieved relatively quickly (within 30 min) in the absence of oxygen, whereas uptake of phenol from solution continues over a long period (up to 40 days) in the presence of oxygen.8 Increasing the adsorbent concentration increases the rate of surface reaction, thus confirming the catalytic role of the carbon surface.8 The rate also increases in the presence of electron-donating groups on the adsorbate molecule in the order ortho > para > meta8,13 and at high pH.8,18 The effect of adsorbent surface characteristics on the irreversible adsorption of phenol has been the subject of recent studies. The presence of acidic surface functional groups containing oxygen decreases the rate of the oxidative coupling reaction.7,18-20 The oxidation of the surface prior to adsorption decreases the extent of irreversibility.5 Surface oxidation increases surface acidity and may inhibit the coupling reaction by quenching the electrons released during the activation and radical formation steps. It is thought that oxygencontaining basic groups play a key role in promoting the irreversible adsorption of phenol19 but that acidic groups may lower the oxidative coupling ability of activated carbons by reducing the effectiveness of basic groups. Metallic species such as Mn are also known to

catalyze oxidative coupling reactions of phenols.21 Increasing the ash content of the carbon (e.g., in the form of Mn) has been reported by some workers to increase the irreversibility5,18,20 while others could find no correlation between adsorptive behavior and the ash and metal contents.19 Efforts toward the regeneration of activated carbon loaded with phenols have often yielded conflicting results. Both solvent extraction and thermal regeneration have been studied by several researchers. Solvent extraction results for a wide variety of solvents including acetone,7 methanol,22 and NaOH solution23 show a significant reduction in capacity after the first regeneration and further decreases upon subsequent regeneration. Thermogravimetric (TGA) analysis and temperature-programmed desorption (TPD) combined with gravimetric analysis or mass spectroscopy has been used to study the thermal regeneration behavior of activated carbons loaded with various compounds.5,24-26 The thermal desorption curve of phenol indicates adsorption at four different sites. Magne and Walker5 showed that solvent extraction could remove phenol corresponding to the first site but had little effect on the remainder of the adsorbed portion, which was suggested as being chemisorbed on the surface. At high temperatures (>1223 K) phenol decomposed but the regeneration of the carbon was not complete, and phenol uptake decreased steadily after subsequent thermal regenerations. After each regeneration, the surface area decreased indicating a clogging of the smaller pores during thermal regeneration at high temperatures. Similar results were reported for activated carbon loaded with chlorophenols.24,25 Thermal regeneration using TGA and TPD coupled with mass spectrometry (TPD-MS) up to 800 °C removed about 50% of the chlorophenols, of which a large fraction were light molecular weight compounds. Another study with substituted phenols (phenol, m-aminophenol, p-cresol, and p-nitrophenol) also yielded similar results.26 The temperature-programmed thermal regeneration could be divided into three steps: (i) 300-425 K where physisorbed phenol desorbs without the release of light gases, (ii) 425-925 K which correspond to chemisorbed phenol desorbed simultaneously as light gases and as “heavy” products; and (iii) >925 K where ring condensation occurs and only light gases (H2, CO, H2O) are detected. Even after heating to 1100 K a substantial residue is left on the carbon. Successive adsorption-regeneration cycles consisting of adsorption from the aqueous phase followed by thermal regeneration result in a continuous loss in adsorption capacity which may even drop to zero after a few cycles.26 Liquid-phase TPD studies using heated water27 and water at subcritical conditions28 have reported desorption profiles very different from gas-phase ones. There was only one peak for the phenol desorption, the regeneration was complete at relatively low temperatures, and 100% adsorption capacity was retained after several regeneration cycles. Adsorption bonding energies determined by differential TPD were found to be similar to hydrogen bonds and were directly related to the electrophilic nature of the substituents. These liquid-phase TPD studies were different from most others mentioned previously. The adsorption equilibrium was not reached during the adsorption step, which was stopped at a predetermined loading or after 1 h; the water used for adsorption was degassed with

