art%3A10.2478%2Fs11532-014-0568-5

1. IntroductionTitanium dioxide is widely applied in aqueous media, because it has excellent optical and electronic properties, low cost, chemical, photonic and thermal stability as well as relatively good reactivity. Thus, it has been considered as a promising photocatalytic material for water and air purification [1-3], conversion of solar energy to electrical via water splitting [1,2,4], adsorption of cations and anions from solutions [5-7], preparation of self-cleaning coatings [1,2], catalyst supports [8]. It is known that its adsorption and catalytic characteristics depend on phase composition, specific surface area and preparation method. At the same time, though TiO2 is extensively studied and widely used in different processes, the relations between structure and useful properties are still poorly understood. Particularly, electokinetic and

adsorption phenomena at the titania/solution interface are practically important properties. For example, surface charge determines not only ion exchange properties of TiO2, but adsorption and destruction of pollutants during photocatalytic processes, as a result [1,9,10].

Adsorption of ions from aqueous solutions, zeta-potential and surface charge were earlier studied only for anatase and rutile [6,11-21]. Particularly, titania samples with different ratio anatase/rutile, specific surface area and particle size were investigated [22-24]. It should be noted that the values of isoelectric point pHiep and point of zero charge pHpzc, summarized in [15], were measured for purified titanias, as a rule. However, commercial TiO2, which are used in real adsorption and photocatalytic processes, contain impurities. Thus, “chloride” and “sulfate” TiO2 can contain up to 0.8 and 2.0 mass.% of impurity anions, respectively [25,26]. In addition,

Central European Journal of Chemistry

Electokinetic and adsorption properties of different titanium dioxides

at the solid/solution interface

* E-mail: [email protected]

Received 10 September 2013; Accepted 12 January 2014

Abstract:

© Versita Sp. z o.o.Keywords: Mono- and biphase TiO2 • Electrical double layer • Surface charge density • Zeta potential • Adsorption of Zn (II) ions

1Institute for Sorption and Problems of Endoecology of NAS of Ukraine, 03164 Kyiv, Ukraine

2Faculty of Chemistry, Maria Curie Skłodowska University, 20-031 Lublin, Poland

Svitlana Khalameida1*, Ewa Skwarek2, Wladyslaw Janusz2, Volodymyr Sydorchuk1, Roman Leboda2, Jadwiga Skubiszewska-Zięba2

Research Article

Six samples of titanium dioxide of different phase compositions and specific surface areas have been characterized by XRD, Raman- and FTIR spectroscopy, adsorption of nitrogen, electrophoresis. Adsorption of Zn(II) ions at the TiO2/NaCl aqueous solution interface as well as the effect of adsorption on the structure of electrical double layer have been studied. The influence of ionic strength, pH and presence of ions on the adsorption of Zn(II) ions at the TiO2/NaCl solution interface have also been investigated. The zeta potential, surface charge density, parameters of adsorption edge pH50% and ∆pH10-90% for different concentrations of basic electrolyte have been determined. Studied unpurified samples showed lower values of isoelectric point pHiep compared with literature data due to the presence of anion impurities. The antibate dependence between pHiep values and particle size has been established. Adsorption of Zn(II) ions using monophase samples is completed at a lower pH than for the biphase TiO2. Appearance of the point CR3 is associated with the charge turnover from positive to negative at high values of pH and formation of Zn(OH)2.

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Cent. Eur. J. Chem. • 12(11) • 2014 • 1194-1205DOI: 10.2478/s11532-014-0568-5

S. Khalameida et al.

carbonate-ions are polluted titania during its preparation through CO2 adsorption from air. Therefore, it’s important to investigate more thoroughly the adsorption of ions on TiO2, which is related to its application in environmental and energy fields.

Electrical double layer (EDL) at the oxide/electrolyte solution interface plays an important role in determining ions distribution. The adsorption of cations can be described using simple models of the electrical double layer such as the constant capacitance model (CCM), or double layer model (DLM) or a more complex one as a triple layer model (TLM)

The following reactions are responsible for the adsorption of divalent cations at the oxide/electrolyte interface:

(1)

(2)

(3)

As evident in Eqs. 1, 2 and 3, the adsorption of cations releases hydrogen ions from hydroxyl groups, and so the increase of pH in the system will favor the adsorption of cations at the metal oxide/electrolyte interface. For characterization of cations adsorption on anatase, the authors [7,20] used the concept of “edge of adsorption” that can be described by such parameters like pH50% (the value of pH when 50% of the initial concentration of cations adsorbs, this parameter characterizes the position of adsorption edge on the pH scale ) and ΔpH10-90% (the range of pH where the adsorption changes from 10% to 90%, it characterizes the slope of the edge) [27].

