Article pubs.acs.org/IECR
Preparation of 137Cs Microspheres for Their Use in Computer Automated Radioactive Particle Tracking Studies Sanjay Kumar Saxena,† K. T. Pillai,‡ Ramu Ram,† and Ashutosh Dash†,* †
Radiopharmaceuticals Division, ‡Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India ABSTRACT: This paper describes a method for the preparation of 137Cs labeled mordenite-coated spheroidal alumina particles [∼ 0.7 mm (ϕ)] for their utility in the computer automated radioactive particle tracking technique for the calibration of detectors. Mordenite was coated successfully, in situ, on alumina microspheres under hydrothermal conditions and characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses. A thorough optimization of experimental parameters such as pH of the adsorbing solution, reaction time, carrier concentration, reaction volume, and temperature, etc., were carried out to arrive at the conditions resulting in optimum impregnation of 137Cs activity into the microspheres. Under the optimized conditions, about 2.7−3.1 MBq (73−85 μCi) of 137Cs could be impregnated on individual microspheres in a reproducible manner. The radioactive microspheres were subsequently covered with a thin layer of polystyrene to prevent the dispersion of 137Cs activity during use.
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INTRODUCTION The technical and economic benefits of the radiotracer technology have been well demonstrated and recognized by the industrial sectors throughout the world.1−3 The use of radiotracer techniques for problem-solving in industries is increasing steadily as more and more potential applications are identified. Computer automated radioactive particle tracking (CARPT) is one of the promising techniques for noninvasive probing of flow in industrial systems such as fluidized beds and bubble columns. The technique is based on the principle of tracking the motion of a single particle as a marker of a specific component of the phase for mapping the velocity field in a flow vessel.4−6CARPT is one of the noninvasive technique that maps the flow field in the whole chemical reactor and provides particle Lagrangian velocities throughout the process column. The CARPT technique is based on the principle of tracking the motion of a single particle as a marker of a specific component of the phase whose mass flow in a vessel is to be mapped. To track the liquids, a small, encapsulated radioactive tracer particle is required, which should float with respect to the liquid phase.7 For tracing particulate solids, the radiotracer particle should be of exactly the same dimensions as the solids to be monitored.6,8−10 The radionuclide chosen for this purpose should preferentially emit gamma rays of adequate energy, have a reasonable half-life, be readily available, and be relatively inexpensive. The gamma energy should be sufficient to transmit through thick metal walls to allow detection outside the vessel or tubing. The long half- life offers the possibilities of using the tracer repeatedly over a reasonable period of time. Several radionuclides such as 46Sc, 198Au, 192Ir, 137Cs, and 131I have been recommended for use in CARPT.11 To obtain reliable results in CARPT studies, regular calibration of the detectors is absolutely essential, with the radioactive particles having similar physicochemical characteristics as those of the particles to be studied under field investigation. We undertook the development of radioactive particles for such studies at the request of another laboratory of our Institute. This requires © 2012 American Chemical Society
judicious selection of a radionuclide, a process to make suitably sized particles, and development of an appropriate technique to incorporate the radioactivity into the particles. In this context, 137Cs, a γ-emitter (662 keV) with a half-life of 30 years is one such radionuclide that can be considered to be ideal for this application. In the quest for a cesium selective matrix, our attention was turned toward mordenite. The unique attributes of mordenite such as its capability to adsorb Cs+ selectively from an aqueous solution,12−15 ability to immobilize 137 Cs on thermal treatment and the radiation stability of the resultant matrix, make it an ideal matrix to incorporate 137Cs. The need for an appropriate process to obtain small particles of zeolite (mordenite) of uniform sizes, prompted us to use a sol− gel process. We have reported the hydrothermal synthesis of zeolite (mordenite) on alumina microspheres using the internal gelation process (IGP).16,17 As a logical extension, here we report the laboratory scale preparation of 137Cs labeled mordenite-coated alumina microspheres for use in CARPT. The studies describe preparation of mordenite-coated alumina microspheres by the sol gel process, optimization of experimental parameters to incorporate the required quantity of 137Cs activity into the microspheres, immobilization of 137Cs on the microspheres, and covering the radioactive microspheres with a thin layer of organic polymer.
