Mesoporous Zirconium Titanium Oxides. Part 2: Synthesis, Porosity

Oct 2, 2008 - ... Nuclear Science and Technology Organisation, Institute of Materials Engineering, New Illawarra Road, Lucas Heights NSW 2234, Austral...
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Langmuir 2008, 24, 12323-12330

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Mesoporous Zirconium Titanium Oxides. Part 2: Synthesis, Porosity, and Adsorption Properties of Beads G. Devlet Sizgek, Erden Sizgek, Christopher S. Griffith, and Vittorio Luca* Australian Nuclear Science and Technology Organisation, Institute of Materials Engineering, New Illawarra Road, Lucas Heights NSW 2234, Australia ReceiVed May 14, 2008. ReVised Manuscript ReceiVed July 28, 2008 Mesoporous zirconium titanium mixed-oxide beads having disordered wormhole textures and mole fractions of Zr (x) ranging from x ) 0.25 to 0.67 have been prepared. The bead preparation method combined the forced hydrolysis of mixtures of zirconium-titanium alkoxides in the presence of long-chain carboxylates with external gelation. Uniformly sized beads could be produced in the size range 0.5-1.1 mm by varying the droplet size and viscosity of the mixed-oxide sol, thus making them suitable for large-scale column chromatographic applications. The beads exhibited narrow pore size distributions with similar mean pore diameters of around 3.7 nm. The specific surface areas of the beads were linked to the Zr mole fraction in the precursor solution and were generally greater than 350 m2/g for x ) 0.5. A combination of scanning transmission electron microscopy and X-ray absorption fine structure analysis indicated that the pore walls of the beads were composed of atomically dispersed Zr and Ti to form a continuous network of Zr-O-Ti bonds. Mass transport in the beads was evaluated by monitoring the kinetics of vanadate and vanadyl adsorption at pH 10.5 and 0.87, respectively.

1. Introduction Recently, we reported on the preparation of zirconium titanium mixed-oxide xerogel powders with 33 mol % Zr (ZrTi-0.33) having wormhole mesoporous texture using cheap porogenic molecules such as long-chain carboxylates including lauric, palmitic, and stearic acids.1 Such mesoporous zirconium titanium oxides have high thermal stability, offer good prospects for modulating adsorption properties through functionalization with suitable phosphonate coupling agents, and once saturated in target radioactive species, can potentially be converted directly to very leach resistant zirconium titanate phases suitable for disposal in a repository.2 In part 1 of this series, we reported the preparation of mesoporous zirconium titanium mixed xerogels and comprehensively demonstrated how the pore dimensions could be very finely tuned over a narrow range of 3 to 4 nm. This offers prospects for tailoring the adsorption properties. In particular, the mass transport kinetics in these materials was investigated using vanadate and vanadyl as probe species. In commercial-scale column chromatographic applications in particular, and even in catalysis, materials are required in the form of particles or beads having diameters of around 1 mm.3 Control of bead diameter is important because the void fraction is one of the parameters influencing ion-exchange column dynamics.4,5 Xerogel powders are most often not suitable, especially for large-scale applications, because column plugging can occur. Aside from column performance, the production costs of such granular materials represent another important consideration in commercial applications; therefore, simple processing strategies for bead production are favored. Traditional granulation methods are cost-prohibitive as a result of the requirement of several additional preparative steps such as precipitation, filtration, * Corresponding author. E-mail: [email protected]. (1) Luca, V.; Bertram, W. K.; Widjaja, J.; Mitchell, D. R. G.; Griffith, C. S.; Drabarek, E. Microporous Mesoporous Mater. 2007, 103, 123. (2) Ringwood, A. E.; Kesson, S. E.; Ware, N. G.; Hibberson, W.; Major, A. Nature 1979, 278, 219. (3) Holl, W. H. Unpublished lecture manuscript; Karlsruhe Research Center, 2004. (4) Lee, I. H.; Kuan, Y. C.; Chern, J. M. J. Hazard. Mater. 2008, 152, 241. (5) Grimes, B. A.; Liapis, A. I. J. Sep. Sci. 2002, 25, 1202.

