Adsorption of Acid Orange 7 on the Surface of Titanium Dioxide

Aug 30, 2005 - Department of Chemistry, University of Patras, GR-26504 Patras, Greece. Received June 1, 2005. In Final Form: July 11, 2005. The adsorp...
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Adsorption of Acid Orange 7 on the Surface of Titanium Dioxide† Kyriakos Bourikas,‡ Maria Stylidi,§ Dimitris I. Kondarides,*,§ and Xenophon E. Verykios§ Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece, and Department of Chemistry, University of Patras, GR-26504 Patras, Greece Received June 1, 2005. In Final Form: July 11, 2005 The adsorption of a model textile azo-dye, Acid Orange 7 (AO7), on the surface of titanium dioxide was extensively investigated in aqueous TiO2 suspensions over wide ranges of AO7 concentrations (1 × 10-4-3 × 10-3 M) and pH values (2-10). Results obtained with the use of a variety of techniques, including potentiometric titrations, adsorption isotherms, adsorption edges, and microelectrophoresis, were used for the description of the “AO7 solution/TiO2 surface” interface. This has been achieved by taking into account the effects of pH on the speciation of the dye in solution and on the nature and population of the surface groups of TiO2. Results could be modeled very well with the use of the recently introduced CD-MUSIC approach. According to the model employed, the TiO2 surface is not considered homogeneous but is characterized by the presence of different types of surface groups, namely singly (TiOH-1/3), doubly (Ti2O-2/3), and triply (Ti3O0) coordinated. Surface complexes are not treated as point charges, but their charge is spatially distributed in the interfacial region. It has been found that adsorption of AO7 on the TiO2 surface occurs to a significant extent only at pH values lower than 7, via the sulfonic group of the azo-dye, through the formation of a bidentate innersphere surface complex. The determination of the adsorption mode of TiO2, which is supported by ex situ FTIR results, as well as of the adsorption constant, Kads, allowed the description of the pH dependency of the AO7 adsorption over large pH and AO7 concentration ranges.

Introduction The photocatalytic oxidation of organic pollutants under aerobic conditions in the presence of semiconductors, such as TiO2, has been the subject of numerous investigations in the last three decades as an emerging, environmentally friendly, and cost-effective method for the treatment of contaminated groundwater and wastewater.1-7 The process is based on the high oxidation potential of active radicals such as OH• and O2-• which are generated over irradiated semiconductor particles. The organic molecules present on the reaction system may react with these oxizing agents to form radicals and other intermediate species, which may be further degraded to eventually yield carbon dioxide and inorganic ions.1-7 Kinetic investigations have shown that the degradation rate depends on parameters related to the incident light energy and intensity, the reactor geometry, the oxidant concentration, as well as the photocatalyst-organic molecule interaction, including the nature and concentration of the organic pollutant and the photocatalyst, the solution pH, and the reaction temperature.7-10 The latter set of parameters affects the extent of adsorption of the organic molecule on the photocatalyst surface through its †

In memory of Maria that she was lost so unexpectedly. * To whom correspondence should be addressed. Tel.: +30-2610991527. Fax: +30-2610-997 853. E-mail: [email protected]. ‡ Department of Chemistry. § Department of Chemical Engineering. (1) Serpone, N., Pelizzetti, E., Eds.; Photocatalysis: Fundamentals and Applications; Willey Interscience: New York, 1989. (2) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol. 1991, 25, 1522. (3) Ollis, D. F., Al-Ekabi, H., Eds.; Photocatalytic Purification and Treatment of Water and Air; Elsevier Science Publishers: Amsterdam, 1993. (4) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. (5) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (6) Mills, A.; Hunte, S. L. J. Photochem. Photobiol. A: Chem. 1997, 108, 1. (7) Herrmann, J.-M. Catal. Today 1999, 53, 115.

equilibrium adsorption constant, Kads. The dependence of degradation rate on Kads reflects the fact that the reaction takes place on the catalyst surface. The investigation of the adsorption processes of organic pollutants on the photocatalyst surface and its dependence on operational parameters is of great importance in elucidating the mechanism of photocatalytic reactions and in formulating appropriate kinetic expressions. Results can also improve the understanding of the interfacial phenomena which take place in photocatalytic reactions. For example, it is of special interest to know whether an adsorbate is adsorbed by a specific chemical interaction, e.g., if it is coordinated innerspherically with surface sites (chemisorbed), or whether it is less specifically adsorbed by electrostatic or hydrophobic interactions with the surface.11 Photocatalytic degradation rates and reaction pathways are expected to be strongly dependent on the specific molecular structure and the nature of the chemical bond between surface and adsorbate. However, only a few investigations have been reported which are focused on the effect of the adsorption to the overall photocatalytic performance.8,12-15 In our previous studies,16-19 we have investigated the photocatalytic degradation of a model azo-dye (Acid Orange 7) in aqueous TiO2 suspensions, with the use of a solar light simulating source. It has been found that complete decolorization and substantial reduction of the chemical oxygen demand (COD) of the solution can be (8) Zhang, F.; Zhao, J.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N. Appl. Catal. B 1998, 15, 147. (9) Sauer, T.; Neto, G. C.; Jose´, H. J.; Moreira, R. F. P. M. J. Photochem. Photobiol. A 2002, 149, 147. (10) Stone, A. T.; Torrents, A.; Smolen, J.; Vasudevan, D.; Hadley, J. Environ. Sci. Technol. 1993, 7, 895. (11) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (12) Chen, H. Y.; Zahraa, O.; Bouchy, M.; Thomas, F.; Bottero, J. Y. J. Photochem. Photobiol. A 1995, 85, 179. (13) Cunningham, J.; Sedlak, P. Catal. Today 1996, 29, 309. (14) Xu, Y.; Langford, C. H. Langmuir 2001, 17, 897. (15) Chun, H.; Yizhong, W.; Hongxiao, T. Appl. Catal. B 2001, 35, 95.

