Langmuir 2000, 16, 3355-3360
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Mo Sorption by Thermally Treated Hydrotalcites J. Serrano,†,‡ V. Bertin,‡ and S. Bulbulian*,†
Instituto Nacional de Investigaciones Nucleares, A. P. 18-1027, Col. Escando´ n, Delegacio´ n Miguel Hidalgo, C. P. 11801, Me´ xico, D. F., Me´ xico, and Universidad Auto´ noma Metropolitana Iztapalapa, Michoaca´ n y la Purı´sima, A. P. 55-532, C. P. 09340, Me´ xico, D. F., Me´ xico Received April 29, 1999. In Final Form: December 13, 1999 MoO42- ions were sorbed in calcined hydrotalcite contained in a column. It was found that 98% of 99mTc formed by 99Mo decay was eluted through the column in the form of pertechnetate. The content of radionuclides was determined by γ-spectrometry, and natural molybdenum was measured by neutron activation analysis. Solids were characterized by thermal analysis, X-ray diffraction, and infrared spectroscopy. Through batch experiments, the hydrotalcite capacity toward molybdate ions (1.12 × 10-3 M) was found to be 3.2 mequiv g-1. It was found that the high molybdate adsorbing capacity of calcined hydrotalcite could be utilized in designing a 99mTc generator made with low specific activity 99Mo-molybdate samples.
Introduction The physical-chemical properties of hydrotalcite (also alluded to in the text as HT) and related compounds have been the subject of attention in recent years.1-4 Hydrotalcites have a chemical composition given by the formula Mg6Al2(OH)16CO3‚4H2O, where Mg2+ and Al3+ ions are randomly distributed in octahedral layers stacked on top of each other forming layered double hydroxides. HT bears a positive electrical charge which in nature is neutralized by carbonate ions located in the interlayer space together with water molecules.5-12 The minimal general formula of HT type compounds is
[M1-x2+Mx3+(OH)2](An-)x/nmH2O
(1)
where M2+ is a divalent cation (Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, or Ca2+), M3+ is a trivalent ion (Al3+, Cr3+, Fe3+, Co3+, Ni3+, or La3+), An- ) CO32-, Cl-, etc., and 0.17 < x < 0.33.11 When HT-type compounds are heated to 500 or 600 °C, they undergo dehydroxylation and decarbonation, the layer structure is destroyed, and a magnesium-aluminum oxide is formed.13 The calcined hydrotalcite (also referred to in this work as CHT) samples can be rehydrated again by contacting the solid with an
aqueous solution containing the desired interlayer anions, thus recovering the original structure.14 Although Drezdzon7 prepared an intercalated organic anion precursor that was subsequently exchanged with the appropriate polymolybdate under mildly acidic conditions to produce Mg12Al6(OH)36(Mo7O24)‚xH2O, as far as we know, the calcined product of hydrotalcite has not been yet investigated as a 99MoO42- sorbent. The calcined product of hydrotalcite can be applied to the removal of 99Mo anionic species from radioactive wastewater, and also, when it is packed in a column it can be useful as a support for 99MoO42-, from which 99mTc can be separated through an elution process. 99 Mo is a radioactive isotope which decays to 99mTc, whose compounds are utilized for medical purposes. Since the latter is very short lived (6 h), it is generally separated immediately from its longer living parent (66 h), 99Mo. This radioisotope can be obtained by 235U neutron fission or by neutron activation of natural molybdenum as follows: 235
U(n, f) 99Mo + fission products + xn (∼ 200 MeV/fission)
or 98
Mo(n, γ)99Mo
†
Instituto Nacional de Investigaciones Nucleares. ‡ Universidad Auto ´ noma Metropolitana Iztapalapa. * To whom correspondence should be addressed. (1) Fetter, G.; Ramos, E.; Olguı´n, M. T.; Bosch, P.; Lo´pez, T.; Bulbulian, S. J. Radioanal. Nucl. Chem. 1997, 221, 63. (2) Kooli, F.; Depege, C.; Ennaqadi, A.; De Roy, A.; Besse, J. P. Clays Clay Miner. 1997, 45, 92. (3) Hansen, H. C. B.; Taylor, R. M. Clay Miner. 1991, 26, 311. (4) Miyata, S. Clays Clay Miner. 1983, 31, 305. (5) Kang, M. J.; Rhee, S. W.; Moon, H. Radiochim. Acta 1996, 75, 169. (6) Shin, H.-S.; Kim, M.-J.; Nam, S.-Y.; Moon, H.-C. Water Sci. Technol. 1996, 34, 161. (7) Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. (8) Cavani, F.; Trifiro, F.; Vaccani, A. Catal. Today 1991, 11, 173. (9) Hermosin, M. C.; Pavlovic, I.; Ulibarri, M. A.; Cornejo, J. Water Res. 1996, 30, 171. (10) Miyata, S.; Kumura, T. Chem. Lett. 1973, 843. (11) Rey, F.; Fornes, V.; Rojo, J. M. J. Chem. Soc., Faraday Trans. 1992, 88, 2233. (12) Reichle, E. T.; Kang, S. Y.; Everhardt, D. S. J. Catal. 1996, 101, 351. (13) Chatelet, L.; Bottero, J. Y.; Yvon, J.; Bouchelaghen, A. Colloids Surf. A 1996, 111, 167.
