Effect of chemical surface heterogeneity on the

PERGAMON

Carbon 38 (2000) 1807–1819

Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon Marcus Franz 1 , Hassan A. Arafat, Neville G. Pinto* Department of Chemical Engineering, ML 0171, University of Cincinnati, Cincinnati, OH 45219, USA Received 7 May 1999; accepted 28 December 1999

Abstract The effects of oxygen-containing groups, particularly carboxylic and carbonyl groups, on the adsorption of dissolved aromatics on ash-free activated carbon have been studied. Adsorption isotherms for phenol, aniline, nitrobenzene, and benzoic acid were generated in both aqueous and cyclohexane media, using carbons with different amounts of surface oxygen groups. It was found that water adsorption, dispersive / repulsive interactions, and hydrogen-bonding were the main mechanisms by which surface oxygen groups influence the adsorption capacity, while donor–acceptor interactions were found not to be significant. The adsorption mechanism was also found to be influenced by the properties of the functional group on the aromatic adsorbate, especially its ability to hydrogen-bond and through its activating / deactivating influence on the aromatic ring.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. Oxidation; C. Adsorption; D. Functional groups, Surface properties

1. Introduction Activated carbon is being used more than ever as an adsorbent in many applications. These cover a wide spectrum of systems including water and wastewater treatment, separations, and hazardous waste treatment. In spite of the large market for activated carbon, the specific mechanisms by which the adsorption of many compounds, especially organics, takes place on this adsorbent are still ambiguous. The complex, heterogeneous nature of the activated carbon surface has led to contradicting mechanisms being proposed in the literature [1]. As a result, the prediction of adsorption capacity is limited to idealized cases [2,3], and the design of most practical systems is semi-empirical in nature. The heterogeneous surface of activated carbon is usually characterized into three main zones: the carbon basal planes, heterogeneous surface groups (mainly oxygen-containing groups), and inorganic ash [4]. The majority of the *Corresponding author. Tel.: 11-513-556-2770; fax: 11-513556-3473. E-mail address: [email protected] (N.G. Pinto). 1 Present address: Freiberg University of Mining and Technology, Freiberg, Germany.

adsorption sites for liquid organics are on the basal planes, which forms more than 90% of the carbon surface. However, the much higher activity of the heterogeneous groups can result in significant effects on the overall adsorption capacity. The exact mechanism by which the heterogeneous surface oxygen groups affect the adsorption capacity is not well understood [5–7], and is the subject of this work.

1.1. Mechanisms The adsorption equilibrium for organics on activated carbon is dependent, to a large extent, on the chemistry of the carbon surface. Heterogeneous oxygen groups have been reported in the literature to play an important role in the process. Coughlin et al. [6] observed a decreasing capacity for phenol adsorption upon increasing surface oxygen content of carbon. They suggested that surface oxygen groups influence phenol adsorption under conditions when the phenol molecules are thought to be adsorbed in a planar position on the basal planes and are held by attractive forces operating over the entire aromatic ring. At higher adsorbate concentrations, phenol molecules get packed more tightly on the surface and are thought to be adsorbed

0008-6223 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00012-9

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M. Franz et al. / Carbon 38 (2000) 1807 – 1819