drying, and sieving. Such procedures can also result in substantial loss of material because dust and particles smaller than the required grain size are also formed. Therefore, mechanically robust spherically shaped particles with the retention of high surface areas and narrow pore size distributions are preferred over powdered materials in many industrial applications, and indeed this represents something of an impediment to the commercial exploitation of many inorganic ion-exchange systems.6,7 An additional but nonetheless extremely important requirement of the granular material is the preservation of mass-transfer efficiency with respect to the powdered materials. Despite the need for macroscopic porous metal oxide beads when contemplating adsorption applications, very few materials of this type have ever been reported even for the much more intensively studied mesoporous silicate systems. In this regard, the millimeter-sized wormhole mesoporous silicate beads prepared by Huo et al. using microemulsion techniques stand out as an exception.8 The objective of the present undertaking was therefore (1) to arrive at a simple method for the synthesis of bulk mesoporous titania-zirconia mixed-oxide materials in spherical form while at the same preserving the basic mesoporosity of the system and (2) to determine the basic transport properties of these mixed ZrxTi1 - xO2 oxide materials as a function of composition by investigating the adsorption of anionic and cationic vanadium species from aqueous solutions. One motivation for this choice of probe species for this class of materials was the eventual possibility of interrogating the chemical environment of the surface-bound species using a wide range of techniques, including 51V solid-state NMR, EPR, Raman spectroscopy and others as in part 1. Apart from vanadates acting as a model species, the adsorption of poly oxo-vanadates on high-surface-area, thermally stable support materials is of particular interest in the important field of selective catalytic reduction.9 To the best of our knowledge, this is the first demonstration of the preparation of large-diameter (0.5-1.1 mm) monodisperse (6) Clearfield, A. Ind. Eng. Chem. Res. 1995, 34, 2865. (7) Clearfield, A. SolVent Extr. Ion Exch. 2000, 18, 655. (8) Huo, Q.; Feng, J.; Schueth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14. (9) Bond, G. C.; Forzatti, P.; Vedrine, J. C. Catal. Today 2000, 56, 329.

10.1021/la801490k CCC: $40.75  2008 American Chemical Society Published on Web 10/02/2008

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mesoporous titanate-based beads. The preparation of a usable engineered form of a mesoporous titanate-based material expands the practical applications of these materials in large-scale columnbased ion exchange or adsorption applications and for fluidized bed catalytic reactions.

2. Experimental Section 2.1. Synthesis. The preparation of titanium-zirconium beads involved several steps. First, a mixed metal alkoxide solution using inexpensive long-chain carboxylate as a porogen was made on the basis of the simple preparation reported by Luca et al.1 Metal oxide precursors titanium(IV) isopropoxide (99.999%) and zirconium(IV) propoxide (70% in propanol) were purchased from Aldrich and used as received. Zirconium(IV) propoxide and titanium(IV) propoxide in the desired molar ratios (Zr/(Zr + Ti) ) 0.25-0.67) were combined. To this solution, a long-chain carboxylate surfactant, palmitic acid (PA) (Sigma, grade II 95%), was added to achieve a metal-to-surfactant ratio of 2. The mixture was then heated to 70 °C to dissolve palmitic acid, which is a solid at room temperature. The alkoxide-surfactant mixture was kept at a 90 °C in a sealed Pyrex bottle for between 12 and 16 h. After cooling to ambient temperature, the mixture was transferred to a Pyrex container and placed in a humidity- and temperature-controlled environment. Typically, relative humidity of 50%, temperature of 30 °C, and air flow of 950 mL min-1 was used for the volatilization of alcohol and partial hydrolysis to initiate the formation of the metal oxide framework. Once the viscosity of the reaction mixture reached between 900 and 1200 cp, the mixture was removed from the humidity chamber and placed in a sealed plastic bottle and left to age at room temperature for at least 2 days. The viscous mixture was then pressure injected as droplets via a flat-tipped stainless steel hypodermic needle into a distilled water bath containing 3 M ammonia and 0.05 g L-1 Tx-100 surfactant. The droplets were gelled externally, retaining the spherical shape as soon as they made contact with the bath. The gelled beads were removed from the bath and washed with distilled water and air dried at room temperature. Removal of the porogen from the dried beads was undertaken by thermal treatment. The initial heating of the beads was critical because of escaping gas from the decomposition of organic material during heating, which can cause fracture and cracking. Therefore, the temperature was ramped from room temperature to 120 at 5 °C min-1, held at this temperature for 2 h before ramping to 450 °C at a rate of 10 °C min-1, and held at this temperature for 4.5 h. The calcined sample was then cooled to room temperature at 10 °C min-1. This temperature was chosen because it was the minimum required to totally eliminate carbon residues from all samples as evidenced by thermal analysis. The samples will henceforth be referred to as ZrTi-x, where x refers to the Zr/(Zr + Ti) mole fraction used in the preparations. 2.2. Characterization. The specific surface area, mean pore diameter, and specific desorption pore volumes were determined by nitrogen physisorption at 77 K using a Micromeretics ASAP 2010 unit. Prior to measurement, samples were degassed at 200 °C. X-ray powder diffraction (XRD) data of the samples were collected on a Scintag X1 diffractometer fitted with a Peltier detector. The diffractrograms were recorded using Cu KR radiation typically over a 2θ range of 0.7-40. Transmission electron microscopy (TEM) was conducted using a JEOL 2000FX11 instrument operating at 200 keV to investigate the nanostructural features. The mixed-oxide beads were ground to fine powders before analysis, and the sample grids were prepared by sonicating the powdered samples in methanol and evaporating 1 drop of this suspension on a carbon-coated grid. X-ray absorption fine structure (EXAFS) spectra were recorded at the Ti K-edge on beamline 20B at the Photon Factory, Tsukuba, Japan, using a Si(111) double monochromator. An estimate of resolution (∆E/E) was calculated from the Darwin width of the Si(111) monochromator and a slit opening of 0.4 mm, and a value of 1.36 eV was obtained. This value was very similar to the peakto-peak first derivative line width obtained for a Ti foil reference