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helium, and there was no drying step between the adsorption and regeneration cycles. All of these factors would minimize the oxidative coupling reaction, which contributes to the irreversible adsorption of phenol. Supercritical fluid extraction has also been studied for the regeneration of activated carbons. DeFilippi et al.10,11 evaluated the supercritical carbon dioxide (SCCO2) regeneration process for activated carbons loaded with organic compounds and achieved 63% regeneration of phenol-loaded activated carbon. They concluded that the SCF regeneration process was economically feasible; however, complete regeneration of activated carbon loaded with phenol could not be achieved. Kander and Paulaitis29 compared the adsorption of phenol on activated carbon from aqueous solution and from supercritical CO2. They found that the phenol was strongly adsorbed on the carbon surface and at equilibrium favored the GAC over supercritical CO2. They suggested that, although SC-CO2 regeneration of GAC loaded with phenol was feasible, a large amount of CO2 would be required for the regeneration. Cyclic adsorption from the aqueous phase followed by regeneration by SC-CO2 showed that after the first regeneration the adsorptive capacity decreased substantially, but subsequent regeneration of the carbon did not significantly reduce the capacity. Tomasko et al.9 found the supercritical regeneration process to be economically viable compared to thermal regeneration, with the additional advantage of maintaining a stable adsorbate capacity. None of the previous studies on supercritical fluid regeneration of activated carbons loaded with phenol described the adsorption from the aqueous phase in detail. It is probable that oxygen was present during the adsorption step and a significant portion of the phenol loaded on the carbon was present as “irreversibly” adsorbed phenol and its oxidative coupling products. This could explain the poor extraction efficiency and the scattered results. Experimental Section Aqueous Phase Adsorption. The adsorption procedure of Vidic et al.3 was followed for preparing the phenol-loaded activated carbon. The oxic and anoxic samples were prepared in the presence and absence of atmospheric oxygen, respectively. The activated carbon (Filtrasorb-400, 12 × 40 mesh, Calgon Carbon, Pittsburgh, PA) was washed in demineralized water and dried under vacuum at 110 °C for 3 days before use. Phenol obtained from Fisher Scientific (loose crystals, g99.0%) was distilled before use. Double-distilled demineralized water buffered at pH 7 with 10 mM phosphate buffer was used for the adsorption. The oxic adsorption was performed by adding 6 g of GAC to a 4-L amber glass bottle containing a 500 mg/L phenol solution. During anoxic adsorption, molecular oxygen was excluded from the system. The GAC (5 g) was purged with nitrogen for several hours before addition to the phenol solution in order to purge molecular oxygen from the carbon. The procedure for the anoxic adsorption was the same as before except that dissolved oxygen was purged from the GAC and water by bubbling nitrogen for several hours before the addition of the buffer and the phenol. Nitrogen was also bubbled through the solution before sealing in order to remove any oxygen from the headspace. The two bottles were sealed and put on a roller mixer for a week; they were then removed from the mixer to prevent grinding

Figure 1. Schematic of the experimental apparatus used for supercritical fluid extraction.

