Technetium extractability

The discovery of the elements 43 and 75 was reported by Noddack et al. in 1925, just seventy years ago. Although the presence of the element 75, rhenium, was confirmed later, the element 43, masurium, as they named it, could not be extracted from naturally occurring minerals. However, in the cyclotron-irradiated molybdenum deflector, Perrier and Segre found radioactivity ascribed to the element 43. This discovery in 1937 was established firmly on the basis of its chemical properties which were expected from the position between manganese and rhenium in the periodic table. However, ten years later in 1937, the new element was named technetium as the first artificially made element. 

Crystals of [Tc(tu)6]Cl3 or [TcCl(tu)5]Cl2 are often employed for the synthesis of technetium(III) complexes. However, since the direct reduction of pertechnetate with excess thiourea in a hydrochloric acid solution yields [Tc(tu)6]3+ in high yield [37], direct use of the aqueous solution of the thiourea complex would be preferable for the synthesis of the technetium(III) complex without isolation of the crystals of the thiourea complex. In fact, technetium could be extracted from the aqueous solution of the Tc-thiourea complex with acetylacetone-benzene solution in two steps [38]. More than 95% extraction of technetium was attained using the following procedure [39] First a pertechnetate solution was added to a 0.5 M thiourea solution in 1 M hydrochloric acid. The solution turned red-orange as the Tc(III)-thiourea complex formed. Next, a benzene solution containing a suitable concentration of acetylacetone was added. After the mixture was shaken for a sufficient time (preliminary extraction), the pH of the aqueous phase was adjusted to 4.3 and the aqueous solution was shaken with a freshly prepared acetylacetonebenzene solution (main extraction). The extraction behavior of the technetium complex is shown in Fig. 6. The chemical species extracted into the organic phase seemed to differ from tris(acetylacetonato)technetium(III). Kinetic analysis of the two step extraction mechanism showed that the formation of 4,6-dimethylpyrimidine-... 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

During recycling of the fuel technetium follows the majority of the other fission products into the waste solutions. After storage for several years, the level of the radioactivity in the waste solutions has fallen sufficiently to allow the extraction... 

Fig. 1. Extraction of technetium from redox process waste ...

Subsequently, solvent extraction was applied to recover the fission product technetium from the residue remaining after the fluorination of irradiated uranium fuel elements . The residue was leached with concentrated aluminum nitrate solution, which was extracted by 0.3 M trilaurylamine in a hydrocarbon diluent. After separation of uranium, neptunium, and aluminum nitrate, technetium was back extracted into a 4 N sodium hydroxide solution. 

Campbell has studied the separation of technetium by extraction with tributyl phosphate from a mixture of fission products cooled for 200 days. Nearly complete separation of pertechnetate is achieved by extraction from 2 N sulfuric acid using a 45 % solution of tributyl phosphate in kerosene. Ruthenium interferes with the separation and is difficult to remove without loss of technetium other radioisotopes can be removed by a cation-exchange process. However, this separation procedure has not been widely applied because of the adverse influence of nitrate. 

Goishi and Libby have investigated the extraction of pertechnetate from alkali solutions with pyridine. Later work showed that a better extraction is obtained using a mixture of sodium hydroxide and sodium carbonate as the aqueous phase. Since the uranyl carbonate complex is not extracted into pyridine, this system may be used for the separation of technetium from uranium. Distribution coefficients of fission products in pyridine are given in Table 4. Substituted pyridine such as 2,4-dimethylpyridine or 4-(5-nonyl)pyridine ) are useful for separating technetium from solutions containing appreciable amounts of aluminum nitrate. 

The extraction with pyridine may be one of the most suitable processes of recovering technetium. The scheme described in Fig. 2 was used at the Oak Ridge National Laboratory . 

Fig. 2. Schem of the extraction of technetium from a waste solution using pyridine ...

The solvent extraction of pertechnetate with cyclohexanone has proved to be an efficient and selective method which can be applied to the separation of Tc from long-lived fission products in the bum-up analysis . The recovery of technetium from the fission products is about 93 %. 

