Adsorption of Aromatic Compounds from Water by Treated Carbon

Highly Selective Adsorption and Separation of Aniline/Phenol from Aqueous Solutions by Microporous MIL-53(Al): A Combined ... Industrial & Engineering...
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Environ. Sci. Technol. 2004, 38, 5786-5796

Adsorption of Aromatic Compounds from Water by Treated Carbon Materials DANIELA M. NEVSKAIA,* EVA CASTILLEJOS-LOPEZ, VICENTA MUN ˜ OZ, AND ANTONIO GUERRERO-RUIZ Departamento de Quimica Inorganica y Quimica Tecnica, Facultad de Ciencias, UNED, P Senda del Rey 9, Madrid 28040, Spain

Carbon materials with different textural and surface chemistry properties have been studied to analyze their behavior in removing aromatic compounds (phenol, o-chlorophenol, p-nitrophenol, aniline, and phenol compound mixtures) from water. A mesoporous high surface area graphite and a microporous activated carbon with (HSAGox and ACox) and without (HSAGT and ACT) oxygen surface groups, were used as adsorbents. Apparent surface areas, surface oxygen groups, and zero points of charge have been determined. The adsorption behavior of single compounds on ACT depends on the relation between the molecular and the pore sizes. The aniline, the nitrophenol, and the chlorophenol interact with the oxygen surface groups of oxidized graphite, while there is no evidence of any type of interaction of the phenol with these groups. The adsorption of the organic compound mixtures on the thermally treated samples is determined by the acid-base character of the adsorbate-adsorbent, whereas on the oxidized carbons, the controlling forces are the specific interactions between organic molecules and the oxygenated groups. Selectivity coefficients for the different mixtures are presented over the entire range of adsorption.

Introduction Waters polluted by organic compounds are very frequently treated by means of adsorption processes in which activated carbons act as adsorbents. So, many studies have been made in order to understand thoroughly the adsorption mechanisms involved in these processes (1-8). The information obtained in this way leads to a better design as well as to an improved efficiency for the activated carbons used as adsorbents. Adsorption capacity of an activated carbon depends on the nature of the adsorbent, the nature of the adsorbate, and the solution conditions (pH, temperature, ionic strength). With respect to the activated carbon surface, the four main components are the carbon basal planes, edges and crystal defects, ash impurities (i.e. metal oxides), and oxygen surface groups. The latter are mainly placed on the edges of the graphitic basal planes. Surface functional groups can be classified as acidic (carboxyl, carbonyl, phenolic, hydroxyl, lactone, anhydride) and basic (chromene- and pyrone-like structures) (9). Despite being a small fraction of the overall carbon surface, the oxygen groups are however * Corresponding author phone: +34-913987241; fax: +34913986697; e-mail: [email protected]. 5786

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very active, exhibiting a significant influence on the adsorption capacity (10). Since phenolic molecules and related compounds are common water contaminants, many attempts have been made to understand how the different ring substituents affect the adsorption (11-20). Thus, for example, Kaneko et al. (20) observed an adsorption trend with the compound’s solubility, while Deng et al. (17) related the uptake with the electron density of the aromatic ring and Moustafa et al. (15) proposed a dependence on pK values. Although the nature of the adsorption driving forces for phenolic compounds is still under discussion, the scales seems to tip toward the π-π argument (19). That is, phenolic compounds (in their uncharged form) are adsorbed on the carbon surface by dispersion forces between the π electrons of aromatic ring and the π electrons of the graphene layers. However, as pointed out by many authors (19, 21-25), the pH of the solution can affect the charge of phenolic compounds and therefore influences the electrostatic interactions between the adsorbent and the adsorbate. It is unusual in wastewaters to find only a sole pollutant. Thus, a better understanding of the multicomponent adsorption from aqueous solutions is needed to improve treatment designs. Competitive adsorption on the adsorbent surface can take place when several aromatic compounds are present in the water (26-29), in such a way that compounds with similar properties (i.e., size, polarity, interaction energy) compete for a limited number of adsorption sites. Studies of multicomponent systems (30-32), even though very important for improving the efficiency of water treatment, are less common, possibly as a consequence of experimental limitations and the difficulty of interpreting isotherms. In a previous paper (33), it has been found that the textural characteristics, the surface inorganic impurities, and the intrinsic oxygen surface groups (in quite low amounts) of the adsorbents affect the selectivity and the adsorption behavior of phenol-aniline mixtures. Here we present a more rigorous study of the effect of oxygen surface groups on the selectivity and adsorption behavior of aromatic compounds. To perform this study, more oxygen groups have been introduced on the adsorbent surfaces by means of a chemical treatment. In addition, the study has been extended to other systems with the purpose of finding how the surface chemistry affects the selectivity and the adsorption behavior of several pairs of adsorbates with different acidity and molecular sizes. Finally, adsorption selectivities for the different systems have been obtained over a large range of organic pollutant concentrations.