Sizgek et al. spectrum. Samples were diluted by mixing boron nitride and were then loaded into sample holders with Kapton windows. Time-resolved synchrotron small angle X-ray scattering (SAXS) measurements were carried out at the ChemMatCARS SAXS instrument on sector 15 of the Advanced Photon Source (Chicago). An irradiating wavelength (λ) of 1.5 Å was used with a camera length of 1840 mm. X-ray scattering data was recorded with a MAR165 CCD detector. All patterns obtained were azimuthally isotropic and were reduced to 1D profiles of intensity versus q (q ) 2π sin θ/λ, where 2θ is the X-ray scattering angle) using the Saxs15id software package.10 Samples were held in thin-walled quartz capillaries (i.d. of 1.5 mm). Care was taken to collect pure water, buffer, and sample signals in the same capillary. In this way, the water signal was used to scale the background-corrected sample data to absolute intensity units. The time-resolved drying experiments were conducted by drawing precursor solution through a 1.5 mm capillary using a peristaltic pump (schematic diagram of experimental setup in Supporting Information). To start the experiments, the solution in the capillary was discharged by changing the direction of the peristaltic pump. While continuing pumping air at a fixed relative humidity and flow rate to the capillary, patterns were recorded at the fastest rate possible, consistent with a good signal-to-noise ratio. 2.3. Batch Kinetic and Adsorption Experiments. The vanadium adsorption kinetics measurements were undertaken using batch contact tests for equilibration times ranging from 5 min to 2 days. Each test utilized 0.1 g of material and a vanadium ion solution volume of 10 mL, giving a solution-to-solid ratio of 100/1. The initial concentration of vanadium ion solution was kept at 185 ppm (3.6 mM). The vanadium ion-containing solution and mixed-oxide beads were placed in contact with each other in 20 mL screw-cap vials. In each series of experiments, samples were mixed on a platform shaker at a constant speed of 150 rpm. Samples were withdrawn at suitable time intervals and filtered through 0.45 µm filters to remove any solid material. Adsorption isotherm experiments were measured at pH 10.5 for initial V concentration changing from 10 to 2500 ppm using the same procedure as that used in batch kinetics experiments. The sample vials were equilibrated on a platform shaker for 48 h at room temperature. The V ion concentration of the samples was measured using ICP-MS analysis. Basic solutions exclusively containing the VO42- anion were prepared using NaVO3 (sodium metavanadate), and the pH was adjusted to 10.2 with NaOH. Acidic solutions containing vanadyl VO2+ as the predominant solution species were prepared from NH4VO3 (ammonium metavanadate) by pH adjustment to 0.87 through the addition of H2SO4.

