Langmuir 2007, 23, 5995-6003
5995
Adsorption of Designed Pyrimidine Derivative Ligands on an Activated Carbon for the Removal of Cu(II) Ions from Aqueous Solution Marı´a D. Gutie´rrez-Valero, M. Luz Godino-Salido, Paloma Arranz-Mascaro´s, Rafael Lo´pez-Garzo´n,* Rafael Cuesta, and Javier Garcı´a-Martı´n Departamento de Quı´mica Inorga´ nica y Orga´ nica, Facultad de Ciencias Experimentales, UniVersidad de Jae´ n, 23071 Jae´ n, Spain ReceiVed September 14, 2006. In Final Form: January 23, 2007
The adsorption of five NR-substituted amino acids with a 5-nitroso-6-oxo pyrimidine as substituent on a commercial activated carbon (AC) has been studied in aqueous solution at several pH values. The adsorption processes of these organic compounds have been analyzed on the basis of the electrolytic behavior of the adsorbates. In all cases, the adsorption process is highly irreversible due to strong π-π interactions between the arene centers of the AC and the pyrimidine residue of the adsorbates. This interaction is consistent with XPS data and HOMO-LUMO theoretical calculations. The adsorption of these organic compounds provides a new route for the functionalization of the AC surface with carboxyl groups. In addition, the adsorption capacity of the AC/organic compound systems for Cu(II) ions in aqueous solution has been studied at different pH values. These systems show an increase of the adsorption capacity for Cu(II) compared to the AC, which is related to the AC functionalization with carboxyl groups due to the adsorbed organic compounds.
Introduction Decontamination of wastewater often implies metal ion recovery processes of which those based on the adsorption of metal ions on adsorbent materials are frequently used. Among these, those based on the ion-exchange methods using selective and strong chelating ion exchangers are likely the most extensively developed,1-3 and also, activated carbons are getting increasing importance.4-7 An important aspect of the adsorption process deals with the physicochemical aspects of the adsorbent-metal interactions.7-9 In the case of activated carbon, among other factors, the adsorption capacity is tightly related to the nature of the functional groups on the carbon surface. Thus, an important aspect in designing suitable activated carbons for metal capture is the control of the chemical surface functionalization. This is a difficult task if the common chemical activation techniques are used. In general, these activation processes are efficient for the preparation of activated carbons with tailored textural characteristics, i.e., surface area and pore size distribution. Nevertheless, they are rather inefficient in the fixation of specific chemical groups. For this purpose, several approaches, including the surface complexation model, have been proposed.10,11 * Corresponding author. E-mail:
[email protected]. Phone:+34-953212186. Fax: +34-953211876. (1) Chiarizia, R.; Horwitz, E. P.; Alexandratos, S. D.; Gula, M. J. Sep. Sci. Technol. 1997, 32, 1-35. (2) Sahni, S. K.; Reedijk, J. Coord. Chem. ReV. 1984, 59, 1-139. (3) Bradshaw, J. S.; Izatt, R. M. Acc. Chem. Res. 1997, 30, 338-345. (4) Brown, P. A.; Gill, S. A.; Allen, S. J. Water Res. 2000, 34, 3907-3916. (5) Csoban, K.; Pa´rka´nyi-Berka, M.; Joo´, P.; Berha, P. Colloid Surf., A 1998, 141, 347-364. (6) Rivera-Utrilla, J.; Ferro-Garcı´a, M. A.; Bautista-Toledo, I. Curr. Top. Colloids Interface Sci. 2002, 5, 191-201. (7) Radovic, L. R., Ed. Chemistry and Physics of Carbon; M. Dekker: New York 2000; Vol 27, pp 243-282. (8) Faur-Brasquet, C.; Reddad, Z.; Kadirvelu, K.; Le Cloirec, P. Appl. Surf. Sci. 2002, 196, 356-365. (9) Alfarra, A.; Frackowiak, E.; Beguin, F. Appl. Surf. Sci. 2004, 22, 84-92. (10) Carrott, P. J. M.; Ribeiro Carrot, M. M.; Nabais, J. M. V.; Ramalho, P. J. P. Carbon 1997, 35, 403-410. (11) Schindler, P. W. Aquatic Surface Chemistry; Wiley: New York 1987.
Chart 1
We have recently reported12,13 a new method to increase the adsorption capacity of activated carbons. This is based on the anchorage of previously designed molecular receptors of metal ions on the carbon surface. These receptors are organic molecules which consist of a pyrimidine aromatic residue, Ar, a Lewis base function, F, and an connecting arm, S, consisting of a polymethylene fragment (Chart 1). The aromatic residue, Ar, favors the anchorage of the organic molecule to the surface of the AC, while the function F acts as the metal receptor. These act as independent units because of the saturated nature of the connecting unit. Previous studies on the adsorption of two of these types of organic compounds (compounds 1 and 2 in Chart 2) on a basic activated carbon (Merck K24504014) have shown that the process is controlled by the plane-to-plane interaction of the arene centers of the activated carbon to the pyrimidine planes.12,13 The irreversible character of the process has been explained on the (12) Garcı´a-Martı´n, J.; Lo´pez-Garzo´n, R.; Godino-Salido, M.; Gutie´rrez-Valero, M.; Arranz-Mascaro´s, P.; Cuesta, R.; Carrasco-Marı´n, F. Langmuir 2005, 21, 6908-6914. (13) Garcı´a-Martı´n, J.; Lo´pez-Garzo´n, R.; Godino-Salido, M.; Cuesta-Martos, R.; Gutie´rrez-Valero, M.; Arranz-Mascaro´s, P.; Stoeckli-Evans, H. Eur. J. Inorg. Chem. 2005, 3093-3103.