For instant use 99mTc is milked from so-called generators as described elsewhere.15-17 Generally these generators consist of columns filled with a suitable sorbent for molybdenum, from which 99mTc can be eluted, while 99Mo remains fixed on the sorbent. After its regeneration, 99mTc can be eluted again. This process can be repeated several times until 99Mo has completely decayed. The most popular 99mTc generator is the one in which alumina is the sorbent material. However, since alumina’s capacity to sorb molybdate ions is very low, only high specific activity of 99MoO 2- ions as separated generally from fission products 4 of 235U can be utilized in this kind of 99mTc generator. Besides, the fission 99Mo production process involves the (14) Miyata, S. Clays Clay Miner. 1975, 23, 369. (15) Tanase, M.; Tatenuma, K.; Ishikawa, K.; Kurosawa, K.; Nishino, M.; Hasegawa, Y. Appl. Radiat. Isot. 1997, 48, 607. (16) El-Kolaly, I. M. J. Radioanal. Nucl. Chem. 1993, 170, 607. (17) El-Absy, A. M. Radiochim. Acta 1991, 55, 23.
10.1021/la9905259 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000
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Serrano et al.
Figure 1. (b) Measurement and standard deviation of remaining MoO42- in the solution after sorption on calcined hydrotalcite (CHT) vs time of contact between the CHT and the MoO42- solution. (0) pH change of the MoO42- solution. [MoO42-]0 ) 1.12 × 10-3 mol/L.
handling of various fission products in a high radiation field. To avoid these unfavorable conditions, we propose to utilize a generator made with hydrotalcite and low specific activity 99Mo. The aim of this work is to study the separation of 99mTc species from 99MoO42- sorbed in hydrotalcite by means of dynamic experiments with the view to obtain in the future a 99mTc generator that can be prepared with low specific activity 99Mo. Materials and Methods
time 2 mL aliquots were taken with a pipet and filtered with 0.45 µm Millipore filters. In these fractions the Mo concentration was determined by NAA. CO2 dissolved in the reaction systems was not eliminated. pH values of sodium molybdate solutions before, during, and after the sorption process were measured with a Methrom Herisau pHmeter (model E520). 99Mo-99mTc sorption dynamic experiments were performed with columns packed with CHT samples and using NaCl solutions for elution of 99mTc. 99 Mo and 99mTc activities eluted through the columns were measured by utilizing the photopeaks of 740 and 140 keV, respectively. Solid samples were characterized by thermal analysis and X-ray diffraction as described elsewhere.19 Transmission infrared spectra were obtained with a Nicolet Fourier transform infrared spectrometer (model 550).