It should be added that except for anatase and rutile, TiO2 still has another modification, namely brookite. It

can exist as an amorphous substance or a combination of several modifications [25]. Zeta-potential, surface charge, and adsorption properties of the latter phases have not been previously studied.

Therefore, the investigations of electrokinetic and adsorption properties of unpurified titanium dioxide which possess different crystal and surface structures, dispersity, and corresponding relationships are the main focus of this paper.

2. Experimental procedure

2.1. ReagentsSix powder-like samples of titania were used which possess different phase compositions: X-ray amorphous,

pure anatase, brookite, and rutile, as well as the mixture of anatase with brookite and rutile. According to manufacturer data, fumed TiO2, analog of P25, contains 80% of anatase and 20% of rutile, and other mixed titania consists of 70% anatase and 30% brookite. The methods of their preparation are presented in Table 1. Samples 2-6 were prepared by the sulfate or chloride methods and contain impurities since all titania samples and their precursor (metatitanic acid TiO(OH)2) were used without additional purification.

2.2. Physical-chemical measurementsThe specific surface area S and other porous structure parameters were calculated from the isotherms of nitrogen adsorption-desorption obtained using an analyzer ASAP 2405N (“Micromeritics Instrument Corp”). The temperature and duration of samples outgassing before measurements were 150˚C and 2h, respectively. A crystal structure of the samples was confirmed by XRD with the aid of the DRON-3 diffractometer (LOMO, Russia) using CuKα radiation and a nickel filter. The FTIR spectra in the range 4000-1400 cm-1 were registered by means of a Perkin–Elmer spectrometer “Spectrum – One” (powder-like mixture with KBr, the mass ratio sample/KBr=1:20 was used). The Raman spectra were recorded using a spectrograph of Renishaw system (Ar laser, 514 nm).

2.3. Electrokinetic measurementsThe ζ potential of the titanium dioxide was determined by means of electrophoresis using a Zetasizer 3000 Standard (Malvern). The concentration of titania in the electrolyte solution was 100 ppm. The suspension of TiO2 was ultrasonificated before measurements. Isoelectric point pHiep was determined from the dependences ζ potential – pH obtained in water and solutions of NaCl as well as in the presence of Zn(II) ions with different concentrations.

The surface charge density σ at the titania oxide/NaCl solution interface was determined by potentiometric titration of the TiO2 suspension. It was carried out in the thermostatic Teflon vessel, in a nitrogen atmosphere free of CO2, at 25ºC.The measurements were performed using a PHM 240 Radiometer Research pH meter with K401 as a glass electrode and G202B as the calomel reference electrode. Potentiometric titration was carried out with the use of the automatic burette Dosimat (Metrohm). The point of zero charge pHpzc was calculated from σ – pH curves.

2.4. AdsorptionThe adsorption of Zn (II) was measured using the radiotracer technique. Zn(II) was chosen as a typical

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Electokinetic and adsorption properties of different titanium dioxides at the solid/solution interface

heavy metal in the environment which was studied comprehensively for anatase [20,21,24]. Particularly, zinc is widely distributed in the Earth’s crust as well as in wastewater as a pollutant. It is also one of the most important trace elements in the human organism. However, few papers have been published in literature which describe Zn adsorption on different titanium oxides.

The specific adsorption of Zn(II) ions at the TiO2

interface was investigated as a function of Zn(II) ions concentration, NaCl concentration and pH. The initial concentration of Zn(II) ions ranged from 1×10-6 to 1×10-3 mol L-1, pH was changed from 3 to 10. Adsorption studies were carried out by means of a potentiometric titration set. The system was calibrated using the buffers of pH = 4, 7 and 10 produced by the Beckman firm. The pH change was made using the NaOH solution of 0.1 mol dm-3 concentration. Radioactivity of the solutions before and after the adsorption process was measured using a liquid scintillation counter LS5000 TD Beckman.