Experimental Section Materials. A commercial activated carbon provided by Norit RS 0.8 (Amersfoort, The Netherlands), here denoted as carbon AC (particle size 0.75-1 mm) was used in this study. Prior to surface treatments, the inorganic impurities were removed with HCl and HF treatments (7). The high surface area graphite (HSAG-300, particle size 15 µm) was provided by Lonza (Switzerland). Surface Treatments. To eliminate surface groups, the demineralized activated carbon (AC) and the as-received graphite (HSAG) were treated at 1173 K for 60 min in a furnace under a helium flow (100 cm3 min-1), at a heating rate of 10 K min-1. The resulting samples were denoted as ACT and HSAGT, respectively. To introduce oxygen, surface groups AC and HSAG were treated with HNO3 (10 wt % concentration). The mixture 10.1021/es049902g CCC: $27.50

 2004 American Chemical Society Published on Web 09/30/2004

(carbon and acid solution) was stirred and heated at 348 K until almost all liquid was evaporated. The ratio of carbon/ HNO3 was 1 g/10 mL. The procedure was repeated three times. The resulting samples were washed with doubly distilled water using a Soxhlet apparatus until there was an absence of NO3- ions in solution. The resulting samples were named ACox and HSAGox, respectively. Adsorbent Characterization. The textural characterization of samples was carried out by N2 adsorption at 77 K. An automatic Micromeritics ASAP 2010 volumetric system was used to obtain the gas adsorption isotherms. The accuracy of these measurements is 5%. The BET equation was applied to the N2 isotherms in order to determine the apparent surface areas. The pore size distributions were obtained by applying the DFT (Density Functional Theory) method to the N2 adsorption isotherms. To determine the zero point of charge (zpc), the electrophoretic mobility (µ) versuss pH of the samples was measured in a Zeta Meter 3.0+ at 298 K. As χa > 1 (χ is the reciprocal of Debye length and a is the particle radius), the Smoluchowski equation (34) µ ) r0ζ/η was applied to obtain the zeta potential (ζ), where η is the viscosity, and r and o are the permittivity of a material and of a vacuum, respectively. The concentration of the sample in each determination was approximately 100 ppm. The pH was adjusted with HCl and NaOH. Suspensions were stabilized 24 h before measurements. The surface groups were determined quantitatively following the Boehm method (35). Also, the surface functional groups of the samples were studied by temperature programmed decomposition (TPD) under vacuum (36). The apparatus used for these experiments consists of a quartz bulb directly attached to a quadrupole mass spectrometer (Balzers QMG 421-C). Adsorption Measurements. To obtain the adsorption isotherms, suspensions of 0.1 g of carbon and 2 mL of bidistilled water were placed in contact with 20 mL water solutions of different known concentrations of phenol, aniline, o-chlorophenol, p-nitrophenol, and phenol and/or aniline/o-chlorophenol/p-nitrophenol mixtures (50/50 molar ratio). The temperature of the suspensions was maintained to 298 K. pH values of the adsorbent-water suspensions were as follows: for thermally treated samples 5.8 (ACT) and 6.1 (HSAGT) and for oxidized samples 4.2 (ACox) and 3.7 (HSAGox). When adsorbates are added, the pH values of the suspensions shifted toward higher values (5-9), always under the pK values of the different compounds. Adsorption equilibrium was reached after 24 h with ellipsoidal stirring (Precision Scientific 360) at 298 K. The stirring rate was 100 rpm. The solids were separated by filtration using a 0.22 µm Millex-GV Millipore filter, and the supernatants were analyzed by UV spectroscopy at 269.5 nm (phenol), 273.2 nm (ochlorophenol), 227.2 nm (p-nitrophenol), and 278.5 nm (aniline) in a Varian CARY 1 spectrophotometer. To measure the more concentrated solution, a quartz path length cell of 10 mm was used, while the low concentrations were measured with a quartz path length cell of 50 mm. The adsorbed amounts q (mol m-2) were calculated on the basis of the concentration changes, determined by UV spectroscopy before and after adsorption. To determine the singlecomponent concentration from the phenol and aromatic compounds mixtures, a two equation system was solved for both compounds using the two overlapping signals from the UV spectra. The equations system are given by:

