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Abstract
The Ph.D. Thesis entitled: “Synthesis and characterization of Nano Zirconia for use in chromatographic 188W/188Re Generator” consists of three chapters; Introduction, Experimental and Results and Discussion.
Chapter 1 (Introduction) includes a brief review of (i) the current status and challenges related to neutron irradiation of tungsten targets in nuclear reactors, in addition to preparation of 188W/188Re generators, (ii) some important synthesis techniques in nanotechnology, (iii) preparation of zirconia and zirconia-contained materials and their applications, (iv) chemistry of tungsten, (v) chemistry of rhenium and (vi) 188W/188Re generators (sublimation, solvent extraction, electrochemical, and chromatographic column generators).
Chapter 2 (Experimental) comprises (i) list of the used chemical reagents, (ii) list of instruments and equipment, (iii) preparation and description of 188W radiotracer solution, (iv) preparation conditions of nano zirconia gels (for hydrothermal and isoamyl alcohol synthesis routes), (v) description of batch distribution studies, (vi) characterization techniques of the synthesized zirconia gels, (vii) description of kinetic and sorption isotherm studies, (viii) 188W breakthrough capacity determination method for zirconia gel chromatographic columns and (ix) description of preparation and performance of 188W/188Re generators.
Chapter 3 (Results and Discussion) represents the obtained results of the studies described in Chapter 2 and their discussion. Batch distribution studies revealed that (as indicated from the determined distribution coefficient values) the optimum sorption of tungstate(VI)-188W was achieved on zirconia gel synthesized via hydrothermal route (at a hydrothermal reaction temperature of 150 °C and gel drying temperature of 350 °C) from Cl- aqueous solution at pH 1.5. On the other hand, zirconia gel synthesized via isoamyl alcohol route and dried at 50 °C achieved the highest distribution coefficient of tungstate(VI)-188W. The optimum zirconia gels synthesized via hydrothermal and isoamyl alcohol routes (gel I and gel II, respectively) were chosen for the subsequent characterization studies. According to XRD studies, crystallite size of gel I was 42 nm and that of gel II was 9 nm. FESEM images indicated an indefinite morphology of gel I and a jelly-like shape of gel II. HRTEM images clarified that the average particle size of gel I was 100 nm while that of gel II was 70 nm. Hydrodynamic light scattering (DLS) studies indicated that Z-average hydrodynamic diameter of gel I was " ~ " 600 nm whereas that of gel II was " ~ " 200 nm with Zeta potentials of 5.95 and 18.4 mV, respectively. As indicated from the pH-metric titration curves of gels I and II, points of zero charge (PZC’s) were found to be pH 3 and pH 4.26 for gels I and II, respectively. As determined from calculations of nitrogen adsorption/desorption chromatographic studies, surface areas were 113.80 and 70.59 m2/g, micropore volumes were 4.3310-2 and 6.0410-3 cm3/g, cumulative mesopore volumes were 0.412 and 0.111 cm3/g and mesopore diameters were 3.44 and 3.91 nm for gels I and II, respectively. Kinetic studies proved that the sorption process of tungstate(VI)-188W on both nano ZrO2 gels (gel I and gel II) followed pseudo 2nd order reaction model with activation energies (Ea’s) of -33.86 and -22.34 kJ/mol, respectively. It was found that sorption isotherms of tungstate(VI)-188W on both nano ZrO2 gels followed Langmuir model. The sorption process in both cases was found to be spontaneous, endothermic and physical in nature with standard enthalpy (H°) and entropy (S°) values of 28.9 and 0.1 kJ/mol for gel I and 52.0 and 0.2 kJ/mol for gel II, respectively. Standard Gibbs free energy (G°) values were found to be -4.3, -6.0 and -8.2 kJ/mol for sorption of tungstate(VI)-188W on gel I at 25, 40 and 60 °C, respectively, whereas they were found to be -6.4, -9.3 and -13.2 kJ/mol for sorption on gel II at 25, 40 and 60 °C, respectively. Breakthrough capacities (Q50%’s) of gels I and II were 73.5 and 153 mg W(VI)/g, respectively. 188W/188Re chromatographic column generators based on gels I and II were successfully prepared and eluted to study their performance. Each generator was eluted for 40 times along 60 days. Rhenium-188 elution yields were 723 and 715 % (at elution flow rate of 2 ml/min) for generators based on gels I and II, respectively, while radionuclidic purity of the eluted 188Re was > 99.99 % in both cases. Radiochemical purities of the eluted 188Re were 95.7-96.9 % and 96.2-98.4 % for generators based on gels I and II, respectively, with eluate pH-value ranging from " ~ "5-7 in both cases. High chemical purities were obtained for 188Re eluates from gel I and gel II generators, where low-concentration Zr impurities were detected in both cases ( 1.9 and 0.6 ppm, respectively).