3. Results and discussion

3.1. Phase compositionThe X-ray diffractograms of the studied titania samples are presented in Fig. 1 and some of their structural characteristics are listed in Table 1. Amorphous specimen and all three pure crystal modifications of TiO2 (monophase samples) as well as two mixtures of anatase with brookite or rutile (biphase samples) are illustrated. According to Sherrer’s formula, the crystallite size Dhkl calculated from the XRD data equals 10-22 nm (Table 1, column 5). At the same time, the size of polycrystalline grains Ds determined using the specific surface area values is 17-80 nm (column 4). The particle size Ds of X-ray amorphous is equal to 7 nm. The comparison of the values of these parameters indicates the degree of primary crystallites aggregation. It can be seen that the discrepancy between the indicated parameters,

namely excess of Ds over Dhkl (taking into account errors of measurements and calculations) is not very large. The exceptions are brookite and fumed samples for which

Table 1. Preparation methods and characteristics of initial titanium dioxide samples.

N Preparation method and firm-producer Phase composition Ds[nm] D[nm]

1 2 3 4 5

1 TiO2 from metalorganic compounds,”Sibur-Chimprom”, Russia X-ray amorphous 7 -

2 Thermodestruction of TiO(OH)2 at 500˚С Anatase 20 10.5

3 Tioxide GB Specialities Ltd, Great Britain Anatase+ brookite 30 13.0

4 TiO2 fumed ”Oriana”, Ukraine Anatase + rutile 55 21.0

5 Thermodestruction of TiO(OH)2 at 700˚С Brookite 80 15.2

6 Thermolysis of aqueous solution of TiCl4 Rutile 23 22.4

Ds - Effective diameter of TiO2 particles calculated in accordance with the formulaDs=(6/ρS)×103 [nm], where ρ – density of titanium dioxide [g cm-3], S – specific surface area [m2 g-1]D - Crystallite diameter of titanium dioxide calculated in accordance with the Sherrer’s equation

Figure 1. X-ray diffractograms for different TiO2: anatase (a), brookite (b), rutile (c), anatase+rutile (d), anatase+brookite (e).

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the grain size is 3-5 times as large as the crystallite size. Consequently, the aggregation of primary crystallites is sufficiently large only for the latter sample.

The Raman spectra generally confirm the phase composition of the studied samples since each of the titania phases has characteristic absorption bands [28-30]. For example, the spectrum of sample 2 from Table 1, which is pure anatase according to XRD, contains only a.b. attributed to anatase, namely 144,

398, 517 and 643 cm-1 (Fig. 2c). Similarly, the peaks of rutile (239, 439 and 612 cm-1) are present on the spectrum for sample 6 (Fig. 2d). At the same time, the spectra of biphase samples 2 and 3 contain a.b. of anatase and brookite, anatase and rutile, respectively (Figs. 2a, 2b). The first has a.b. with the maxima at 153 and 632 cm-1 which are closer to brookite than anatase. The second contains a.b. at 145 cm-1 (anatase) along the peaks centered at 239, 449 and 612 cm-1 which are attributed to rutile. The bands assigned to anatase are revealed in the spectrum obtained for amorphous X-ray titania (sample 1) which is not presented. However, their intensity is much smaller than for the well crystallized anatase (Fig. 2c). It is apparent that sample 1 contains incipient fine-crystalline anatase which is not detected by the XRD-measurements.

3.2. Surface structureThe FTIR spectra in the range 4000-1400 cm-1 which characterize the structure of surface are shown in Fig. 3 (numbers of spectra correspond to the sample numbers in Table 1). All spectra contain the broad band between 3750 and 2000 cm-1 which is assigned to the fundamental stretching vibration of hydroxyl groups: free and/or bonded [31,32]. This a.b. consists of several components, as a rule [31,33-35]. However, since the spectra were recorded without prior thermo-evacuation of the samples, this broad band is split into components only for fumed titania (spectrum 4), namely at 3427, 3336 and 3257 cm-1. These absorption bands can be attributed to symmetric and antisymmetric νOH

modes of molecular water coordinated to Ti4+, weakly adsorbed water molecules, and H-bonded hydroxyl groups [31,32,34]. The value of absorption in this range (which may be a measure of the OH-groups content) for all samples, except for pure rutile, is approximately proportional to their specific surface area. At the same time, it is obvious that the OH-groups content also depends on the phase composition of titania. The spectra of X-ray amorphous titania and the samples containing anatase reveal a.b. around 1620-1630 cm-1, which [31,32,34,35], is attributed to the bending mode of adsorbed water. Besides, intensive a.b. centered at 1440-1450 cm-1 is present in the spectra of samples enriched with brookite and rutile. This band is usually assigned to the vibrations of adsorbed CO2 [34]. Therefore, these samples contain a large amount of carbonate impurities on the surface.

