1992 sami-el-waer-1992

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Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 157, No. 1 (1992) 3-14 SORPTION OF TECHNETIUM ON INORGANIC SORBENTS AND NATURAL MINERALS S. EL-WEAR,* K. E. GERMAN,** V. F. PERETRUKHIN** *Ta/ura Nuclear Researche Center, Tripoli (Libya) **Institute of Physical Chemistry of Acad. Scs USSR Leninski prospect, 31 Moscow, 117 915 (USSR) (Received November 5, 1990) The sorption behavior of the pertechnetate anion in various solid-solution systems under aerobic conditions and pH 1.3-12.5 has been investigated. Batch techniques were employed. On most of natural minerals only surface adsorption occurs. Rs-values were no larger than 2.0 mlo g= 1. Adsorption on various natural minerals and rocks such as sandstone, basalt, granite, pyrite, peat and others are comparaed with the analogous processes on artificial inorganic sorbents: titanium oxides (thermoxide-34, thermoxide-3), erystaline cadmium sulfide, zirconium phosphate, and complex inorganic sorbents: antimony oxide - silicon oxide - phosphorus pentoxide, antimony oxide - silicon oxide - aluminium oxide, lithium oxide - manganese oxide - aluminium oxide - water, lithium oxide - titanium oxide - chromium oxide - water. For comparison the sorption of Tc on some organic sorbents was included. The solubility of T% S7 in water was measured to be 0.257 g/t. It has been shown that preliminary irradiation of sorbents such as sandstone, peat and humic acid by -/-rays with doses not less than 10 ~ rad results in the decrease of To(VII) sorption. Introduction All over the world no less than 19000 TBq of 99Tc had already been produced in nuclear reactors up to 1990. As considered, 10% of this amount has been dissipated in the environment) Estimations for the last years show that around 160 TBq of 99Tc has been released during atmospheric atomic bomb testing) On the other hand, as was mentioned by GARCIA-LEON, the continuous increase of 99 roTe.generator s in nuclear medicine should be given attention) Although some authors consider this contribution of 99Tc to the environment to be negligible, 4 to our mind it should be taken into account that all this technetium penetrates the human surrounding through the human body. So, ecological aspects of Tc-chemistry now are of increasing importance, and consist of problems of To-determination, 3 - 7 volatilization,S-11 solubility measurement and leaching, 12-17 sorption,1 s-20 and the diffusion of Tc.21,22 Many problems in Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

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Technetium sorption by anionexchange resins, inorganic sorbents, natural minerals and rocks. Effect of radiation. Precipitstion of sulfide. Solubility of sulfide.

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  • 1.Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 157, No. 1 (1992) 3-14SORPTION OF TECHNETIUM ON INORGANIC SORBENTS AND NATURAL MINERALS S. EL-WEAR,* K. E. GERMAN,** V. F. PERETRUKHIN***Ta/ura Nuclear Researche Center, Tripoli (Libya) **Institute o f Physical Chemistry o f Acad. Scs USSR Leninski prospect, 31 Moscow, 117 915 (USSR) (Received November 5, 1990) The sorption behavior of the pertechnetate anion in various solid-solution systems under aerobic conditions and pH 1.3-12.5 has been investigated. Batch techniques were employed. On most of natural minerals only surface adsorption occurs. Rs-values were no larger than 2.0 m l o g= 1. Adsorption on various natural minerals and rocks such as sandstone, basalt, granite, pyrite, peat and others are comparaed with the analogous processes on artificial inorganic sorbents: titanium oxides (thermoxide-34, thermoxide-3), erystaline cadmium sulfide, zirconium phosphate, and complex inorganic sorbents: antimony oxide - silicon oxide - phosphorus pentoxide, antimony oxide - silicon oxide - aluminium oxide, lithium oxide - manganese oxide - aluminium oxide - water, lithium oxide - titanium oxide chromium oxide - water. For comparison the sorption of Tc on some organic sorbents was included. The solubility of T% S7 in water was measured to be 0.257 g/t. It has been shown that preliminary irradiation of sorbents such as sandstone, peat and humic acid by -/-rays with doses not less than 10 ~ rad results in the decrease of To(VII) sorption.Introduction All over the world no less than 19000 T B q o f 9 9 T c had already been p r o d u c e d in nuclear reactors up to 1990. As considered, 10% o f this a m o u n t has been dissipated in the e n v i r o n m e n t ) Estimations for the last years show that around 160 T B