10.1021/la051434g CCC: $30.25 © 2005 American Chemical Society Published on Web 08/30/2005

Adsorption of Acid Orange 7

achieved with satisfactory rates with the use of optimal operational parameters.16 The photocatalytic reaction pathways have been examined in detail and a TiO2mediated photodegradation mechanism has been proposed on the basis of quantitative and qualitative determination of intermediate compounds.17-19 The effect of incident light energy (UV/vis versus visible) on the apparent decolorization rate of AO7 solutions has been also examined in order to quantify the extent to which the visible lightinduced photosensitization mechanism contributes to the overall photocatalytic degradation of azo dyes by solar light radiation.19 The objective of the present work is to investigate in detail the mechanism of the AO7 interaction with TiO2 photocatalyst in order to provide further insight into the photocatalytic oxidation of organic pollutants. This is achieved by studying the speciation of the dye in solution and the surface chemistry of TiO2 as functions of pH. The interaction of these two components is used to explain results of adsorption experiments, which include adsorption edges and adsorption isotherms. Modeling of the adsorption data provides a clear picture of the kind of the interaction between AO7 and the TiO2 surface. The aim is to identify the mode of adsorption of AO7 on TiO2 and to determine its surface concentration as a function of pH and dye concentration in solution. These findings will then be used to develop an appropriate kinetic model and to determine kinetic constants by fitting results of AO7 decolorization rate obtained under variable experimental conditions. Modeling of the adsorption data is achieved with the use of the recently introduced CD-MUSIC (charge distribution-multisite complexation) model,20 which emphasizes the importance of the structure of both the photocatalyst surface and the adsorbed species. Surface complexes are not treated as point charges but are considered to have a spatial distribution of charge in the interfacial region. The model has been applied in several oxy-anion systems20-24 as well as in systems related to adsorption of organic matter.25,26 In most cases, its predictions are consistent with physically realistic surface complexes found by spectroscopy. Experimental Section Materials. Titanium dioxide used as a photocatalyst was kindly supplied by Degussa Co. (P-25) and was used as received. According to the manufacturer, P-25 is mainly anatase, with a specific surface area of 50 m2‚g-1 and a mean particle size of 30 nm. It is the most commonly used titania in catalysis and colloid chemistry and its patch wise structure of the surface is mainly anatase, having a small amount of rutile (less than 10%).27,28 Acid Orange 7 was purchased from Fluka Chemical Co. and was (16) Kiriakidou, F.; Kondarides, D. I.; Verykios, X. E. Catal. Today 1999, 54, 119. (17) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Appl. Catal. B 2003, 40, 271. (18) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Int. J. Photoenergy 2003, 5, 59. (19) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Appl. Catal. B 2004, 47, 189. (20) Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 179, 488. (21) Venema, P.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 183, 515. (22) Venema, P.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 192, 94. (23) Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1999, 210, 182. (24) Rietra, P. J. J.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1999, 218, 511. (25) Geelhoed, J. S.; Hiemstra, T.; Van Riemsdijk, W. H. Environ. Sci. Technol. 1998, 32, 2119. (26) Filius, J. D.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1997, 195, 368.

Langmuir, Vol. 21, No. 20, 2005 9223 used as received. It is a nonbiodegradable synthetic azo-dye, with a molecular formula of C16H11N2O4SNa, widely used in the textile industry. Deionized water was produced by a Milli-Q system and used systematically. Equilibrium Adsorption Experiments. Adsorption isotherms of AO7 on TiO2 were obtained at room temperature and constant pH (2, 5.7, or 10) by determining the amount of the dye adsorbed on the surface of titanium dioxide as a function of its concentration in the solution, at equilibrium. For this, a fixed weight of TiO2 (200 mg) was added in a series of 100 mL aliquots of aqueous solutions of AO7 of variable concentration, in the range of 1 × 10-4-3 × 10-3 M. The suspensions were left overnight (12 h) in the dark under continuous stirring in thermostated vessels. The vessels were equipped with a pH control system involving a glass/saturated calomel electrode (Metrhom) and a dosimeter. This pH control system allowed for the automatic adjustment of the pH during the adsorption process by adding to the vessels aqueous solutions (0.1M) of either NaOH or HNO3. Nitrogen was bubbled into the vessel during the adsorption to prevent dilution of atmospheric CO2, which would bring about a change in the pH. Then, the samples were filtered using a 0.45 µm filter and the concentration of the dye in the filtrate was determined by measuring the absorbance at 485 nm. The amount of the dye molecules adsorbed on TiO2, Nads (mol/ g), was calculated from the following equation:

Nads )

