ZrO2

Article pubs.acs.org/est

Uranyl-Sorption Properties of Amorphous and Crystalline TiO2/ZrO2 Millimeter-Sized Hierarchically Porous Beads Maryline Chee Kimling,† Nicholas Scales,‡ Tracey L. Hanley,‡ and Rachel A. Caruso*,†,§ †

Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia § CSIRO, Materials Science and Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia ‡

S Supporting Information *

ABSTRACT: Hierarchically porous TiO2/ZrO2 millimetersized beads were synthesized using a sol−gel templating technique, and investigated for suitability as radionuclide sorbents using uranyl as a radionuclide-representative probe. The bead properties were varied by altering either composition (22, 36, and 82 wt % Zr in the Ti/Zr composite) or calcination temperature (500 or 700 °C). Uranyl adsorption was higher for the crystalline beads (surface area: 52−59 m2 g−1) than the amorphous beads (surface area: 95−247 m2 g−1), reaching a maximum of 0.170 mmol g−1 for the 22 wt % Zr sample. This was attributed to the higher surface hydroxyl density (OH nm−2), presence of limited microporosity, and larger mesopores in the crystalline beads. Mass transport properties of the crystalline beads were not compromised by the large bead diameter: sorption rates comparable to those reported for powders were achieved and rates were higher than exclusively mesoporous reported systems, thereby highlighting the importance of pore hierarchy in designing materials with improved kinetics. Chemical stability of the sorbent, an important property for processes involving corrosive effluents (e.g., radioactive waste), was also assessed. Crystalline beads displayed superior resistance against matrix leaching in HNO3. Stability varied with composition: the 22 wt % Zr sample demonstrated the highest stability.

■

INTRODUCTION The containment of highly radioactive/toxic waste is a topic of interest and major concern.1 To date, borosilicate glass matrices have been used to entrap the waste which then remains at storage facilities.1,2 Proposed plans for long-term disposal are immobilization of the radionuclides in a solid host followed by burial in deep geological sites (repositories).3 Due to the health and biological hazards that these elements represent, it is necessary that these hosts are resistant to radiation and environmental degradation over long time periods. However, studies have shown potential problems that borosilicate glasses could face in such repositories.3−5 For instance, extensive disintegration of the matrix, over a period of two weeks, was observed when it was subjected to water/steam at ∼300 °C and high pressure conditions ∼300 bar; conditions that are likely to be encountered in such repositories.4 Thereby increasing risks of radionuclides leaching from the matrix, thus directing research to nonsiliceous or ceramic counterparts. Of the various inorganic substrates possible, TiO2 and ZrO2 are considered as ideal candidates in view of their high radiolytic stability and very low solubility in an aqueous-based environment.3,6−9 Recently, mixed TiO2/ZrO2 matrices have been examined.10−16 Binary metal oxides benefit from superior attributes over their pure metal oxide analogues, e.g. improved © 2012 American Chemical Society

surface areas, surface acidities, thermal, mechanical, and radiolytic stabilities.14,17−19 The chemical composition of the matrix also affords conversion into a highly durable inert dense ceramic phase, following saturation with the waste,3,13,20 thus providing permanent radionuclide confinement. TiO2/ZrO2 particulate materials with controlled mesoporosity were synthesized using alkylcarboxylate surfactants as templates.13,21 This process was adapted to produce mesoporous (∼3.7 nm) millimeter-sized (0.5−1.1 mm) TiO2/ZrO2 beads.17 The macroscopic size resulted in slower uranyl-sorption kinetics than those obtained for mesoporous (4.3 nm) particulate (particle size of >80 μm) materials.12 The transport properties of the macroscopic-sized beads were improved by incorporating another level of porosity by employing a macroporous polyacrylonitrile (PAN) template.11,12 TiO2/ZrO2 matrices are also suitable platforms for incorporating highly stable surface organic functionalities for the selective capture of target radionuclides, through robust, hydrolytically stable phosphonate linkages.10,12,15 Received: Revised: Accepted: Published: 7913