q o f99Tc hasbeen released during atmospheric atomic b o m b t e s t i n g ) On the other hand,as was m e n t i o n e d by G A R C I A - L E O N , the continuous increase o f 99 roTe.generator s in nuclear medicine should be given a t t e n t i o n ) A l t h o u g h some authors consider this c o n t r i b u t i o n o f 99Tc to the e n v i r o n m e n t to be negligible, 4 to our m i n d it should be taken into account that all this t e c h n e t i u m penetrates the h u m a n surrounding through the h u m a n b o d y . So, ecological aspects o f Tc-chemistry n o w are o f increasing importance, and consist o f p r o b l e m s o f To-determination, 3 - 7 volatilization,S-11 solubility m e a s u r e m e n t and leaching, 12-17 sorption,1 s-20 and the diffusion o f Tc.21,22 Many problems inElsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

2. S. EL-WEARet al.: SORPTIONOF TECHNETIUMON INORGANICSORBENTS ecological aspects of Tc behavior have not yet been investigated. Only little information is available about the sorption of Tc in pure natural minerals of inorganic and organic nature.In,21 This fact make~ it difficult to predict the behavior of Tc in the environment. The behavior of Tc on some minerals and soils is of great importance, particularly in peats, which is one of the main ingredients of the soil in Eastern Europe, especially in the region of Chernobyl contamination. This problem has not been studied at all. So we thought it necessary to concentrate on the following: (1) investigation of To-behavior in natural waters in contact with a large number of natural minerals, (2) modeling of Te-behavior in artificial minerals and inorganic sorbents in comparison with organic sorbents, and (3) investigation of To-behavior on irradiated natural sorbents.Experimental Technetium was obtained from the firm "Isotope", USSR, in the form of KTcO4 and transformed into NaTcO4 by means of cation exchange in the Laboratory of Radiochemical Research, Institute of Physical Chemistry, Academy of Sciences of the USSR. 2 aMinerals The sorption of 99To was studied on the artificial minerals, natural minerals and organic ion-exchangers. The artificial minerals studied were: Thermoxide-34, Thermoxide-3, GSK, ZrP, SPSC, SKK-AL, ISMA, ISTH-1, and Polysorb-1 (all from USSR), and some inorganic sorbents from different countries: Chromosorb-P, Chromosorb-W (from USA), and Celite-545 and Celite-C-22 (from UK). The natural minerals were: sandstone, feldspar, bauxite, basalt, megrele, phosphorite, peat, pyrite and kaoline. All specimens used-were obtained from deposits in the USSR. The results were compared with the sorption of Tc on organic anion exchange resins: AV-17 and VP-1AP (Table 6). All the minerals were crushed and sieved. The fraction between 6 0 - 8 0 mesh was chosen. Prior to use, the particle fraction was washed several times with distilled water until no dust was visible in washing water. Thereafter air dried solids were used in the experiments.Sorption experiments For preparation of the pH 1.27, 6.46 and 12.6 solutions, 132 mg of NaTcO4 was dissolved in one liter of dilute HNO3, distilled water and dilute NaOH, respectively. For measuring the sorption ratio Rs, batch sorption experiments were performed by shaking 1 g of the crushed and sieved minerals with 10 ml of Tc solution with dif-4 3. S. EL-WEARet al.: SORPTION OF TECHNETIUMON INORGANICSORBENTS ferent concentrations. Varying the contact time from 1 - 2 0 days for pH 1.27, from 1 - 1 3 days for pH 6.46 and from 1 - 7 days for pH 12.7 and 1 - 2 months for natural minerals and other minerals.Measurements of solubility For explanation of sorption behavior of Tc on sulfuoric minerals it was necessary to get the exact value of Tc2 $7 solubility, but this value was not available from the literature. So we synthesized Te2 $7 for measuring its solubility. Equimolar quantities of Na2S and NaTeO4 were stirred in a beaker for 30 minutes. The following reaction explains the formation of To2 $7: 8H20 + 7Na2S + 2NaTcO4 a9 Tc2S7, + 16NaOHThe precipitate was centrifuged and washed 10 times with distilled water and in each washing was centrifuged at 8000 r.p.m, for 30 minutes. Distilled water was added to the precipitate with stirring and the initial pH was 7. Five hours later the solution became violet due to the formation of unknown intermediate technetium sulfides, the UV-VIS spectrum is show0 in Fig. 1. &35200- 1.6I.D - 1.4- 1.219250 - 1.0J.,I1II38343026, 22181/-,13)-xlO-Scm-1Fig. 1. UV-VIS spectrum of supernatant solution of technetium sulfide in water After three days of stirring the solution became colorless and the pH value was 2.35 due to-the following reversible reaction: Tc2S7 + 8 H 2 0.' 