of the GAC and allowed to equilibrate for another week at room temperature (22 ( 2 °C). The liquid was then decanted off, and the GAC was dried under vacuum at 40 °C. The phenol loading was determined by measuring the phenol in the liquid solution after adsorption and also by comparing the weight of the GAC before and after adsorption. The phenol concentration was determined by measuring the absorbance at 270 nm using a UV-vis spectrophotometer (Varian Cary-1E). A small amount of GAC loaded with carbon-13 labeled phenol (99% [13C6]phenol, Cambridge Isotope Labs, Andover, MA) under oxic and anoxic conditions was prepared for detailed TPD analysis. For these, 500 mg of GAC was added to 100-mL solutions of 435 mg/L [13C6]phenol in buffered water. A system blank was also prepared by adding GAC to clean buffered water as a control. The adsorption and drying steps were as described above. Supercritical Fluid Extraction. A schematic of the experimental setup used for supercritical fluid extraction experiments is shown in Figure 1. The apparatus consisted of an SFX-220 extractor and two 260-D syringe pumps (ISCO). A known weight of GAC preloaded with phenol was placed in the extraction vessel. Supercritical fluid, either pure CO2 (bone dry, 99.99% liquid carbonic) or a cosolvent mixture, was pumped through the vessel at a constant temperature and pressure. A fused-silica restrictor controlled the flow. The flow rate varied between 1.2 and 1.7 mL/min (at experimental conditions) but was fairly constant during a single extraction. The extract was trapped in a round-bottomed flask containing water. A condenser at the top of the flask minimized loss by volatilization. A second trap was placed after the condenser to collect any volatilized solute. The amount of phenol extracted was determined by discrete sampling and without interrupting the extraction. The trap was changed and analyzed for phenol at regular intervals by measuring the UV absorbance at 270 nm. The GAC was also weighed before and after each experiment to check for mass balance closure. Cosolvent extractions were performed with mixtures of acetic acid (glacial, Fisher Scientific) and SC-CO2. The cosolvent mixtures were prepared by mixing known amounts of CO2 and cosolvent using two syringe pumps. Temperature-Programmed Desorption (TPD). Temperature-programmed desorption studies were conducted as a final phase of this work. Both freshly prepared GAC and samples from the same batch of GAC that was used for the SFE experiments over a period of 6 months were analyzed. The TPD system consisted of a quartz sample tube, furnace, gas chromatograph (GC),

3092 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 Table 1. Activated Carbon Samples Used in This Study sample

loading, mmol of phenol/g of GAC

liquid-phase conc, mmol of phenol/L

2.01 2.27 1.91

3.01 1.90 3.12

0.79 0.84 0.0

0.05 0.03 0.0

anoxic, [12C6]phenol oxic, [12C6]phenol anoxic batch no. 2, [12C6]phenol anoxic, [13C6]phenol oxic, [13C6]phenol oxic, blank

Figure 3. Supercritical fluid extraction of phenol from oxic and anoxic GAC with pure CO2 at 36 °C and 141 bar.

Figure 2. Aqueous phase adsorption isotherms for phenol on GAC under oxic and anoxic conditions. Data are from Vidic and Suidan12 at 21 °C; this study was conducted at room temperature (22 ( 2 °C).

and quadrupole mass spectrometer (MS engine) as previously described.30 The sample tube consisting of 1/ - and 1/ -in. quartz tubing (total volume 7 mL) was 4 2 connected to a four-port isolation valve to maintain helium flow during sample loading; quartz wool was used to pack the sample. The helium flow was maintained at 30 cm3/min. All lines after the sample tube were kept at 175 °C to prevent condensation and were of 1/8- and 1/16-in. o.d. to keep the residence time as small as possible. The GAC sample was placed in the sample tube and kept under a helium flow for 5 h at room temperature before the start of each TPD cycle in order to remove physically adsorbed oxygen and water. A three-step computer-controlled temperature program was applied during the desorption procedure. The sample was first maintained at 25 °C for 10 min; the temperature was then ramped at a rate of 10 °C/min up to 680 °C where it was held for an additional 20 min. The desorbed components were analyzed by GC-MS (HP 5890 Series II, HP 5989A mass engine, HP 9000 CHEM Station computer) equipped with a fused-silica column (Supelco, 30 m, 0.25 mm i.d.). The GC oven temperature was 180 °C; the GC-MS interface temperature was 250 °C. A mass range between 5 and 250 was applied in total ion mode for the desorption experiments; the selective ion mode was used for preliminary experiments. Results and Discussion Adsorption. Table 1 shows a list of the different GAC samples used in this study with the total phenol loading and the corresponding equilibrium liquid-phase phenol concentrations. The results of the batch adsorption experiments were consistent with the results obtained by Vidic and Suidan12 as shown in Figure 2. The same kind of GAC and a similar adsorption procedure were used with the exception that in this study the adsorption was conducted at room tempera-