April 11, 2012 June 6, 2012 June 14, 2012 June 27, 2012 dx.doi.org/10.1021/es3011157 | Environ. Sci. Technol. 2012, 46, 7913−7920

Environmental Science & Technology

■

To date, studies have been limited to amorphous systems and to a particular TiO2/ZrO2 composition, as exemplified above. An amorphous system is advantageous for generating materials with high surface areas, potentially leading to enhanced adsorption capacities. However, limited diffusion of an adsorbate through the pores may occur, potentially resulting in reduced adsorption capacities, due to the overall sorbent structure. Pore enlargement (e.g., through a crystallization process) would be an option to alleviate these diffusion issues. Recently, Ide et al.15 conducted in situ monitoring of the functionalization process of TiO2/ZrO2 matrices with a bisphosphonic-based ligand, containing a UV chromophore for the monitoring processes. Studies showed the potential of employing crystalline supports over amorphous analogues in achieving high levels of surface functionalization. Additionally, chemical composition variation can alter the properties of the materials, consequently influencing the material performance in applications;16,18,22 these variations should also be explored. Despite the recent progress made in assessing TiO2/ZrO2 materials as sorbents, additional improvement requires further studies. To fully harness the functionality of a material to a particular application, studies examining the correlation of the material properties with its performance are essential. Pore structure also influences the adsorption behavior of a sorbent material, effecting its performance.12,14,15,23,24 Hierarchically porous materials, possessing macro-, and meso- and/ or micropores are becoming increasingly popular:25 e.g., in the fields of separation12 or catalysis23 where fast mass transport of the bulk solution, as provided by the presence of macropores, is as important as obtaining high loadings that can be achieved by micro and/or mesoporous networks, which result in high surface areas. The external dimension and shape of the final materials also need to be tailored to best suit the final application. For instance, in large-scale adsorption-separation/catalysis-chromatography-related applications, larger-sized particles are highly advantageous due to the ease of handling and recovery or reduced issues involving clogging that are generally associated with micro/nanoparticles.26 Furthermore, uniformly sized particles, preferably of spherical shape, are desirable for optimizing regular flow and column efficiency.27 Sol−gel templating is a resourceful tool for synthesizing diverse porous inorganic materials with different chemical compositions,28,29 dimensions, pore architectures,24 pore sizes,28 or geometrical shapes28,30−33 that include spherical particles29,34 through the careful selection of templates.33 For instance, porous alginate beads as sacrificial templates for the formation of porous inorganic-based monodisperse beads were recently reported.34,35 In this study, hierarchically porous TiO2/ZrO2 millimetersized beads were synthesized via a sol−gel templating technique using alginate beads as a sacrificial template. The effect of varying Ti/Zr composition (22, 36, and 82 wt % Zr) or crystallinity (amorphous or crystalline) resulted in differences in properties such as surface area, surface hydroxyl group, and pore accessibility. The uptake capacity and sorption kinetics of uranyl (a radionuclide-representative probe) were investigated. As these materials have potential as sorbents for nuclear waste separation in harsh aqueous media, the influence of varying the above-mentioned parameters on the chemical stability of the materials in nitric acid solution was also studied.