V∆C w

(1)

where ∆C ) C0 - Ceq (mol/L) is the decrease in the molarity of AO7 in solution after reaching equilibrium, with C0 and Ceq being the initial and equilibrium concentration of AO7, respectively, V (L) the volume of the aqueous solution and w (g) the mass of the photocatalyst. Similar experiments were conducted to determine the effect of pH of the solution on the amount of dye adsorbed on the photocatalyst surface at constant initial dye concentrations (50, 100, or 300 mg/L). For this purpose, the adsorbed amount of the azo-dye was obtained by mixing 70 mL of aqueous solution of AO7 with 750 mg/L of TiO2. The pH of the solution was adjusted (using either NaOH or HNO3) to the desired value (between 1.5 and 8) before addition of TiO2. The suspension was left in the dark under continuous stirring for about 1 h. The values of pH at equilibrium of the suspension were measured and the extent of adsorption of the azo-dye on TiO2 was evaluated in terms of color removal (absorbance at 485 nm) after centrifugation and filtration of the aliquots, following the procedure described above. Potentiometric Titration of an AO7 Aqueous Solution. Potentiometric titration of an aqueous solution of AO7 was carried out as follows: A known amount of AO7 (30 mg) was added to 100 mL of deionized water, and the solution was purged with N2 to remove dissolved CO2. The pH of the solution was adjusted to a value of ≈11 using a solution of NaOH (0.1 M). Then, the titration was carried out by adding HNO3 (0.1 M) with an automatic microburet (Radiometer Copenhagen ABU901 Autoburet) at a constant temperature of 25 °C. The pH value was recorded after each addition of the acidic solution as a function of its volume. Potentiometric Titrations of TiO2 Suspensions. The potentiometric titrations were performed under N2 atmosphere and constant temperature (25.0 ( 0.1 °C) for three suspensions containing the same amount of the immersed oxide but with different ionic strengths (0.001, 0.01, or 0.1 M NaNO3). Each suspension was equilibrated for 20 h in order to reach an equilibrium pH value. A small amount of a base, (NaOH; 1M) was then added to deprotonate a significant part of the surface sites rendering the surface negative and then the suspension was titrated by adding small volumes of an HNO3 0.1 M aqueous solution. The pH value was recorded after each addition of the acidic solution as a function of its volume. A similar titrating procedure was followed for the blank solution. Details concerning (27) Spanos, N.; Georgiadou, I.; Lycourghiotis A. J. Colloid Interface Sci. 1995, 172, 374. (28) Contescu, C.; Popa, V. T.; Schwarz, J. A. J. Colloid Interface Sci. 1996, 180, 149.

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the setup and the procedure used as well as the method for the determination of the surface charge, σο, can be found elsewhere.29 Microelectrophoresis. The ζ potential of TiO2 and AO7/ TiO2 suspensions containing 300 mg AO7 L-1 were determined using a Zetasizer 5000 (Malvern Instruments Ltd) microelectrophoresis apparatus, at a constant temperature of 25 °C. Sufficiently dilute suspensions of the oxide were prepared, with a constant ionic strength of 0.01 M. The pH of the suspensions was adjusted by the addition of small quantities of 0.1 M HNO3 or NaOH solutions. FTIR Spectroscopy. FTIR spectra were obtained employing a Nicolet 740 spectrometer equipped with a TGS detector and a KBr beam splitter.30 The sample used to investigate the mode of adsorption of AO7 on the photocatalyst surface was prepared as follows: the TiO2 sample (750 mg/L) was equillibrated with a 300 mg/L solution of AO7, filtered, dried in air at 70 °C, and then stored in the dark to prevent degradation of the adsorbed compounds. The sample, in finely powdered form, was placed into a sample holder and its spectrum was obtained employing a DRIFT cell (Spectra Tech) with a 32-scan data acquisition at a resolution of 4 cm-1. Titanium dioxide powder, subjected to a similar pretreatment (suspended in water, filtered, dried, etc.) was used as a background reference. The spectrum of the AO7 powder was obtained without any pretreatment. All spectra were obtained at room temperature, with the powders being exposed to the atmosphere.

Model Description Multisite Complexation Model. Metal oxides are characterized by the presence of different types of surface groups. The multisite complexation (MUSIC) model takes into account this surface heterogeneity. Variation in types of surface oxygens is due to the different number of coordinating metal ions. The charge on these surface oxygen species can be found by applying the Pauling Bond Valence concept,31 in which the charge of the central metal ion is distributed over the coordinating oxygens. The structure of titanium oxide consists of Ti4+ filled oxygen octahedra. The oxygens in the bulk of the solid are triply coordinated (Ti3O0), receiving from each Ti4+ a bond valence of 2/3 (each Ti4+ ion distributes its charge over six surrounding oxygens). The surface oxygens may be singly, doubly, and triply coordinated, depending on the number of the coordinating Ti4+ ions. Their coordination is lower than those of the bulk because of some missing bonds. The binding of one or two protons can compensate the missing charge. In the case of singly coordinated groups, three species can be defined: TiO-4/3, TiOH-1/3 and TiOH+2/3 . It has been shown that the constants of the two 2 consecutive protonation steps differ very strongly.32-34 For the singly coordinated Ti groups, this implies that only the second protonation step is of relevance for the charging behavior. On the basis of the above concepts, the charging behavior of the titania surface can be described by the following protonation reactions of singly and doubly coordinated groups: KH1