A radiochemical procedure is proposed for the determination of technetium activities from mixed fission products of uranium and thorium. The chief decontamination step is the extraction of TcO into a tetrapropylammonium hydroxide-bromoform mixture from 4.0 M NaOH solutions. Decontamination factors of 10 with chemical yields of 50-70% have been obtained. 

Distillation methods using sulfuric acid are the most efficient for isolating technetium produced by neutron irradiation of kilogram amounts of molybdenum. Boyd et al. have used this method to separate technetium from pure molybdenum which had been irradiated for one year. In this case for each gram of molybdenum 6 ml of concentrated surfuric acid are added and about 75 % of technetium is passed into the distillate. When double the amount of acid is added, nearly 90 % of technetium are found in the distillate. More than 98 % of technetium are extracted after two distillations. 

The extraction of technetiiun by different organic solvents is used in numerous separation and concentration procedures. Technetium is extracted as pertechnetate or, in lower oxidation states, as a complex compound. TcO can be extracted by the following main types of reactions ... 

With all solvents studied including cyclohexanol, methyl ethyl ketone and cyclohexanone, heptavalent technetium is extracted most effectively from sodium sulfate and weakest from sodium nitrate or sodium perchlorate solutions. The data in Fig. 5 appear to be consistent with those on the solubilities of various sodium salts in pure tri-H-butyl phosphate. For example, the solubility of Na SO in TBP is extremely small compared with NaClO. ... 

The slight extractability of TcO from perchlorate solutions and the almost non-extractability by non-polar solvents in used for the re-extraction of technetium into the aqueous phase by either shaking the organic phase with perchlorate solution or by diluting the extractant with a non-polar solvent. As shown in , after 3-4 fold dilution of methyl ethyl ketone by hexane, the distribution coefficient in the system organic solvent/water is only about 6x 10 . 

As already mentioned in section 2.1 pertechnetate may be efficiently extracted by pyridine from alkaline solutions Since pyridine derivates are less soluble in the aqueous phase than pyridine, they extract technetium more efficiently even from nitrate solutions. For example, the distribution coefficients of technetium in the extraction from a 2 M (NH )2COj solution with a high nitrate concentration by pyridine and 2-methylpyridine are 7.5 and 242, respectively . Higher distribution coefficients can be achieved by using 3-methyl- or 4-methyl-pyridines. The pyridine derivates are the most promising reagents for the extraction of technetium from nitrate solutions. 

In addition to pertechnetate various complex compounds of lower valent technetium can be extracted (Table 7). 

Salaria et al. suggest the separation of technetium and rheniiun as cupfer-ronates of pertechnetate and perrhenate which have different solubilities in chloroform. Pertechnetate can partially be extracted from 4 M H SO by shaking the solution with chloroform, which is pre-equilibrated with a solution of 1 % cupfer-ron in 4 M H SO. The main part of perrhenate remains unextracted. 

An efficient method has been developed by Pozdnyakov and Spivakov . In alkaline solution pertechnetate, in contrast to perrhenate, is reduced by hydrazine sulfate. After reduction technetiiun is no more extracted by methyl ethyl ketone. The distribution coefficient of technetium is by a factor up to 2500 smaller than that of rhenium. 

Procedure Hydrazin sulfate is added to a mixture of pertechnetate and perrhenate in 3-6 N NaON or KOH until its concentration is about 3 x 10 M. The solution is stirred and, after 10 min, rhenium is extracted by an equal voliune of methyl ethyl ketone. For complete separation of rhenium from technetium the extraction must be repeated 2-3 times. After a twofold extraction 99% of technetium and only 0.8 % of rhenium remain in the aqueous phase. 

Kiba et al. propose the separation of technetium from rheniiun by carbon tetrachloride extraction using potassium ethyl xanthate as reducing agent. 

Procedure A solution of NH TcO (2 ml) in 0.1 M aqueous ammonia containing Tc ( 10 ppm) is acidified by the addition of 1.5 N HCl (2 ml). To this mixture a 0.5 M freshly prepared aqueous solution of potassium xanthate (1 ml) is added. The mixture is placed in a separatory funnel and mixed by rotating the funnel. Then CCl is added (5 ml) and the mixture is shaken for 20 min. More than 99 % of technetium can be extracted into CCl while rheniiun entirely remains in the aqueous phase. 