10.1021/la0626959 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/28/2007
5996 Langmuir, Vol. 23, No. 11, 2007
Gutie´ rrez-Valero et al. Chart 2
basis of the electronic properties of the interacting moieties, i.e., the arene centers of the activated carbon and the pyrimidine residue, Ar, of the molecular receptors. Moreover, the anchored organic compounds provided the efficient functionalization of the carbon surface. A consequence of this functionalization is the increase of the adsorption capacity of the AC-organic compound system toward Zn(II), Cd(II), and CrO42- ions.13,12 To get further insight into these systems, we have extended these studies to a set of five derivatives (compounds 3-7 of Chart 2) analogous to compounds 1 and 2. The aim is to show that, as the electronic properties of the aromatic moieties of compounds 1 and 2 remain in the new compounds (receptors), the adsorption in the previously used basic carbon (Merck K24504014) is also irreversible. Moreover, an increase of the adsorption capacity of the AC-receptors for metal ions should be expected in comparison to that of the AC. This is because the anchorage of the new compounds to the carbon surface produces the introduction in the system of basic functional groups. Production of compounds 3-7 was carried out by substitution of four L-amino acids (alanine, methionine, serine, and valine) and the dipeptide glycylglycine, respectively, at the C2 position of the compound 4-amino-1-methyl-2-methoxy-5-nitroso-6-oxopyrimidine (Chart 2). The synthesis methods have already been described.14-16 Thus, in this paper, we report the adsorption of receptors 3-7 on the AC at different pH. Moreover, the adsorption capacities of the carbon and the carbon-receptor adsorbents to Cu(II) have also been measured at different pH values. The results were analyzed, taking into account the reactivities of the receptor/ metal ion systems in aqueous solutions. Experimental Section As already commented, compounds 3-7 were synthesized following a previously reported method, and their molecular structures were solved by single-crystal X-ray diffraction methods.14-16 (14) Low, J. N.; Lo´pez, M. D.; Arranz Mascaro´s, P.; Cobo Domingo, J.; Godino Salido, M.; Lo´pez Garzo´n, R.; Gutie´rrez, M. D.; Melguizo, M.; Ferguson, G.; Glidewell, C. Acta Crystallogr. 2000, B56, 882-892. (15) Arranz-Mascaro´s, P.; Godino-Salido, M. L.; Lo´pez-Garzo´n, R.; Gutie´rrezValero, M. D.; Moreno, J. M. Polyhedron 1999, 18, 793-797. (16) Low, J. N.; Quesada, A.; Glidewell, C.; Fontecha, M. A.; Arranz, P.; Godino, M. L.; Lo´pez, R. Acta Crystallogr. 2002, E58, o942-o945.
Optimum geometries of the molecules were calculated from the conformations corresponding to the global minimum by using the MM2 algorithm (ChemBats3D, CambridgeSoft Corp.).17 The geometry was optimized without any constraints, allowing all atoms, bonds, and dihedral angles to change simultaneously. The final RMS gradient was less than 0.001 kcal·mol-1·Å-1 for all the minimized structures. Extended Hu¨ckel molecular orbital (EHMO) analyses for idealized geometries of the compounds were obtained with the HYPER-CHEM software.18 The XPS spectra of the AC and of the carbon-receptor systems were obtained in a VG-Microtech Multilab electron spectrometer, by using the Mg KR (1253.6 eV) radiation of twin anode in the constant analyzer energy mode with pass energy of 50 eV. The pressure of the analysis chamber was maintained at 5·10-10 mB. The binding energy and the Auger kinetic energy scale were regulated by setting the C1s transition at 284.6 eV. The accuracy of BE values was (0.2. The spectra obtained after background signal correction were fitted to Lorentzian and Gaussian curves19 in order to obtain the number of components, the BE values of each peak, and the peak areas. The UV-vis spectra of the receptors (compounds 3-7) in aqueous solutions (0.1 M KCl) were recorded on a Perkin-Elmer Lambda 19 spectrophotometer. HCl and KOH were used to adjust the pH values, which were measured on a Crison 2002 micro-pH meter. The protonation data of compounds 4, 5, and 7 (in aqueous solutions 0.1 M KCl) have already been reported.20,21 In the case of compounds 3 and 6, the speciation as a pH function in 0.1 M KCl aqueous solution was obtained by titrating a 10-3 M acidic (HCl) solution of the compounds with a KOH standardized solution. The experiment for each of the compounds was repeated three times, and the obtained data were processed with the Hyperquad and Best programs.22,23 A detailed description of the experimental procedure was previously reported.24 (17) Dudek, M. J.; Ponder, J. W. J. Comput. Chem. 1995, 16, 791. (18) HyperChem, release 4.5 for Windows; Hypercube, Inc.: Waterloo, Ontario, Canada, 1995. (19) Kwok, R. XPSPEAK, XPS peak fitting program; University of Hong Kong, 2000. (20) Lo´pez-Garzo´n, R.; Arranz-Mascaro´s, P.; Godino-Salido, M. L.; Gutie´rrezValero, M. D.; Cuesta, M.; Moreno, J. M. Inorg. Chim. Acta 2003, 355, 41-48. (21) Lo´pez-Garzo´n, R.; Godino-Salido, M. L.; Arranz-Mascaro´s, P.; Fontecha, M. A.; Gutie´rrez-Valero, M. D.; Cuesta, R.; Moreno, J. M.; Stoeckli-Evans, H. Inorg. Chim. Acta 2004, 357, 2007-2014. (22) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739-1753. (23) Martell, A. E.; Motekaitis, R. J. Determination and Use of Stability Constants; VCH: New York, 1992. (24) Godino-Salido, M. L.; Gutierrez-Valero, M. D.; Lo´pez-Garzo´n, R.; MorenoSa´nchez, J. M. Inorg. Chim. Acta 1994, 221, 177-181.
