ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2009, Vol. 83, No. 7, pp. 1208–1211. © Pleiades Publishing, Ltd., 2009. Original Russian Text © T.I. Tikhomirova, S.S. Kubyshev, A.V. Ivanov, P.N. Nesterenko, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 7, pp. 1360–1364.
PHYSICAL CHEMISTRY OF SURFACE PHENOMENA
Sorbent Based on Aluminum Oxide Modified with Sodium Sulfonate T. I. Tikhomirova, S. S. Kubyshev, A. V. Ivanov, and P. N. Nesterenko Faculty of Chemistry, Moscow State University, Moscow, Russia e-mail:
[email protected] Received June 23, 2008
Abstract—The adsorption of Tiron (sodium sulfonate) on aluminum oxide surface was studied, and optimum conditions for the modification of aluminum oxide surface were determined. The sorption of copper(II) on aluminum oxide modified with Tiron was studied. It was shown that the sorption of copper occurs by the mechanism of complex formation with the modifier (Tiron). The composition of the surface complex and its stability constant were determined: Cu : Tiron = 1 : 1, log β = 14.0 ± 0.4. DOI: 10.1134/S0036024409070280
INTRODUCTION Aluminum oxide is an available, mechanically strong, and chemically and hydrolytically stable porous material with a large specific surface area, which can be used as a support for the preparation of new sorbents. Extensive use of aluminum oxide is limited by the complexity of modifying its surface with various organic compounds. With triethoxysilanes and trichlorosilanes as modifiers, molecules of organic compounds are grafted to the surface by several Si–O–Al bonds. The hydrolytic stability of these bonds is not high; therefore, this method of modification is usually used to fix hydrophobic and aminopropyl groups [1]. Considerable attention has recently been focused on the chemisorption modification of aluminum oxide surface by various organic compounds. For different longchain modifiers, it was found [2] that the strength of coverage increased in the series carboxylic acids < alkylphosphonic acids < hydroxamic acids. It is evident that the stability of coverage is proportional to the conventional stability constant of the aluminum complex with the corresponding ligand. Pyrocatechol derivatives are strong complexing agents with respect to aluminum. It is our opinion that various derivatives of pyrocatechol containing 1,2-dihydroxyphenyl anchoring groups can be used for modifying the surface of Al2O3. Many of such compounds, including Tiron, 3,4-dihydroxyphenylalanine, dopamine, pyrogallol, pyrocatechol violet, and pyrogallol red, form stable complexes with Al3+ ions, which favors stable modification of support surfaces. On the other hand, the presence of other functional groups in the modifiers enhances the applicability of these sorbents to sorption concentration and chromatography of metal ions. This work deals with the
modification of aluminum oxide surface with Tiron (4,5-dihydroxy-m-benzenedisulfonic acid disodium salt). EXPERIMENTAL Reagents and solutions. In surface modification, solutions of Tiron (disodium 4, 5-dihydroxy-m-benzenedisulfonate) of ch. d. a. (pure for analysis) grade from Reakhim were used. The sorption of copper was studied using a 1.0 mg/l standard solution of Cu2+ (GSO Ekoanalitika), a 0.1% picramine-epsilon (ch. d. a. grade) solution, and 0.1 and 1 M solutions of HCl and NaOH prepared from standard solutions (Germed, Germany). Instruments. The optical density of solutions was measured on a KFK-2 photoelectric colorimeter and an SF-46 spectrophotometer (LOMO, St. Petersburg); test tubes were shaken on a Sky Line mechanical vibratory mixer (ELMI, Riga). The acidity of solutions was measured by a pH-121 universal millivoltmeter with a glass electrode and a silver-chloride reference electrode. The diffuse reflectance spectra of sorbent samples were recorded on a Spektroton colorimeter (OKBA Khimavtomatika, Chirchik). The specific surface area, average diameter of pores, and pore volume were determined by the nitrogen adsorption method using an ASAP2400 porosimeter (Micromeritics Instrument Corp., USA). Sorbent preparation. γ–Aluminum oxide (Merck, Germany) was used as a support; particle size: 28% of particles were smaller than 63 µm, and more than 72% of particles were larger than 63 µm. Aluminum oxide was sieved through a sieve with 60 µm meshes. The specific surface area measured by the BET method was found to be 138 m2/g, and the average diameter of pores, 8 nm. In order to remove dust and aluminate impurities, the selected fraction was subjected to sedimentation from a 0.01 M HNO3 solution. The sedimen-
1208
SORBENT BASED ON ALUMINUM OXIDE MODIFIED WITH SODIUM SULFONATE
tation time was 1 h. Sedimentation was performed several times with washing the sorbent with distilled water to transparency and neutral reaction of the supernatant. The sorbent was filtered on a glass filter, washed with distilled water to neutral reaction, and dried in air.