3. Results and Discussion 3.1. Characterization. In the as-produced air-dried state (Figure 1a), the mesoporous beads were transparent after drying, becoming opaque after calcination (Figure 1b). The zirconium titanate beads obtained by the simple method presented here were of extremely uniform size and could be easily prepared in different sizes within the range of 0.5-1.2 mm by varying the droplet size and viscosity of the mixed-oxides sol. Successful granulation was dependent on sufficient hydrolysis of the mixed alkoxide/porogen mixture and subsequent aging. The importance of these steps cannot be understated; however, the system appeared to be tolerant of small changes in relative humidity for the hydrolysis step, aging time, and viscosity. The effect of varying the Zr molar fraction x at constant metal/ surfactant molar ratio (2:1) on the porosity of the ZrTi-x mixed oxides was in the first instance gauged from the nitrogen adsorption-desorption isotherms (Figure 2). The isotherms were all of type IV according to the IUPAC classification,11 and all compositions had a pronounced H2 hysteresis loop that is usually (10) Cookson, D.; Kirby, N.; Knott, R.; Lee, M.; Schultz, D. J. Synchrotron Radiat. 2006, 13, 440.

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Figure 1. Photographic images of (a) as-prepared and (b) calcined (450 °C) beads (ZrTi-0.33 and (Ti + Zr)/PA)2).

Figure 2. N2 adsorption-desorption isotherms for zirconium titanates with different Zr mole fractions calcined at 450 °C: (a) ZrTi-0.25, (b) ZrTi-0.33, (c) ZrTi-0.5, (d) ZrTi-0.55, (e) ZrTi-0.67.

associated with capillary condensation in mesoporous channels. As the amount of Zr increased, the H2 hysteresis loops became less pronounced. However, the relative pressures corresponding to the hysteresis loops were essentially identical, indicating similar average pore diameters for all samples. Pore size distributions (PSDs) determined using non-localized density functional theory (NL-DFT) are shown in Figure 3. These data confirm that all of the materials had PSDs with half-widths of about 2.5 nm, which were typically centered at around 3.7 nm. Similar clustering of the PSDs was obtained using BJH theory. However, as previously observed for variable-pore-diameter xerogels in part 1, the NLDFT mean pore diameters were somewhat larger than those determined using the BJH theory, which were typically around 2.8 nm. Therefore the textural characteristics of the ZrTi-x beads were similar to those of the xerogel powders (x ) 0.33) addressed in part 1 and those reported by our group previously.1 The specific surface areas (SBET) of the materials showed a slight but steady increase with increasing zirconium content, reaching a peak at x ) 0.5 and then decreasing again (Figure 4). These results were extremely reproducible, so the trends can be regarded as definitive. However, the pore volumes of the materials (11) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.

Figure 3. Pore size distribution of zirconium titanates for various Zr mole fractions as calculated from N2 adsorption data using NL-DFT.

Figure 4. BET surface area and pore volume as a function of the mole fraction of Zr in mixed oxides calcined 450 °C. The size of the data markers represents the estimated uncertainty.

decreased only very slightly with increasing zirconium content. Such small variations are still considered to be beyond the limit of uncertainty. The average pore diameters behaved similarly within experimental uncertainty. The X-ray diffraction (XRD) patterns of the ZrTi-x beads calcined at 450 °C (Figure 5) exhibited distinct correlation peaks in the low-angle region at about 2θ ) 1.5°. No higher-angle features were observed for any of the compositions studied at this calcination temperature, indicating that neither titania, zirconia, nor mixed-oxide phases crystallized. This is consistent with what was observed previously for related zirconium titanium oxide xerogel powders.1 The X-ray diffraction patterns of the

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Figure 5. X-ray powder diffraction patterns of zirconium titanate spheres calcined at 450 °C. Figure 7. TEM and EDS analyses of selected samples calcined at 450 °C.