TiOH-1/3 + H+ 798 TiOH2+2/3 KH2

Ti2O-2/3 + H+ 798 Ti2OH+1/3

(2) (3)

Protonation of the uncharged triply coordinated groups, Ti3O0, is not possible in the normal pH range. Therefore, these groups are considered inert in normal conditions. (29) Lycourghiotis, A. Acidity and Basicity of Solids; Fraissard, J., Petrakis, L., Eds.; NATO ASI Series, Kluwer Academic Publishers: Dordrecht, 1994; Series C: Volume 444, pp 415-444. (30) Chafik, T.; Kondarides, D. I.; Verykios, X. E. J. Catal. 2000, 190, 446. (31) Pauling, L. J. Am. Chem. Soc. 1929, 51, 1010.

A detailed description of the MUSIC model can be found elsewhere.32-34 CD-MUSIC Approach. An extension of the abovementioned bond valence concept to surface complexation forms the basis of the CD-MUSIC model.20 The charge of surface complexes is distributed over two electrostatic planes. A schematic representation of this idea is given in Figure 1. The charge of the surface groups is located at the surface plane (0 plane). Adsorbed ions are distinguished in innersphere and outersphere complexes. The innersphere complexes are assumed to have a spatial distribution of the charge. Part of their charge is attributed to the surface (since not all ligands of the adsorbed complex are common with the surface groups) and the remaining of the part is attributed to the first plane (1-plane), at a certain distance from the surface. It should be mentioned here that, as has been suggested,35-37 only the singly coordinated surface groups are reactive for innersphere complex formation of the anions. The outersphere complexes of electrolyte ions are placed at a position determined by the minimum distance of approach of hydrated ions to the oxide surface. They form ion pairs with the surface groups without having common ligands with them. They are located in the outer plane (see Figure 1) and treated as point charges in the CD model, like the counterions of the diffuse part of the double layer (DDL). In the case of absence of surface complexation, the model becomes identical with the basic Stern layer model, and the compact part of the double layer is characterized by the well-known Stern capacitance (C). Innersphere surface complexes divide the Stern layer in two parts, the inner layer with C1 capacitance and the outer one with C2 capacitance. These are related to the overall Stern layer capacitance according to the following:

1 1 1 ) + C C1 C2

(4)

The above-presented electrostatic approach is known as the three plane (TP) model.20,38 Model Calculations. Calculations were carried out with ECOSAT, a computer code for the calculation of chemical equilibria.39 The solution and the surface equilibria used are presented in the Appendix. Results and Discussion AO7 Solution Chemistry. The solution chemistry of AO7 can be described by the following equilibria: K1

[HL-] [L2-][H+]

(5)

K2

[H2L] [HL-][H+]

(6)

L2- + H+ 798 HL-, K1 ) HL- + H+ 798 H2L, K2 )

According to the above equilibria, three forms of the azo(32) Hiemstra, T.; Van Riemsdijk, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989, 133, 91. (33) Hiemstra, T.; De Wit, J. C. M.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (34) Hiemstra, T.; Venema, P.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 184, 680. (35) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A. Geochim. Cosmochim. Acta 1993, 57, 2251. (36) Russell, J. D.; Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature 1974, 248, 220. (37) Torrent, J.; Barro´n, V.; Schwertmann, U. Soil Sci. Soc. Am. J. 1990, 54, 1007. (38) Venema, P.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 181, 45.

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Figure 1. Schematic representation of the metal oxide/solution interface according to the TP model. Surface groups are coordinated with metal ions of the solid phase and the corresponding charge is located in an electrostatic plane (0-plane). The surface groups may form innersphere complexes with adsorbed ions. The surface-oriented ligands of innersphere complexes are also located in the surface plane (0-plane). A bidentate surface complex has two common ligands, a monodentate surface complex one. The solution directed ligands of the innersphere complexes are located in the inner plane of the compact part of the double layer (1-plane). The charge of the central ion of the innersphere complexes is distributed over both electrostatic planes. Pair forming ions are treated as point charges and placed in the outer plane (2-plane). The space between a set of electrostatic planes is characterized by a capacitance.

dye are present in its aqueous solution: the H2L, the deprotonated HL-, and the doubly deprotonated L2-. Their relative concentration is mainly determined by the pH of the solution. The values of the protonation constants K1 and K2 were calculated by fitting the experimental titration data (obtained at 25 °C), using equilibria (5) and (6) (Figure 2). The calculated values are the following: log K1 ) 10.6 and log K2 ) 1, in good agreement with values reported in the literature.40,41 It must be mentioned that the value of K2 is not so accurate since the titration data are not particularly sensitive for the logK2 value. In the calculations we used activity coefficients estimated with the Davies equation (constant ) 0.2). The speciation of AO7, determined with the use of the calculated values for the equilibrium constants, is presented in Figure 3 as a function of the pH of the solution. It is observed that in the pH range of 2-9 only the deprotonated form, HL-, of the dye is practicaclly present in the solution. At higher pH values, the relative concentration of the doubly deprotonated form, L2-, increases (39) Keizer, M. G.; Van Riemsdijk, W. H. ECOSAT; technical report of the Department of Soil Science and Plant Nutrition; Wageningen University: Wageningen, The Netherlands, 1998. (40) Herrera, F.; Lopez, A.; Mascolo, G.; Albers, P.; Kiwi, J. Appl. Catal. B 2001, 29, 147. (41) Bandara, J.; Mielczarski, J. A.; Kiwi, J. Langmuir 1999, 15, 7670.

