Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon

Transcription

1 PERGAMON Carbon 38 (2000) Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon 1 * Marcus Franz, 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 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. Oxidation; C. Adsorption; D. Functional groups, Surface properties 1. Introduction adsorption sites for liquid organics are on the basal planes, which forms more than 90% of the carbon surface. Activated carbon is being used more than ever as an However, the much higher activity of the heterogeneous adsorbent in many applications. These cover a wide groups can result in significant effects on the overall spectrum of systems including water and wastewater adsorption capacity. The exact mechanism by which the treatment, separations, and hazardous waste treatment. In heterogeneous surface oxygen groups affect the adsorption spite of the large market for activated carbon, the specific capacity is not well understood [5 7], and is the subject of mechanisms by which the adsorption of many compounds, this work. especially organics, takes place on this adsorbent are still ambiguous. The complex, heterogeneous nature of the 1.1. Mechanisms activated carbon surface has led to contradicting mechanisms being proposed in the literature [1]. As a result, the The adsorption equilibrium for organics on activated prediction of adsorption capacity is limited to idealized carbon is dependent, to a large extent, on the chemistry of cases [2,3], and the design of most practical systems is the carbon surface. Heterogeneous oxygen groups have semi-empirical in nature. been reported in the literature to play an important role in The heterogeneous surface of activated carbon is usually the process. characterized into three main zones: the carbon basal Coughlin et al. [6] observed a decreasing capacity for planes, heterogeneous surface groups (mainly oxygen-con- phenol adsorption upon increasing surface oxygen content taining groups), and inorganic ash [4]. The majority of the of carbon. They suggested that surface oxygen groups influence phenol adsorption under conditions when the *Corresponding author. Tel.: ; fax: phenol molecules are thought to be adsorbed in a planar position on the basal planes and are held by attractive address: neville.pinto@uc.edu (N.G. Pinto). forces operating over the entire aromatic ring. At higher 1 Present address: Freiberg University of Mining and Technology, adsorbate concentrations, phenol molecules get packed Freiberg, Germany. more tightly on the surface and are thought to be adsorbed / 00/ $ see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 1808 M. Franz et al. / Carbon 38 (2000) in a vertical orientation (end-on position). In this case, oxygen groups, were involved in the adsorption mechainteractions among adsorbed phenol molecules become nism of phenol on activated carbon. significant and very little influence of surface oxygen on The present study was undertaken to further investigate the adsorption capacity was reported. To explain this they the influence of the heterogeneous surface oxygen groups proposed two major influences: (i) the chemisorbed oxy- on the adsorption mechanism. By using a variety of gen removes electrons from the p-electron system of the aromatic adsorbates with different functional groups, and carbon basal planes, creating positive holes in the conduc- studying their adsorption behavior from cyclohexane and tive p-band of the graphitic planes. This would lead to water solutions on carbons with different amounts of weaker dispersive interactions of the phenol p-electron oxygen groups, additional insight was obtained on the system with the p-band of the basal planes; (ii) Coughlin influence of surface oxygen. 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 2. Experimental was also proposed by Dubinin [8]. However, the data presented by Coughlin et al. contradicts this argument, 2.1. Materials since at high phenol concentration, there appears to be no Kureha LP spherical Bead Activated Carbon (BAC) (0.5 influence of oxygen sites. mm diameter), purchased from Kureha Chemical Industry Mattson et al. [7] proposed a different mechanism based Company (NY, USA), was used in this study. The LP on the role of basic carbonyl oxygen groups. Using carbon, which is made from petroleum pitch, is ash-free. infrared internal reflection spectroscopy (IRS), they Previous tests on this carbon [12] have indicated that attempted to gain information about the state of bonding polymerization of aromatics such as phenol and aniline do for different nitrophenols. Based on their results, they not occur on the LP-carbon surface, even under oxic suggested that the hydrogen of the hydroxyl group of solution conditions. The carbon was conditioned upon phenol is involved in an intracomplex hydrogen bond with receiving by boiling in deionized water for 1 h, then drying the surface oxygen complex. They concluded, however, in an oven at 1108C for 24 h. This carbon will be referred that this hydrogen bond interaction is small. They exto as LP-DI. To study the influence of surface oxygen plained that the aromatic ring interaction with the surface groups on adsorption, the LP-DI carbon was oxygenated oxygen, namely with the carbonyl group, contributes the with air in a tubular furnace at 3508C. A 3-g sample was major influence through a donor acceptor interaction, with placed in a quartz container, which was then placed in the the carbonyl groups acting as the electron donor and the furnace for 60 min under a constant flow of air. The aromatic ring of aromatic compounds acting as the accepsample was then cooled to room temperature in air. The tor. Further, Mattson et al. proposed that once the carbonyl oxygenated carbon will be referred to as LP-Air. Desites are exhausted, the aromatic compounds start to form oxygenated carbon was obtained by heating the LP-DI donor acceptor complexes with the basal planes. carbon to 8508C in the tubular furnace under a constant Mahajan et al. [9] suggested that the formation of water flow of nitrogen for 1 h. It was then cooled to room clusters by secondary water adsorption on the carboxylic temperature under a flow of nitrogen and stored in a groups hinders the migration of phenols into smaller pores nitrogen atmosphere. This carbon will be referred to as and to the basal planes and decreases the availability of LP-N 2. Previous analyses in our laboratory have shown active sites on the basal planes. This effect eventually leads that these treatments change the content of both carbonylto a drop in adsorption capacity as the amount of oxygen and carboxyl-type oxygen groups on the surface [13], as groups increases. More recent studies have also shown shown in Table 1. This was confirmed using linear evidence of the water adsorption effect [5,10,11]. Muller temperature programmed desorption (LTPD) experiments, and Gubbins [11] have shown, using Monte Carlo molecuwhich are described elsewhere [12]. lar simulations, that water adsorption is enhanced by All of the organic compounds used in this study were surface oxygen groups. A similar conclusion has been purchased from Fisher Scientific (Pittsburgh, PA) in the reported by Lee and Reucroft [10]. highest purity available, and no further purification was To isolate the water adsorption effect, Leng [12] used performed. cyclohexane as an adsorption medium for phenol. He found that the adsorption capacity of phenol on oxygenated carbon increases when adsorption takes place from cyclo Point of zero charge (PZC) measurements hexane solution, which is the opposite to the effect of oxygen groups in aqueous medium. These observations led To quantify the point of zero charge (PZC), 0.1 g of LP Leng to conclude that hydrogen-bonding between the ( carbon was added to 0.02-l solutions of 0.1 N NaCl, whose OH) group on the phenol molecule and the oxygen groups initial ph had been adjusted with NaOH or HCl. The on the surface, and water adsorption around the carboxylic containers were sealed and placed on a shaker for 24 h,