Article

EXPERIMENTAL SECTION

Materials. Alginic acid sodium salt (≥2000 cP viscosity of a 2% solution, mannuronic/guluronic acid ratio of 1.56), titanium(IV) isopropoxide (97%), and zirconium(IV) propoxide (70% in 1-propanol) were obtained from Sigma-Aldrich. Ethanol, isopropanol, and CaCl2·2H2O (AR) were obtained from Chem-Supply. UO2(NO3)2·6H2O and HNO3 solution (69%) were purchased from Merck. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩcm. Synthesis of Template Beads. Calcium alginate (CaAlg) beads were formed by dripping a 1 wt % sodium alginate solutionas detailed previously34(from tubing with a diameter of 2 mm) to a Ca2+ bath (0.27 M), using a peristaltic pump (Gilson Minipuls 3) set at a speed of 35 rpm. Beads were cured for 24 h prior to washing with water to remove excess Ca2+. Synthesis of TiO2/ZrO2 Beads. Adapting a reported templating procedure,34 the CaAlg beads underwent solvent exchange from water to ethanol to isopropanol prior to infiltrating with a 70%/30% (w/w) metal alkoxide precursor/ isopropanol solution. The mixed metal alkoxide solutions were combined to achieve a relative concentration of 22, 36, and 82 wt % Zr to Ti. The infiltrated gel beads were then transferred to a 1:1 (v/v) water/isopropanol solution. The composite CaAlg/ inorganic beads were dried at room temperature for 2 days, then dried in an oven at 60 °C for 6 h. The template was removed by calcining the composite beads at 500 or 700 °C for 5 h (heating rate of 1.5 °C min−1). Sample nomenclature used is TiZrx-y where x is the Zr content (wt %) in the Ti/Zr composite (i.e., Zr/(Zr + Ti) × 100%) and y is the calcination temperature (°C). Chemical Stability Studies. HNO3 solutions (5 mL) at concentrations of 0.0001, 0.01, 0.1, 1, or 2 M were poured into polypropylene vials containing the whole beads (0.05 g). Beads were stirred in the respective solutions for 24 h on an orbital shaker set at 135 rpm then filtered through 0.45-μm syringe filters (Supor membrane, from PALL). The supernatants were collected for analysis using a Thermo X Series inductively coupled plasma mass spectrometer (ICP-MS). The percentage elemental (Ti, Zr) release was calculated according to eq. S1 in the Supporting Information (SI). Uranyl-Sorption Studies. The loading capacity and rate of uranium uptake onto the TiO2/ZrO2 beads were tested under acidic conditionspH 3.8, adjusted using 0.01 M HNO3. For adsorption capacity studies, uranium-containing solutions (5 mL) at varying concentrations of 10, 25, 50, 75, 100, 125, 150, 175, 200, 300, 400, or 500 ppm were added to 0.05 g of whole beads (V/m = 100 mL g−1), stirred on a shaker set at 135 rpm for 48 h, then filtered through 0.45-μm filters (Polyethersulfone membrane, from Sartorius Stedim Biotech). Adsorption of uranyl on the membranes was negligible, as determined using standardized solutions. The supernatants were analyzed for uranium content with a Bruker 820-MS ICP-MS instrument. The data were fitted with a Langmuir36,37 or a Freundlich model37 (SI). For kinetics studies, a procedure similar to that above was used to monitor uranium uptake (at an initial concentration of 100 ppm) onto the beads with time: 5, 10, 20, 30, 60, 120, 240, 480, 960, 1440, and 2880 min. The data were fitted with a pseudo-second-order kinetic model38 (SI). 7914

dx.doi.org/10.1021/es3011157 | Environ. Sci. Technol. 2012, 46, 7913−7920

Environmental Science & Technology

Article

Table 1. Properties of the TiO2/ZrO2 Beads sample

BET surface area (m2 g−1)

TiZr22-500 TiZr36-500 TiZr82-500 TiZr22-700

247 235 95 52

± ± ± ±

1 3 2 1

21.2 15.1 11.8 14.2

59 ± 1 54 ± 1

TiZr36-700 TiZr82-700

crystal sizea (nm)

± ± ± ±

pore sizeb 50 nm)40 in the calcined beads. Furthermore, the onset of high N2 uptake at low relative pressures indicates the presence of micropores ( TiZr36 > TiZr22. At pH 3.8, UO22+ is expected to be the dominant species based on thermodynamic calculations.49−51 In aqueous solutions, UO22+ forms aquocomplexes, with UO2(H2O)52+ identified as the most stable complex. Hydration can take place in both the primary or secondary hydration sphere. Hence, the final size of the hydrated molecules can vary (a diameter of ∼1 nm has been reported).52 Thus limited diffusion of the complex is expected in pores of comparable dimensions. The higher amount of micropores present in TiZr22-500 and TiZr36-500 samples 7917