ture (not thermostated) with larger samples. The temperature for the new data points was 22 ( 2 °C versus 21 °C. The temperature difference may account for the small difference in the actual amount adsorbed. However, the relative adsorption capacities for oxic and anoxic cases are consistent, and a significantly higher loading is achieved under oxic conditions. Supercritical Fluid Extraction. Supercritical fluid extractions with CO2 were performed on both the oxic and the anoxic samples. The SFE conditions, 36 °C and 141 bar, used for most extractions were chosen to be similar to Kander and Paulaitis29 for ease of comparison with their adsorption data. Figure 3 shows the extraction of phenol from the oxic and anoxic carbons. The percent phenol extracted based on the total phenol uptake from solution is plotted as a function of the cumulative amount of CO2 used normalized by the weight of clean (phenol-free) GAC. The extraction profiles of the two batches of anoxic carbon are similar, with about 60% total removal of phenol by the end of the experiment. The oxic carbon, which had a higher total uptake of phenol during aqueous adsorption, showed a much lower degree of extraction. At the end of the experiment, only 34% of the loading was desorbed as phenol. This compares surprisingly well with exhaustive acetone extraction reported by Kilduff and King,18 who showed that only about 35% of the phenol adsorbed under similar conditions can be removed. The SFE results emphasize the effect of the conditions of adsorption on the regenerability of activated carbon loaded with phenols. The extent of desorption is reduced by 25% depending on whether the adsorption was conducted in the presence or absence of air. Contrary to earlier beliefs, the low degree of extraction by SFE in the oxic case does not suggest that SC-CO2 is a poor solvent but that phenol is irreversibly adsorbed in the presence of oxygen. The SFE performance compares very well with previously reported solvent extraction results for phenol adsorbed under similar conditions.18 Qualitative mass spectra of the extracts showed the presence of dimers including 2,2′-dihydroxybiphenyl and phenoxyphenols (ortho and para) in all cases. The effect of increasing the temperature of extraction at constant CO2 density is shown in Figure 4. The SFE was performed at two conditions, 36 °C, 141 bar and 50 °C, 209 bar, which were chosen so the CO2 density was constant (18.1 mol/L). There was a minimal improvement in extraction at the higher temperature. This is surprising because, although the interaction with the

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Figure 4. Effect of temperature on the supercritical fluid extraction of phenol from oxic GAC with pure CO2. The density of SC-CO2 at both conditions is constant at 18.1 mol/L.

Figure 5. Effect of the addition of acetic acid as the cosolvent on the supercritical fluid extraction of phenol from oxic GAC.

surface is not very dependent on temperature over these small ranges, the adsorption equilibrium of phenol between SC-CO2 and GAC is much more sensitive to temperature.29 Cosolvent extraction of the phenol was performed to determine the effect of increased polarity of the SCF solvent. Acetic acid was chosen as the cosolvent since it is very polar, and the carboxylic group is expected to interact favorably with phenol and possibly attack any adsorbate-surface bonds arising from chemisorption. Acetic acid is soluble in CO2 over a wide range of concentrations at the conditions of operation used in this study.31 It has also been found to be 100% reversibly adsorbed by activated carbon and can be easily removed by SC-CO2.11 It is of practical importance that the regeneration of GAC with cosolvent mixtures of acetic acid and CO2 does not result in loading of the carbon surface with the cosolvent. The results of the cosolvent extraction at about 4 and 8% (mole basis) concentrations of acetic acid are shown in Figure 5. The percent phenol extracted is plotted versus the amount of CO2 used; the cosolvent amount is not included in the x-axis. The addition of acetic acid had little impact on the overall extraction of phenol. In fact, there is little difference between SC-CO2, CO2 with acetic acid cosolvent, and liquid acetone18 as solvents with respect to the final extraction efficiency. This is somewhat surprising considering the wide range of density and cohesive energy exhibited by these three solvents and indicates that the solvent power is not the limiting factor for this particular extraction. However, there are significant differences between extraction

Figure 6. Dimers (molecular weight 186) of phenol detected during TPD of aged oxic and anoxic GAC.