3 Table 1 Quantification of surface oxygen groups on LP-carbon [12] M. Franz et al. / Carbon 38 (2000) Carbon CO2 CO Total Surface area covered (mmol/ g) (mmol/ g) (mmol/ g) by oxygen complex 2 (m / g) LP-DI LP-N LP-Air 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 The LP carbon was previously analyzed in our laboradistribution tory [12] and was found to be ash-free. Ash constituents (metals) usually give hydrophilic sites on the carbon A Micromeritics Gemini 2360 (Norcross, GA, USA) surface. The absence of these constituents enables the BET apparatus was used to measure the surface area of the study of the influence of oxygen-containing hydrophilic carbons used. The surface area measurements were re- groups. peated twice for each carbon and the experimental error for It has been found that the oxygenation process to form these measurements was found to be in the range 2 to 5%. LP-Air from LP-DI carbon increases the oxygen-contain- Surface area measurements were performed with both ing complexes on the carbon surface by 110%, while degassed and non-degassed carbon samples. Degassing deoxygenation of LP-DI to LP-N2 reduces the amount of was achieved by placing the sample under a flow of helium these groups by 40% [12]. Using linear temperature for 2 h at 1008C. Pore size distributions were measured programmed desorption [12] (LTPD) and FT-IR specusing a Micromeritics ASAP 2010 machine with nitrogen, troscopy [13], the type of oxygen groups on the surface utilizing the Barrett, Joyner, and Halenda (BJH) method were probed in earlier studies for LP-DI, LP-Air and (ASTM D ). 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 2.4. Adsorption isotherms measurements CO2 evolved corresponds to the amount of carbonyl and carboxylic groups on the surface, respectively. Based on Equilibrium isotherms were determined using the bottle their conclusions it can be seen that the quantity of both point method, following the ASTM standard procedure carboxylic and carbonyl-type oxygen groups increased (ASTM D a). Adsorption isotherms were generated with oxygenation and decreased with deoxygenation. using cyclohexane or buffered aqueous solution as the BET surface areas obtained are shown in Table 2. The adsorption solution medium. All aqueous isotherms were measurements were made with both degassed and nonmeasured in controlled (buffered) ph solutions, at ph 7.0, degassed carbons. On comparing surface areas of the to exclude any effects of ph variations on the solubility of degassed LP samples, it is seen that the oxidation leads to the adsorbate or charge of the carbon surface. The buffer an increase of 32% for LP-Air compared to LP-DI. This was prepared using 0.05 M Na2HPO 4/ H3PO4 in deionized large difference is attributed to the partial activation of water. A UV-spectrophotometer (Shimadzu, UV160U) was carbon during the oxygenation process at high temperature. used to measure the adsorbate concentration in the solution. 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 2 Carbon Surface area (m / g) Difference of surface area between PZC Degassed Not degassed degassed and non-degassed surfaces (%) LP-DI LP-Air LP-N