dx.doi.org/10.1021/es3011157 | Environ. Sci. Technol. 2012, 46, 7913−7920

Environmental Science & Technology

Article

0.25−2 mm) achieved comparable qmax = 0.12 mmol g−1 (28.3 mg g−1) at pH 3, V/m = 1000 mL g−1.53 Note that the latter two examples have significantly higher surface areas than the crystalline beads from this study, again highlighting the lack of correlation between qmax and surface area. Uranyl Sorption Kinetics Studies. The rates of adsorption of UO22+ into the porous crystalline beads were also investigated. Similar equilibrium uptake capacities (98%) were achieved for TiZr22-700 and TiZr36-700 beads, but TiZr22-700 overall and initial sorption rates (k and h0) were approximately double that of TiZr36-700 beads (Figure 3c and Table 3). The higher adsorption rates of TiZr22-700 beads

relative to TiZr82-500 sample would limit access to hydrated UO22+ by size. The above argument was strengthened by comparing the crystalline with the amorphous beads. Values of qmax were higher for the crystalline beads, despite featuring lower surface areas (Figure 4a). Access to the pores for subsequent

Table 3. UO22+ Adsorption Pseudo-Second-Order Kinetic Constants for Crystalline TiO2/ZrO2 Beadsa pseudo-second-order constant −2

−1

qe × 10 (mmol g ) k (g mmol−1 min−1) h0 × 10−4 (mmol g−1 min−1) R2

Figure 4. Dependency of (a) surface area, (b) hydroxyl content (OH g−1), and (c) hydroxyl density (DOH) on monolayer uranyl adsorption capacity (qmax), obtained for the corresponding TiZrx-500 and TiZrx700 beads, at pH 3.8, V/m = 100 mL g−1.

a

TiZr22-700

TiZr36-700

4.2 ± 0.1 0.42 ± 0.05 7±1 0.988

4.3 ± 0.1 0.23 ± 0.03 4±1 0.988

[UO22+] = 100 ppm, pH 3.8, V/m = 100 mL g−1.

could possibly be attributed to the larger mesopore size of this sample (i.e., 13.8 nm against 11.4 nm (TiZr36-700)). As the surface area did not vary substantially, it is unlikely to contribute significantly to these differences in results. However, we could expect other surface properties arising from the differences in Ti/Zr compositions or crystallinity between these two samples to have some effect, as previously noted. The adsorption kinetics of the alginate-templated TiO2/ZrO2 beads were compared with those of reported TiO2/ZrO2 (33 mol % Zr) matrices,11,12,17 which were examined for the uptake of UO22+ at pH 3.8, V/m = 100 mL g−1. The mesomacroporous alginate-templated beads had adsorption rates ∼2.5−10 times higher than exclusively mesoporous amorphous PAN-templated TiO2/ZrO2 beads (2.8 nm; k = 0.08 g mmol−1 min−1, h0 = 2.05 × 10−5 mmol g−1 min−1),12,17 whereas h0 sorption rates were similar to those of mesoporous amorphous TiO2/ZrO2 xerogels (4.28 nm, 6.74 × 10−4 mmol g−1 min−1).12 Powder-like materials are expected to have higher adsorption rates due to shorter path lengths for diffusion. However, current findings demonstrate that bulk materials (beads in this case) can achieve efficiencies similar to particulate-like materials by introducing a pore hierarchy. The crystalline beads herein prepared displayed h0 rates similar to hierarchically porous PAN-templated beads which featured significantly larger (by ∼3−4 times) surface areas.11,12 However, the alginatetemplated beads showed k rates ∼2−5 times lower than the PAN-templated beads.11,12 The PAN-templated beads had larger micrometer-sized macropores and smaller mesopores (4−6 nm) in contrast to the smaller macropores (hundreds of nanometers) and bigger mesopores (11−14 nm) of the current beads. Although a pore hierarchy is desirable in generating materials with efficient mass transport properties, the mesopore−macropore size ratio also needs to be well tuned to achieve effective kinetic properties. This study demonstrates that sol−gel templated mesomacroporous TiO2/ZrO2 crystalline beads are better suited as sorbents in nuclear waste processing over the corresponding amorphous beads. Pore accessibility and higher hydroxyl group densities are major parameters contributing to the increased adsorption capacity. The beads display high adsorption kinetics,