Figure 6. X-ray diffraction patterns of as-dried and calcined (450 °C) zirconium titanates.

as-synthesized and calcined ZrTi-0.33 sample are shown in Figure 6. The as-synthesized material gave a single relatively broad low-angle reflection, which upon removal of the organic by calcination increased in intensity and shifted in d-spacing from 33.0 to 60.6 Å. The increase in d-spacing on calcination has been observed and commented upon previously1 and was further elaborated on in part 1 of this series. Clearly, such a shift to higher spacing on calcination and removal of the carboxylate cannot be viewed as resulting from the expected shrinkage of an ordered unit cell as happens in conventional ordered mesoporous materials obtain through surfactant templating. It is our hypothesis that this system displays domain growth or coarsening with the retention of self-similarity following a temperature excursion. The findings from XRD analysis are supported by TEM analysis. TEM examination of the present samples (Figure 7) accompanied by EDS elemental analysis has shown that all of the materials had homogeneous distributions of Zr and Ti throughout with no macroscopic phase segregation, as was also the case in our previous studies.1 The limitation of TEM analysis for gleaning information on elemental homogeneity is the limited spatial resolution (1-10 nm) of the technique. However, it is clear that at least on the length scale of the analysis beam these materials are homogeneous. To obtain more information on the element distribution on the atomic scale, we investigated the samples calcined at 450 °C by X-ray absorption spectroscopy. The first aspect that needs to be pointed out is that the pre-edge XANES of the sample with ZrTi-0.5 is quite different from the pre-edge XANES of nanocrystalline anatase (Figure 8a). The ZrTi-0.5 sample has a relatively weak pre-edge structure and certainly does not possess

Figure 8. XANES spectra of nanocrystalline anatase and ZrTi-0.5 (a) and K- and R-space EXAFS at the Ti K-edge of (b) nanocrystalline anatase and (c) mesoporous ZrTi-0.5 calcined at 450 °C.

any multiplet structure such as that observed in titania polymorphs of amorphous titania.12 (12) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650.

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Table 1. Characterization Data for Zirconium Titanate Beads Calcined at Various Temperatures 450°

550°

600°

650° 42.0 76.8 75.0 79.9

ZrTi-0.25 ZrTi-0.33 ZrTi-0.5 ZrTi-0.67

318.6 325.9 355.8 339.5

SBET (m2/g) 266. 9 144.1 279.1 194.4 291.7 194.1 282.0 118.3

ZrTi-0.25 ZrTi-0.33 ZrTi-0.5 ZrTi-0.67

0.32 0.31 0.30 0.28

Vpore (cm3/g) BJH Desorption 0.28 0.23 0.29 0.25 0.27 0.26 0.24 0.16

0.12 0.21 0.18 0.15

ZrTi-0.25 ZrTi-0.33 ZrTi-0.5 ZrTi-0.67

2.91 2.82 2.71 2.81

Dave (nm) BJH Desorption 3.13 4.61 3.02 3.43 2.83 3.25 2.81 3.73

8.47 7.61 5.90 5.18

In Figure 8b, c are shown the K- and R-space EXAFS of the ZrTi-0.5 sample in comparison with a sample of nanocrystalline anatase. There are obvious qualitative differences in both the Kand R-space data that make it absolutely clear that significant changes have occurred in the Ti coordination environment of the mixed oxide compared with that of the pure anatase nanoparticles or, for that matter, that of amorphous titanium oxides.12 The EXAFS data of both the titania nanoparticles and the ZrTi-0.5 sample have been modeled using the same fitting methodology as employed previously for such systems, and the fitting parameters are reported in Table 4. The parameters for the anatase nanoparticles are in agreement with expectations and previously reported values. However, the spectrum of the ZrTi-0.5 sample clearly required a different fitting model, and the large relative intensity of the second Fourier transform peak suggested that the outer shell would have a different composition from the O and Ti scattering found in titania materials. In fact, the data was well modeled using a shell of Zr atoms at a distance of 3.68 Å. (See the fits of Figure 8b and the parameters of Table 4.) The EXAFS data therefore suggests that Zr next-nearest neighbors are present in the wall structure of the mesoporous beads, affording a significant proportion of Ti-O-Zr bonds, thus supporting the TEM analysis. The thermal stability of the variable-composition ZrTi-x (x ) 0.25-0.67) phases were assessed by calcinination at various temperatures. Figure 9 summarizes the results obtained from XRD measurements for the thermally treated samples. For ZrTi0.33 and ZrTi-0.5 (Figure 9b, c), low-angle peaks were observed up to 600 °C. No peaks from crystalline phases could be observed in the high-angle region of the corresponding X-ray diffractrograms, implying retention of the mesoporous texture. The surface area and porosity data in Table 1 confirm the retention of porosity up to 600 °C. However, for ZrTi-0.67 and ZrTi-0.25 at 600 °C (Figure 9a, d) there was a marked reduction in the intensity of the low-angle peaks whereas for ZrTi-0.67 at this calcination temperature a high-angle peak at 2θ ) 30.9° was also present as a result of the crystallization of a ZrO2-like phase. The results suggest that for samples ZrTi-0.25 and ZrTi-0.67 partial degradation of the mesoporous structure commences at 600 °C. No early loss of porosity occurred for intermediate compositions where the crystallization of TiO2 or ZrO2 is usually suppressed.13 Time-resolved capillary thin film drying experiments of asprepared and partially hydrolyzed precursor solutions as a function of humidity were carried out using synchrotron radiation in an attempt to shed some light on the mechanism of formation of (13) Hirano, M.; Nakahara, C.; Ota, K.; Inagaki, M. J. Am. Ceram. Soc. 2002, 85, 1333.

