School of Chemistry - Theses
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ItemSol-gel template synthesis of porous metal oxide beads for sequestration of radionuclides( 2011)Meeting growing energy demands and the need to seek for alternative, cleaner and sustainable sources of energy in the fight to reduce the negative environmental impacts of greenhouse gas emissions (from fossil fuel-related energy schemes) could potentially present itself in the form of nuclear energy. However, several challenges remain regarding the management of the waste generated from nuclear power plants. One of the current practices adopted for the management of spent nuclear fuel is through immobilization in borosilicate glass matrices, which then remain at existing storage facilities. The future of nuclear waste containment is now tending towards the long-term disposal of the immobilized waste in deep geological sites. Durable and stable host matrices are required towards maximizing the life-span of repositories but also towards reducing risks of leaching of the radionuclides. Studies have shown the limitations of borosilicate glass matrices for such purposes, hence urging research towards non-siliceous, inorganic materials which have shown better prospects. The life-span of the repository can be further improved if recycling of the spent fuel is practised. Partitioning of radioactive materials allows reduction in volume of the waste and toxic radionuclides. Most importantly, it allows the recycling of fissile materials, which can be re-used for energy production. In this thesis, hierarchically porous inorganic (TiO2 or mixed TiO2/ZrO2) millimeter-sized beads were synthesized. Their suitability as radionuclide (selective) adsorbents was probed. The synthesis of structurally defined inorganic beads was achieved via a sol-gel templating technique which involved the inorganic coating of preformed template beads, with defined structural properties, and their subsequent removal by calcination. A range of polymeric beads were developed in-house and subsequently examined as sacrificial templates for the synthesis of the inorganic beads. Porous poly(vinylidene fluoride-co-hexafluoropropylene) beads were produced via phase separation processes. This involved the dripping of a polymeric solution into a coagulation bath. Polymeric beads of variable pore architectures were attained by altering the nature of the baths: Aqueous-based baths at room temperature or a liquid N2 bath yielded cellular or radial tubular porous structures, respectively; a combination of both structures was obtained for baths at 2-5 °C. Rigid bead structures were obtained at higher polymer concentrations. Despite the structurally sound scaffold of the polymeric beads, they were not suitable as templates. The resulting inorganic beads were mechanically weak. Robust hierarchically porous carbon-metal oxide beads were obtained by carbonizing the beads in an inert atmosphere. The porous structure of the initial scaffold was preserved in the final beads. Porous agarose beads were prepared using a non-conventional method whereby a polymeric solution (agarose dissolved in dimethyl sulfoxide) was dripped into liquid N2. The beads featured an interconnected macroporous fibrous structure and mesopore sizes which varied with the content of agarose. The regularity (shape and size) of the beads improved at higher agarose contents. Similar characteristics were imparted in the mixed TiO2/ZrO2 beads. The addition of a secondary metal oxide (ZrO2) component to TiO2 influenced the properties of the beads: The highest surface areas were achieved at a 25 wt% Zr regardless of the agarose content; increasingly denser porous structures were obtained at higher Zr contents and the pure oxides displayed larger interparticulate mesopores than the mixed oxides. Yet, the inorganic beads lacked sufficient mechanical stability. Mechanically robust inorganic (TiO2 or mixed TiO2/ZrO2) beads were produced using alginate beads as templates. The variable structural properties of the alginate beads (obtained by altering the bead synthesis, e.g. concentration of sodium alginate, NaAlg, and calcium ions, Ca2+, in the gelation bath) were used to control the properties of the TiO2 beads. The surface area of the TiO2 beads reached maxima using template beads prepared in a 0.27 M Ca2+ bath. A 1 wt% NaAlg-templated system yielded anatase beads whereas semi-crystalline TiO2 beads were obtained for the 2 wt% NaAlg-templated system. The TiO2 (anatase) beads displayed high surface areas of 170 ± 10 m2 g-1 (calcined at 450 °C) and beads calcined at 700 °C remained anatase with a decrease in surface area to 46.1 ± 0.5 m2 g-1. The functionalization of the TiO2 beads was studied using an amino-bisphosphonate molecule. The solution pH and surface area of the TiO2 beads influenced the loading. Hierarchically porous TiO2/ZrO2 beads were prepared with varying parameters (such as surface area, pore size and surface hydroxyl groups), attained through varying Ti/Zr composition (22, 36 and 82 wt% Zr) or crystallinity (amorphous or crystalline), achieved by varying calcination temperatures (500 or 700 °C). The effect of the above parameters on the uptake a uranyl, and the hydrolytic and radiolytic properties of the beads were assessed. Higher adsorption capacities were obtained for the crystalline beads. This was attributed to the higher surface hydroxyl density, the presence of limited microporosity and the larger mesopores in the crystalline beads, relative to the amorphous beads. The pore hierarchy of the crystalline beads resulted in improved uranyl adsorption rates over reported exclusively mesoporous beads and comparable rates to reported xerogel powders. The crystalline beads displayed superior hydrolytic properties than the amorphous beads (22 wt% Zr sample showed the highest resistance to matrix leaching in HNO3 solutions). The crystalline beads were stable to amorphization when subjected to γ-irradiation. TiO2/ZrO2 beads were functionalized with amino-bisphosphonates and investigated for the selective uptake of radiotracers: actinide (239Pu) and lanthanides (152Sm, 153Eu, 158Gd) in HNO3 media. The two amino-bisphosphonate molecules studied both comprised of a bisphosphonic group, used as anchor points to the metal oxide support, and a pendant group (either -NH2 (Alen) or -N(CH3)2 (Alenmod)). Both amino-bisphosphonate molecules displayed higher affinities towards Pu, thereby affording separation from the lanthanides. However, Alenmod-grafted molecules displayed superior extraction performance. The nature of the porous bead support onto which the organic moieties were grafted also influenced the adsorption process. The uptake of the radiotracers was more selective towards the functionalized crystalline TiO2/ZrO2 beads than towards the functionalized amorphous TiO2/ZrO2 beads. The larger porous structure of the crystalline matrix was believed to facilitate the access of the radionuclides to the grafted organic moieties. The organic loading did not direct the selective uptake process. The hydrolytic stability of the Ti or Zr matrix was further improved under the presence of the grafted organic moieties.