log D 3
2
Procedure of sorption measurements in the static regime. Weighed samples of the sorbent (in the sorption of Tiron, weighed samples of the support) were placed in 15-ml graduated test tubes, and a standard solution of the sorbate was added. The desired acidity was obtained using HCl or NaOH. The volume was brought to 10 ml with distilled water. Test tubes were shaken on the vibratory mixer until equilibrium was established. The sorbate concentration in the aqueous phase was determined by spectrophotometry. The sorbate content in the sorbent phase was determined as the difference between the initial and equilibrium concentrations. The Tiron concentration was determined using a calibration curve constructed for a 10–3 M Tiron standard solution at 290 nm. Copper concentration was determined photometrically using the reaction with picramine-epsilon [3].
2
0.4
0.2
0
0.001
OH
0.005 caq, M
Fig. 2. Sorption isotherms of Tiron on alumina from aqueous solutions (msorb = 0.1 g) at pH (1) 2 and (2) 6.3.
can therefore adsorb Tiron as a result of electrostatic interactions [5], HO HO O S O– O
O
O S – O O
H H H H H H H H O+ O+ O+ O+ + Al O Al O Al O Al+ .
OH
Al O Al+ O Al
Sorption can also occur as a result of the specific interaction of hydroxyl surface groups with the aromatic ring of Tiron. For example, it is known [4] that chemically modified silica sorbents contain residual silanol groups, which can form hydrogen bonds with aromatic rings and phenol groups [4]. Tiron with two sulfo groups in its molecule is a fairly strong acid and is present as anions even in acid media. In acid and neutral solutions, alumina surface contains a significant amount of positively charged Brönsted acidic sites and RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
0.003
....
HO
2
....
HO
10 pH
1
....
O–
8
csorb, mmol/g
O– S
6
....
O O S O
4
Fig. 1. The dependence of the sorption of Tiron on medium acidity (msorb = 0.1 g, cT = 10–4 M).
RESULTS AND DISCUSSION Aluminum oxide surface has sites of three types, Lewis, Brönsted acidic, and Brönsted basic sites [1], and can therefore interact with different compounds by different mechanisms. The sorption of Tiron on aluminum oxide surface can occur on Lewis acidic sites by the complex formation mechanism with the formation of a cation exchanger,
1209
There remain two free phenol groups on sorbent surface, which can form complexes with transition metal ions (see above). In the sorption of Tiron on alumina, the time of the establishment of sorption equilibrium is no longer than 10 min. Over the pH range 4–8, the sorption value is virtually constant; it slightly increases in more acidic media (Fig. 1). In acid solutions, alumina can dissolve, and, at higher pH values (>9), it can produce aluminates. Moreover, in the alkaline region, the reagent can be oxidized. Over the pH range of measurements, Tiron is present as the H2R2– doubly charged anion; the K1
Vol. 83
No. 7
2009
1210
TIKHOMIROVA et al.
log D
2.5
1.5
2
3
4
pH
Fig. 3. pH dependence of the sorption of copper(II) (msorb = 0.1 g, cCu = 1.5 × 10–4 M).