Adsorption of Pyrimidine Ligands on ActiVated Carbon A granulated activated carbon (AC), Merck K24504014, was used as adsorbent. The elemental analysis, surface area, porosity, nature of oxygen-containing groups, and proton isotherms (to obtain the point of charge zero, pHPZC) have been obtained as reported.12 The volume of micropores was calculated from the application of the Dubinin-Radushkevich equation to the N2 adsorption at 77 K. The micropore average width, Lo, was obtained from the empirical expression suggested by Stoeckli.25 For the study of the reactivity of the receptors with Cu(II), potentiometric titrations of metal/ligand mixtures were carried out at 298.1 ( 0.1 K in KCl 0.1 M. For this purpose, a 713 Metrohm pH-mV meter, equipped with a combined glass electrode and connected to a Metrohm 765 Dosimat autoburette (1 mL ( 0.001) was used. The experimental procedure has been described elsewhere.24 Typically, (1-1.5) × 10-3 M ligand concentrations and 1:1 ligand/metal molar ratios were employed in the potentiometric measurements. At least four titration experiments (150 data points each), were carried out in the pH range between 2.5 and 10. The HYPERQUAD software22 was used to calculate the equilibrium constants from the emf data. The adsorption isotherms of receptors 3-7 on the AC were obtained at 298.1 K at various initial pH which were adjusted with KOH or HCl. In all of the experiments, 0.0500 g of carbon was added to a plastic flask containing 50 mL of the appropriate adsorbate solution. The suspensions were continuously shaken in a water bath at constant temperature until the equilibrium was reached (15R.C. days), which was previously determined by kinetic measurements. The initial and equilibrium concentrations of the adsorbates were determined by UV spectroscopic measurements at λ values of the isosbestic points. After the adsorption experiments, the adsorbents were carefully filtered, washed with doubly distilled water, and dried in a desiccator. Then, they were added to a 100 mL plastic flask containing 50 mL of doubly distilled water to obtain the desorption isotherms. The suspensions were continuously shaken in a water bath until the equilibrium time was reached (7 days). The concentrations of the desorbed compounds were measured by UV spectroscopy. The adsorption isotherms of Cu(II) (as CuCl2) in aqueous solution, on the AC, and the AC-receptor adsorbents were obtained, at 298.1 K. Typically, 0.0500 g of the adsorbent was added to a 100 mL plastic flask containing 50 mL of the appropriate aqueous solution of the adsorbate. The experiments were carried out at several pH values, which were initially adjusted with KOH or HCl. To carry out the adsorption of Cu(II) on the AC-receptor adsorbents, it was necessary to previously prepare the AC receptors. These were prepared by previous adsorption experiments of the corresponding receptor solutions on the AC, at the maximun ligand concentrations corresponding to 100% of irreversible adsorption. When AC-receptor adsorbents were used to adsorb Cu(II), it was also checked that, in the equilibrium solutions, none of the ligands were desorbed.
Results and Discussion Characterization of the Adsorbates. The structural and electronic data of the adsorbates together with their electrolytic behavior are basic parameters which are needed to explain the adsorption of the receptors on the AC, as has been shown12 in the case of compounds 1 and 2. For this reason, this section is devoted to these aspects. The structural similarities between the aromatic moieties of compounds 3-7 and that of 1 and 2 (see Chart 2) also suggest similar electronic properties in such moieties, which are the main interacting sites with the AC. The electron withdrawing character of the C5NO and C6O substituents, determine short C5-N and N-O bond lengths in C5NO groups and relatively longer bond distances in the HNaminoac-C2N3C4N fragment. This indicates strong polarization of the aromatic moieties of all of the compounds with the negative charge at the former and the positive one at the second.14,16 This fact reduces the basicity of the N3ring atom. In fact, this atom remains
Langmuir, Vol. 23, No. 11, 2007 5997 Table 1. Protonation Constants (log K) of the Compounds (0.1 M KCl, 298.1 K) log K reaction
3
/ H + H+ + [HL]- / [H2L] H+ + [H2L] / [H3L]+ +
a
L2-
[HL]-
3.63 1.95
4
5
12.2a
12.1a
3.19 2.23
3.36 1.62
6
7
3.05 2.00
10.39 3.61 2.02
Values obtained by UV-vis measurements.