3.3. Porous structureAll titania samples are high-dispersed powders. They possess the values of specific surface area S, calculated according to the BET method, between 19 and

Figure 2. Raman spectra for different TiO2: anatase+brookite (a), anatase+rutile (b), anatase (c) rutile (d).

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242 m2 g-1. Besides, the particles, of which all powders consist, have different porosity. Thus, sorption pore volume Vs, calculated from the N2 adsorption-desorption isotherms at relative pressure p/p0 close to 1 (Fig. 4), is equal to 0.05-0.56 cm3 g-1. The porosity is mainly presented by mesopores. The most probable diameter of the mesopores dpsd determined from the curves of pore size distribution is equal to 3-28 nm (Table 2). The samples 4 and 5 (fumed TiO2 and pure brookite)

obviously contain macropores along with mesopores. Low values of S and Vs support this (Fig. 4). The form of capillary-condensation hysteresis loop indicates different shapes of pores in the titania samples. At the same time, the range of p/p0, in which the hysteresis loop is located, shows the degree of mesopores uniformity. Thus, it is evident that sample 3 (mixture of anatase and brookite) possesses the most homogeneous porous structure.

Figure 3. FTIR spectra for different TiO2: amorphous (1), anatase (2), anatase+brookite (3), anatase+rutile (4), brookite (5), rutile (6).

Table 2. Chosen structural parameters of the TiO2 samples.

Parameters of porous structureSample of TiO2

Anatase+rutile

Amorphous Anatase+brookite

Anatase Brookite Rutile

BET surface area S [m2 g-1] 28 242 55 90 19 66

Langmuir surface area[m2 g-1]

36 311 70 116 24 84

Sorption pore volume Vs [cm3 g-1] (0.05) 0.56 0.24 0.11 0.09 0.08

BJH cumulative desorption volume of pores between 1.7 and 300 nm diameter [cm3 g-1] or volume of mesopores

(0.06) 0.58 0.25 0.12 0.10 0.08

Average pore diameter - 4Vs/S [nm] 6.9 9.2 17.7 4.8 18.6 4.9

BJH adsorption on the average pore diameter (4Vme /S) [nm]

12.6 8.1 16.3 4.5 21.3 4.6

BJH desorption on the average pore diameter (4 Vme /S) [nm]

12.9 7.6 14.2 3.9 19.0 3.8

Diameter of mesopores from the PSD curve dPSD [nm]

- 6.4 28.0 3.7 24.3 3.7

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3.4. Electrokinetic propertiesAlthough the properties of the electrical double layer at the TiO2/electrolyte solution interface are frequently studied, there are controversies concerning the position of the point of zero charge pHpzc and the isoelectric point pHiep. The obtained results are mostly compatible for different forms in the pH range [15]. The same characteristics are presented in Table 3 for the TiO2 adsorbents presented in this paper.

It is known that pHiep for TiO2 depends on its phase composition [1,7,13,15,22]. All obtained values are lower than those for purified samples of titania presented in the literature [15], namely ≤ 5. Among them, brookite has the most acidic surface with the pHiep equaling 2.8. This means that the surface is negatively charged at pH > pHiep. The latter is important for practical use of unpurified TiO2. For example, photodegradation of some dyes as pollutants is performed at pH = 4-5 [36,37]. The increase of their adsorption by the titania surface can improve photocatalytic properties of TiO2 catalyst. The presence of anionic impurities is also the main reason for low pHiep quantities.

The particle size can also influence the pHiep as the authors of [22] have shown for anatase and rutile. Indeed, the value of pHiep for anatase specimens decreases when the particle size increases. A similar correlation was obtained for titania samples of a different crystal structure which were studied in this work. Thus, brookite possessing the maximal particles size, namely 80 nm, has minimal pHiep magnitude. Contrary, the maximal value of pHiep was obtained for X-ray amorphous TiO2. Tables 1 (column 4) and 3 (column 3) also show that only the sample composed of anatase and brookite does not fit into this relationship. The differences between the pHpzc and pHiep values for all samples may be also caused by anion impurities that influence the zeta potential measurements based on electrophoresis.