in a vertical orientation (end-on position). In this case, interactions among adsorbed phenol molecules become significant and very little influence of surface oxygen on the adsorption capacity was reported. To explain this they proposed two major influences: (i) the chemisorbed oxygen removes electrons from the p-electron system of the carbon basal planes, creating positive holes in the conductive p-band of the graphitic planes. This would lead to weaker dispersive interactions of the phenol p-electron system with the p-band of the basal planes; (ii) Coughlin et al. also suggested that water clusters, mainly forming on the acidic carboxylic groups through H-bonding, hinder the penetration of aromatic molecules into the micropores, which represent a large fraction of the surface area; this was also proposed by Dubinin [8]. However, the data presented by Coughlin et al. contradicts this argument, since at high phenol concentration, there appears to be no influence of oxygen sites. Mattson et al. [7] proposed a different mechanism based on the role of basic carbonyl oxygen groups. Using infrared internal reflection spectroscopy (IRS), they attempted to gain information about the state of bonding for different nitrophenols. Based on their results, they suggested that the hydrogen of the hydroxyl group of phenol is involved in an intracomplex hydrogen bond with the surface oxygen complex. They concluded, however, that this hydrogen bond interaction is small. They explained that the aromatic ring interaction with the surface oxygen, namely with the carbonyl group, contributes the major influence through a donor–acceptor interaction, with the carbonyl groups acting as the electron donor and the aromatic ring of aromatic compounds acting as the acceptor. Further, Mattson et al. proposed that once the carbonyl sites are exhausted, the aromatic compounds start to form donor–acceptor complexes with the basal planes. Mahajan et al. [9] suggested that the formation of water clusters by secondary water adsorption on the carboxylic groups hinders the migration of phenols into smaller pores and to the basal planes and decreases the availability of active sites on the basal planes. This effect eventually leads to a drop in adsorption capacity as the amount of oxygen groups increases. More recent studies have also shown evidence of the water adsorption effect [5,10,11]. Muller and Gubbins [11] have shown, using Monte Carlo molecular simulations, that water adsorption is enhanced by surface oxygen groups. A similar conclusion has been reported by Lee and Reucroft [10]. To isolate the water adsorption effect, Leng [12] used cyclohexane as an adsorption medium for phenol. He found that the adsorption capacity of phenol on oxygenated carbon increases when adsorption takes place from cyclohexane solution, which is the opposite to the effect of oxygen groups in aqueous medium. These observations led Leng to conclude that hydrogen-bonding between the (– OH) group on the phenol molecule and the oxygen groups on the surface, and water adsorption around the carboxylic

oxygen groups, were involved in the adsorption mechanism of phenol on activated carbon. The present study was undertaken to further investigate the influence of the heterogeneous surface oxygen groups on the adsorption mechanism. By using a variety of aromatic adsorbates with different functional groups, and studying their adsorption behavior from cyclohexane and water solutions on carbons with different amounts of oxygen groups, additional insight was obtained on the influence of surface oxygen.

2. Experimental

2.1. Materials Kureha LP spherical Bead Activated Carbon (BAC) (0.5 mm diameter), purchased from Kureha Chemical Industry Company (NY, USA), was used in this study. The LP carbon, which is made from petroleum pitch, is ash-free. Previous tests on this carbon [12] have indicated that polymerization of aromatics such as phenol and aniline do not occur on the LP-carbon surface, even under oxic solution conditions. The carbon was conditioned upon receiving by boiling in deionized water for 1 h, then drying in an oven at 1108C for 24 h. This carbon will be referred to as LP-DI. To study the influence of surface oxygen groups on adsorption, the LP-DI carbon was oxygenated with air in a tubular furnace at 3508C. A 3-g sample was placed in a quartz container, which was then placed in the furnace for 60 min under a constant flow of air. The sample was then cooled to room temperature in air. The oxygenated carbon will be referred to as LP-Air. Deoxygenated carbon was obtained by heating the LP-DI carbon to 8508C in the tubular furnace under a constant flow of nitrogen for 1 h. It was then cooled to room temperature under a flow of nitrogen and stored in a nitrogen atmosphere. This carbon will be referred to as LP-N 2 . Previous analyses in our laboratory have shown that these treatments change the content of both carbonyland carboxyl-type oxygen groups on the surface [13], as shown in Table 1. This was confirmed using linear temperature programmed desorption (LTPD) experiments, which are described elsewhere [12]. All of the organic compounds used in this study were purchased from Fisher Scientific (Pittsburgh, PA) in the highest purity available, and no further purification was performed.

2.2. Point of zero charge ( PZC) measurements To quantify the point of zero charge (PZC), 0.1 g of LP carbon was added to 0.02-l solutions of 0.1 N NaCl, whose initial pH had been adjusted with NaOH or HCl. The containers were sealed and placed on a shaker for 24 h,


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Table 1 Quantification of surface oxygen groups on LP-carbon [12] Carbon

CO 2 (mmol / g)

CO (mmol / g)

Total (mmol / g)

Surface area covered by oxygen complex (m 2 / g)

LP-DI LP-N 2 LP-Air

0.131 0.060 0.251

0.846 0.522 1.799

0.977 0.582 2.050

48.8 29.1 102.4

after which the pH was measured. The PZC occurs when there is no change in the pH after contact with the carbon.