adsorption to occur was likely to be more important, as the crystalline beads featured larger mesopores and lacked the particulate structure seen for the amorphous beads. Surface hydroxyl groups, which can act as adsorption sites to bind UO22+,41−43 also contribute to the adsorption process. A nonlinear dependency between OH content and qmax was observed (Figure 4b). However DOH displayed a stronger influence (Figure 4c): increasing qmax values were obtained with increasing DOH except for the TiZr82-500 sample. It could be argued that accessibility of UO22+ to the OH sites could have been restricted, due to its particulate, porous nature, generating reduced qmax. However, this phenomenon would be expected to be more significant in both TiZr22-500 and TiZr36-500 beads where higher amounts of micropores were observed. Further analysis is required to fully understand this observation. For example, the mechanism of UO22+ adsorption could vary on the different bead surfaces due not only to the surface hydroxyl groups but also to variation in the co-ordination to surface Ti and Zr sites. Binding of UO22+ on rutile, at pH ∼3, was reported to occur through an inner-sphere bidentate surface complexation, involving two different types of OH sorption sites.41,42 In another study, the adsorption of UO22+ on zircon at pH ∼3, proceeded through an inner-sphere tridentate surface complexation involving two and one sorption sites from the silicate and zirconium matrices, respectively.43 In all, the higher surface hydroxyl group densities featured in the crystalline beads largely contributed to the higher adsorption capacities obtained, over the amorphous beads. The current TiZrx-500 beads had lower qmax than previously reported bimodal porous amorphous TiO2/ZrO2 beads (215 m2 g−1, 0.14 mmol g−1, pH 3.8, V/m = 100 mL g−1).12 The substantially lower microporosity in the reported beads is the most likely reason for the observed difference. Conversely, the TiZrx-700 beads (of lower surface area: 52−59 m2 g−1) achieved qmax comparable to such beads.12 High surface area mesoporous TiO2/SiO2 particulates (170−1271 m2 g−1, 2.46− 6.14 nm) attained a lower binding capacity of 0.035−0.066 mmol g−1 at pH 5.10, V/m = 100 mL g−1.47 Whereas high surface area granulated activated carbons (965−1200 m2 g−1, 7918