3.5. Adsorption of Zn(II)To evaluate the Zn(II) adsorption, it is necessary to take into account that zinc ions in the water solutions form the following complexes with the hydroxyl groups: ZnOH+, Zn (OH)2(aq), Zn(OH)3

- and Zn(OH)4-2 [38].The

analysis of the concentration of the Zn(II) species as a function of pH shows that up to pH 8.5 the dominant ions are Zn2+

(aq), whereas above pH 8.5 the hydrocomplexes Zn(OH)2(aq) predominate. The percentage of ZnOH+ ions is small, reaching a maximum which corresponds to a value of several percent at pH=8.5. Amorphous zinc hydroxide can appear in the studied system at higher concentrations of zinc ions [39].

It should be noted that adsorption of Zn(II) ions as a function of pH on all investigated samples of TiO2 runs similarly to other simple metal oxides, namely adsorption value “A” increases with the pH growth and becomes almost complete at pH=7-9 depending on phase composition of TiO2 samples (Fig. 5). The sharp increase of the cation adsorption from 0% to 100% with the increase of pH of the electrolyte as much as 1, 2 units is observed. Their values are sufficiently close for all studied samples and equal to 2.4-2.9 μmol m-2 (Table 3) which is consistent with literature data [7]. This means that each zinc ion can be adsorbed on one hydroxyl group. Obviously, different porous structure of studied samples does not influence the zinc adsorption. The question is whether all the samples contain only mesopores and macropores that do not make diffusion difficulties.

The adsorption process can be characterized by the adsorption edge, which is described by two parameters: pH50% and ΔpH10-90% [27]. These characteristics are presented in Tables 4 and 5. The values of the parameter which characterize the position of the adsorption edge

Table 3. Some parameters of titania samples.

N Phase composition

pHiep pHpzc A [μmol m-2]

1 2 3 4 5

1 Amorphous 5.0 7.1 2.9

2 Anatase 4.45 5.67 2.6

3 Brookite 2.80 6.3 2.4

4 Rutile 3.4 5.96 2.5

5 Anatase + brookite 4.13 6.13 2.6

6 Anatase + rutile 3.25 5.9 2.5

Figure 4. Isotherms of nitrogen adsorption-desorption for different TiO2: amorphous − ■ (1), anatase+brookite − ▲ (2), anatase − ○ (3), brookite − ●(4), rutile − × (5), anatase+rutile− ∇ (6).

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(pH50%) of the investigated samples are divided into two groups: monophase and biphase titania (Fig. 5). The observed differences can be associated with the peculiarities of the structure of the biphase samples. These complex systems are obviously not a simple mechanical mixture of constituent phases. Therefore, their properties do not conform to the rule of additivity and

show the synergistic effect. For example, in the studies [40,41] the catalytic activity of two-phase titania (P-25 consisting of anatase and rutile, and nanocrystalline TiO2 containing anatase and brookite) with the destruction of organic pollutants is much greater than the activity of the individual phases and their mechanical mixtures.

Tables 4 and 5 show that the adsorption edge characterized by pH50% increases from the lowest initial concentration to the highest initial concentration of zinc for all titania samples. Similarly, the adsorption edge slope characterized by the parameter ∆pH10%-90% decreases from the lowest initial concentration to the highest initial concentration of zinc.

3.6. Influence of Zn (II) ions on ζ potentialFig. 6 presents the exemplary surface charge density as a function of pH in the presence of Zn(II) ions for the amorphous TiO2/1×10-3mol L-1 NaCl solution–Zn(II) ions system. It is evident that values above pH 5 show that the content of negatively charged groups on the TiO2

surface significantly increases with the increasing Zn(II) ions concentration. At the same time, a low concentration of Zn(II) causes small changes in the density of surface charge.

The literature data show that the specific adsorption cations may cause big changes in the ζ potential value [6,7,13]. This effect, as can be seen in Figs. 7-12, in the studied systems is also significant, especially for high concentrations of metal ions and at pH>6 [10]. The ζ potential increasing with the increase of Zn(II) concentration is a general trend observed for all studied TiO2 samples. The growth of ζ potential takes place with a pH value of about 8 which approximately corresponds to the saturation of the adsorption isotherms (Fig. 5).