3. Results and discussion

3.1. Characterization of activated carbon 2.3. Measurements of surface area and pore size distribution A Micromeritics Gemini 2360 (Norcross, GA, USA) BET apparatus was used to measure the surface area of the carbons used. The surface area measurements were repeated twice for each carbon and the experimental error for these measurements was found to be in the range 2 to 5%. Surface area measurements were performed with both degassed and non-degassed carbon samples. Degassing was achieved by placing the sample under a flow of helium for 2 h at 1008C. Pore size distributions were measured using a Micromeritics ASAP 2010 machine with nitrogen, utilizing the Barrett, Joyner, and Halenda (BJH) method (ASTM D4641-94).

2.4. Adsorption isotherms measurements Equilibrium isotherms were determined using the bottle point method, following the ASTM standard procedure (ASTM D3860-89a). Adsorption isotherms were generated using cyclohexane or buffered aqueous solution as the adsorption solution medium. All aqueous isotherms were measured in controlled (buffered) pH solutions, at pH 7.0, to exclude any effects of pH variations on the solubility of the adsorbate or charge of the carbon surface. The buffer was prepared using 0.05 M Na 2 HPO 4 / H 3 PO 4 in deionized water. A UV-spectrophotometer (Shimadzu, UV160U) was used to measure the adsorbate concentration in the solution.

The LP carbon was previously analyzed in our laboratory [12] and was found to be ash-free. Ash constituents (metals) usually give hydrophilic sites on the carbon surface. The absence of these constituents enables the study of the influence of oxygen-containing hydrophilic groups. It has been found that the oxygenation process to form LP-Air from LP-DI carbon increases the oxygen-containing complexes on the carbon surface by 110%, while deoxygenation of LP-DI to LP-N 2 reduces the amount of these groups by 40% [12]. Using linear temperature programmed desorption [12] (LTPD) and FT-IR spectroscopy [13], the type of oxygen groups on the surface were probed in earlier studies for LP-DI, LP-Air and LP-N 2 . The LTPD results are summarized in Table 1 [12]. In a detailed study of surface oxygen complexes, Otake and Jenkins [15] have argued that the amount of CO and CO 2 evolved corresponds to the amount of carbonyl and carboxylic groups on the surface, respectively. Based on their conclusions it can be seen that the quantity of both carboxylic and carbonyl-type oxygen groups increased with oxygenation and decreased with deoxygenation. BET surface areas obtained are shown in Table 2. The measurements were made with both degassed and nondegassed carbons. On comparing surface areas of the degassed LP samples, it is seen that the oxidation leads to an increase of 32% for LP-Air compared to LP-DI. This large difference is attributed to the partial activation of carbon during the oxygenation process at high temperature. This takes place by partial gasification of the carbon to produce CO and CO 2 . As a result of this, more micropores

Table 2 Surface area and PZC for LP-carbons Carbon

LP-DI LP-Air LP-N 2

Surface area (m 2 / g)

Difference of surface area between

Degassed

Not degassed

degassed and non-degassed surfaces (%)