unprotonated in aqueous solutions of these ligands even at pH values below 2 (see below). The neutral forms of compounds 4, 5, and 7 behave as biprotic species, H2L, in aqueous solution, in the pH range 2-10.5. The two acidic protons are those of the carboxyl group of the amino acidic residue and the C2ringNH group. The corresponding protonation processes are H+ + HL- / H2L and L2- + H+ / HL-, for the carboxyl group and the C2ringNH, respectively (see Table 1). In spite of the occurrence of various basic groups in 4, 5, and 7 neutral molecules, only protonations at the N atom of the C5NO group are observed at pH values below ca. 4.21 The acid-base behavior of compounds 3 and 6 is similar to that of compounds 4, 5, and 7. Protonation of C5NO group takes place at pH below ca. 4, producing typical UV and visible spectral changes which are shown in Figure 1 for compound 6. Similarly, the deprotonation of the C2-NH group produces UV and visible spectral changes, at pH values higher than ca. 10 (Figure 1). The protonation of the carboxylate group starts at pH of ca. 5.5 and is completed at pH of ca, 1.5. This process, which is not detected by UV spectroscopy, was determined potentiometrically. The protonation constant values for the described processes are summarized in Table 1. The species distribution plots as a function of pH22,23 are very similar for the five studied compounds. The typical behavior is shown for compound 5 in Figure 2. It is seen that the monocationic form is the major species at pH < 2, whereas the neutral form H2L is the most abundant in the ca. 2-3.5 pH window. The monoanionic form, which starts to appear at a pH of ca. 2, is the only species existing at pH values above 5. Study of the Adsorption of the Receptors. The AC used as adsorbent (Merck K24504014) has already been characterized,12 and the analytical and textural data are summarized in Table 2. This table shows the existence of a significant amount of mesoand macropores and a well-developed nitrogen surface area, which means that the sample has a large volume of micropores. In addition, this material has negligible amount of N (0.75%) and a small amount of O (4.25%), which appears as oxygen functional groups: mainly basic quinone and carbonyl groups and very small amounts of acidic lactone and phenolic groups.12 This is supported by the XPS diagram of the AC (Figure 3). The O1s band is composed of two main signals due to saturated oxygen functions (COH and -O- groups) at 533.9 eV and unsaturated ones (carbonyl, carboxyl and quinone groups) at 531.7 eV. A shoulder in the main band at 537.1 eV is assigned to a water signal. Thus, the reported basicity of this carbon (the pHPZC value is 8.3) was explained on the basis of the predominance of basic arene centers which are protonated by the reaction26
CΠ + H3O+ / CΠ-H3O+ (25) Stoeckli, F. In Porosity in carbons-characterization and applications; Patrick, J., Ed.; Arnold: London, 1995; pp 67-92. (26) Leon y Leon, C. A.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992, 30, 797-810.
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Figure 1. pH dependence of the adsorption spectra of compound 6 in the UV ([6] ) 5·10-5 M, 0.1 M KCl) and visible (inset, [6] ) 10-3 M, 0.1 M KCl) regions: (a) acidic solution; (b) basic solution.
Figure 2. Species distribution vs pH for compound 5 (H2L) in aqueous solution (0.1 M KCl) at 298.1 K. Table 2. Elemental Analysis and Textural Characteristics of the Carbon12 analysis (%) C
H
N
SBET Vmic Vmes Vmac Sext rpm ash Oa (m2/g) (cm3/g) (cm3/g) (cm3/g) (m2/g) (Å) (%)
94.7 0.3 0.75 4.25 1000 a
0.38
0.14
0.24
62
8.3 0.35
Obtained by difference.
The adsorption isotherms of compounds 3-7 on the AC at various pH values are collected in Figure 4. They are of L-type according to Giles’s classification27 and are similar to these of compounds 1 and 2.12 This suggests that the adsorbates are flatadsorbed through the pyrimidine planes to the arene centers of the AC. Even if XPS characterizes the surface only to a depth of a few nanometers and in dry conditions, the data obtained by this technique provide relevant information in relation to the adsorption model. The comparison of the N1s bands of the XPS diagrams of the compounds to those of the corresponding ACcompound also supports this fact (Figure 3a and Table 3). To do that, it is assumed that the contribution to these signals of the heteroatomic nitrogen of the AC is negligible (in fact, such a signal does not appear in the XPS diagram of the AC). Thus, in the case of compounds 3-6, the N1s bands are composed of a signal due to the five aromatic nitrogen atoms (the energy values ranging between 400.2 and 400.5 eV). In the case of compound 7, there is an additional contribution of the amide nitrogen atom. (27) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J. Chem. Soc. 1960, 3973-3993.
In all the cases, the N1s bands in the spectra of the AC-compounds are significantly shifted to higher energies relative to the corresponding compounds, as is seen in Figure 3a and Table 3 for compound 3 and AC-compound 3. This effect, which is consistent with some electron donor behavior of the arene centers of the carbon to the aromatic residues of the compounds (Ar, in Chart 1) can explain the highly irreversible character of the adsorption processes which will be commented on afterward. The O1s bands of the potassium salts of compounds 3-6 are composed of two signals. The first one (at ca. 531.6 eV) is due to COO-, and the second (at ca. 530.9 eV) is assigned to C6O and C5NO, respectively. In addition, there is a signal due to H2O (at ca, 532.9 eV) in some cases. Figure 3 shows those signals for the potassium salt of compound 3 for illustration. By comparing the XPS O1s band of the AC-compound 3 sample with those of the AC and of the potassium salt of compound 3, two interesting facts are seen. First, it is observed that both of the compound O1s signals are shifted to higher energy values: by 1.0 eV, the C6O and C5NO signal; and by 0.9 eV, the COO- signal. This points out the increase of electronic density of the oxygen atoms due to π electron donation from arene centers to the pyrimidine, in the case of the former. In the second, this is due to the breaking of the supramolecular assembly in the solid adsorbate (in which the carboxylate groups acts as strong electron donors as ligands to K+ ions and/or in hydrogen bonding), when this is anchored to the carbon. Second, the lack in the XPS spectrum of AC-compound 3 adsorbent of the water signal in the O1s band, which appears in the AC spectrum at ca. 537.1 eV due to water molecules anchored to the arene centers, strongly suggests the removal of water induced by the compound anchorage to such centers. In spite of the existence of different compound species in aqueous solutions and the heterogeneity of the adsorbent surface, the existence of a predominant adsorption mechanism in all cases (namely, π-π interactions between the arene centers and the pyrimidine moieties) explains the fitting to Langmuir-type functions of all the isotherms (R2 varies between 0.893 and 0.982). In addition to this, there are other adsorption models which take into consideration heterogeneity effects of adsorption.28 Nevertheless, by assuming an adsorptive mechanism based on the above-described plane-to-plane interactions, a qualitative account of the Langmuir maximum adsorption capacities of compounds 3-7 (Table 4) can be carried out by assuming that the adsorbent(28) Bansal, R. C.; Goyal, M. ActiVated Carbon Adsorption; CRC Press: Boca Rato´n, 2005; Chapter 2.