Figure 2. Titration of an AO7 solution with HNO3 at 25 °C. Solid line corresponds to the calculated curve based on equilibrium equations (5) and (6).

with increasing pH, and this species becomes the predominant one above pH 12 (Figure 3). Determination of Primary Charging Parameters. Modeling of the adsorption data requires first that the charging behavior of titania is known. By modeling surface titration data, important parameters of the double layer properties can be determined, like the overall capacitance and the ion pair formation constants.

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Figure 3. Speciation of AO7 in aqueous solution at 25 °C as a function of pH.

The charging mechanism of titanium oxide is described by the protonation reactions of singly, TiOH-1/3, and doubly, Ti2O-2/3, coordinated groups (eqs 2 and 3). As already mentioned, triply coordinated surface groups, Ti3O0, are inert in the normal pH range and therefore are not expected to influence the titration curves. The log KH values of the protonation reactions 2 and 3 have been estimated recently based on the MUSIC model and the bond valence principle.34 The difference in these values has been found relatively small. Because of the uncertainty, we simplified protonation processes by assuming equal values for the log KH of reactions 2 and 3. For this case, the log KH values are equal to the PZC (point of zero charge) value of the oxide,20,34 if symmetrical ion pair formation is assumed. The formation of ion pairs can be described by the following reactions: KNa+

TiOH-1/3 + Na+ 798 TiOH-1/3 - Na+ KNO

-

TiOH2+2/3 + NO3- 798 TiOH2+2/3 - NO33

KNa+

Ti2O-2/3 + Na+ 798 Ti2O-2/3 - Na+ KNO

(7) (8) (9)

-

Ti2OH+1/3 + NO3- 798 Ti2OH+1/3 - NO3- (10) 3

According to these reactions, the Na+ and NO3- ions (counterions) are retained electrostatically on the oxide surface forming ion pairs with the negative and positive surface sites, respectively. Their role is the partial neutralization of the surface charge. In the present study, we use asymmetrical ion pair formation due to a small difference between PZC (6.5) and IEP (isoelectric point) (6.8).42 This leads to the adoption of a value of 6.6 for the protonation constants log KH1 and log KH2, slightly higher than the PZC. For the calculation of the surface charge, a value for the site density (Ns) is also required. Calculated Ns values for the different crystal faces of both anatase and rutile can be found in ref 31. The 010 and 011 crystal phases are often assumed to be the predominant faces of anatase.43 However, the 001 face can be also found.44 Since the picture is not so clear and the variation in site densities for (42) Bourikas, K.; Hiemstra, T.; Van Riemsdijk, W. H. Langmuir 2001, 17, 749. (43) Kostov, I. Mineralogy; Oliver and Boyd Ltd: Edinburgh, 1968. (44) Parfitt, G. D. In Progress in Surface and Membrane Science; Cadenhead, D. A., Danielli, J. F., Eds.; Academic Press: New York, 1976; Vol. 11, p 181.

Figure 4. Charging behavior of titanium dioxide (Degussa P-25) at three electrolyte (NaNO3) concentrations. Data points correspond to experimental data and lines correspond to the calculated curves, using the Basic Stern model. Parameter values are given in the text.

Figure 5. Variation of the concentration of the various surface hydroxyls of titanium oxide with the solution pH.

different crystal faces is not large (between 5.2 and 7.0 nm-2 for each type of surface groups), we used an intermediate value of 5.6 for both singly and doubly coordinated sites per nm2. These values have been determined recently in a similar study concerning the adsorption of molybdates on the surface of titania.45 A good description of the titration data was obtained (Figure 4) by using a value of 0.9 F m-2 for the overall Stern capacitance and asymmetric ion pair formation, with log KNa+ ) -0.5 and log KNO3- ) -1.3. It should be mentioned that these values are in excellent agreement with corresponding mean values calculated after fitting a large number of titration data of titanium oxide found in the literature.42 Based on the above-calculated values of the charging parameters, the variation of the concentration of the various surface groups of titanium oxide with the pH can be calculated. This variation is presented in Figure 5. It is observed that, as the pH decreases, the concentration of the protonated form of both singly and doubly coordinated surface groups, TiOH2+2/3 and Ti2OH+1/3, increases at the expense of that of the nonprotonated forms, TiOH-1/3 and Ti2O-2/3, causing the development of a positive potential on the titanium oxide surface. The opposite occurs when the pH increases. Determination of Adsorption Parameters. The charging behavior of titania, as determined in the previous paragraph, is expected to influence strongly its adsorption (45) Bourikas, K.; Hiemstra, T.; Van Riemsdijk, W. H. J. Phys. Chem. B 2001, 105, 2393.

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Figure 8. Proposed adsorption mode of AO7 on the surface of TiO2. Figure 6. Adsorption of AO7 on titanium oxide as a function of pH, at three different initial dye concentrations (50, 100, or 300 mg/L; TiO2 concentration: 750 mg/L; room temperature). Data points represent experimental data and solid lines correspond to calculated curves.