1112 1465 1032

1044 1132 988

6.5 29.4 4.5

PZC

7.15 2.8 9.2

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are generated, as will be discussed later. The surface areas of LP-DI and LP-N 2 are, in contrast, close to each other, with the LP-N 2 area being 7% less than for LP-DI. This slight reduction in area can be attributed to the loss of oxygenated sites that formed part of the original surface area, or the possible collapse of some of the micropores due to the high temperature treatment of LP-N 2 . The reduction can also be attributed to the formation of graphitized surface. However, the latter explanation is less likely at the deoxygenation temperature used [14,15]. On comparing degassed and non-degassed surfaces, it is seen that the degassed surfaces consistently have a larger BET area. The difference is highest for oxygenated carbon (LP-Air) and lowest for deoxygenated carbon (LP-N 2 ). Surface oxygen has been postulated to be responsible for attracting water molecules [9], which form clusters through H-bonds around hydrophilic oxygen groups (such as carboxylic- and phenolic-type groups) and lead to the blockage of smaller pores, reducing the accessible surface area. Although the difference in surface area for degassed and non-degassed LP-N 2 is around the upper limit of the experimental error, the larger difference for LP-DI and LP-Air and the consistent trend of increasing difference with surface oxygenation support this argument. The cumulative pore size distribution for the three carbons is displayed in Fig. 1. The vast majority of the pore volume was found to be in the micropore (less than 2 nm) range and much of the remainder in the mesopore (2 to 50 nm) range. Macropores constituted a very small fraction of the total pore volume. This is as expected for this high surface area carbon. It is also observed from Fig. 1 that LP-DI and LP-N 2 have similar pore size distributions with slightly less micropores in LP-N 2 . This can be attributed to the collapse of some of the micropores at

the high treatment temperature of LP-N 2 , as discussed earlier. On the other hand, the oxygenation to LP-Air affects the pore size distribution more clearly. This has also been reported by others [16,17]. More mesopores, and many more micropores are generated as a result of the oxygenation process. As mentioned in the discussion on BET surface area, this increase in meso- and micropores is attributed to the partial activation of the carbon during the oxygenation process. This is consistent with higher BET area obtained for LP-Air. It should be noted that the oxygenation process did not affect the macroporous portion of the carbon, which is consistent with the results of Moreno-Castilla et al. [17]. Finally, measurements of point of zero charge (PZC) for all three carbons are presented in Table 2. It is observed from these data that PZC decreases with increasing surface oxygenation.

3.2. Mechanism of adsorption In order to study the effect of heterogeneous surface oxygen groups on the adsorption capacity, adsorption isotherms were generated for different aromatic adsorbates on the LP carbons. Presented in Fig. 2 are adsorption isotherms for phenol on LP-Air, LP-DI, and LP-N 2 . Also shown is the best-fit Freundlich isotherm characterization of the experimental data, presented by the solid line. These isotherms were generated in aqueous medium buffered at pH 7 and at 238C, and are normalized for the differences in surface area (Table 2). The normalization was performed to minimize the influence of changes in surface area, enabling a clearer comparison of the effects of chemical heterogeneity. All normalizations involved dividing the amount adsorbed by the total surface area measured for the

Fig. 1. Pore size distribution for LP carbon.


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Fig. 2. Effect of surface oxygen on adsorption of phenol on LP carbon in aqueous solution.

degassed carbon (Table 2), rather than non-degassed carbon. Considering that the difference between degassed and non-degassed surface area is due to enhanced water adsorption on the latter, the choice of area used for normalization is logical for isotherms measured in cyclohexane. For isotherms measured in water, on the other hand, an argument could be made for using the nondegassed surface area, on the basis that it is the accessible area in this case. This was, however, not the choice for the following two reasons. Firstly, water adsorption is a consequence of the chemical heterogeneity, and is therefore an effect that is of interest. Normalization with the non-degassed area would obscure at least part of the pore blocking influence brought about by the presence of surface groups. Secondly, it is known that the adsorption of a component from the gas phase, as is the case for BET measurements, is not in general similar to that from its liquid phase. Thus, the use of the non-degassed area cannot be expected to correctly represent the accessible area in liquid adsorption. In order to illustrate the trends observed, the amount of phenol adsorbed at selected equilibrium concentrations was used. In all cases the comparison was made at 20 mg / ml (q20 ). For isotherms that were essentially parallel, this is the only comparison made, since it is representative of the behavior over the entire range studied. In cases where the isotherms are not parallel a comparison is also made at 80 mg / ml (q80 ), which is at the higher end of the concentration range studied. Shown in Fig. 3a are the values of q20 for phenol for LP-Air, LP-DI, and LP-N 2 , obtained from the isotherms in Fig. 2; since the isotherms are essentially parallel in this case, only q20 has been shown. Fig. 3a indicates a clear decrease in adsorption capacity as the amount of surface oxygenation is increased. Using the adsorption capacity for