rates, with CO2 + acetic acid giving the highest rate of removal. Two moles of CO2 with 4% acetic acid removed the same amount of phenol as 5 mol of pure CO2. This suggests the possibility of facilitated transport with the addition of acetic acid, and extraction rates are the subject of continuing work. Increasing the concentration of the acetic acid does not have any significant effect, leading to the conclusion that the cosolvent effect saturates at 4 mol % or less. Kander and Paulaitis29 had suggested that SCF regeneration of GAC loaded with phenols may not be economical since a large amount of CO2 would be required. This problem can be overcome by the addition of a small amount of acetic acid as cosolvent. Temperature-Programmed Desorption. The TPD of oxic and anoxic GAC in the selective ion mode confirmed the presence of dimers, specifically, 2,2′dihydroxybiphenyl, 4,4′-dihydroxybiphenyl, 2-phenoxyphenol, and 4-phenoxyphenol on the carbon surface. These are common products of the oxidative coupling of phenol.16,21 The dimers were detected on the GAC surface in both the oxic and anoxic cases as shown in Figure 6. These dimers were also identified qualitatively by GC-MS in the SCF extracts of both samples. However, no trimers were detected. The relative abundance was lower in the anoxic case, and the dimers were desorbed from two sites on the oxic GAC compared to only one site on the anoxic GAC. This indicates that the irreversible adsorption of phenol proceeds, albeit at a slower rate, even in the absence of molecular oxygen. It should be mentioned here that the TPD analysis was conducted on these GAC samples about 6 months after they were prepared. It is probable that during this time the surface reaction may have continued, resulting in the presence of significant quantities of higher molecular weight products in the anoxic sample. However, the desorption of phenol dimers at two distinct temperatures from the oxic carbon compared to only one peak seen in the TPD of anoxic GAC suggests that, even though the polymerization of the phenol molecule can occur over time in the absence of oxygen, there is an additional mechanism for the reaction in the presence of oxygen. The oxic GAC was then compared with freshly prepared anoxic GAC to determine the effect of oxygen on thermal desorption of phenol and its reaction products (Figure 7). The TPD analysis was performed in the total ion mode to detect a wider range of compounds. As shown in Figure 7a, the thermogram of phenol has at least four peaks for both the oxic and anoxic cases, indicating that phenol is adsorbed on four energetically

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Figure 7. TPD thermograms of (a) phenol and (b) dibenzofuran from GAC: the aged oxic sample compared with the fresh anoxic sample.

distinct sites with different adsorption strengths. The first two peaks (100-400 °C) correspond to physisorption, while the higher temperature peaks (>400 °C) indicate chemisorption. A larger fraction of the phenol is more strongly adsorbed in the oxic GAC as shown by the relative size of the highest temperature peak. The reaction products (dimers) were represented by dibenzofuran (DBF), which was the most abundant high molecular weight compound observed in the total ion mode. Figure 7b shows the desorption profiles of DBF for the oxic and anoxic GAC. Dibenzofuran was also detected by Abuzaid and Nakhla16,17 in solvent extracts of oxic GAC. Desorption of DBF was observed at temperatures greater than 400 °C. Since the boiling point of DBF is 287 °C, desorption at these high temperatures indicates that the DBF is either chemisorbed or present on the surface as some other species (e.g., 2,2′-dihydroxybiphenyl), which is strongly adsorbed and undergoes dehydration during desorption at these elevated temperatures. The sharp rise of the DBF peak corresponds to the second sudden drop in the total signal for both the oxic and anoxic cases. In TPD analysis, sharp drops in the signal are usually indicative of endothermic reactions, which lower the temperature of the sample, causing an overall reduction of the signal. It is probable that DBF is the dehydration product of 2,2′-dihydroxybiphenyl. The effectiveness of SFE for the removal of phenol adsorbed at different sites and specific compounds extracted by SC-CO2 is revealed by TPD analysis of GAC samples before and after extraction. The effect of SFE on the freshly prepared anoxic sample is shown in Figure 8. The effect of extraction on the higher molecular weight fraction detected as DBF is shown in Figure 8b. A similar comparison for the oxic case is shown in Figure 9. For both the oxic and anoxic cases, SFE could remove most of the physisorbed phenol. The chemisorbed portion, however, was affected little for the

Figure 8. Effect of SFE on the TPD of phenol from anoxic GAC: (a) phenol; (b) dibenzofuran.