Figure 7. Adsorption of AO7 on titanium oxide as a function of the equilibrium concentration, at three different pH values. Data points represent experimental data and solid lines correspond to calculated curves.

capacity for the AO7 ions. A highly positive surface charge at low pH values enhances the adsorption capacity for anions, like HL-. Therefore, the pH can be considered as the most important parameter influencing the adsorption isotherm. The correct description of the pH dependency of adsorption can give direct information about the structure of the adsorbed species. The dependence of AO7 adsorption on the solution pH was investigated by performing adsorption edge measurements and adsorption isotherms. The results are summarized in Figures 6 and 7, respectively (data points). Figure 6 represents the amount of the azo-dye adsorbed (in µmol/g) versus the equilibrium pH of the suspension. The adsorption edges were obtained at three different initial concentrations of AO7 (50, 100, or 300 mg/L). It is observed that the amount of adsorbed dye on the TiO2 surface decreases with increasing solution pH and becomes negligible for pH values higher than 7. The adsorption isotherms of AO7 on TiO2 at three different pH values (pH 2, 5.7 or 10) are presented in Figure 7, in which the amount of the dye adsorbed on the surface of TiO2 is plotted as a function of its concentration in solution, at equilibrium. It is again observed that the extent of adsorption of the azo-dye on the TiO2 surface decreases rapidly with increasing pH and becomes negligibly small at pH 10. It is also observed that the amount of the dye adsorbed increases with increasing equilibrium concentration and reaches a plateu. All isotherms showed a type of L-shape according to the classification of Giles et al.46 The L-shape isotherms indicate that there is no

Table 1. Surface Parameters (TP Model) Used for the Description of the Adsorption Data parameter

value

parameter

value

m-2)

1.1 5.0 6.6 -0.5

log KA NS (nm-2) log Kads

-1.3 5.6 25.7

C1 (F C2 (F m-2) log KH log KC

strong competition between the solvent and the dye to occupy the TiO2 surface sites.47 Moreover, this type of isotherm implies localized, Langmuir-type adsorption of the AO7 species in the innerplane developed in the region of the “aqueous solution/titania surface” interface with too weak, if any, lateral interactions.48,49 The CD-MUSIC approach with the three-plane model option was applied to calculate the corresponding curves. The value of the outer capacitance C2 was fixed at 5 F m-2, based on previous reported results from related studies.20-26 The value for C1 (1.1 F m-2) was calculated using eq 4, based on the overall capacitance C of 0.9 F m-2, found from the modeling of the titration curves. The adsorption data were analyzed assuming various types of surface complexes. However, a very good fitting was achieved (see solid lines in Figures 6 and 7) only in the case where a bidentate innersphere surface complex had been assumed. Specifically, the adsorption reaction is defined as Kads

2TiOH2+2/3 + HL- 798 Ti2HL1/3 + 2H2O

(11)

According to the above adsorption-reaction, the ions HLof the AO7 are adsorbed on the surface of titanium oxide via reaction with two protonated singly coordinated groups, TiOH2+2/3, forming bidentate innersphere complexes. A schematic representation of this complex is given in Figure 8. It should be mentioned that the charge of the adsorbed complex is equally distributed between the titania surface and the first plane of the double layer. The calculated values for the adsorption constant as well as for the other surface parameters used in the description of the adsorption data are compiled in Table 1. The lines in Figures 6 and 7 were calculated using this parameter set. The finding that the AO7 upon its adsorption on the titania surface forms innersphere complexes is further corroborated by the electrokinetic results of Figure 9, (46) Giles, C. H.; Silva, A. P. D.; Easton, I. A. J. Colloid Interface Sci. 1974, 47, 766. (47) Muruganandham, M.; Swaminathan, M. Solar Energy Mater. Solar Cells 2004, 81, 439. (48) Spanos, N.; Vordonis, L.; Kordulis, Ch.; Lycourghiotis, A. J. Catal. 1990, 124, 301. (49) Giles, C. H.; Smith, D.; Huitson, A. J. Colloid Interface Sci. 1974, 47, 755.

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Figure 9. Variation of zeta potential with pH for (a) a TiO2 suspension and (b) an AO7/TiO2 suspension (AO7 concentration: 300 mg/L; ionic strength: 0.01 M; room temperature). Data points represent experimental data and solid lines correspond to calculated curves assuming a distance of 1 nm between the shear plane and the head end of the DDL. Table 2. Maximum Coverage (%) of the Singly Coordinated Surface Groups of Titanium Oxide by AO7 pH