A1Ph ) a1Ph,jCPh + a1,jCj A2j ) a2Ph,jCPh + a2,jCj where j denotes aniline, o-chlorophenol, or p-nitrophenol; A1,Ph (269.5) and A2,j (j ) 279.8, 273.2, or 227.2) are their respective absorbances; a1Ph,j, a1,j, a2Ph,j, and a2,j are constants;

FIGURE 1. Pore size distribution calculated by DFT method.

TABLE 1. Structure Characteristics of Activated Carbons and Graphites DFT pore vol (cm3 g-1) sample

SBET (m2 g-1)

total

NO2Ph. Relationship between this trend and the adsorbate acidity (NO2Ph > ClPh > Ph >An) has not been found. Likewise the aromatic ring p-electron density of the compounds (An > Ph > ClPh > NO2Ph) cannot be related with the adsorption trend.

FIGURE 4. Adsorption isotherms of single compounds.

TABLE 3. Some Chemical Properties and Molecular Size of Adsorbates

a

compd

solubility (mol L-1)

pKa

a (Å)a

b (Å)b

c (Å)c

σt (Å2)d

σt (Å2)e

phenol (Ph) o-chlorophenol (ClPh) p-nitrophenol (NO2Ph) aniline (An)

0.998 0.221 0.122 0.397

9.8 8.5 7.2 4.6f

3.96 3.97 4.59 4.05

3.19 3.57 3.19 3.19

4.11 4.11 4.11 4.11

39.7 44.5 46.0 40.5

26.2 29.3 26.2 26.2

Molecular half length.

b

Molecular half width. c Thickness of the molecule.

d

Face-down orientation. e Stand-up orientation. f pKb.

TABLE 4. Adsorbed Amounts q (µmol m-2) of Single Aromatic Compounds and Degree of Coverage θ (%) Assuming That All Molecules Are in a Face-down or Stand-up Orientation compd HSAGT HSAGox ACT ACox

q θ (%) face down q θ (%) face down θ (%) stand up q θ (%) face down q θ (%) face down