4 1810 M. Franz et al. / Carbon 38 (2000) are generated, as will be discussed later. The surface areas the high treatment temperature of LP-N 2, as discussed of LP-DI and LP-N2 are, in contrast, close to each other, earlier. On the other hand, the oxygenation to LP-Air with the LP-N2 area being 7% less than for LP-DI. This affects the pore size distribution more clearly. This has slight reduction in area can be attributed to the loss of also been reported by others [16,17]. More mesopores, and oxygenated sites that formed part of the original surface many more micropores are generated as a result of the area, or the possible collapse of some of the micropores oxygenation process. As mentioned in the discussion on due to the high temperature treatment of LP-N 2. The BET surface area, this increase in meso- and micropores is reduction can also be attributed to the formation of attributed to the partial activation of the carbon during the graphitized surface. However, the latter explanation is less oxygenation process. This is consistent with higher BET likely at the deoxygenation temperature used [14,15]. area obtained for LP-Air. It should be noted that the On comparing degassed and non-degassed surfaces, it is oxygenation process did not affect the macroporous porseen that the degassed surfaces consistently have a larger tion of the carbon, which is consistent with the results of BET area. The difference is highest for oxygenated carbon Moreno-Castilla et al. [17]. (LP-Air) and lowest for deoxygenated carbon (LP-N 2). Finally, measurements of point of zero charge (PZC) for Surface oxygen has been postulated to be responsible for all three carbons are presented in Table 2. It is observed attracting water molecules [9], which form clusters through from these data that PZC decreases with increasing surface H-bonds around hydrophilic oxygen groups (such as oxygenation. carboxylic- and phenolic-type groups) and lead to the blockage of smaller pores, reducing the accessible surface 3.2. Mechanism of adsorption area. Although the difference in surface area for degassed and non-degassed LP-N2 is around the upper limit of the In order to study the effect of heterogeneous surface experimental error, the larger difference for LP-DI and oxygen groups on the adsorption capacity, adsorption LP-Air and the consistent trend of increasing difference isotherms were generated for different aromatic adsorbates with surface oxygenation support this argument. on the LP carbons. Presented in Fig. 2 are adsorption The cumulative pore size distribution for the three isotherms for phenol on LP-Air, LP-DI, and LP-N 2. Also carbons is displayed in Fig. 1. The vast majority of the shown is the best-fit Freundlich isotherm characterization pore volume was found to be in the micropore (less than 2 of the experimental data, presented by the solid line. These nm) range and much of the remainder in the mesopore (2 isotherms were generated in aqueous medium buffered at to 50 nm) range. Macropores constituted a very small ph 7 and at 238C, and are normalized for the differences in fraction of the total pore volume. This is as expected for surface area (Table 2). The normalization was performed this high surface area carbon. It is also observed from Fig. to minimize the influence of changes in surface area, 1 that LP-DI and LP-N2 have similar pore size dis- enabling a clearer comparison of the effects of chemical tributions with slightly less micropores in LP-N 2. This can heterogeneity. All normalizations involved dividing the be attributed to the collapse of some of the micropores at amount adsorbed by the total surface area measured for the Fig. 1. Pore size distribution for LP carbon.