2HTcO4 + 7 H 2 SThe solution was centrifuged and the solubility of the precipitate was measured to be 0.257 g/l. 4. S. EL-WEAR et al. : SORPTION OF TECHNETIUM ON INORGANIC SORBENTSMeasurement of radioactivity Samples of 20/A were taken from each batch and dried on a paper disc and the /3-activity of 99 Tc was measured by an NRQ-605 a - / 3 - 7 autometic instrument (Tesla, Czechoslovakia), equipped with a stilbene scintillation/3-detector. The discrimination level of the pulse was chosen to provide the best efficiency for measuring the soft/3-emission of 99 Tc. Relative technique was used.Irradiation technique An experiment has been done on sorption of Tc on irradiated minerals; sandstone, peat and humic acid were 7-irradiated with a 6~ source for 20 hours. The absorbed dose was 107 Rad at room temperature (See Table 8).Calculations The sorption ratio R s and R's was used in this work to describe the sorption properties of the minerals. The following equations give the ratio of R s and R's: AsolidVsolution(1)R s -Asolutionwhere A s o l i dVsolidactivity of the dried sorbent, A s o l u t i o n - activity of the supernatant solution, V s o l u t i o n -- volume of supernatant solution in ml, Vsoli d - weight of the dried sorbent in g. -V R's=C1 - C2--(2).WC2where V - volume of the solution, W - weight of solid material used, C1 - initial activity per ml of a given radioactive Tc in solution, C2 - activity per ml for the solution after contact. For the calculation of the solubility of Tc2S7, we used the following equation: Is CS~- -IoVo "- -Vs~Co(3) 5. S. EL-WEAR et al.: SORPTION OF TECHNETIUM ON INORGANIC SORBENTSwhere Cs - solubility of Tc2 $7, Is - counts of Tc in solution, Io - counts of Tc in standard, Vs - aliquot volume o f Tc solution, Vo - aliquot volume of Tc standard, Co - concentration o f Tc in standard. m C 0 ~vwhere m - mass of Tc in standard, v - volume of standard.Results and discussionThe sorption experiments on 99 Tc were carried out with the minerals listed in Tables 1, 5, 6 and 7. As can be seen in Tables 2, 3 and 4, the study of 99Tc sorption on artificial minerals at pH of 1.27, 6.46, and 12.7 was carried out. As we have seen from formulas 1 and 2, the values o f R's and Rs represent the molecular sorption and the ionic sorption, respectively. The ionic sorption ratios o f the artificial minerals Rs, were very small at all pH values. Small value appears on Thermoxide-34 at pH 6.46, being equal to 3.2. The GSK mineral showed R's values of 2.2, 1.25 and 0.57 at pH's of 1.27, 6.46, J and 12.7, respectively. Those values o f R s are due to the formation of a black precipitate of Tc2 $7. The precipitation increases with decreasing pH. Though the solubility product of CdS itself (SP = 3.6 9 10 - 2 9 in natural media is lower than the SP o f Tc2S 7 at p H 2.35).In acidic media a partial dissolution o f GSK (CdS) occurs, providing the increase of S2 - ion concentration. The R's-values are small too, but still higher than the ion exchange which is represented by Rs-values. As is shown in Tables 2, 3 and 4, the R's-values are large in neutral media and decrease in acidic and alkaline media. T h e smallest Rs-values were obtained for the artificial inorganic minerals listed in Table 5. From Table 6, it can be seen that Rs-values are very large on the organic sorbents, AV-17 and VP-1AP, the largest one being observed at pH 6.46, which is equal to 1.2 9 l0 s ml 9 mg -1 6. S. EL-WEAR et al.: SORPTION OF TECHNETIUM ON INORGANIC SORBENTS Table 1 Formulas o f some minerals Mineral Thermoxide-34 Thermoxide-3 GSK ZrP SPSC SKK-AI ISMA ISTHFormula Crystalline TiO 2 (high density) Crystalline TiO~ (low density) Crystalline CdS ZrO 3 9 n P 3 0 * 9 k H 2 0 S b 3 0 s 9 nSiO 2 9 m P 2 0 s A b 3 0 s 9 nSiO 3 9 m M e O 9 k i l O o A I 3 0 t n L i 3 0 9 m M n O 3 9 1A130 t 9 kH=O nLi=O 9 mTiO= 9 1CRO3 9 k H 3 0Table 2 Sorption o f Tc o n artificial minerals a t pH 1.27 Mineral Thermoxide-34Thermoxide-3Contact time, dayR~4 8 20< 0.054< 0.05 0.3 2.948 20 GSK48 20 ZrP48 20Rs after 90 days 2.99 10-12.2o 10 "30.3 5.33.2 7.3 11.0 2.5 1.9 3.92.24.09 10 "39 10 -3SPEC4 8 20 pyrite > megrele kaoline. Table 8 shows the difference in the value o f Rs, for the non-irradiated 9 8. S. EL-WEAR et al.: SORPTION OF TECHNETIUM ON INORGANIC SORBENTS Table 4 Sorpfion of Tc on artificial minerals at pH 12.7 MineralC0ntaef time, dayR~Rs after 90 daysThermoxide-341 2 3 70.36 0.02 0.731.0 o 10-4Thermoxide-31 2 3 70.36 0.54 1.34 1.84.0 o 10 -4GSK1 2 3 70.13 0.85 2.160.57ZrP1 2 3 7< 0.1 0.1 1 0.9SPSC1 2 3 7< < <