% coverage

pH

% coverage

2 3 4 5 6

86 72 52 31 14

7 8 9 10

4 1 0.2 0.02

where the zeta-potential of the TiO2 particles is plotted as a function of pH in the presence and absence of AO7. It is observed that the adsorption of the dye causes a strong shift of the zeta potential toward lower values over the entire pH range. It is worth noting that the shift of the iep is equal to about 2 pH units. This precludes a weak interaction of AO7 with the surface of titanium oxide and supports the fact that AO7 is adsorbed strongly on it forming innersphere surface complexes. At this point, it should be mentioned that a quantitative description of zeta potentials is accompanied by uncertainties. An important reason is the theoretical complexity of the transformation of experimental electromobility data to zeta potentials, for nonideal particle suspensions. The IEP can be considered as the most reliable experimental result. The zeta potential is considered equivalent with the potential at the head end of the diffuse double layer. Adopting this assumption, the trend of a decrease in the zeta potential in the presence of negatively charged adsorbed species as well as the shift in the IEP can be predicted well as can be seen in Figure 9. However, the absolute zeta potential values could not be accurately predicted. The experimental values are quite lower compared to the calculated ones. This has been already mentioned in the literature.42 Lower potentials can be calculated if the shear plane is apparently located at a certain distance from the head end of the DDL.42,50 Following this empirical approach, we described fairly well the electrokinetic data assuming a distance of 1 nm between the shear plane and the head end of the DDL (see Figure 9). This is in accordance with similar results found for several (hydr)oxides.20,42,50 Having determined the adsorption of AO7 on the surface of titanium oxide as well as the adsorption constant Kads, the pH dependency of the AO7 adsorption over a large pH (50) Hiemstra, T.; Yong, H.; Van Riemsdijk, W. H. Langmuir 1999, 15, 5942.

Figure 10. Ex situ FTIR spectra obtained from AO7 powder and from AO7 adsorbed on TiO2.

and initial AO7 concentration range can be described. Moreover, an estimation of the surface coverage of the titanium oxide surface by the ions of AO7 can be made. The maximum coverage of the singly coordinated surface groups is given in Table 2. It is observed that as pH decreases the maximum coverage that can be achieved at each pH value increases. This is expectable in view of the results of Figure 5 and the adsorption reaction 11. According to the later, the AO7 is adsorbed on the positively charged groups, TiOH2+2/3, the concentration of which increases as the pH decreases, at the expense of the corresponding negatively charged groups, TiOH-1/3 (Figure 5). Thus, at low pH values more positive groups are available for the AO7 adsorption and the maximum coverage of the total singly coordinated surface groups increases (Table 2). At this point, it should be mentioned that in addition to pH, which strongly influences adsorption, the coverage of the surface is significantly affected by the potential of the interfacial region. Thus, the accumulation of the negatively charged ions of the AO7 in this region causes the development of a negative charge, which inhibits further adsorption of the ions of the dye. Moreover, results presented in Table 2 show that at pH values close to 2 the adsorption of AO7 on the surface of titanium oxide reaches its maximum value. This is because almost all of the available singly coordinated surface groups have been covered by the ions of the dye. This means that the interaction with the singly coordinated titania surface sites reaches its physical maximum slightly below pH 2. The results presented above are in accordance with the results of spectroscopic studies performed to determine the structure of the adsorbed AO7 species on TiO2. The FTIR spectra of AO7 powder and of AO7 adsorbed on TiO2 are presented in Figure 10. Several bands can be distinguished in the region of 2000-1000 cm-1, a detailed assignment of which has been reported in a previous study.17 Briefly, the intense band at 1508 (or 1514) cm-1 is attributed to the azo-bond of the dye, the bands at 1599 and 1452 cm-1 are characteristic of phenyl ring vibrations, the band at 1618 (or 1624) cm-1 is due to a combination of phenyl ring vibrations with stretching of the CdN group, the band at 1555 (or 1558) cm-1 is related to the NHNdC group, the bands at 1256 and 1223 cm-1 are assigned to v(C-N) and v(N-N) stretching vibrations, respectively, and the bands at 1130 and 1047 cm-1 are due to the coupling between the benzene mode and vs(SO3)17 (and references there in). It should be noted that the absence of bands at 1198 cm-1 [vs(SO3)] and 1304 cm-1 [vas(SO3)] provides strong evidence that the AO7 molecule adsorbs on the titania surface via the two oxygen atoms of the

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Table 3. Surface Speciation for the Adsorption of AO7 on Titanium Oxide Surface dissolved component surface component electrostatic component H+

surface species TiOH-1/3 Ti2O-2/3 TiOH2+2/3 Ti2OH+1/3 TiOH-1/3 - Na+ Ti2O-2/3 - Na+ TiOH2+2/3 - NO3Ti2OH+1/3 - NO3Ti2HL1/3 sum ∑1 ) H+(t)-OH-(t) ∑2 ) Na+(t) ∑3 ) NO3-(t) ∑4 ) L2-(t) ∑5 ) FANS,1 ∑6 ) FANS,2 ∑7 ) FA/F (σ0 - ∑zjFNS,j) ∑8 ) FA/F σ1 ∑9 ) FA/F σ2 σ0 ) C1(Ψ0 - Ψ1) σ1 ) C2(Ψ1 - Ψ2) - C1(Ψ0 - Ψ1) 1 σ2 ) C2(Ψ2 - Ψ1) ( x80000rRT 2

x∑

0 0 1 1 0 0 1 1 3 ∑1 (A-1) (A-2) (A-3) (A-4) (A-5) (A-6) (A-7) (A-8) (A-9) (A-7a) (A-8a)