dx.doi.org/10.1021/es3011157 | Environ. Sci. Technol. 2012, 46, 7913−7920

Environmental Science & Technology

Article

coordinating organic functionalities. ACS Appl. Mater. Interfaces 2010, 2, 3436−3446. (11) Drisko, G. L.; Kimling, M. C.; Scales, N.; Ide, A.; Sizgek, E.; Caruso, R. A.; Luca, V. One-pot preparation and uranyl adsorption properties of hierarchically porous zirconium titanium oxide beads using phase separation processes to vary macropore morphology. Langmuir 2010, 26, 17581−17588. (12) Sizgek, G. D.; Griffith, C. S.; Sizgek, E.; Luca, V. Mesoporous zirconium titanium oxides. Part 3. Synthesis and adsorption properties of unfunctionalized and phosphonate-functionalized hierarchical polyacrylonitrile-F-127-templated beads. Langmuir 2009, 25, 11874− 11882. (13) Luca, V.; Bertram, W. K.; Widjaja, J.; Mitchell, D. R. G.; Griffith, C. S.; Drabarek, E. Synthesis of mesoporous zirconium titanates using alkycarboxylate surfactants and their transformation to dense ceramics. Microporous Mesoporous Mater. 2007, 103, 123−133. (14) Drisko, G. L.; Imperia, P.; de los Reyes, M.; Luca, V.; Caruso, R. A. Size matters: Incorporation of poly(acrylic acid) and small molecules into hierarchically porous metal oxides prepared with and without templates. Langmuir 2010, 26, 14203−14209. (15) Ide, A.; Drisko, G. L.; Scales, N.; Luca, V.; Schiesser, C. H.; Caruso, R. A. Monitoring bisphosphonate surface functionalization and acid stability of hierarchically porous titanium zirconium oxides. Langmuir 2011, 27, 12985−12995. (16) Drisko, G. L.; Luca, V.; Sizgek, E.; Scales, N.; Caruso, R. A. Template synthesis and adsorption properties of hierarchically porous zirconium titanium oxides. Langmuir 2009, 25, 5286−5293. (17) Sizgek, G.; Sizgek, E.; Griffith, C.; Luca, V. Mesoporous zirconium titanium oxides. Part 2: Synthesis, porosity, and adsorption properties of beads. Langmuir 2008, 24, 12323−12330. (18) Schattka, J. H.; Shchukin, D. G.; Jia, J. G.; Antonietti, M.; Caruso, R. A. Photocatalytic activities of porous titania and titania/ zirconia structures formed by using a polymer gel templating. Chem. Mater. 2002, 14, 5103−5108. (19) Reddy, B. M.; Khan, A. Recent advances on TiO2-ZrO2 mixed oxides as catalysts and catalyst supports. Cat. Rev.-Sci. Eng. 2005, 47, 257−296. (20) Ringwood, A. E.; Kesson, S. E.; Ware, N. G.; Hibberson, W. O.; Major, A. Synroc process - Geochemical approach to nuclear waste immobilization. Geochem. J. 1979, 13, 141−165. (21) Griffith, C. S.; Sizgek, G. D.; Sizgek, E.; Scales, N.; Yee, P. J.; Luca, V. Mesoporous zirconium titanium oxides. Part 1: Porosity modulation and adsorption properties of xerogels. Langmuir 2008, 24, 12312−12322. (22) Manriquez, M. E.; Lopez, T.; Gomez, R.; Navarrete, J. Preparation of TiO2-ZrO2 mixed oxides with controlled acid-basic properties. J. Mol. Catal. A: Chem. 2004, 220, 229−237. (23) Lee, Y. J.; Kim, Y. W.; Jun, K. W.; Viswanadham, N.; Bae, J. W.; Park, H. S. Textural properties and catalytic applications of ZSM-5 monolith foam for methanol conversion. Catal. Lett. 2009, 129, 408− 415. (24) Drisko, G. L.; Cao, L.; Kimling, M. C.; Harrisson, S.; Luca, V.; Caruso, R. A. Pore size and volume effects on the incorporation of polymer into macro- and mesoporous zirconium titanium oxide membranes. ACS Appl. Mater. Interfaces 2009, 1, 2893−2901. (25) Su, B. L., Sanchez, C., Yang, X. Y., Eds. Hierarchically Structured Porous Materials: From Nanoscience to Catalysis, Separation, Optics and Life Science; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011. (26) Clearfield, A. Inorganic ion exchangers, past, present, and future. Solvent Extr. Ion Exch. 2000, 18, 655−678. (27) Seko, M.; Takeuchi, H.; Inada, T. Scale-up for chromatograpicseparation of para-xylene and ethylbenzene. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 656−661. (28) Zhou, J. F.; Zhou, M. F.; Caruso, R. A. Agarose template for the fabrication of macroporous metal oxide structures. Langmuir 2006, 22, 3332−3336. (29) Deshpande, A. S.; Shchukin, D. G.; Ustinovich, E.; Antonietti, M.; Caruso, R. A. Titania and mixed titania/aluminum, gallium, or

and mechanical and chemical stability, which are also important attributes in this field of application. The spherical form and macroscopic size of the materials are well suited for chromatography-related applications, and allow for easy handling and recovery of the materials.

■

ASSOCIATED CONTENT

S Supporting Information *

Calculations for Langmuir, Freundlich, and pseudo-secondorder kinetics fits, surface hydroxyl, and carbon content; Freundlich parameters (uranyl-sorption studies); figures for optical images of beads, TGA, XRD, HRTEM, SEM, nitrogen sorption, and elemental release. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. Dehong Chen for conducting TEM analysis, and Ms. Cheryl McHugh and Ms. Sally Taylor for ICP-MS measurements (chemical stability studies). Dr. Andrea O’Connor provided access to the Instron 5848 MicroTester Unit and Ms. Silvia Leo assisted with the mechanical strength measurements. The Electron Microscopy Unit, Bio21 Institute, The University of Melbourne provided electron microscopy access. This research was financially supported by an Australian Research Council (ARC) Discovery Project (DP0877428), Australian Institute of Nuclear Science and Engineering (AINGRA09101 and AINGRA10120), and an Albert Shimmins Fund Writing-up Award. R.A.C. is a recipient of an ARC Future Fellowship (FT0990583).