5 M. Franz et al. / Carbon 38 (2000) 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 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. adsorption on the latter, the choice of area used for On the other hand, the capacity of the deoxygenated LP-N 2 normalization is logical for isotherms measured in cyclo- is 148% that of LP-DI. This trend, which has been hexane. For isotherms measured in water, on the other previously observed by Coughlin and Ezra [6], Mattson et hand, an argument could be made for using the non- al. [7], and Leng [12], indicates the strong effect of surface degassed surface area, on the basis that it is the accessible oxygen groups on the adsorption mechanism. area in this case. This was, however, not the choice for the The donor acceptor mechanism for the effect of oxygen following two reasons. Firstly, water adsorption is a groups, as proposed by Mattson et al. [7], proposes an consequence of the chemical heterogeneity, and is there- interaction between the carbonyl oxygen groups (donor) fore an effect that is of interest. Normalization with the and the phenol s aromatic ring (acceptor). According to non-degassed area would obscure at least part of the pore this mechanism, it is expected that increasing the amount blocking influence brought about by the presence of of oxygen groups should lead to an increase in adsorption surface groups. Secondly, it is known that the adsorption capacity by creating additional carbonyl sites for the of a component from the gas phase, as is the case for BET donor acceptor interactions. Mattson et al. [7] proposed measurements, is not in general similar to that from its that oxygenation of carbon surface transforms the carbonyl liquid phase. Thus, the use of the non-degassed area cannot groups to other types of oxygen-containing groups, such as be expected to correctly represent the accessible area in carboxylic-type groups, which leads to the observed deliquid adsorption. crease in capacity with surface oxygenation. However, as In order to illustrate the trends observed, the amount of discussed earlier (Table 1), oxygenation of LP carbon phenol adsorbed at selected equilibrium concentrations was leads to an increase of both carbonyl and carboxylic used. In all cases the comparison was made at 20 mg/ml oxygen groups. Based on these results, it is believed that ( q 20). For isotherms that were essentially parallel, this is the donor acceptor mechanism, if present, is not the the only comparison made, since it is representative of the dominant mechanism for phenol adsorption. behavior over the entire range studied. In cases where the It is proposed that the mechanism behind the observed isotherms are not parallel a comparison is also made at 80 decrease in capacity with surface oxygenation is water mg/ml (q 80), which is at the higher end of the con- adsorption. As water comes in contact with the carbon centration range studied. surface, it adsorbs on the hydrophilic, polar oxygen Shown in Fig. 3a are the values of q20 for phenol for groups, particularly the carboxylic groups, located at the LP-Air, LP-DI, and LP-N, obtained from the isotherms in entrance of the carbon pores. Being highly accessible and 2 Fig. 2; since the isotherms are essentially parallel in this hydrophilic, these oxygen groups provide ideal sites for case, only q20 has been shown. Fig. 3a indicates a clear water clusters to build up by hydrogen-bonding. Taking decrease in adsorption capacity as the amount of surface into consideration that the majority of the porous structure oxygenation is increased. Using the adsorption capacity for of LP carbon is micro- or mesoporous (Fig. 1), a hydration