Figure 9. Effect of SFE on the TPD of phenol from oxic GAC: (a) phenol; (b) dibenzofuran.

anoxic carbon and not at all for the oxic carbon. For the anoxic GAC SC-CO2 completely removes the phenol on the first site and a large portion of that on the second site but has little effect on the most strongly adsorbed phenol. The extraction of phenol corresponding to the first two peaks is similar for the oxic case. However, none of the phenol corresponding to the latter two peaks is removed. In fact, the relative size of the peak at 520 °C is apparently enhanced after extraction. The untreated sample showed a sharp drop in the signal at 540 °C; this may again be due to an endothermic

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Figure 10. Removal of phenol by SFE from oxic and anoxic GAC. Comparison of the extraction efficiency based on the total amount adsorbed from solution and on the abatement of the phenol peak as observed by TPD of untreated and post-SFE samples.

reaction (e.g., dehydration of 2,2′-dihydroxybiphenyl to DBF). The SFE process extracts the weakly bound dimer, and the peak seen at 520 °C vanishes after extraction. It is possible that SFE removes the reactants for this reaction, causing a reduction of the intensity of the endothermic reaction. In the absence of the corresponding temperature reduction in the extracted sample, the signal drops more gradually. The reduction of the DBF peak after SFE of the oxic GAC is shown in Figure 9b. Abatement of the initial portion of the DBF peaks was observed for both the oxic and anoxic cases. No DBF was detected in the extracts, while a significant amount of 2,2′-dihydroxybiphenyl was present; therefore, it can be inferred that the DBF peak corresponds to dehydrated dimer. The DBF peak in the post-SFE oxic GAC continues to rise until the end of the TPD experiment, suggesting that a significant portion of the dimer is still present on the surface after extraction by SC-CO2 followed by thermal desorption up to 680 °C. This shows how strongly the oxidative coupling products of phenol are adsorbed on the surface of activated carbon. An interesting fact brought out by the TPD results is a comparison of the removal of phenol from activated carbon by SFE based on total uptake from the solution and that based on the abatement of the phenol peak as determined by TPD analysis before and after regeneration by SFE. This is shown in Figure 10. About 60% abatement of the integrated phenol peak was observed after extraction of the anoxic GAC with SC-CO2. This is consistent with the extraction of phenol as measured by UV analysis of the extract (60.9%), suggesting that most of the phenol adsorbed is removed as phenol during thermal regeneration and, though the dimers are present on the surface, their relative abundance is much less than phenol. For the SFE-treated oxic GAC there is a 55% reduction of the amount of phenol desorbed during the duration of the temperature program, which is much higher than the actual fraction of the phenol uptake removed by SFE (34%). This is a strong indication that a significant portion of the phenol uptake in the presence of oxygen is irreversibly adsorbed and does not desorb but starts to fragment at high temperatures (>550 °C). In addition, this also suggests that more oxidative coupling products may be present on the surface on the oxic carbon than can be observed via TPD analysis. An isotope study was conducted to conclusively dif-

Figure 11. TPD thermograms of (a) phenol and (b) dibenzofuran from oxic and anoxic GAC. Labeled isotope study ([13C6]phenol) of freshly adsorbed phenol.