The different behavior of technetium and rhenium may arise because Re (VII) is not reduced by xanthic acid to the same oxidation state as Tc (VII). Other suitable extracting solvents are chloroform, 1.1.1-trichloroethane and isopropyl ether. 

Appreciable separation of pertechnetate from molybdate is achieved by pyridine as extracting agent However, the high boiling point of pyridine complicates the recovery of technetium by steam distillation of pyridine. Therefore, this method is rarely used. 

Technetium and molybdenum can also be separated by extracting molybdate with ethyl ether from a medium 1.2 M in both NH SCN and HCl Mo (VI) and Tc (VII) undergo reduction and complex formation in this medium. After extraction pure technetium is left in the aqueous phase in 80% yield. 

Pertechnetate and molybdate are reduced in acetic acid media by p-thiocresol and form complex compounds. As mentioned above the technetitun compound can be extracted by chloroform. Since the blue molybdenum complex is insoluble in this solvent separation of technetium from molybdenum can be achieved " . 

Tetra-n-heptylammonium iodide in benzene extracts TcO and MoO from an aqueous phase of pH 10.5. However, TCO4 is extracted much better. A separation factor of about 14 has been found. Thus, only a few cycles of solvent extraction are required to separate efficiently technetium from molybdenum . 

The extraction of TcO with methyl ethyl ketone, acetone, and pyridine results in a ruthenium decontamination factor of about 10 . Another effective separation method is based on the extraction of technetium as triphenylguanidinium pertechnetate from sulfuric acid by means of chlorex ()S-chloroethyl ether). Pertechnetate can be re-extracted with 3 N NH OH solution . 

A method has been developed for the determination of technetium-99 in mixed fission products by neutron activation analysis Tc is separated from most fission products by a cyclohexanone extraction from carbonate solution, the stripping into water by addition of CCI4 to the cylohexanone phase, and the adsorption on an anion exchange column. Induced Tc radioactivity is determined using X-ray spectrometry to measure the 540 and 591 keV lines. The sensitivity of the analysis under these conditions is approximately 5 ng. The method has been successfully applied to reactor fuel solutions. 

This amount of thiocyanate is sufficient for both complete reduction and complex formation. Reduction is allowed to proceed for 30 to 45 s after the addition of the thiocyanate. A bright red color can readily be observed at a technetium (VII) concentration of 0.1 ng per ml. Acetone (6 ml) is then added and the volume of the solution mixed and adjusted to 10 ml with distilled water. At this point, the color has generally developed to less than 50% of its final intensity. Quartz 1-cm glass-stoppered cells are filled with the technetiiun solution and placed in a 20 °C water-cooled spectrophotometer. The extinction will approach a maximum intensity in 1 to 3 h. The maximiun extinction occurs at 510 nm with a molar extinction coefficient and standard deviation of 47,500 + 500 in 60 vol. % of the acetone-aqueous medium. An additional examination of the analysis may be carried out by extract-... 

Foster et al. have developed a method for determining technetium in dissolved nuclear fuel solutions. Tetrapropylammonium pertechnetate is doubly extracted from a basic medium into chloroform and the colored technetium (V) thiocyanate complex is formed in the chloroform phase by the addition of sulfuric acid, potassium thiocyanate and tetrapropylammonium hydroxide. The colored complex absorbs at 513 nm, has a molar extinction coefficient of 46,000 and is stable for several hours. Of more than 50 metals studied, none impairs measurements at ratios less than 100 to 1 mol with respect to technetium. Most anions do not disturb the determination of technetiiun. The standard deviation for a single determination is 0.09 fig over the range of 1 to 20 fig of technetium. 

ReO , and Hg, however, only slightly. CrOj forms a similar complex, but the technetium complex can be separated from the chromium complex by extracting the former into carbon tetrachloride. According to Fujinaga et al. the method can be improved by the use of 1 M hydrochloric acid instead of sulfuric acid. 

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