LP-DI as the reference, it is found that the adsorption capacity for phenol on LP carbon decreases to only 58% of that for LP-DI when the carbon is oxygenated to LP-Air. On the other hand, the capacity of the deoxygenated LP-N 2 is 148% that of LP-DI. This trend, which has been previously observed by Coughlin and Ezra [6], Mattson et al. [7], and Leng [12], indicates the strong effect of surface oxygen groups on the adsorption mechanism. The donor–acceptor mechanism for the effect of oxygen groups, as proposed by Mattson et al. [7], proposes an interaction between the carbonyl oxygen groups (donor) and the phenol’s aromatic ring (acceptor). According to this mechanism, it is expected that increasing the amount of oxygen groups should lead to an increase in adsorption capacity by creating additional carbonyl sites for the donor–acceptor interactions. Mattson et al. [7] proposed that oxygenation of carbon surface transforms the carbonyl groups to other types of oxygen-containing groups, such as carboxylic-type groups, which leads to the observed decrease in capacity with surface oxygenation. However, as discussed earlier (Table 1), oxygenation of LP carbon leads to an increase of both carbonyl and carboxylic oxygen groups. Based on these results, it is believed that the donor–acceptor mechanism, if present, is not the dominant mechanism for phenol adsorption. It is proposed that the mechanism behind the observed decrease in capacity with surface oxygenation is water adsorption. As water comes in contact with the carbon surface, it adsorbs on the hydrophilic, polar oxygen groups, particularly the carboxylic groups, located at the entrance of the carbon pores. Being highly accessible and hydrophilic, these oxygen groups provide ideal sites for water clusters to build up by hydrogen-bonding. Taking into consideration that the majority of the porous structure of LP carbon is micro- or mesoporous (Fig. 1), a hydration

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Fig. 3. Comparison of effect of surface oxygen on adsorption capacity for phenol in aqueous (a) and cyclohexane (b) solution.

cluster can effectively reduce the accessibility and affinity of the phenol molecules to the inner pore structure. Additional support for the proposed effect of water adsorption is obtained in the BET surface areas presented in Table 2. As was discussed earlier, BET measurements without degassing the carbon to remove the adsorbed moisture show a reduction in surface area by as much as 29% for oxygenated carbon. Since this indicates that the adsorbed water is capable of reducing the accessibility of the smaller N 2 molecules in BET measurements, water clusters are believed to be effective in hindering the larger phenol molecules as well. There are additional theoretical and experimental data confirming the influence of water adsorption. We have recently shown [5] with heat of wetting data, obtained by flow microcalorimetry, that there is significantly more water adsorption on LP-Air than on LP-DI. Also, recently Muller and Gubbins [11] have calculated, using molecular simulations, that a slight increase in the amount of oxygen groups will lead to a remarkable increase in water cluster formation. They have further shown that these clusters are formed in a 3dimensional configuration capable of totally blocking the carbon pores. Adsorption isotherms were also generated for phenol on LP-Air, LP-DI, and LP-N 2 using cyclohexane as the solvent. Fig. 4 shows the isotherms obtained at 238C. q20 and q80 for these isotherms are plotted in Fig. 3b. As in Fig. 3a, the capacity of LP-DI is used as a reference. It is

observed from Fig. 3b that the oxygenated carbon has a 47% higher capacity than LP-DI at 20 mg / ml and 23% at 80 mg / ml. On the other hand, the deoxygenated LP-N 2 has 26% and 12% higher capacity than LP-DI at 20 mg / ml and 80 mg / ml, respectively. Using cyclohexane as the adsorption medium eliminates the water adsorption mechanism. Moreover, cyclohexane acts as a partial isolator of electrostatic interactions of phenol with the carbon surface, since the dielectric constant for cyclohexane (´ 52.0243) is much lower than that of water (´ 580.1) [18]. The higher adsorption capacity for LP-N 2 as compared to LP-DI is contrary to what can be expected on the basis of the donor–acceptor mechanism, as proposed by Mattson et al. [7]; in the absence of water adsorption, it is expected that LP-DI should have a higher adsorption capacity since it has more carbonyl groups on the surface. Additionally, the higher adsorption capacity for LP-Air, compared to LP-DI, in cyclohexane provides evidence for the important influence of water adsorption. In the absence of water, the relative capacities of LP-Air and LP-DI are the opposite of that in Fig. 3a. The observed trends in Fig. 3b can be explained on the basis of two effects: (i) the influence of oxygen groups on the dispersive / repulsive interactions between the basal planes and the adsorbed molecules; and (ii) hydrogenbonding between surface oxygen groups and functional group(s) attached to the benzene ring of the adsorbed molecule.