Adsorption of Pyrimidine Ligands on ActiVated Carbon
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Figure 3. XPS data of compound 3: (a) N1s signals and (b) O1s signals.
adsorbate affinity is determined by both electrostatic and nonelectrostatic potentials.12,29 The highest values of the maximum adsorptive capacities from the Langmuir equation, Xm, for compounds 3-7 are found at pH 3.5 (Table 4). At this pH, most of the solutes (more than 50% in all cases) are found as monoanions, e.g., they are counterions to the H3O+ groups anchored to the arene centers (the target positions of the adsorbates). Thus, attractive electrostatic interactions are expected, aside from the π-π interactions cited above. At pH values 7 and 8, at which 100% of all of the adsorbates are in monoanionic forms, the maximum adsorption capacities for all of the adsorbates decrease in relation to pH ) 3.5. This is due to the absence of electrostatic interaction due to the negligible charge existing on the carbon surface at the former pH. Thus, only nonelectrostatic interactions between the adsorbent and the adsorbates are expected. Moreover, as the electronic properties of both the adsorbent (the AC) and the adsorbates do not change from pH 7 to 8, the maximum adsorption capacities for each of the compounds are maintained in this pH range.12 At pH 9.5, the compounds are in monoanionic forms, namely, they are co-ionic with the negatively charged groups existing on the carbon surface at this pH. These charges are located at the oxygen groups existing at the edges of the graphite layers, e.g., far away from the adsorbate molecules which are located at the arene centers. This probably diminishes the expected repulsive interactions between co-ionic species and explains the negligible differences among Xm values at pH 9.5, 7, and 8. Moreover, the conjugation of the oxygen functional groups with the negative charges and the arene centers of the carbon surface probably enhance their π basic character, favoring the adsorbentadsorbates interactions. In the case of compound 6, a slight decrease in Xm value is observed at pH 9.5. This is probably due to the increase of the solubility of the highly hydrophilic serine residue in basic medium. (29) Mu¨ller, G.; Radke, C. J.; Prausnitz, J. M. J. Phys. Chem. 1980, 84, 369376.
The AC surface areas covered by the adsorbates (MCA, Table 4) were obtained from their molecular areas calculated for standard conformations by using the three-dimensional data of the molecules determined with ChemBats3D software (cf. ExperimentalSection).Theproductofthemolecularareas(m2·molecule-1), the Avogadro’s number (in molecule·mmol-1), and the maximum adsorption capacities, Xm (mmol·g-1 of carbon) obtained from the Langmuir isotherms (Table 4) renders the MCA values of Table 4. The values range between 175 m2 and 585 m2, i.e., they largely exceed the meso- and macropore surfaces (62 m2, Table 2). This fact indicates that diffusion of the adsorbate molecules to micropores with pore dimension (rpm ) 8.3 Å, Table 2) larger than the smallest molecular dimension (Table 4) occurs. This behavior has also been reported for the adsorption of compounds 1 and 2.12 The adsorption from solution on AC has been described by several models which take into account factors such as heterogeneity of adsorbent surface, solubility of the solute, the relative solvent/solute sizes, and the affinity of both of them for the AC surface.30-32 The last factor has been related, in the case of water as the solvent, to the influence of micropore filling by water on the adsorption of organic solutes.33,34 As commented in the Experimental Section, the desorption of these compounds has also been studied. In all cases, the desorption percentages of compounds 3-7 are very small, which indicate the highly irreversible character of the adsorption, although this is slightly dependent on the pH. The smallest average values of desorption percentages are found at the lowest pH ) 3.5 due to the low solubilities of the neutral forms of the adsorbates existing at this pH. This value is 1.8% for ligand 4. The highest desorption (30) Bansal, R. C.; Goyal, M. ActiVated Carbon Adsorption; CRC Press: Boca Rato´n, 2005; Chapter 3. (31) Radovic, L. R., Ed. Chemical and Physics of Carbon; M. Dekker: New York, 2000; Vol 27, pp 290-360. (32) Alcan˜iz-Monge, J.; Linares-Solano, A.; Rand, B. J. Phys. Chem. B 2001, 105, 7998-8006. (33) Brennan, J. K.; Bandosz, T. J.; Thomson, K. T.; Gubbins, K. E. Colloids Surf., A 2001, 187-188, 539-568. (34) Seredych, M.; Gierak, A. Colloids Surf., A 2004, 245, 61-67.