phenol

chlorophenol

nitrophenol

aniline

3.00 71 2.92 70 46 3.05 73 2.00 48

3.06 82 4.52 121 80 2.95 79 2.12 57

3.14 87 5.00 138 79 2.33 65 2.04 57

2.62 64 5.25 128 83 2.96 72 2.30 56

The differences in the maximum adsorbed amounts between Ph, ClPh, and An are in the 4% range (Table 4). Thus, it can be said that the three compounds are adsorbed in approximately equal amounts despite their difference in molecular sizes or pK values. In the case of nitrophenol, the difference in the uptake (23%) is high enough to be significant and to merit a further explanation. The ACT sample exhibits an important amount of micropores of size about 5 Å (Figure 1). The greater length of the nitrophenol (Table 3) in comparison with the other molecules would be able to produce a greater steric hindrance for adsorption in these micropores (or at least in a fraction of them), and consequently, the quantity adsorbed of the nitrophenol is small as compared to the other compounds. This is confirmed by the fact that at lower degrees of coverage (where the filling of smaller, greater energy pore sizes is taking place) the difference between the adsorption isotherm of the nitrophenol and the adsorption isotherms of the rest of the compounds is more pronounced (Figure 4, ACT logarithmic scale). This is indicative of the inaccessibility of p-nitrophenol by the smaller micropores. This hypothesis is also supported by the evidence that on the graphite surface

(Figure 4 HSAGT), where only mesopores are presented (Figure 1), the adsorbed amounts of the nitrophenol are very similar to those of the chlorophenol and the phenol molecules. Therefore, in the case of the thermally treated microporous activated carbon, the main factor that affects the adsorption behavior is the relation between the porous structure of the adsorbent and the molecular size of the used adsorbate. However, the differences in acidity or π-electron density of these compounds are not important enough to be reflected in the adsorption behavior. When the organic compounds are adsorbed on the ACox, two main facts are observed: a change in the adsorption trend and a decrease in the adsorbed amounts with respect to the ACT sample. Assuming that the oxygen surface groups produce the acidification of the carbon surface and that their presence is better tolerated by the more basic aniline than by the more acid-substituted phenols, the compound uptake order (at low degrees of coverage: An > Ph > ClPh > NO2Ph) follows closely the basicity or aromatic ring π-electron density trend (Figure 4 ACox). At high degrees of coverage the difference in the uptakes between the phenol, the chlorophenol, and the nitrophenol is within the accuracy range VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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in rather similar amounts at high equilibrium concentrations. The fact that the adsorbed amounts on the ACox decrease in comparison with the ACT cannot be attributed to the oxygen surface groups because the adsorbed aniline is also decreased.

FIGURE 5. Possible arrangement of the molecules onto oxidized graphite. (5%), while the aniline is adsorbed in higher amounts (13%). Thus, in the case of the ACox sample, the influence of the surface chemistry is shown again (i.e., higher amount of aniline adsorption), especially at low coverage. As a consequence of the oxidizing treatment, textural modifications have been produced. The mean micropore size of the activated carbon becomes larger (Figure 1) and the mesopore percent also increases. These changes reduce the differences in the uptakes and so, all phenolic compounds are adsorbed

Adsorption on Graphites. On mesoporous graphites, the surface chemistry is expected to be more important for the adsorption behavior than are the textural properties. At high concentrations, the thermally treated graphite shows (Figure 4 HSAGT) a similar behavior for the phenol, the chlorophenol and the nitrophenol (5% of difference) and a lower uptake for aniline (13% of difference), the most basic compound. Therefore, the graphite surface has less affinity for the more basic molecule of aniline than for the more acid-substituted phenols. However, it seems that the difference in pK values (Table 3) of the substituted phenols is not high enough to result in changes in the amount adsorbed on this type of surface. In the case of the oxidized graphite, the adsorption trend (An > NO2Ph > ClPh > Ph) cannot be explained by the aromatic compounds (Figure 4 HSAGox) basicity trend. It is also notable that the adsorbed amounts, except for the case

FIGURE 6. Isotherms of adsorption of one of compounds (open symbols) with respect to the mixture on ACT. The projection on the Y-Z axis represents the isotherm of the compound with respect to its equilibrium concentration. The projection on the X-Y axis informs about the concentration of both compounds in the liquid phase. 5790