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Fig. 4. Effect of surface oxygen on adsorption of phenol on LP carbon in cyclohexane solution.

3.2.1. Effect of dispersive /repulsive interactions The physisorption of aromatics on activated carbon takes place mainly through dispersive interactions between the aromatic molecules and the carbon basal planes. These dispersive interactions are basically in the form of van der Waals interactions. It is documented in the literature [14] that heterogeneous oxygen groups attract and localize the electrons of the basal planes, hence, forming partially positive ‘islands’ in the basal planes. On the other hand, the functional group attached to the aromatic adsorbate can activate or deactivate the benzene ring to which it is attached [19]. Activating groups act as electron donors, which create a partially negative benzene ring by pushing the electrons toward the ring. Deactivating groups attract the electrons and produce a partially positive ring. Since the benzene ring has a larger size compared to the functional group, the interaction of the benzene ring with the surface basal planes is more effective in the adsorption mechanism. The hydroxyl group (–OH) is an activating group [19]. This means that the aromatic ring of the phenol molecule has a partial negative charge. When phenol adsorbs on LP-Air, the oxygen groups, which localize the electrons and create a positive island in the basal planes, will increase the adsorption capacity relative to LP-DI, as is experimentally observed (Fig. 3b). The opposite should hold for the deoxygenated LP-N 2 . However, that data show a higher capacity for LP-N 2 compared to LP-DI (Fig. 3b). This suggests that additional factors play a role. It is also important to note that cyclohexane, which acts as a partial electrostatic isolator, can reduce the effectiveness of dispersive interactions. In other words, it is unlikely that the 47% increase in capacity of LP-Air, as indicated by the q20 values, compared to LP-DI, can be solely due to the attractive interactions between the

benzene ring of phenol and the positive islands in the basal planes. To explain the large difference, H-bonding is proposed as a significant mechanism.

3.2.2. Hydrogen-bonding mechanism It is proposed that aromatic compounds with a functional group capable of H-bonding, such as phenol, do so with carboxylic and carbonyl oxygen groups on the surface. Consequently, increasing surface oxygen content leads to a higher adsorption capacity for phenol since the number of sites available for H-bonding is increased. The effect of H-bonding is clearer in cyclohexane solution, where the absence of water molecules (also capable of H-bonding) eliminates competition for the available oxygen sites. It is proposed that much of the enhanced capacity of LP-Air is due to H-bonding. It is, however, not clear why the adsorption capacity for LP-N 2 was found to be higher than that for LP-DI. Both H-bonding and dispersive interactions point to a lower capacity. It is unlikely that the significant difference observed is solely due to experimental error. Further investigations to explain this observation are planned. It is interesting to observe that while H-bonding can take place in both cyclohexane and aqueous media, the effect of H-bonding cannot be seen in the adsorption of phenol in aqueous medium (Fig. 3a). Water adsorption is clearly dominant. Taking into consideration that water adsorption and H-bonding of phenol take place on the same oxygen groups, which are hydrophilic by nature, it is to be expected that water molecules are much more competitive in adsorbing on these groups than the more hydrophobic phenol molecules. In order to establish the importance of H-bonding and dispersive / repulsive interactions, the adsorption of another

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Fig. 5. Effect of surface oxygen on adsorption of aniline on LP carbon in cyclohexane solution.