csorb, µmol/g 1
500
300 2
3
4
100 0
0.4
0.8
1.2
1.6 caq, µM
Fig. 4. Sorption isotherms of copper on various sorbents (msorb = 0.1 g, pH 6.0): (1) sorbent modified with Tiron (0.5 mmol/g) in acid media, (2) aluminum oxide, (3) sorbent modified with Tiron (0.145 mmol/g) at pH 8, and (4) sorbent modified with Tiron (0.27 mmol/g) at pH8.
and K2 values of phenol groups are 7.62 and 12.50, respectively [6]. Sorption isotherms of Tiron on aluminum oxide at different pH values are presented in Fig. 2. The isotherm obtained at pH 6.3 exhibits two saturation steps. It is likely that the first isotherm region corresponds to the formation of a monolayer. For the first plateau, the area occupied by one molecule (nm2) can be calculated by the formula 21
S sp × 10 S i = ----------------------, at N A where Ssp is the specific surface area of the support (138 m2/g), at is the specific sorption value for the
close-packed monomolecular adsorption layer (for the first plateau of the isotherm, 0.27 mmol/g), and NA is the Avogadro constant. Calculations yield 0.86 nm2, which suggests that the Tiron molecule is aligned parallel to the sorbent surface, because, for example, the area occupied by the phenol molecule is 0.4 nm2 [7]. The isotherm obtained at pH 2 has quite a different shape; this isotherm is described by the Langmuir equation. The maximum capacity is 0.5 mmol/g, which is 1.5 times larger than the maximum capacity at pH 6.3. These data lead us to suggest that, at pH 6 (in the region of the first plateau), sorption predominantly occurs because of the specific interaction of aluminum oxide surface with the aromatic ring of Tiron. At pH 2, sorption occurs thanks to the electrostatic interaction of Tiron sulfo groups with acidic surface sites (see above). The Tiron molecule is then likely perpendicular to the sorbent surface. It is evident that the Tiron concentration on the sorbent surface must be higher than that in the case of molecules aligned parallel to the surface. The presence of the second saturation step in the isotherm at pH 6 can be explained by a change in the orientation of Tiron molecules adsorbed on the surface caused by a different character of their binding with the surface. Under the conditions of our experiments (pH 2–6), the sorption of Tiron on a Lewis acidic site with the formation of a complex with aluminum is unlikely; otherwise, the sorption of the regent would decrease as the acidity of solutions increases, because the complex of aluminum with Tiron is formed at pH 4–8 [8]. The sorption of copper(II) on alumina oxide modified with Tiron. In the case of the most probable mechanism of sorption of Tiron (pH 2–6), its complex forming groups remain free and can be used for the sorption of metal ions, which form stable complexes with Tiron. We selected copper(II), which is noted for simple behavior in solutions, as such a metal. A study of the sorption of copper(II) on alumina modified with Tiron must provide a deeper insight into the mechanism of sorption of modifying reagents. In the case of Tiron binding with alumina through phenol groups, only sulfo groups remain free, and the sorption of metal cations from solution can occur by only the ion exchange mechanism. In the sorption of Tiron by electrostatic interaction of its sulfo groups with Brönsted acid sites of alumina surface or specific interaction of the aromatic ring with sorbent surface, hydroxyl groups remain free. The sorption isotherms of copper(II) (Fig. 4) were obtained on sorbents containing different amounts of the modifier; type I sorbents were obtained at pH 6, and type II sorbents, at pH 2. The sorption isotherms on type I sorbents are described by the Langmuir equation and have the following features: as the organic modifier concentration grows, the Henry distribution coefficient increases from 4.0 × 102 for aluminum oxide to 2.7 × 103 for the sorbent with the highest modifier concentra-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 83