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FIGURE 7. Isotherms of adsorption of one of compounds (open symbols) with respect to the mixture on ACox. The projection on the Y-Z axis represents the isotherm of the compound with respect to its equilibrium concentration. The projection on the X-Y axis informs about the concentration of both compounds in the liquid phase. of the phenol (that remains constant), are higher on the oxidized graphite (Table 4) by up to a factor of 2 (aniline) in comparison with their adsorption on the thermally treated graphite. The high increase in adsorbed amounts of the chlorophenol, the nitrophenol, and the aniline is attributed to their specific interactions with the graphite oxygen surface groups. In fact, as a consequence of the introduction of oxygen groups, the surface hydrophobicity is changed. The surface is more hydrophilic, allowing more water molecules to be adsorbed (41). As the water molecules have a polar nature, they will preferably hydrogen-bond with the oxygen surface groups rather than adsorb on the graphite surface. The increase in the chlorophenol, the nitrophenol, and the aniline uptakes signifies that these aromatic compounds are able to displace the water molecules from the adsorbent surface in this case even more than when the oxygen groups are absent. This fact corroborates the hypothesis that some kind of aromatic compound-oxygen surface group interactions are originated. Nevertheless, it seems reasonable to suppose that

the specific interactions could have a different nature for the case of the basic aniline than for the acid chlorophenol and nitrophenol molecules. Surface Coverage and Molecular Orientation. With the purpose of knowing if the adsorbent surface is totally or partially covered by the adsorbate molecules, as well as if some changes in the molecular orientation are taking place during the adsorption, experimental and theoretical uptakes are compared using the following equations:

θ)

qexp qt

qt (µmol m-2) )

(1) 1026 σt N A

(2)

where qexp and qt are the experimental and theoretical maximum adsorbed amounts (µmol m-2), respectively; NA is the Avogadro constant; and θ is the degree of coverage. VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Isotherms of adsorption of one of compounds (open symbols) with respect to the mixture on HSAGT. The projection on the Y-Z axis represents the isotherm of the compound with respect to its equilibrium concentration. The projection on the X-Y axis informs about the concentration of both compounds in the liquid phase. The degrees of coverage vary from 64 to 87% for thermally treated adsorbents and fall to 48-57% for oxidized activated carbon (Table 4). Furthermore, from the degrees of coverage it is reasonable to conclude that the aromatic molecules are adsorbed mainly in a flat orientation. In the case of the oxidized graphite, the degrees of coverage of the chlorophenol, the nitrophenol, and the aniline are higher than 100% considering a stacked orientation of the molecules. This indicates the possibility of a bilayer formation or a change in the orientation of the adsorbed molecules. Bertoncini et al. (41) reported that phenol adsorption on a graphite surface on a stand-up orientation. However, bilayer formation has not been found for phenolic compounds in the consulted literature. So, assuming that the chlorophenol, the nitrophenol, and the aniline are adsorbed in a stand-up orientation, around 80% of the surface coverage is reached, while the phenol covers only about 46% of the surface. Nevertheless, the fact that the phenol uptake scarcely varies when the oxygenated groups are introduced (Table 4) suggests that interactions of the phenol with these groups do not take place and no change of orientation for the adsorbed molecules is produced. Also an intermediate situation can 5792

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be considered. A 100% of the coverage could be reached, with the adsorbed molecules linked to oxygen surface groups in a stand up arrangement (63% for aniline, 65% for nitrophenol, and 51% for chlorophenol) and the molecules adsorbed on the surface free of oxygen groups in a stacked orientation (37% for aniline, 35% for nitrophenol, and 49% for chlorophenol). The changes in the adsorption orientation support the hypothesis of a specific interaction between the organic molecules and the graphite oxygen surface groups. On this point, the possible disposition of the organic compounds adsorbed on the surface of the HSAGox can be inferred. As mentioned previously, the oxidizing treatment on the surface of the graphite results in several types of groups, and the acidities of these functional groups are very different. Thus, the carboxylic acids have a value of pK of about 4, lactones close to 6, whereas the phenolic groups have a pK value of approximately 9 (42). In the case of aromatic compounds, the aniline is clearly a base whereas the nitrophenol and the chlorophenol are acids of varying strength. Therefore, it is reasonable to suppose that the aniline will tend to interact with the most acid groups on the graphite surface (Figure 5).