H-bonding aromatic molecule was studied. Shown in Fig. 5 are the adsorption isotherms for aniline on LP-Air, LP-N 2, and LP-DI in cyclohexane at 238C. Shown in Fig. 6 are the q20 and q80 values for aniline, as obtained from Fig. 5. It is clearly seen that the adsorption capacity increases with surface oxygenation. The capacity for LPAir was 156% that of LP-DI at q20 and 122% at q80 , while the capacity of LP-N 2 is only 87% that of LP-DI at q20 and 90% at q80 . This observed trend agrees with the proposed mechanisms. In the absence of water adsorption, the adsorption of aniline is most strongly affected by Hbonding between the amine functional group (–NH 2 ) of aniline and surface oxygen. It is also affected by the change in dispersive interactions due to surface oxygenation. Aniline, like phenol, has a strong activating group [19]. This causes the benzene ring to have a partial negative charge. Following the same argument as for phenol, increasing the amount of oxygen-containing groups on the surface enhances the adsorption of aniline by increasing the attractive dispersive interactions. Both the H-bonding and the dispersive interaction mechanisms act

toward increasing the adsorption capacity of aniline as the surface oxygen groups are increased, consistent with experimental observations. The adsorption of nitrobenzene, a compound with a deactivating group that does not H-bond, was also studied. Figs. 7 and 8 show the adsorption isotherms for this compound from aqueous solution buffered at pH 7 and from cyclohexane solution, respectively. The isotherms were generated at 238C for LP-DI, LP-Air, and LP-N 2 . Shown in Fig. 9a and b are the q20 and q80 values obtained from Figs. 7 and 8, respectively. According to the earlier discussion, in aqueous solution water adsorption is dominant, which should cause the adsorption capacity to decrease as the amount of surface oxygen groups is increased. This is exactly the trend observed in Fig. 9a; the adsorption capacity of LP-Air is only 60% that of LP-DI, while capacity for LP-N 2 is 108% that for LP-DI. The trends for nitrobenzene in cyclohexane are, in contrast, totally different from that observed for phenol or aniline. As shown in Fig. 9b, the capacity for nitrobenzene dropped as the amount of oxygen groups on the surface was

Fig. 6. Effect of surface oxygen on q20 and q80 capacities for aniline in cyclohexane solution.


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Fig. 7. Effect of surface oxygen on adsorption of nitrobenzene on LP carbon in aqueous solution.

increased for both q20 and q80 . The nitro (–NO 2 ) functional group of nitrobenzene is not capable of H-bonding. Therefore, in cyclohexane medium, water adsorption and H-bonding mechanisms are eliminated. Hence, dispersive / repulsive interactions should dominate. Being a deactivating group, the nitro group withdraws electrons from the aromatic ring, making it partially positive. Therefore, repulsive electrostatic interactions of the adsorbate with the basal planes will increase as the amount of surface oxygen groups is increased. This should result in a reduced capacity for nitrobenzene as the amount of oxygen groups is increased, which is exactly the trend observed in Fig. 9b. An additional conclusion can be reached from the data in Fig. 9b. The higher adsorption capacity for LP-N 2 compared to LP-DI indicates that the donor–acceptor

mechanism, as proposed by Mattson et al. [7], is not likely to be influential. If it was, the capacity for LP-Air and LP-DI would be higher than that for LP-N 2 , since the donor–acceptor interaction is much stronger than van der Waals interactions. The adsorption of benzoic acid on LP carbon was also studied. Adsorption isotherms in aqueous solution at pH 11.6 and pH 3 were obtained previously in our laboratory by Leng [12], and are plotted in Figs. 10 and 11, respectively. Isotherms for benzoic acid in cyclohexane are shown in Fig. 12. q20 values obtained from Figs. 10–12 are plotted in Fig. 13, along with q80 values for cyclohexane isotherms. It is observed from Fig. 13a that the adsorption capacity for benzoic acid on LP carbon decreases as the amount of

Fig. 8. Effect of surface oxygen on adsorption of nitrobenzene on LP carbon in cyclohexane solution.

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Fig. 9. Comparison of the effects of surface oxygen on the adsorption capacity for nitrobenzene in aqueous (a) and cyclohexane (b) solution.

oxygen groups is increased at pH 11.6. This agrees with expectations based on the proposed mechanisms. At the high pH of 11.6, the majority of the carboxylic surface oxygen groups will exist in the dissociated carboxylate form (–COO 2). This will enhance water adsorption, and cause a larger difference in capacity between LP-DI and LP-Air (53%) than observed correspondingly with phenol (42%) and nitrobenzene (40%) at pH 7.0. Another key consideration is electrostatic interactions. Since all three carbons have PZC values less than 11.6, at this pH, the overall charge of the LP carbon surface is negative, and the more oxygenated surfaces have a stronger negative charge.