No. 7
2009
SORBENT BASED ON ALUMINUM OXIDE MODIFIED WITH SODIUM SULFONATE
tion. Sorption is, however, lower than the maximum sorption capacity in the plateau region. It is likely that, on alumina, the sorption of copper occurs by the ion exchange mechanism and, on modified sorbents, by the complex formation mechanism. The high capacity of initial alumina can be explained by a higher concentration of hydroxyl groups compared with complex forming groups of modified sorbents. The sorption isotherm of copper(II) on type II sorbents is also described by the Langmuir equation; however, the largest capacity of the sorbent is then significantly higher than that for type I sorbents, which can be explained by a higher modifier concentration on sorbent surface. In the sorption of copper, modified sorbents turn yellow–green, and aluminum oxide, blue, as is typical of copper ions. A comparison between the diffuse reflectance spectra of Tiron-modified sorbents treated with solutions of copper(II) and the absorption spectra of an aqueous solution of the copper(II) complex with Tiron showed that all the spectra have a maximum in the region of 430 nm, which is characteristic of the copper complex with Tiron [9]. The conclusion can be drawn that the sorption of copper(II) on the sorbent modified with Tiron occurs by the complex formation mechanism according to the reaction 2+
Cu aq + nH 2 R s
+
[Cu(R)n] + 2n H aq ,
where H2R is the doubly charged Tiron anion form. The equilibrium constant of the reaction is written as + 2n
[ CuR n ] [ H ] -n K = --------------------------------2+ [ Cu ] [ H 2 R ] or, in the logarithmic form, +
log K = log D + 2n log [ H ] + n log [ H 2 R ] , and, accordingly, log D = 2npH – log K' . From the slope of the log D = f(pH) dependence, the component ratio of the complex was obtained, n = 1. The stability constant of the complex formed on sorbent surface
[ CuR ] s β = ------------------------------. 2+ [ Cu ] aq [ R ] s was estimated. The [CuR]s and [Cu2+]aq values were determined experimentally, and [R]s was calculated from the total concentration of the ligand and the K1 and K2 values for Tiron. The result obtained, log β = 14.0 ± 0.4, is in close agreement with the data [6] on the complex in aqueous solutions ( log β = 14.25). To summarize, over the solution pH range studied, the adsorption of Tiron on alumina surface occurs predominantly as a result of electrostatic interactions. Because of the high stability constant of the copper complex with Tiron, the sorbent can be used for sorption concentration of copper at low concentrations followed by copper determination in both the sorbent phase (for example, by X-ray fluorescence analysis) and in the concentrate after desorption. REFERENCES 1. G. V. Lisichkin, A. Yu. Fadeev, A. A. Serdan, et al., Chemistry of Grafted Surface Compounds (Fizmatlit, Moscow, 2003) [in Russian]. 2. J. P. Folkers, C. B. Gorman, P. E. Labinis, et al., Langmuir 11, 813 (1995). 3. V. N. Podchainova and L. N. Simonova, Analytic Chemistry of Elements: Copper (Nauka, Moscow, 1990) [in Russian]. 4. O. A. Filippov, T. I. Tikhomirova, G. I. Tsizin, and Yu. A. Zolotov, Zh. Anal. Khim. 58, 454 (2003) [J. Anal. Chem. 58, 398 (2003)]. 5. C. Pagnoux, J. Ceram. Process. Res. 3 (1), 10 (2002). 6. A. E. Martell and R. M. Smith, “NIST Critically Selected Stability Constants of Metal Complexes,” NIST Standard Reference Database, 46, v. 8.0 (2004). 7. A. M. Koganovskii, N. A. Klimenko, T. M. Levchenko, and I. G. Roda, Adsorption of Organic Substances from Water (Khimiya, Leningrad, 1990) [in Russian]. 8. T. Kiss, I. Sovago, and R. B. Martin, J. Am. Chem. Soc. 111, 3611 (1989). 9. A. K. Madjumdar and C. P. Savariar, Anal. Chim. Acta 21, 53 (1959).
SPELL: OK
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
1211
Vol. 83
No. 7
2009