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Figure 4. Adsorption isotherms of compounds 3-7 at several pH values and at 298.1 K. Table 3. Binding Energy Valuesa (eV) for the O1s and N1s Core Level Spectra O1s compound sample
H2O
N1s carbon
COO- C6O + C5NO -COH + -O- CdO
AC 537.1 compound 3 533.0 532.1 AC-compound 3 533.0 AC-compound 3-Cu2+ 532.3 533.8
531.6 532.6 532.9
533.9
531.7
533.9 533.9
400.3 531.7 401.0 531.7 401.3
a The variation of the last digit is (2 units in all values (see Experimental Section).
percentages (around 10%) were obtained at pH ) 9.5 due to the higher solubilities of the monoanionic adsorbates. The irreversibility of these processes also support the contention that there are strong interactions between the pyrimidine planes of the compounds and the arene centers of the AC, which are consistent with the electron-donor behavior from the arene centers to the acid pyrimidine moieties (see XPS data discussed above). The possibility of π-π electron donation between the AC and the adsorbates is conditioned (besides other factors as their symmetries) by the energy differences between HOMO and LUMO orbitals of both of them. In accordance with the two-
state adsorption model,35 it has been pointed out that irreversible adsorption occurs for aromatic adsorbates if the energy differences between the HOMO and LUMO orbitals (EHOMO - ELUMO) are smaller than 7.34 eV.36 For this reason, an OO.MM. calculation has been carried (see Experimental Section) of modelized adsorbent surfaces and the adsorbates. Adsorbent surfaces were modelized by taking into account the oxygen functional groups existing in the AC. Then, the average values of the surface areas interacting with each of the compound molecules were calculated as the ratios between the surface area of the carbon and that covered by the compounds (Chart 3). The energies of the HOMO and LUMO orbitals of the adsorbates and the AC are summarized in Table 5. The very small energy differences between HOMOadsorbent and LUMOadsorbates found in all cases support the existence of π-π interactions between the arene centers of the AC acting as donors and the aromatic residues of the ligands which behave as acceptors. Such strong interactions also explain the observed shifting of the N1s signals of the aromatic nitrogen atoms in the XPS of AC-compound samples compared to those (35) Atkins, P. W. Physical Chemistry, 3rd ed.; Oxford University Press: Oxford, 1986. (36) Tamon, H.; Okazaki, M. J. Colloid Interface Sci. 1996, 179, 181-187.
Adsorption of Pyrimidine Ligands on ActiVated Carbon
Langmuir, Vol. 23, No. 11, 2007 6001
Chart 3. Cluster Models of the Adsorbent Surface12
Table 4. Molecular Dimensions (D)a, Calculated Molecular Areas (XZ)a, Adsorption Equilibrium Constants (KL, langmuir), Langmuir Maxima Adsorption Capacities (Xm), and Maxima Covered Areas (MCA) for the Ligands Xm (mmol g-1)
KL D (Å) compound 3 4 5 6 7 a
X 6.4 5.5 6.8 6.4 6.4
Y
Z
3.5 7.3 4.3 8.7 5.5 8.8 3.4 9.1 3.8 12.1
XZ (m2 molec-1) 10-19
4.7 × 4.8 × 10-19 6.0 × 10-19 5.8 × 10-19 7.8 × 10-19
pH
MCA (m2)
pH
pH
3.5
7.0
8.0
9.5
3.5
7.0
8.0
9.5
8(1) 9(2) 20(1) 6.8(8) 4(1)
6.2(7) 5.3(4) 20(2) 7(1) 6.0(8)
6.1(7) 6.0(7) 14(1) 5.1(9) 9(1)
4.6(8) 7.8(7) 14(2) 11.2(9) 5.0(5)
1.01(4) 0.91(4) 0.83(1) 1.00(4) 1.25(9)
0.86(4) 0.67(2) 0.70(2) 0.68(3) 0.66(3)
0.85(3) 0.65(2) 0.72(1) 0.75(4) 0.68(2)
0.84(6) 0.61(2) 0.68(3) 0.50(1) 0.81(3)
3.5
7.0
8.0
9.5
285(11) 241(11) 240(8) 238(17) 264(12) 194(6) 189(6) 176(6) 300(4) 253(7) 259(4) 243(11) 351(14) 239(10) 264(14) 175(3) 585(42) 311(14) 318(9) 381(14)
Theoretical values (see Experimental Section).