The second general feature is the presence of a charge reversal point (CR2) which indicates the overloading of the electrical double layer owing to cations adsorption [42,43]. Table 6 shows that the value of CR2 depends on the kind of TiO2 and concentration of Zn(II) ions. Therefore, some peculiarities were found for the investigated samples of titania which will be discussed below.

Thus, the effect of Zn(II) ions concentration on the zeta potential of TiO2 (amorphous)/ 1×10-3 mol L-1 NaCl solution is presented in Fig. 7. As can be seen only for 1×10-3 mol L-1 Zn(II) ions concentration a charge reversal point (CR2) at pH=6.8 is observed. Interestingly the zeta potential has solely positive values within pH 3-8 when the content of Zn ions equals 1×10-3 mol L-1.

The influence of Zn(II) ions concentration on the zeta potential of TiO2 (anatase)/1×10-3 mol L-1 NaCl solution interface is presented in Fig. 8. CR2 is 7.70 for Zn(II) ions concentration 1×10-4 mol L-1 and shifts towards lower pH

Figure 5. Isotherms of Zn(II) ions adsorption for different TiO2: amorphous (a), anatase (b), rutile (c), anatase+rutile (d), anatase+brookite (e), brookite (f).

Figure 6. The surface charge of TiO2 (amorphous) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ▬ − 0, ◊ − 1×10 -6 mol L-1, ○− 1×10 -5 mol L-1, Δ − 1×10-3 mol L-1.

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values, namely 6.50, when the concentration of Zn(II) increases to 1×10-3 mol L-1. Moreover, CR3 is associated with charge turnover from positive to negative at pH 9.2 and is the isoelectric point for Zn(OH)2 [31]. This means that the whole surface of TiO2 may be covered with zinc hydroxide at such concentrations of Zn(II) ions.

The effect of Zn(II) concentration on the zeta potential of TiO2 (brookite)/1×10-3 mol L-1 NaCl solution is presented in Fig. 9. The lowest concentrations cause the zeta potential increase in the whole pH range for this TiO2 modification and the point of charge turnover CR2 is observed even at 8.8 for the concentration of Zn(II) ions 1×10-5 mol L-1. This point shifts towards 7.6 and 6.8 for the concentrations of Zn(II) ions 1×10-4 mol L-1 and 1×10-3 mol L-1, respectively.

For pure rutile the Zn concentration has the noticeable effect on the zeta potential at the value

1×10-5 mol L-1 for which CR2 appears at pH=8.5 (Fig. 10). For the concentrations of 1×10-4 and 1×10-3 mol L-1 of Zn(II) ions CR2 is the same and equals about 7.4.

The following peculiarities for the biphase TiO2 samples are observed. Fumed titania, a mixture of anatase and rutile, has low (negative) values of zeta potential in a wide pH range, namely 3-7, for all concentrations of Zn ions (Fig. 11). This is different for both pure components which once again confirm their complex structure. The same can be said about the second biphase sample, namely the anatase and brookite mixture (Fig. 12). Mentioned herein features of biphase system as well as their adsorption properties described above can be associated with interaction of nanocrystals of components and structure of interfacial zone [40,41,44].

Table 4. Parameters of Zn(II) adsorption for monophase samples.

Amorphous

Concentration 1 ×10-6 mol L-1 1 ×10-5 mol L-1 1 ×10-3 mol L-1

pH50% 5.31 6.43 7.54

∆pH10%-90% 1.72 1.53 1.3

Anatase


pH50% 6.7 6.92 7.11

∆pH10%-90% 4.48 3.84 3.65

Brookite


pH50% 8.76 10.21 12.18

∆pH10%-90% 0.79 0.19 -3.39

Rutile


pH50% 4.78 5.11 7.23

∆pH10%-90% 2.45 2.24 1.84

Table 5. Parameters of Zn(II) adsorption for biphase samples.

Anatase+ Brookite


pH50% 7.49 8.65 9.86

∆pH10%-90% 4.25 2.82 1.67

Anatase+ Rutile


pH50% 6.79 7.76 8.67

∆pH10%-90% 2.81 2.65 1.6

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Figure 7. The potential ζ of TiO2 (amorphous) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ○− 0, ● − 1×10-5 mol L-1, ■ − 1×10-4 mol L-1, ♦ − 1×10-3 mol L-1.

Figure 8. The potential ζ of TiO2 (anatase) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ◊ − 0, ♦ − 1×10-6 mol L-1, ● − 1×10-5 mol L-1, ■ − 1×10-4 mol L-1, ▲− 1×10-3 mol L-1.