ferentiate between carbon-containing groups from the GAC backbone and from the phenol. Carbon-13-labeled phenol ([13C6]phenol) was adsorbed onto GAC. Because of the small amounts of labeled phenol used, the total phenol loading for the oxic and anoxic carbons was similar (0.84 and 0.79 mmol/g, respectively). TPD-MS analysis showed that all the carbon atoms in the desorbed phenol and dimers came from the labeled phenol, indicating that the GAC backbone did not directly participate in the reaction. Comparisons of the phenol and DBF desorption profile for the oxic and anoxic GAC are shown in Figure 11. There were substantial differences between these TPD results and those reported previously (Figures 6-9) for GAC that had been stored for 6 months. The TPD analysis of these samples was performed directly after adsorption and drying. As before, phenol molecules desorbed at four different temperatures and similar peaks were found in both the oxic and anoxic cases, but the temperature corresponding to these peaks was lower for the oxic carbon. Desorption of DBF also occurs at a lower temperature for the oxic GAC as shown in Figure 11b. For the aged carbon the phenol peaks in the oxic case shifted toward higher temperatures, and a larger fraction of the phenol was more strongly bound. Our interpretation of this phenomenon is that the presence of molecular oxygen affects the initial adsorption of phenol on the activated carbon surface and causes the adsorbed phenol molecules to be in an “activated” or “destabilized” state. We postulate that this activated phenol is initially less strongly adsorbed, hence the lower desorption temperature from fresh GAC. However, over time it can react to form dimer species or it can diffuse along the surface to more stable adsorption sites. Because this activated state is not as stable as phenol adsorbed in the absence of oxygen, we see more reaction products and lower temperature desorption of phenol in the oxic case than in the anoxic case. Thus, if the carbon is stored in air, one sees

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reaction or surface diffusion of phenol to more stable sites even in the anoxic carbon. This has also been observed by Magne and Walker,5 who found that exposure of the activated carbon to air after adsorption changed the shape of the thermal desorption curve and some of the physisorbed phenol became more strongly adsorbed. The conclusion to be drawn is that oxygen in the liquid or gas phase can take part in the oxidative coupling reactions which can continue during the drying and storage time of the carbon after adsorption and that the regenerability of carbon loaded with phenol will decline with time. In addition, the role of the activated carbon surface should not be understated. As shown by Kilduff and King,18 the reversible adsorption can be altered dramatically by surface pretreatment. Though our [13C6]phenol results show that the activated carbon surface does not directly take part in the oxidative coupling, some reaction occurs even in the anoxic case. We believe that the presence of molecular oxygen serves to enhance the concentration of activated phenol on the carbon surface rather than being solely responsible for the reactive species. The very stable sites for phenol adsorption are a result of the structure of activated carbon and will result in a finite amount of irreversible adsorption regardless of the presence or absence of molecular oxygen. One can only hope to achieve an extraction of the weakly bound phenol and some of the dimer species. In this regard, supercritical extraction is as effective and efficient as a liquid solvent extraction. Conclusion The coupled SFE and TPD-MS studies together with a detailed knowledge of the aqueous phase adsorption not only reveal information about intermolecular interactions in the fluid phase but also clearly describe the nature and relative abundance of species present on the carbon surface and their effect on regenerability via SFE. The results show that there is a substantial difference between adsorption from an aqueous solution in the presence and absence of molecular oxygen. This difference in the adsorption characteristics is manifested even more dramatically in the desorption behavior of phenol from GAC. The regenerability of the oxic GAC with SFE was found to be almost 50% less than that of the anoxic GAC. However, SFE is as effective for the regeneration of the oxic GAC as exhaustive solvent extraction with acetone. In addition, by using a small amount of acetic acid as the cosolvent, the rate of extraction is significantly enhanced, allaying previous fears that SFE may not be economically feasible due to large solvent requirements. The TPD-MS results show that phenol is adsorbed onto F-400 activated carbon in four energetically distinct sites and only the two lowest energy sites are subject to desorption via supercritical extraction. Phenol also reacts to form dimers (predominantly dihydroxybiphenyls and phenoxyphenols), which further react upon heating to form dibenzofuran. The isotope experiments show that the carbon backbone does not directly participate in these reactions. Chemisorption of phenol and the formation of strongly adsorbed polymers via oxidative coupling are the cause of the irreversible adsorption. The aging of contaminated GAC also significantly affects the regenerability. The presence of molecular oxygen serves to “activate” or “destabilize” the adsorbed phenol, thereby facilitating its reaction. This activated phenol reverts to a more stable state through either reaction or possible surface diffusion during storage in air.