Hydrotalcite, Mg6Al2(OH)16CO3‚4H2O (HT), was prepared in the laboratory as described previously by Sato et al.:18 1000 mL of 0.25 mol L-1 AlCl3 and 0.75 mol L-1 MgCl2‚6H2O aqueous solution were added dropwise to 1000 mL of 0.5 mol L-1 Na2CO3 and 2.5 mol L-1 NaOH under vigorous stirring. Both solutions were previously heated and maintained at 60 °C during the stirring. Once produced, hydrotalcite was separated by filtration and then washed by dialysis with deionized water until no chloride was detected with the help of silver nitrate aqueous solution. The hydrotalcite was dried at room temperature in an open dish for 5 days and then again at 80 °C in air for 3 h, crushed in an agatha mortar, and finally sieved to obtain HT grain size of 333 µm. The x value in the general formula (1) of the HT sample compound prepared was 0.25. The calcined product CHT was obtained by heating HT at 500 °C in air for 18 h. This was the product utilized as sorbent of MoO42- ions. On the other hand, 99MoO42- of high specific activity in aqueous solution was acquired as a fission product from Nordion International. Low specific activity 99Mo in aqueous solution was prepared by mixing the high specific activity 99Mo with natural Na2MoO4 solution. Natural molybdenum (Na2MoO4) was also utilized for batch experiments. The molybdenum content in the batch experiment solution was determined by neutron activation analysis (NAA). Aliquots of 1 mL of the solution containing molybdenum were neutron irradiated for 5 s in the pneumatic tube facility of the TRIGA Mark III nuclear reactor with an approximate neutron flux of 1013 n cm-2 s-1. The photopeak of 0.192 MeV from 101Mo produced by the nuclear reaction 100Mo(n, γ)101Mo was utilized to determine the 101Mo radioactivity by using a gamma spectrometer that was set up with a Ge/hyperpure solid-state detector and a computerized multichannel analyzer. All anion sorption isotherms were determined following the same procedure: batchwise experiments were carried out in 500 mL flasks at 25 °C, CHT samples were added to the nonradioactive 1.12 × 10-3 M Na2MoO4 aqueous solution, and the resulting suspensions were continuously shaken for 4 days. From time to
MoO4 Sorption by Batch Experiments. Figure 1 shows the time dependence of MoO42- sorption by the CHT samples, as well as the pH change, studied by batch experiments. The initial Na2MoO4 solution concentration was 1.12 × 10-3 M ([MoO42- ]0), and the corresponding initial pH value was 5.2. The figure shows typical results obtained with a grain size of 333 µm. Once calcined, CHT samples were shaken with the MoO42- aqueous solution, and the lamellar structure of hydrotalcite was recovered with MoO42- and some CO32- ions (coming from the CO2 dissolved in the molybdate solutions) as neutralizing ions, forming the MoO42--exchanged HT, also alluded to in the text as MoO4-HT. As can be seen in the figure, the MoO42concentration in the solution decreased continuously for increasing contact time between the initial solid phase CHT and the molybdate solution down to a minimum and then raised slightly until the molybdate ion equilibrium concentration was reached in about 75 h. The statistical deviations of the measurements are shown in the Figure 1. The minimum in the molybdate ion concentration corresponds to the maximum sorption of molybdate ions on CHT, while the raising of its concentration corresponds to a molybdate ion desorption probably from the external surface of hydrotalcite grains. The molybdate equilibrium sorption value (3.25 ( 0.4 mequiv of Mo/g) was higher than the sorption corresponding to the anion exchange capacity (2.1 mequiv/g) of HT samples, which is probably due to an additional adsorption on the external surfaces
(18) Sato, T.; Fujita, H.; Endo, T.; Shimada, M.; Tsunashima, A. React. Solids 1988, 5, 219.
(19) Olguı´n, M. T.; Bosch, P.; Acosta, D.; Bulbulian, S. Clays Clay Miner. 1998, 46, 567.
Results and Discussion 2-
99Mo
Sorption by Hydrotalcites
Table 1.
99Mo-Molybdate
Sorption by CHT at Different Initial pH Valuesa 99MoO 24
99MoO 24
sorption by CHT (%) 92.74 95.41 87.82
a
Langmuir, Vol. 16, No. 7, 2000 3357
pH0
pHeq
3 5 7
9.5 9.6 9.9
sorption by CHT (%) 82.66 67.85 27.00
Table 2. elution no.
eluted 9mTc (%)
eluted 99Mo
aq NaCl solution (mL)
1 2 3 1 2 3 1 2 3
98.49 66.19 63.24 99.80 90.00 93.00 76.73 47.40 40.09