Table 5. HOMO and LUMO Energies of the Adsorbent Model and the Receptors (eV) character model
HOMO
LUMO
C28 C28O C28Lactone C28Quino 3 4 5 6 7
-10.95 -10.91 -10.95 -11.19 -11.53 -12.24 -12.15 -12.23 -12.18
-10.14 -10.39 -10.14 -10.70 -10.53 -10.62 -10.63 -10.59 -10.65
for compounds only (Figure 3). The same behavior has been reported in the study of the adsorption of compounds 1 and 2 on the same AC. The similar behavior of compounds 1 and 2 and that of compounds 3-7 of this work can be explained on the basis of the similar electronic features of their aromatic residue, in spite of the different substituents existing at C2. This is because none of these substituents are conjugated with their (similar) pyrimidine moieties. Complex Formation in Aqueous Solution. The studies of ligand-Cu(II) complex formation are needed to determine the pH conditions at which the basic functions of the receptors are operative to fit the experimental conditions in the adsorption studies. Moreover, they are very important, as the coordination
behavior of the free ligands is needed to explain the coordination capacities to Cu(II) when they are adsorbed on the AC. The complex speciation of ligand/Cu(II) mixtures in aqueous solution ([ligand]/[metal] ) 1/1) was determined by using a potentiometric method.22-24 The complex formation equilibria, the corresponding constants, and the species distribution for the five systems, obtained from the data of Table 6, are very similar. This suggests similar coordination behavior for the five compounds. Table 6 and Figure 5, respectively, show the data for compound 5 as an example. At low values of pH, the basic carboxylate groups of the amino acidic residues are protonated, which means that they cannot act as coordinating positions.37 Moreover, the basic atoms of the C6O and C5NO groups of the pyrimidine moieties are the only available metal-binding sites. This favors the formation of mononuclear complexes of the [Cu(H2L)]2+ type, at these low pH values. Nevertheless, these types of complexes are hardly detected by the potentiometric data in the pH range 2-6 due to the very low proton affinities of C6O and C5NO groups (see Table 1). In fact, only in the case of the Cu2+/ligand 3 system was the above complex detected potentiometrically. (37) Martell, A. E.; Smith, R. M.; Motekaitis, R. J. NIST Critically Selected Stability Constants of Metal Complexes Database, version 7; Texas A & M University, College Station, TX, 2003.
6002 Langmuir, Vol. 23, No. 11, 2007
Gutie´ rrez-Valero et al.
Figure 5. Species distribution of the compound 5/CuII system in aqueous solution as a function of pH ([5] ) [CuII] ) 10-3 M, 0.1 M KCl, 298.1 K).
Figure 7. Adsorption isotherms of Cu2+aq at 298.1 K and at several pH values: (a) on AC; (b) on AC-ligand 4. Figure 6. Visible spectra of the compound 4/CuII system at pH ) 2 and several [4]/[CuII] molar ratios ([4] ) 10-3 M, 0.1 M KCl).
Nevertheless, the [Cu(H2L)]2+, however, can be detected by UV measurements. As already pointed out, the formation of these complexes implies interaction of Cu(II) ions with C6O and/or C5NO pyrimidine groups. The binding of the metal ion to the nitrogen atom of NO blocks the σ nonbonding electron pair of this atom. A consequence of this is that the band at ca. 525 nm of the visible spectra of the ligands due to the σNO f π transition disappears.21 This can be seen in Figure 6 for the ligand 4-Cu(II) system, as an example. A similar change is observed for the other ligand-Cu(II) systems. Deprotonation of the carboxyl group of the amino acid in ligand/Cu(II) systems occurs at pH smaller than in the absence of the metal ion (ca. 2). Thus, the formation of mononuclear cationic complexes of type [Cu(HL)(H2L)]+ and/or [Cu(HL)]+ occurs at pH clearly less than 2 (see Figure 5). These type of
complexes contain the HL- ligands coordinated through the carboxylate group. The optimum pH for the occurrence of single carboxylate coordination of the ligands is around 4.5 in all of the ligand/Cu(II) systems. At this pH, the single species in solution is the [Cu(HL)]+ complex. Increasing amounts of Cu(HL)OH neutral complex and [CuOH]+ species appear in the media at pH values above 4.5. Nevertheless, the [Cu(HL)]+ concentration decreases up to pH near 7 at which the main species existing in the media are CuOH+ and Cu(OH)2. Adsorption of Cu(II) from CuCl2 Aqueous Solutions. The adsorption of Cu(II) from aqueous solution, on the AC and the AC-ligand (3-7) systems, was carried out to get insight into the effect on the adsorption capacity of the AC produced by the anchorage of the ligands. The adsorption isotherms were obtained at pH values of ca. 2.5, 4.5, and 6.0 for each of the adsorbents. These pH values were selected by taking into account the species distribution of the different ligand-Cu(II) systems (Figure 5), which exhibit a maximum at pH around 4 in all cases.