Also, the carboxylic functional group of benzoic acid (pKa 54.2 [18]) will exist in the negative dissociated (– COO 2) form. Repulsive electrostatic interactions between the dissociated benzoic acid and the negative carbon surface will further reduce the adsorption capacity with the effect being strongest for the most oxygenated carbon. Electrostatic interactions were not considered in the previous discussion for phenol, since phenol (pKa 59.99 [18]) will exist in the neutral, non-dissociated, form at pH 7.0, the pH used in those studies. At pH 3, the capacity does decrease with increased surface oxygenation (Fig. 13b), but the difference is much smaller than that at pH 11.6. At

Fig. 10. Effect of surface oxygen on the adsorption of benzoic acid on LP carbon in aqueous medium at pH 11.6, from Leng [12].


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Fig. 11. Effect of surface oxygen on the adsorption of benzoic acid on LP carbon in aqueous medium at pH 3, from Leng [12].

pH 3, a smaller amount of water adsorption is expected, since the carboxylic groups on the surface are essentially in neutral form. Additionally, at this low pH, benzoic acid exists in the neutral form and no electrostatic interactions are expected. When benzoic acid is adsorbed from cyclohexane, Hbonding and dispersive / repulsive interactions are expected to be the dominant mechanisms. The carboxylic group of benzoic acid is deactivating, which should give a lower capacity at higher surface oxygen content. On the other hand, the carboxylic group of benzoic acid is capable of H-bonding, which should give a higher capacity with surface oxygenation. Considering the nature of these two types of bonding, one expects H-bonding to dominate over the dispersive / repulsive interactions. Therefore, the net

effect of these two mechanisms, working in opposite directions, is expected to be an increase in capacity with surface oxygenation. Fig. 13c shows, however, that LP-Air has a lower capacity than LP-DI. This trend may be due to the H-bonding characteristics of benzoic acid. The carboxylic group of benzoic acid is capable of H-bonding through two sites: the carboxylic–hydrogen and the carbonyl–oxygen atoms. It is possible that a benzoic acid molecule, H-bonded to the surface, can H-bond with another benzoic acid molecule and form a chain or a cluster, which could lead to pore blockage, similar to water adsorption. On the other hand, Fig. 13c shows that LP-N 2 has 5% lower capacity than LP-DI when q20 values are compared, but 11% more capacity when q80 values are compared. Considering the above discussion, LP-N 2 is

Fig. 12. Effect of surface oxygen on the adsorption of benzoic acid on LP carbon in cyclohexane solution.

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Fig. 13. Comparison of the effects of surface oxygen on the adsorption capacity for benzoic acid in aqueous (a) and cyclohexane (b) solution.

consistently expected to have a higher capacity than LPDI. Additional research is planned to explore this confusing behavior further.

Acknowledgements This work was supported by the National Science Foundation GOALI Grant No. BES 9930739, co-sponsored by Calgon Carbon Corporation and E.I. DuPont deNemours. This support is gratefully acknowledged.

4. Conclusions Adsorption isotherms for phenol, aniline, nitrobenzene, and benzoic acid have shown that surface oxygen groups have a significant effect on the mechanism of adsorption of liquid aromatics on activated carbon. Surface oxygen groups, particularly carboxylic groups, are believed to adsorb water, creating water clusters through H-bonding, which reduces the accessibility and affinity for aromatic adsorbates, and thus reduces the adsorption capacity. Oxygen groups, on the other hand, can enhance the adsorption capacity in the absence of water, by forming H-bonds with the aromatics. Based on experimental data, it is also proposed that surface oxygen groups can affect adsorption through dispersive / repulsive interactions by attracting and localizing the electrons of the basal planes of the carbon surface. For aromatics with activating functional groups, such as phenol and aniline, this enhances the adsorption capacity. For aromatics with deactivating groups, such as benzoic acid, this reduces the capacity. Finally, the data suggest that the donor–acceptor mechanism cannot be an effective adsorption mechanism for aromatics on activated carbon.

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