■

REFERENCES

(1) Gonzalez, A. J. The safety of radioactive waste management. IAEA Bull. 2000, 42. (2) Zorpette, G.; Stix, G. Nuclear waste: The challenge is global. Spectrum, IEEE 1990, 27 (18−24), 48. (3) Ringwood, A. E.; Kesson, S. E.; Ware, N. G.; Hibberson, W.; Major, A. Immobilization of high-level nuclear-reactor wastes in synroc. Nature 1979, 278, 219−223. (4) McCarthy, G. J.; White, W. B.; Roy, R.; Scheetz, B. E.; Komarneni, S.; Smith, D. K.; Roy, D. M. Interactions between nuclear waste and surrounding rock. Nature 1978, 273, 216−217. (5) Ringwood, T. Immobilization of radioactive-wastes in synroc. Am. Sci. 1982, 70, 201−207. (6) Weber, W. J.; Ewing, R. C.; Catlow, C. R. A.; Rubia, T. D.; Hobbs, L. W.; Kinoshita, C.; Matzke, H.; Motta, A. T.; Nastasi, M.; Salje, E. H. K. Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium. J. Mater. Res. 1998, 13, 1434−1484. (7) Gong, W. L.; Lutze, W.; Ewing, R. C. Zirconia ceramics for excess weapons plutonium waste. J. Nucl. Mater. 2000, 277, 239−249. (8) Heimann, R. B.; Vandergraaf, T. T. Cubic zirconia as a candidate waste form for actinides - Dissolution studies. J. Mater. Sci. Lett. 1988, 7, 583−586. (9) Adelhelm, C.; Bauer, C.; Gahlert, S.; Ondracek, G. TiO2-A ceramic matrix. In Radioactive Waste Forms for the Future; Lutze, W., Ewing, R. C., Eds.; Elsevier: New York, 1988; pp 393−426. (10) Griffith, C. S.; Reyes, M. D. L.; Scales, N.; Hanna, J. V.; Luca, V. Hybrid inorganic-organic adsorbents Part 1: Synthesis and characterization of mesoporous zirconium titanate frameworks containing 7919