6 1812 M. Franz et al. / Carbon 38 (2000) 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. observed from Fig. 3b that the oxygenated carbon has a 47% higher capacity than LP-DI at 20 mg/ ml and 23% at Additional support for the proposed effect of water ad- 80 mg/ml. On the other hand, the deoxygenated LP-N 2 sorption is obtained in the BET surface areas presented in has 26% and 12% higher capacity than LP-DI at 20 mg/ml Table 2. As was discussed earlier, BET measurements and 80 mg/ ml, respectively. Using cyclohexane as the without degassing the carbon to remove the adsorbed adsorption medium eliminates the water adsorption mechamoisture show a reduction in surface area by as much as nism. Moreover, cyclohexane acts as a partial isolator of 29% for oxygenated carbon. Since this indicates that the electrostatic interactions of phenol with the carbon surface, adsorbed water is capable of reducing the accessibility of since the dielectric constant for cyclohexane ( ) is the smaller N2 molecules in BET measurements, water much lower than that of water ( 580.1) [18]. clusters are believed to be effective in hindering the larger The higher adsorption capacity for LP-N2 as compared phenol molecules as well. There are additional theoretical to LP-DI is contrary to what can be expected on the basis and experimental data confirming the influence of water of the donor acceptor mechanism, as proposed by Mattson adsorption. We have recently shown [5] with heat of et al. [7]; in the absence of water adsorption, it is expected wetting data, obtained by flow microcalorimetry, that there that LP-DI should have a higher adsorption capacity since is significantly more water adsorption on LP-Air than on it has more carbonyl groups on the surface. Additionally, LP-DI. Also, recently Muller and Gubbins [11] have the higher adsorption capacity for LP-Air, compared to calculated, using molecular simulations, that a slight LP-DI, in cyclohexane provides evidence for the important increase in the amount of oxygen groups will lead to a influence of water adsorption. In the absence of water, the remarkable increase in water cluster formation. They have relative capacities of LP-Air and LP-DI are the opposite of further shown that these clusters are formed in a 3- that in Fig. 3a. dimensional configuration capable of totally blocking the The observed trends in Fig. 3b can be explained on the carbon pores. basis of two effects: (i) the influence of oxygen groups on Adsorption isotherms were also generated for phenol on the dispersive/ repulsive interactions between the basal LP-Air, LP-DI, and LP-N2 using cyclohexane as the planes and the adsorbed molecules; and (ii) hydrogensolvent. Fig. 4 shows the isotherms obtained at 238C. q bonding between surface oxygen groups and functional 20 and q80 for these isotherms are plotted in Fig. 3b. As in group(s) attached to the benzene ring of the adsorbed Fig. 3a, the capacity of LP-DI is used as a reference. It is molecule.

7 M. Franz et al. / Carbon 38 (2000) Fig. 4. Effect of surface oxygen on adsorption of phenol on LP carbon in cyclohexane solution Effect of dispersive/repulsive interactions benzene ring of phenol and the positive islands in the basal The physisorption of aromatics on activated carbon planes. To explain the large difference, H-bonding is takes place mainly through dispersive interactions between proposed as a significant mechanism. the aromatic molecules and the carbon basal planes. These dispersive interactions are basically in the form of van der Hydrogen-bonding mechanism Waals interactions. It is documented in the literature [14] It is proposed that aromatic compounds with a functhat heterogeneous oxygen groups attract and localize the tional group capable of H-bonding, such as phenol, do so electrons of the basal planes, hence, forming partially with carboxylic and carbonyl oxygen groups on the positive islands in the basal planes. On the other hand, surface. Consequently, increasing surface oxygen content the functional group attached to the aromatic adsorbate can leads to a higher adsorption capacity for phenol since the activate or deactivate the benzene ring to which it is number of sites available for H-bonding is increased. The attached [19]. Activating groups act as electron donors, effect of H-bonding is clearer in cyclohexane solution, which create a partially negative benzene ring by pushing where the absence of water molecules (also capable of the electrons toward the ring. Deactivating groups attract H-bonding) eliminates competition for the available oxythe electrons and produce a partially positive ring. Since gen sites. It is proposed that much of the enhanced the benzene ring has a larger size compared to the capacity of LP-Air is due to H-bonding. It is, however, not functional group, the interaction of the benzene ring with clear why the adsorption capacity for LP-N2 was found to the surface basal planes is more effective in the adsorption be higher than that for LP-DI. Both H-bonding and mechanism. The hydroxyl group ( OH) is an activating dispersive interactions point to a lower capacity. It is group [19]. This means that the aromatic ring of the phenol unlikely that the significant difference observed is solely molecule has a partial negative charge. When phenol due to experimental error. Further investigations to explain adsorbs on LP-Air, the oxygen groups, which localize the this observation are planned. electrons and create a positive island in the basal planes, It is interesting to observe that while H-bonding can take will increase the adsorption capacity relative to LP-DI, as place in both cyclohexane and aqueous media, the effect of is experimentally observed (Fig. 3b). The opposite should H-bonding cannot be seen in the adsorption of phenol in hold for the deoxygenated LP-N 2. However, that data aqueous medium (Fig. 3a). Water adsorption is clearly show a higher capacity for LP-N2 compared to LP-DI (Fig. dominant. Taking into consideration that water adsorption 3b). This suggests that additional factors play a role. and H-bonding of phenol take place on the same oxygen It is also important to note that cyclohexane, which acts groups, which are hydrophilic by nature, it is to be as a partial electrostatic isolator, can reduce the effective- expected that water molecules are much more competitive ness of dispersive interactions. In other words, it is in adsorbing on these groups than the more hydrophobic unlikely that the 47% increase in capacity of LP-Air, as phenol molecules. indicated by the q20 values, compared to LP-DI, can be In order to establish the importance of H-bonding and solely due to the attractive interactions between the dispersive/ repulsive interactions, the adsorption of another