Na+ NO3- L2- TiOH-1/3 Ti2O-2/3 e-FΨ0/RT e-FΨ1/RT e-FΨ2/RT 0 0 0 0 1 1 0 0 0 ∑2

0 0 0 0 0 0 1 1 0 ∑3

0 0 0 0 0 0 0 0 1 ∑4

1 0 1 0 1 0 1 0 2 ∑5

0 1 0 1 0 1 0 1 0 ∑6

0 0 1 1 0 0 1 1 1.5 ∑7

0 0 0 0 0 0 0 0 -0.5 ∑8

0 0 0 0 1 1 -1 -1 0 ∑9

log K 0 0 log KH log KH log KNa+ log KNa+ log KNO3- + log KH log KNO3- + log KH log Kads

Ci(e-ziFΨ2/RT - 1) (A-9a)

i

sulfonate group of the dye (Figure 8). On the other hand, Bauer et al.51 proposed that AO7 is adsorbed on the surface of TiO2 via the above two oxygen atoms of the sulfonate group and the oxygen of the hydrogen form of AO7. They suggested that the CdO group interacts with the titania surface giving a new band, which, however, appears at the unusually low value of 1280 cm-1 and seems doubtful in view of the present results. Our modeling results presented above are in very good agreement with our spectroscopic observations as well as with the results of Bandara et al.,41 who proposed the same structure for the adsorbed AO7. Specifically, they concluded that azo dye adsorption on iron, titanium, and aluminum oxides occurs via the sulfonic group of the dye, forming a bidentate complex with the surface oxygens. However, in this work, we succeeded for the first time in this system to relate spectroscopic information with the description of macroscopic adsorption data using the CDMUSIC model, which incorporates physically realistic surface complexes. Results of the present study allow for the determination of the adsorption constant of AO7 on TiO2, Kads, over wide pH and initial AO7 concentration ranges and, consequently, determination of the amount of the adsorbed dye at any set of experimental conditions. This information could prove to be very useful in many applications such as, for example, in the development of detailed kinetic models describing the dependence of the photocatalytic degradation rate of AO7 on solution pH and CAO7. However, care should be taken when doing this since, for some photocatalyzed reactions, adsorption/desorption equilibria are not established under irradiation of TiO2, because the substantial reactivity of adsorbed active species (electrons, holes, hydroxyl radicals, etc.) causes a continuous displacement from equilibrium of the adsorbed reactant concentration.52 Conclusions The present study shows that the adsorption of AO7 on titanium dioxide surface can be described very well by the (51) Bauer, C.; Jacques, P.; Kalt, A. Chem. Phys. Lett. 1999, 307, 397. (52) Ollis, D. F. J. Phys. Chem. B 2005, 109, 2439.

CD-MUSIC model, over wide pH and AO7 concentration ranges. In this model, the titanium dioxide surface is not considered homogeneous but is characterized by the presence of different types of surface groups, namely singly (TiOH-1/3), doubly (Ti2O-2/3), and triply (Ti3O0) coordinated. Surface complexes are not treated as point charges but their charge is spatially distributed in the interfacial region. Moreover, the adsorption of AO7 on the TiO2 surface occurs via the sulfonic group of the azo-dye through the formation of a bidentate innersphere surface complex. The determination of the adsorption reaction of AO7 on the surface of titanium dioxide as well as of the adsorption constant, Kads, allows the description of the pH dependency of the AO7 adsorption over a large pH and initial AO7 concentration range. The adsorption of AO7 occurs to a significant extent only at pH values lower than 7 and reaches its maximum at pH close to 2. Appendix In Table 3, the formation of each surface species is defined in terms of components (columns), being components in solution, surface components (surface groups), and electrostatic components (exp(-FΨi/RT)), with i ) 0, 1, and 2, standing for the corresponding planes. The concentration S (mol l-1) of a surface species can be calculated reading the table horizontally and using the following general expression:

[Ck]n ∏ k

[S] ) 10logK

k

(A)

where Ck is the component’s concentration (mol l-1) and nk the coefficient given in the table. The expression of logK is given in the last column of Table 3 as a function of various log K values whose meaning and value can be found in the text (see Table 1). All log K values are based on intrinsic constants, adjusted for activity corrections in the case of I * 0. The activity coefficients were estimated with the Davies equation (constant ) 0.2). The calculation of the coefficients for the electrostatic components has also been described in the text (see CD-MUSIC approach). The parameters in the summation terms in Table 3 are F the solid solution ratio (kg l-1), A the specific surface area (m2 kg-1), F the Faraday constant (C mol-1), σ0, σ1

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and σ2 the charge (C m-2) in respectively the 0-, 1-, and 2-plane, zj the charge of the surface reference groups TiOH-1/3 and Ti2O-2/3, NS,j the site densities (mol m-2) of the corresponding surface groups, Ψ0, Ψ1, and Ψ2 the electrostatic potential (V) of respectively the 0-, 1-, and 2-plane, C1 the capacitance (C V-1 m-2) of the layer between the 0- and 1-plane, C2 the capacitance (C V-1 m-2) of the layer between the 1- and 2-plane, 0 the absolute dielectric constant (C V-1 m-2), r the relative dielectric constant, R the gas constant (J mol-1 K-1), T the absolute temper-

Bourikas et al.

ature (K), and Ci the concentrations of the dissolved electrolyte solution species with valence zi. Values of the above-mentioned parameters can be found in the text (Table 1). Acknowledgment. We thank European Social Fund (ESF), Operational Program for Educational and Vocational Training II (EPEAEK II), and particularly the program PYTHAGORAS, for partially funding this work. LA051434G