These results point to the importance of understanding both surface phenomena and fluid phase phenomena in developing separation schemes based on adsorption/ desorption and point out the need for evaluation of sample preparation when reporting supercritical extraction results. For the specific case of activated carbon loaded with phenol, the fact is brought out that the removal of phenol from carbon is inherently difficult. The nonregenerability is not due the ineffectiveness of SFE but due to the irreversible nature of the adsorption itself which does not lend itself to complete regeneration even upon exhaustive desorption at high temperatures or with strong organic solvents. Acknowledgment Financial support provided for this work from the National Science Foundation through Grants CTS9409786 and CTS-9412544 is gratefully acknowledged. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The Ohio State University is acknowledged for the University Fellowship received by R.H. Literature Cited (1) Bansal, R. C.; Donnet, J.-B.; Stoeckli, F. Active Carbon; Marcel Dekker Inc.: New York, 1988. (2) Singh, B. N.; Rawat, N. S. Comparative Sorption Studies of Toxic Phenols on Flyash and Impregnated Flyash. J. Chem. Technol. Biotechnol. 1994, 61, 307. (3) Vidic, R. D.; Suidan, M. T.; Traegner, U. K.; Nakhla, G. F. Adsorption Isotherms: Illusive Capacity and Role of Oxygen. Water Res. 1990, 24, 1187. (4) Snoeyink, V. L.; Weber, W. J.; Mark, H. B. Sorption of Phenol and Nitrophenol by Active Carbon. Environ. Sci. Technol. 1969, 3, 918. (5) Magne, P.; Walker, P. L., Jr. Phenol Adsorption on Activated Carbons: Application to the Regeneration of Activated Carbons Polluted with Phenol. Carbon 1986, 24, 101. (6) Yonge, D. R.; Kelnath, T. M.; Poznaska, K.; Jiang, Z. P. Single-Solute Irreversible Adsorption on Granular Activated Carbon. Environ. Sci. Technol. 1985, 19, 690. (7) Grant, T. M.; King, C. J. Mechanism of Irreversible Adsorption of Phenolic Compounds by Activated Carbon. Ind. Eng. Chem. Res. 1990, 29, 264. (8) Cooney, D. O.; Xi, Z. Activated Carbon Catalyzes Reaction of Phenolics during Liquid-Phase Adsorption. AIChE J. 1994, 40, 361. (9) Tomasko, D. L.; Leman, G. W.; Eckert, C. A. Pilot Scale Study and design of a Granular Activated Carbon Regeneration Process Using Supercritical Fluids. Environ. Prog. 1993, 12, 208. (10) DeFilippi, R. P.; Krukonis, V. J.; Robey, R. J.; Modell, M. Supercritical Fluid Regeneration of Carbon for Adsorption of Pesticides; EPA Report EPA-600/2-80-054; EPA Industrial Environmental Research Laboratory: Research Triangle Park, NC, 1980. (11) DeFilippi, R. P.; Robey, R. J. Supercritical Fluid Regeneration of Adsorbents; EPA Report EPA-600/2-83-038; EPA Industrial Environmental Research Laboratory: Research Triangle Park, NC, 1983. (12) Vidic, R. D.; Suidan, M. T. Role of Dissolved Oxygen on the Adsorptive Capacity of Activated Carbon for Synthetic and Natural Organic Matter. Environ. Sci. Technol. 1991, 25, 1612. (13) Vidic, R. D. Oxidative Coupling of Phenols on Activated Carbon: Fundamentals and Implications. Ph.D. Dissertation, University of Cincinnati, Cincinnati, OH, 1992. (14) Vidic, R. D.; Suidan, M. T.; Brenner, R. C. Oxidative Coupling of Phenols on Activated Carbon: Impact on Adsorption Equilibria. Environ. Sci. Technol. 1993, 27, 2079. (15) Vidic, R. D.; Suidan, M. T.; Sorial, G. A.; Brenner, R. C. Molecular Oxygen and the Adsorption of PhenolssEffect of Functional Groups. Water Environ. Res. 1993, 65, 156.

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Received for review December 30, 1997 Revised manuscript received March 5, 1998 Accepted March 9, 1998 IE970936P