8 1814 M. Franz et al. / Carbon 38 (2000) 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. toward increasing the adsorption capacity of aniline as the 5 are the adsorption isotherms for aniline on LP-Air, surface oxygen groups are increased, consistent with LP-N2, and LP-DI in cyclohexane at 238C. Shown in Fig. experimental observations. 6 are the q20 and q80 values for aniline, as obtained from The adsorption of nitrobenzene, a compound with a Fig. 5. It is clearly seen that the adsorption capacity deactivating group that does not H-bond, was also studied. increases with surface oxygenation. The capacity for LP- Figs. 7 and 8 show the adsorption isotherms for this Air was 156% that of LP-DI at q20 and 122% at q 80, while compound from aqueous solution buffered at ph 7 and the capacity of LP-N is only 87% that of LP-DI at q and from cyclohexane solution, respectively. The isotherms % at q. This observed trend agrees with the proposed were generated at 238C for LP-DI, LP-Air, and LP-N mechanisms. In the absence of water adsorption, the Shown in Fig. 9a and b are the q20 and q80 values obtained adsorption of aniline is most strongly affected by H- from Figs. 7 and 8, respectively. According to the earlier bonding between the amine functional group ( NH 2) of discussion, in aqueous solution water adsorption is domianiline and surface oxygen. It is also affected by the nant, which should cause the adsorption capacity to change in dispersive interactions due to surface oxygen- decrease as the amount of surface oxygen groups is ation. Aniline, like phenol, has a strong activating group increased. This is exactly the trend observed in Fig. 9a; the [19]. This causes the benzene ring to have a partial adsorption capacity of LP-Air is only 60% that of LP-DI, negative charge. Following the same argument as for while capacity for LP-N2 is 108% that for LP-DI. The phenol, increasing the amount of oxygen-containing trends for nitrobenzene in cyclohexane are, in contrast, 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 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 q and q capacities for aniline in cyclohexane solution

9 M. Franz et al. / Carbon 38 (2000) Fig. 7. Effect of surface oxygen on adsorption of nitrobenzene on LP carbon in aqueous solution. increased for both q20 and q 80. The nitro ( NO 2) func- mechanism, as proposed by Mattson et al. [7], is not likely tional group of nitrobenzene is not capable of H-bonding. to be influential. If it was, the capacity for LP-Air and Therefore, in cyclohexane medium, water adsorption and LP-DI would be higher than that for LP-N 2, since the H-bonding mechanisms are eliminated. Hence, dispersive/ donor acceptor interaction is much stronger than van der repulsive interactions should dominate. Being a deactivat- Waals interactions. ing group, the nitro group withdraws electrons from the The adsorption of benzoic acid on LP carbon was also aromatic ring, making it partially positive. Therefore, studied. Adsorption isotherms in aqueous solution at ph repulsive electrostatic interactions of the adsorbate with the 11.6 and ph 3 were obtained previously in our laboratory basal planes will increase as the amount of surface oxygen by Leng [12], and are plotted in Figs. 10 and 11, groups is increased. This should result in a reduced respectively. Isotherms for benzoic acid in cyclohexane are capacity for nitrobenzene as the amount of oxygen groups shown in Fig. 12. q20 values obtained from Figs are is increased, which is exactly the trend observed in Fig. 9b. plotted in Fig. 13, along with q80 values for cyclohexane An additional conclusion can be reached from the data isotherms. in Fig. 9b. The higher adsorption capacity for LP-N2 compared to LP-DI indicates that the donor acceptor 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.