dx.doi.org/10.1021/es3011157 | Environ. Sci. Technol. 2012, 46, 7913−7920

Environmental Science & Technology

Article

indium oxide spheres: Sol-gel/template synthesis and photocatalytic properties. Adv. Funct. Mater. 2005, 15, 239−245. (30) Stein, A.; Li, F.; Denny, N. R. Morphological control in colloidal crystal templating of inverse opals, hierarchical structures, and shaped particles. Chem. Mater. 2008, 20, 649−666. (31) Shchukin, D. G.; Caruso, R. A. Inorganic macroporous films from preformed nanoparticles and membrane templates: Synthesis and investigation of photocatalytic and photoelectrochemical properties. Adv. Funct. Mater. 2003, 13, 789−794. (32) Drisko, G. L.; Wang, X. D.; Caruso, R. A. Strong silica monoliths with large mesopores prepared using agarose gel templates. Langmuir 2011, 27, 2124−2127. (33) Kimling, M. C.; Caruso, R. A. Templating of macroporous or swollen macrostructured polymers. In Hierarchically Structured Porous Materials: From Nanoscience to Catalysis, Separation, Optics, Energy, and Life Science; Su, B.-L., Sanchez, C., Yang, X.-Y., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011; pp 131−172. (34) Kimling, M. C.; Caruso, R. A. Sol-gel synthesis of hierarchically porous TiO2 beads using calcium alginate beads as sacrificial templates. J. Mater. Chem. 2012, 22, 4073−4082. (35) Abramson, S.; Meiller, C.; Beaunier, P.; Dupuis, V.; Perrigaud, L.; Bee, A.; Cabuil, V. Highly porous and monodisperse magnetic silica beads prepared by a green templating method. J. Mater. Chem. 2010, 20, 4916−4924. (36) Langmuir, I. The constitution and fundamental properties of solids and liquids. Part I Solids. J. Am. Chem. Soc. 1916, 38, 2221− 2295. (37) Messina, P. V.; Schulz, P. C. Adsorption of reactive dyes on titania-silica mesoporous materials. J. Colloid Interface Sci. 2006, 299, 305−320. (38) Ho, Y. S. Review of second-order models for adsorption systems. J. Hazard. Mater. 2006, 136, 681−689. (39) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: the Physics and Chemistry of Sol-Gel Processing; Academic Press, 1990. (40) El Haskouri, J.; de Zarate, D. O.; Guillem, C.; Latorre, J.; Caldes, M.; Beltran, A.; Beltran, D.; Descalzo, A. B.; Rodriguez-Lopez, G.; Martinez-Manez, R.; Marcos, M. D.; Amoros, P. Silica-based powders and monoliths with bimodal pore systems. Chem. Commun. 2002, 330−331. (41) Perron, H.; Roques, J.; Domain, C.; Drot, R.; Simoni, E.; Catalette, H. Theoretical investigation of the uranyl ion sorption on the rutile TiO2(110) face. Inorg. Chem. 2008, 47, 10991−10997. (42) Vandenborre, J.; Drot, R.; Simoni, E. Interaction mechanisms between uranium(VI) and rutile titanium dioxide: From single crystal to powder. Inorg. Chem. 2007, 46, 1291−1296. (43) Lomenech, C.; Simoni, E.; Drot, R.; Ehrhardt, J. J.; Mielczarski, J. Sorption of uranium(VI) species on zircon: Structural investigation of the solid/solution interface. J. Colloid Interface Sci. 2003, 261, 221− 232. (44) Zou, H.; Lin, Y. S. Structural and surface chemical properties of sol-gel derived TiO2-ZrO2 oxides. Appl. Catal., A 2004, 265, 35−42. (45) Baisden, P. A.; Choppin, G. R. Nuclear waste management and the nuclear fuel cycle. Radiochemistry and Nuclear Chemistry,[Ed. Sándor Nagy], in Encyclopedia of Life Support Systems (EOLSS), Developed under the Auspices of the UNESCO; Eolss Publishers: Oxford, UK; 2008, 9, 17. (46) Aust, U.; Benfer, S.; Dietze, M.; Rost, A.; Tomandl, G. Development of microporous ceramic membranes in the system TiO2/ZrO2. J. Membr. Sci. 2006, 281, 463−471. (47) Shin, Y. S.; Burleigh, M. C.; Dai, S.; Barnes, C. E.; Xue, Z. L. Investigation of uranyl adsorption on mesoporous titanium-based sorbents. Radiochim. Acta 1999, 84, 37−42. (48) Pavel, C. C.; Walter, M.; Popa, K. Improvement of retention capacity of ETS-10 for uranyl ions by porosity modification and their immobilization into a titanosilicate matrix. J. Mater. Chem. 2008, 18, 3342−3346. (49) Lefevre, G.; Kneppers, J.; Fedoroff, M. Sorption of uranyl ions on titanium oxide studied by ATR-IR spectroscopy. J. Colloid Interface Sci. 2008, 327, 15−20.

(50) Chisholm-Brause, C. J.; Berg, J. M.; Matzner, R. A.; Morris, D. E. Uranium(VI) sorption complexes on montmorillonite as a function of solution chemistry. J. Colloid Interface Sci. 2001, 233, 38−49. (51) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley and Sons: New York, 1976. (52) Tsushima, S.; Suzuki, A. Hydration numbers of pentavalent and hexavalent uranyl, neptunyl, and plutonyl. J. Mol. Struct.-Theochem. 2000, 529, 21−25. (53) Mellah, A.; Chegrouche, S.; Barkat, M. The removal of uranium(VI) from aqueous solutions onto activated carbon: Kinetic and thermodynamic investigations. J. Colloid Interface Sci. 2006, 296, 434−441.

7920

dx.doi.org/10.1021/es3011157 | Environ. Sci. Technol. 2012, 46, 7913−7920