FIGURE 9. Isotherms of adsorption of one of compounds (open symbols) with respect to the mixture on HSAGox. The projection on the Y-Z axis represents the isotherm of the compound with respect to its equilibrium concentration. The projection on the X-Y axis informs about the concentration of both compounds in the liquid phase. In the case of phenolic compounds, two options are possible. Either the interaction takes place through the hydroxilyc group or by means of another substituent (nitro or chloro groups). Since the phenol does not interact with the oxygen surface groups, it seems possible that the OH ring groups of the nitrophenol and the chlorophenol do not react with either of the surface groups. Therefore, is proposed that these two molecules interact with the graphite oxygen groups through their chloro or nitro groups (Figure 5). Stoeckli et al. (43, 44) have demonstrated that the adsorption mechanism of the phenol is the same for nonporous and microporous carbons. Therefore the decrease of the quantities adsorbed on ACox noted previously could be due to micropores preventing (due to steric hindrance) the molecules of the adsorbate from standing on the surface of the activated carbon. Adsorption of Mixtures. Figures 6-9 show the partial adsorption isotherms of the phenol-aromatic compound systems with respect to the concentration (Ceq/Cs) in the liquid phase of both components of the mixture for both the heated and the oxidized activated carbons and graphites.

The total uptakes, the percent corresponding to each compound of the mixtures, and the surface coverages are given in Table 5. Although the initial mixtures are 50% in each compound, the data show that the adsorbed amounts of each component are very different. Adsorption on Activated Carbons. On the thermally treated activated carbon, the most acid component of the mixtures is adsorbed in large amounts (see Figure 6). However, in case of the phenol-nitrophenol system, the phenol uptakes are higher than the nitrophenol ones at low coverage (see also selectivities in Figure 10) due to the inaccessibility of the last of the smaller micropores. On the oxidized activated carbon (ACox, Figure 7), the preferential adsorption of compounds is affected by factors such as the acid-basic character and specific interactions. The adsorption behavior of the phenol-aniline mixture is clearly directed by the acidbasic character. That is, more basic molecules of the aniline are retained to a greater extent on the acid sites of the oxidized carbon than are the more acid phenol molecules. In the phenol-chlorophenol mixture again the most basic compound is adsorbed more at high equilibrium concentrations. VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 10. Adsorption selectivities of phenol on adsorbent surfaces. However, at lower degrees of coverage, more chlorophenol is adsorbed (as shown in Figure 10) due to its specific interactions with oxygen surface groups. In the case of the phenol-nitrophenol mixture, the specific interactions are again the driving force for the adsorption. That is, the more acid nitrophenol is adsorbed in greater quantities than is the more basic phenol on the acid activated carbon surface. Therefore, when a mixture is adsorbed, the surface chemistry can play a more important role in the process than when a single compound is adsorbed. Adsorption on Graphites. In the case of the thermally treated graphite (Figure 8), as for the ACT carbon, higher uptake occurs for the most acid component of the mixture. 5794

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That is, more phenol is adsorbed from the mixtures than aniline, although less than chlorophenol and nitrophenol. The fact that more nitrophenol than phenol is adsorbed on the mesoporous surface from the phenol-nitrophenol mixture confirms the hypothesis of a steric hindrance on the activated carbon surface. On the oxidized graphite (HSAGox, Figure 9), more basic aniline is adsorbed in higher amounts than is the phenol, which can be attributed to specific bonds of the basic molecules with the acid groups of this carbon. However, in the case of the phenol-chlorophenol and the phenolnitrophenol mixtures, the more acid compounds (chlorophenol or nitrophenol) are adsorbed to a greater extent on