Figure 9. The potential ζ of TiO2 (brookite) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ◊ − 0, ♦ − 1×10-6 mol L-1, ● − 1×10-5 mol L-1, ■ − 1×10-4 mol L-1, ▲− 1×10-3 mol L-1.

Figure 10. The potential ζ of TiO2 (rutile) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ○ − 0, ♦ − 1×10-6 mol L-1, ● − 1×10-5 mol L-1, ■ − 1×10-4 mol L-1, ▲− 1×10-3 mol L-1.

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4. ConclusionsComparative investigations of electrokinetic and adsorption properties for the series of mono- and biphase unpurified titania samples in the aqueous medium were first carried out. All examined samples, which possessed different phase composition, specific surface area, and particle size, had significant concentrations of acidic groups on their surfaces due to the presence of anionic impurities. The latter was the reason that negative values of the zeta potential were observed at a

Figure 11. The potential ζ of TiO2 (anatase+rutile) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ○ − 0, ♦ − 1×10-6 mol L-1, ● − 1×10-5 mol L-1, ■ − 1×10-4 mol L-1, ▲− 1×10-3 mol L-1.

Figure 12. The potential ζ of TiO2 (anatase+brookite) as a function of pH in the presence of Zn(II) ions. Zn (II) ions concentration: ◊ − 0, ♦ − 1×10-6 mol L-1, ● − 1×10-5 mol L-1, ■ − 1×10-4 mol L-1, ▲− 1×10-3 mol L-1.

Table 6. The values of charge reversal points for TiO2 samples.

Nr Samples Zn (II) ions Concentration

[mol L-1]

CR2 CR3

1Anatase+rutile

0

1×10-6

1×10-5 9.95

1×10-4 6.93 (11.8)

1×10-3 7.50 (9.8)

2 Amorphous

0

1×10-5

1×10-4 6.78

1×10-3 (9.67)

3 Anatase+brookite

0

1×10-6

1×10-5 8.28

1×10-4 6.63 (10.3)

1×10-3 5.74 (9.7)

4 Anatase

0

1×10-6

1×10-5

1×10-4 7.7

1×10-3 6.5 9.2

5 Brookite

0

1×10-6

1×10-5 8.8

1×10-4 7.6

1×10-3 6.8 (9.6)

6 Rutile

0

1×10-6

1×10-5 8.5

1×10-4 7.5

1×10-3 7.5 9.3

Values in parentheses are obtained by extrapolating

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pH ranging from 3 to 9 and pHiep. The lowest pHiep value equaling 2.80 was observed for brookite. As a result, pHpzc and pHiep differed significantly. This may have a practical importance, since the pH value, at which the adsorption of cations and azo-dyes was possible, was significantly shifted to the acidic range. Antibate relationship between particle size and pHiep, established earlier for anatase, was confirmed for titania of different phase composition. Besides, some peculiarities of different TiO2 modifications were established. There was a difference in the adsorption of Zn(II) ions for mono- and biphase samples: isotherms of adsorption were shifted towards lower pH values for the former compared with the latter. This distinction may have been due to a higher specific surface area (therefore, higher concentration of adsorption sites) for monophase TiO2, and peculiarities of biphase samples structure.

At the same time, the increase of Zn(II) concentration caused a shift of the adsorption edge position, characterized by the pH50% parameter, towards higher pH values. The adsorption of zinc increased with the increasing pH and was almost complete at a high pH. A complete uptake of Zn(II) occurred in the pH range of 7-9.

Changes in the adsorption edge slope were revealed as a decrease from the lowest initial concentration to the highest initial concentration of zinc for all titania samples. The effect of the presence of Zn(II) ions on the zeta potential for different forms of TiO2 in NaCl solution was significant. For the low initial concentration of metal ions, the reduction of zeta potential value was observed at a pH>6, and at higher concentration the adsorption of Zn(II) ions shifted the pHiep towards higher pH values.The high concentration of adsorbing Zn(II) cations caused the overcharging of the compact part of the electrical double layer and shifts the position of CR2 point. The appearance of the point CR3 which was associated with the charge turnover from positive to negative at pH 9.2 and formation of Zn(OH)2 covering were found for pure anatase.

AcknowledgementsThis work was supported by the European Community under the Marie Curie International Research Staff Exchange Scheme (IRSES), Project No 230790.

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