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EXPERIMENTAL SECTION Materials. Reagents such as nitric acid, aluminum nitrate, carbon tetrachloride (CCl4), urea, hexamethylenetetramine, etc. were of analytical grade and were procured from E. Merck (Germany) and BDH (England). Mordenite used in this investigation was synthesized in situ using no organic templates. Cesium-137 as 137CsNO3 was available in-house. Solutions Received: Revised: Accepted: Published: 4485
January 23, 2012 March 6, 2012 March 6, 2012 March 6, 2012 dx.doi.org/10.1021/ie300205p | Ind. Eng. Chem. Res. 2012, 51, 4485−4492
Industrial & Engineering Chemistry Research
Figure 1. Experimental arrangement for the calcination of
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Cs-loaded mordenite-coated alumina microspheres.
Chemical Stability. The chemical stability of the microspheres was assessed in several mineral acids and bases, such as HCl, HNO3, NaOH, and NH4OH. A weighed amount of the microsphere material (1 g) was placed in 50 mL of the solvent of interest and kept for 24 h with continuous shaking at room temperature. Subsequently it was filtered, and the level of Al3+ and Si4+ metal ions in the filtrate was determined by ICP−AES. XRD, SEM, and EDS Analysis. The shape and surface morphology of the mordenite-coated alumina microspheres were examined by SEM. The quality of the mordenite-coated surfaces was examined by XRD analysis. Chemical composition of the surface was studied by EDS analysis of a dummy microsphere prepared in an identical manner as of the active microspheres. Determination of 137Cs Adsorptive Parameters. Several experiments were carried out for the optimization of the adsorptive parameters such as reaction time, pH of the feed, reaction temperature, reaction volume, and inactive Cs concentration to obtain the best possible yield. In this study, a solution containing ∼37 kBq (1 μCi) of 137Cs was taken. The pH of the resulting reaction solution was adjusted to the desired level with the help of dilute NaOH or HNO3 and shaken for a predetermined time at the desired temperature. The amount of 137Cs taken up by the substrate was estimated as the difference in activity of the solution before and after adsorption, as measured by gamma spectrometry from the counts acquired under the 662 keV photo peak. The percent adsorption (%) was calculated using the relationship
were prepared from doubly demineralized water obtained by passing distilled water through a Millipore Milli-Q water purification system. Mordenite-coated alumina microspheres were prepared as per the procedure described. The X-ray diffraction (XRD) patterns were recorded on a Philips X-ray diffractometer (model PW 1710) with Ni-filtered Cu Kα radiation, using silicon as the external standard. The XRD patterns were analyzed by comparison with the reported ones. Scanning electron microscope, model Vega MV-2300T/40, supplied by M/s TESCAN, Czechoslovakia, was used for scanning electron microscopy (SEM) analysis. The chemical composition was obtained by energy dispersive X-ray spectroscopy (EDS) analysis (Oxford, model INCA E350). Determination of trace level of metal contaminations was done using inductively coupled plasma−atomic emission spectroscopy (ICP−AES JY-238, Emission Horiba Group, France). A HPGe coaxial photon detector system (Canberra Eurisys, France) coupled to a multichannel analyzer with a 0.5 keV resolution and range from 1.8 keV to 2 MeV and a 152Eu standard source for energy and efficiency calibration was used for radioactivity measurements of individual radiolabeled particles. All other GR/AR grade chemicals were procured from local manufacturers in India. Preparation of Mordenite-Coated Alumina Microspheres. Mordenite-coated alumina microspheres [∼0.7 mm (ϕ)] were prepared using the sol−gel technique. In brief, alumina gel spheres were first prepared by an internal gelation process (IGP) in which the feed solution of aluminum nitrate containing urea and hexamine was dispersed as droplets into hot silicone oil under optimized conditions.16 The gel spheres were degreased with CCl4, washed with dilute NH4OH, dried at 100 °C, and finally calcined at 700 °C. Subsequently, the alumina−mordenite composite was synthesized by treating 5 g of alumina spheres with 18 mL of 0.8 M sodium silicate solution ([Si] = 1.6 M) in Teflon-lined pressure vessel (capacity 24 mL) at 150 °C for 100 h and thereafter kept at ambient temperature for another 100 h.17
% adsorption =
Ai − A f 100 Ai
(1)
where Ai and Af are the initial and final radioactivity of 137Cs, respectively. Preparation of 137Cs Microspheres. The preparation of radioactive microspheres required for the intended application was carried out using mordenite-coated alumina microspheres prepared by the sol gel method under the optimized conditions. 4486
dx.doi.org/10.1021/ie300205p | Ind. Eng. Chem. Res. 2012, 51, 4485−4492
Industrial & Engineering Chemistry Research
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uniform sizes, mostly spherical in morphology, and contained very little broken particles. Chemical Stability. It was observed that the microspheres were insoluble in water; dilute mineral acids and alkali (up to 4 M) as