TABLE 5. Adsorbed Amounts q from Mixturesa and Degree of Coverage θ (%) phenol + chlorophenol sample HSAGT HSAGox ACT ACox a

q θ (%) face down q θ (%) face down θ (%) stand up q θ (%) face down q θ (%) face down

phenol + nitrophenol

phenol + aniline

total

Ph

ClPh

total

Ph

NO2Ph

total

Ph

An

3.02 78 4.67 112 74 2.81 73 2.52 70

28.0 20 35.9 30 20 34.1 23 59.5 36

72.0 58 64.1 82 54 65.9 50 40.5 34

3.01 81 5.13 140 81 2.73 73 2.18 58

23.7 17 11.0 13 9 36.0 24 34.0 18

76.3 64 89.0 127 72 64.0 49 66.0 40

2.86 69 5.10 124 81 2.96 72 2.85 69

55.9 38 18.3 22 15 56.3 40 40.2 27

44.1 31 81.7 102 66 43.7 32 59.8 42

Expressed in µmol m-2 for total and in percent related with each compound.

this acidic surface. This behavior again indicates that both molecules interact specifically with the graphite oxygen surface groups. Tentatively, it can be assumed that the adsorbed species are aligned as represented in Figure 5. Surface Coverage and Molecular Orientation. As in the case of single-compound adsorption on activated carbons and on the thermally treated graphite, both mixture compounds are adsorbed in a face-down orientation. The range of degree of coverage of the mixtures (from 58 to 81%) is situated between those of the corresponding single compounds (Tables 4 and 5). In the case of the oxidized graphite, if stacked orientation is admitted, the degree of coverage exceeds 100% in all mixtures. Considering a stand-up orientation of the adsorbed compounds, the surfaces coverage ranges between 74 and 81%. Selectivity. The adsorption selectivity in solution is characterized by the selectivity coefficient k (45, 46):

molar fraction of component i qi/(qi + qj) in the carbon adsorbed mixture ) k) molar fraction of component i CLi/(CLi + CLj) in the aqueous solution mixture (3) Figure 10 represents the selectivity coefficients of the phenol in the three mixtures on activated carbons and graphites. Phenol-Aniline Mixture. The phenol-aniline system behaves as expected. That is, on the more basic surface (ACT or HSAGT) the phenol is selectively adsorbed, while on the more acid surface (ACox and HSAGox), the aniline is preferred instead of phenol. Furthermore, the selectivity is higher toward the favored compound at lower equilibrium concentrations. This observation is reasonable because the surface chemistry has more influence on the adsorption behavior at a low degree of surface coverage. Phenol-Chlorophenol Mixture. This system has less difference in selectivity values among the compounds in the mixture. Notice that these two compounds have nearly the same pK values. Over practically the entire range of concentrations used, the chlorophenol is slightly preferred by the adsorption sites as compared to the phenol. Phenol-Nitrophenol Mixture. On the ACT, the preference for phenol adsorption could be attributed to the relationship between the size of the molecules and the size of the pores. This hypothesis is confirmed by the fact that on the HSAGT surface, where only mesopores are present, the nitrophenol is more selectively adsorbed. On the ACox and HSAGox surfaces, the selectivity to the nitrophenol molecule is explained by the specific interactions produced between the nitrophenol compound and the oxygen surface groups. This selectivity is greater for the HSAGox because it has a greater amount of oxygenated groups per unit area. This study has shown that, by unit weight, the ACT is the adsorbent that removes the most aromatic compounds from the water. But by unit area, the best adsorbent could be the

oxidized graphite. The activated carbons have more advantages when the organic molecules have a sufficiently small size so that they can avoid the steric hindrance during the adsorption. However, the HSAGox allows molecules of greater size to adsorb and also in a faster way. In fact, we have verified in our laboratory that the adsorption kinetic of graphites is 40 times faster than that of the activated carbon.

Acknowledgments The authors acknowledge the financial support from the UNED under project 2000 I + D no. 46 and Spanish Ministry of Science and Technology under project MAT2002-04189C02-02. Also E.C.-L. acknowledges a predoctoral research grant from UNED.

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Received for review January 20, 2004. Revised manuscript received July 30, 2004. Accepted August 17, 2004. ES049902G