10 1816 M. Franz et al. / Carbon 38 (2000) 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 This agrees with Also, the carboxylic functional group of benzoic acid expectations based on the proposed mechanisms. At the (pka54.2 [18]) will exist in the negative dissociated ( high ph of 11.6, the majority of the carboxylic surface COO 2 ) form. Repulsive electrostatic interactions between oxygen groups will exist in the dissociated carboxylate the dissociated benzoic acid and the negative carbon 2 form ( COO ). This will enhance water adsorption, and surface will further reduce the adsorption capacity with the cause a larger difference in capacity between LP-DI and effect being strongest for the most oxygenated carbon. LP-Air (53%) than observed correspondingly with phenol Electrostatic interactions were not considered in the previ- (42%) and nitrobenzene (40%) at ph 7.0. Another key ous discussion for phenol, since phenol (pka59.99 [18]) consideration is electrostatic interactions. Since all three will exist in the neutral, non-dissociated, form at ph 7.0, carbons have PZC values less than 11.6, at this ph, the the ph used in those studies. At ph 3, the capacity does overall charge of the LP carbon surface is negative, and the decrease with increased surface oxygenation (Fig. 13b), more oxygenated surfaces have a stronger negative charge. but the difference is much smaller than that at ph 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].

11 M. Franz et al. / Carbon 38 (2000) 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, H- bonding 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 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 car- bonyl 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 H-bonding, which should give a higher capacity with adsorption. On the other hand, Fig. 13c shows that LP-N 2 surface oxygenation. Considering the nature of these two has 5% lower capacity than LP-DI when q20 values are types of bonding, one expects H-bonding to dominate over compared, but 11% more capacity when q values are 80 the dispersive/ repulsive interactions. Therefore, the net compared. Considering the above discussion, LP-N is 2 Fig. 12. Effect of surface oxygen on the adsorption of benzoic acid on LP carbon in cyclohexane solution.

12 1818 M. Franz et al. / Carbon 38 (2000) 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 LP- DI. Additional research is planned to explore this confusing behavior further. 4. Conclusions Acknowledgements This work was supported by the National Science Foundation GOALI Grant No. BES , co-sponsored by Calgon Carbon Corporation and E.I. DuPont de- Nemours. This support is gratefully acknowledged. Adsorption isotherms for phenol, aniline, nitrobenzene, and benzoic acid have shown that surface oxygen groups References have a significant effect on the mechanism of adsorption of liquid aromatics on activated carbon. [1] Martin RJ, Ng WJ. Chemical regeneration of exhausted Surface oxygen groups, particularly carboxylic groups, activated carbon-i. Water Res 1984;18:59. [2] Sontheimer H, Crittenden J, Summers RS. Activated carbon are believed to adsorb water, creating water clusters for water treatment, Germany: DVGW-Forschungsstelle, through H-bonding, which reduces the accessibility and affinity for aromatic adsorbates, and thus reduces the [3] Weber W, McGinley P, Katz L. Sorption phenomena in adsorption capacity. Oxygen groups, on the other hand, subsurface systems: concepts, models and effects on concan enhance the adsorption capacity in the absence of taminant fate and transport. Water Res 1991;25(5): water, by forming H-bonds with the aromatics. Based on [4] Puri B. In: Walker P, editor, Chemistry and physics of experimental data, it is also proposed that surface oxygen carbon, vol. 6, New York: Marcel Dekker, groups can affect adsorption through dispersive/ repulsive [5] Arafat H, Franz M, Pinto N. Effect of salt on the mechanism interactions by attracting and localizing the electrons of the of adsorption of aromatics on activated carbon. Langmuir basal planes of the carbon surface. For aromatics with 1999;15(18): activating functional groups, such as phenol and aniline, [6] Coughlin R, Ezra F. Role of surface acidity in the adsorption of organic pollutants on the surface of carbon. Environ Sci this enhances the adsorption capacity. For aromatics with Technol 1968;2(4): deactivating groups, such as benzoic acid, this reduces the [7] Mattson JS, Mark HB, Malbin MD, Weber WJ, Critten JC. capacity. Surface chemistry of active carbon: specific adsorption of Finally, the data suggest that the donor acceptor mecha- phenol. J Colloid Interface Sci 1969;3(1): nism cannot be an effective adsorption mechanism for [8] Dubinin MM. In: Walker PL, editor, Chemistry and physics aromatics on activated carbon. of carbon, vol. 2, New York: Marcel Dekker, 1966, p. 51.

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