Table 6. Stability Constants of the Observed CuII-Ligand Complexes (log K) (0.1 M KCl, 298.1 K) log K reaction + H2L / [Cu(H2 Cu2+ +HL- + H2L / [Cu(HL)(H2L)]+ Cu2+ +HL- / [Cu(HL)]+ Cu2+ + HL- + OH- / Cu(HL)OH Cu2+ + OH- / [CuOH]+ Cu2+ + 2 OH- / Cu(OH)2 Cu2+
L)]2+
3
4
5
6.41 3.16 10.86 -8.46 -14.62
6.39 3.47 11.51 -8.48 -14.96
6
7
2.77 3.82 11.36 -9.15 -15.87
6.54 3.30 11.88 -14.59
5.62 3.18 12.08
Adsorption of Pyrimidine Ligands on ActiVated Carbon
Langmuir, Vol. 23, No. 11, 2007 6003
Table 7. Maximum Cu(II) Adsorption Capacities (mmol g-1) on AC and AC-Ligand and Langmuir Constants, KL (in brackets), Obtained by Fitting the Experimental Data to Langmuir Equation adsorbent pH
AC-ligand 3
2.5
0.082(5) [11(2)] 0.16(1) [28(2)] 0.141(9) [29(7)]
4.5 6.0
AC-ligand 4 0.093(3) [3.4(2)] 0.123(2) [373(42)] 0.114(5) [160(44)]
AC-ligand 5
AC-ligand 6
AC-ligand 7
AC
0.04(1) [1.4(5)] 0.15(1) [13(2)] 0.108(3) [73(5)]
0.123(7) [147(14)] 0.16(1) [36(6)] 0.15(2) [11(3)]
0.024(6) [11(5)] 0.09(2) [6(2)] 0.10(2) [6(2)]
0.09(2) [3(1)] 0.14(2) [129(25)] 0.124(3) [144(22)]
The adsorption isotherms of Cu(II) on the AC are collected in Figure 7. The fit of the data to the Langmuir equation rendered Xm values of 0.024, 0.09, and 0.10 mmol/g, at pH 2.5, 4.5, and 6.0, respectively. These reflect an increase in metal adsorption as the pH rises, which is due to a decrease in competition between proton and metal species by the surface active sites. With the low proton and Cu(II) affinities by the oxygen-containing groups of the carbon,37,38 it is expected that most of the Cu(II) is adsorbed via π-d interactions between the π orbitals of the arene centers (the active sites for H+ in this AC)38 and the d orbitals of the metal. Moreover, among the oxygen functional groups in this carbon, a certain pH-dependent affinity of Cu(II) for carboxyl functions can only be expected in the pH range studied. Nevertheless, the number of carboxyl groups of this AC is negligible,12 although they can appear as a consequence of the lactone hydrolysis39 due to the high equilibrium time (3 days) of the adsorption isotherms. Even so, the total amount of carboxyl groups is negligible compared to the Xm values for Cu(II) adsorption. For the adsorption of Cu(II) on the AC-ligands, the adsorbents were prepared by adsorption of the ligands on the AC, at pH ) 7 and at the highest concentration of each ligand corresponding to complete irreversible adsorption (see Experimental Section). Thus, the five AC-ligand adsorbents used have different amounts of ligand per gram of AC. For this reason, the comparison of their maximum adsorption capacities has no physical meaning. All the equilibrium solutions of the adsorption runs were analyzed by UV and visible spectroscopy to check that the ligands were not desorbed during the experiments. Figure 7 shows the isotherms at three different pH values of the adsorption of Cu(II) on the AC-ligand 4 system, as an example. The experimental data fit to Langmuir-type functions, with R2 factor values ranging between 0.970 and 0.992, in all cases. This allows one to calculate the maximum metal adsorption capacities of AC and AC-ligand adsorbates. The results are collected in Table 7. It is evident from these data that the preadsorption of the ligands on the AC clearly increases the adsorption capacity for Cu(II) by a factor which depends on the pH. The N1s bands in the XPS spectra of AC-ligand-Cu(II) solid samples are similar to those of the corresponding nonmetallated adsorbents as is seen in Figure 3 for the AC-ligand 3-Cu(II) sample. The same is observed for O1s. Moreover, the signal due to C6O and C5NO oxygen atoms, at 532.9 eV, remains unaffected in relation to the AC-ligand 3 sample. Nevertheless, the COOsignal is shifted 0.8 eV at higher energy values, suggesting a (38) Biniak, S.; Pakula, M.; Szymansky, G. S.; Swiatkowski, A. Langmuir 1999, 15, 6117-6122. (39) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley-Interscience: New York, 1992; pp 378-381.
Cu(II)-COO- interaction. This means that the improvement in the adsorption capacity for Cu(II) on the AC-ligand adsorbents compared to the AC is due to the presence of the ligands on the carbon surface. The trends in the maximum adsorption capacities of the ACligand adsorbents with the pH are consistent with the corresponding species distribution plots data. This can be seen in the ligand 5/Cu(II) system (Figure 5). The lowest complexation capacity at pH 2.5 is due to minimal carboxyl deprotonation (ca. 10% for ligand 5, see Figure 2). Thus, this also supports the contention that the other a priori available functions of the ligand (C5NO-C6O) for Cu(II) complexation are blocked due to the plane-to-plane ligand anchorage. For this reason, the metal ion is not allowed to coordinate to these functions. The maximum adsorption capacity is observed at pH 4.5 due to the availability of all the carboxylate groups for complexation of Cu(II). A small decrease in the Cu(II) adsorption capacity is observed at pH 6.0 compared to pH 4.5, which is probably due to the competitive formation of CuOH+ soluble species.
Conclusions Compounds 3-7 exhibit analogous molecular topologies than previously studied compounds 1 and 2. The pyrimidine moieties of all of them also exhibit similar electronic features (properties). These are derived from similar distribution of the π electronic cloud in all of them. This is due to the aliphatic nature of the separator moieties S which does not allow conjugation with the pyrimidine residue, Ar. It has been shown that there is a markedly irreversible adsorption of the ligands on the arene centers of the AC. This is caused by the complementary soft Le¨wis acid behavior of the pyrimidine moiety and the soft basic nature of the arene centers of the AC. Through this adsorption mechanism, the anchorage of compounds 3-7 provides a route to develop COO- functionalities on the carbon surface. In addition, the adsorption of compounds 4, 5, and 7 also introduces -CH2-S-CH2-, -CH2OH, and amide functions, respectively. The functionalized ACs exhibit enhanced Cu(II) adsorption capacities in relation to the unfunctionalized AC, due to the existence of COO- species in the former. The trends in the adsorption capacities with pH are directly related to the relative carboxylate complexing ability observed in the aqueous solutions of the different ligand/Cu(II) systems. These results open a new route to obtain a more thermodynamically efficient functionalization of the carbon surface through the design of new ligands of this class. Acknowledgment. The authors thank the Spanish Ministerio de Educacio´n y Ciencia for financial support (Proyecto PPQ 2000/1667). LA0626959