the bead materials. Figure 10 shows a selection of time-dependent small-angle X-ray diffraction patterns of the as-prepared zirconium titanate precursor solution (ZrTi-0.33) drying in a capillary in a flow of air at 30% relevant humidity. SAXS patterns at other relative humidities and of partially hydrolyzed precursor solutions are provided as Supporting Information. Within the first 30 s of drying, a single broad reflection with a d spacing of 10.2 Å was observed. As drying progressed, this reflection moved to lower angles with the final d spacing reaching 25 Å. Just prior to this, after about 50 s of drying, two reflections could be discerned with d spacings of 19.6 and 13.2 Å. The series of transformations on drying indicates a clear evolution of the structure. The initial 10.2 Å is about what would be expected for a palmitic acid molecule and therefore may be indicative of a lamellar phase. As drying progressed, the d spacing more or less doubled, which we believe is consistent with the formation of tubular pores because these would have a diameter corresponding to two palmitic acid molecules plus some amount corresponding to the zirconium titanium oxide pore walls. TEM of the dried material clearly does not support a lamellar structure on drying but rather a disordered bicontinuous wormhole texture. 3.2. Vanadium Batch Kinetics and Adsorption. In an attempt to probe and compare the ion-exchange properties of the variable-composition mixed oxide beads, in particular, the pore accessibility, and also to facilitate comparison with the data of part 1 on xerogels, vanadium oxo-ion uptake as a function of time was studied under both basic (pH 10.2) and acidic (pH 0.87) conditions with an initial vanadium concentration of 185 ppm (3.6 mM). As stated in part 1, these two pH values at the vanadium concentrations used here should afford solutions containing exclusively anionic vanadate and cationic vanadyl species, respectively. More precisely, speciation diagrams for vanadium14,15 suggest that at low pH values (pH ∼3 or lower) vanadium(V) exists as the relatively stable dioxocation, VO2+, whereas at pH 10 small orthovanadate oxoanions, VO3(OH)2-, predominate. This was confirmed using 51V solution NMR as was the total absence of small oligomers such as V2O72-. The amount of vanadium uptake was calculated using the methodology of Ho et al.,16 which was also adopted in part 1 of this series. We remind the reader of the equation based on the difference in vanadium ion concentration in aqueous solution initially and at time t of the adsorption

qt )

V(Ci - Ct) m

(1)

where qt is the amount of vanadium adsorbed per unit amount of adsorbent (mmol/g) at time t (min), V is the volume of the solution (L), Ci is the initial vanadium ion concentration (mmol/L), Ct is the vanadium ion concentration at time t of adsorption (mmol/L), and m is the mass of adsorbent material (g) added to the solution. The best fit to the vanadium adsorption data was obtained using the pseudo-second order kinetic model. The pseudo-second-order reaction rate expression can be written as

qt )

kqe2t (1 + kqet)

(2)

where k is the pseudo-second order rate constant, qe the amount adsorbed at equilibrium, and kqe2(h0) is the initial sorption rate. Equation 2 was solved using a least-squares routine (downhill (14) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976. (15) Peacock, C. L.; Sherman, D. M. Geochim. Cosmochim. Acta 2004, 68, 1723. (16) Ho, Y. S.; McKay, G. Water Res. 2000, 34, 735.

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Table 2. Comparison of Pseudo-Second-Order Kinetic Values (pH, 10.2; initial V concentration, 3.6 mM) for ZrTi-0.33 in Powder and in Spherical Beads samples

particle size (µm) pseudo-second-order rate, k initial sorption rate, h0