Extraction pertechnetate

Extraction of tetrahedral pertechnetate anion from aqueous solutions using several crown ethers is well known. The coextraction of cesium (or strontium) and technetium from nuclear waste by calix[4]arene-crown-6 has been reported from alkaline media. Although technetium in its common pertechnetate form does not complex directly with crown ethers, pertechnetate extraction may be facilitated by crown ethers as the coanion of sodium (for alkaline nitrate waste). Pertechnetate at trace levels in the waste may be more than a 1000-fold more extractable than the smaller nitrate anion in ion-pair extraction processes.87... 

Calixarene crown-6 compounds, which are neutral extractants like crown ethers, are able to coextract technetium with cesium. Tests carried out with several calix-arene-crown ethers (MC7, MC8, MC14, BC2, BC5, BC8, and BC10) show that the extraction of technetium, present in the aqueous phase at a concentration 10 5 M, is enhanced as the cesium concentration in the aqueous phase increases from 10 5 to 10-2 M. As expected, an increase of nitrate concentration prevents pertechnetate extraction in competition with nitrate anion. The extraction of technetium is only appreciable when the nitric acid does not exceed 1 M. Distribution ratios DCs (close to 8) are comparable for the various calixarenes. However, a decrease of extraction is observed for naphtho derivatives.88 89... 

Beer, P D., Hopkins, P K., McKinney, J. D., Cooperative halide, perrhenate anion-sodium cation binding and pertechnetate extraction and transport by a novel tripodal tris(amido benzo-15-crown-5) ligand. Chem. Commun. 1999, 1253-1254. 

The primarily electrostatic receptors described in Section V.C have shown both synthetic sophistication and the development of solvent shielded, hydrophobic, electron-deficient cavities. This research has yielded potentially useful pertechnetate extraction agents as well as receptors containing functioning metal centers, which possess potential for future technological application. 

ITie dependence of the pertechnetate extraction with cyclohcxanol on the concentration of acid, initially in the aqueous phase, demonstrates the rapid extraetion increase upon the addition of small amounts of aeid. After a maximum extraction coefficient is reached, an exponential decrease sets in (Fig. 7.4.A). Curves similar to those in Fig. 7.4.A were also observed with eyelohexanone, tri-/t-butyl phosphate (TBP), and with solutions of TBP in a liquid hydrocarbon. 

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-... 

Early in 1970, Few et al. [10] radiolabelled polystyrene particles for a mucociliaiy clearance study. The radiolabelled aerosols were produced by a spinning-disk generator. The technique involves the key steps of extracting sodium pertechnetate (Na " Tc04) into chloroform as tetraphenylarsonium pertechnetate, followed by evaporation of the chloroform. A solution of polystyrene is added to the radioactive residue and dispersed Scheme 1). This technique has subsequently been adopted by... 

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 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 %. 

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 ... 

Fig. 4. Extraction of pertechnetate from aqueous solutions with cyclohexanol as a function of acid concentration ...

Table 6. Extraction of pertechnetate from various aqueous solutions at 25...

The dependence of the extraction coefficient of pertechnetate on the salt concentration and kind of anions being an aqueous solutions is shown in Fig. 5. 

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. 

Extraction with a solution of methyltricaprylammonium chloride in chloroform results in nearly quantitative isolation of pertechnetate from aqueous media, ranging from 4 M sulfuric acid or 9 M hydrochlorid acid to pH 13 . A 1 1 pertechne-tate-organic cation adduct seems to be formed at any pH an excess of the organic reagent is only necessary if extraneous anions can compete with pertechnetate. 

Pertechnetate in neutral and alkaline media can be extracted into solutions of tetra-alkylammonium iodides in benzene or chloroform. With tetra-n-heptylammo-nium iodide (7.5 x 10 M) in benzene distribution coefficients up to 18 can be obtained . A solution of fV-benzoyl-iV-phenylhydroxylamine (10 M) in chloroform can be used to extract pertechnetate from perchloric acid solution with a distribution coefficient of more than 200, if the concentration of HCIO is higher than 6 M The distribution of TcO between solutions of trilauryl-ammonium nitrate in o-xylene and aqueous solutions of nitrate has been measured. In 1 M (H, Li) NOj and 0.015 M trilaurylammonium nitrate the overall equilibrium constant has been found to be log K = 2.20 at 25 °C. The experiments support an ion exchange reaction . Pertechnetate can also be extracted with rhodamine-B hydrochloride into organic solvents. The extraction coefficient of Tc (VII) between nitrobenzene containing 0.005 %of rhodamine-B hydrochloride and aqueous alcoholic " Tc solution containing 0.0025 % of the hydrochloride, amounts to more than 5x10 at pH 4.7 . 

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

Since the extraction behavior of pertechnetate is nearly equal to that of perrhenate, the separation of both elements is difficult and often involves reduction of Tc (VII) to lower oxidation states. 

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. 

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. 

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 " . 

The separation of technetiiun from ruthenium involves major difficulties due to the presence of a large number of oxidation states and ionic forms of ruthenium, some of which are capable of being extrated with technetiiun. However, the separation is accomplished by the extraction of pertechnetate with pyridine in 4 N NaOH In alkaline media ruthenium is reduced by the organic solvent to lower valences and is not extracted. The extraction of ruthenium from the aqueous phase can be achieved only in the presence of an oxidant in the solution (e.g. a hypochlorite oxidizing to RuOj which is extracted with pyridine). 

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 . 

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. 

Microgram amounts of pertechnetate can be determined by measuring the extinction of its colored complex with toluene-3,4-dithiol in 2.5 N hydrochloric acid after extraction into carbon tetrachloride . One hour must be allowed for the development of the color. The molar extinction coefficient at 450 nm is 15,000. Beer s law is followed over the range of 1.5 to 16.5 fig Tc per ml. The overall error does not exceed a standard deviation of 5%. Because many cations interfere, an initial separation of technetiiun is necessary. 

Jassim, T. M. Fridemo, L. LUjenzin, J. O. in T. M. Jassim, Co-Extraction of Pertechnetate with some Metal Nitrates in TBP-Nitric Acid Systems. Diss. Chalmers Techn. Univ., Gothenburg (1986). 

For a given ionic strength, //, depends on the nature of the coextracted anion A-. To allow the formation and extraction of the neutral complex, the coextracted mineral anion A- has to lose part (or all) of its hydration shell. The smaller the hydration energy of the mineral anion is, the easier is its transfer to the organic phase, and thus the higher is the affinity of the solvation extractant toward trivalent 4/ and 5/ elements (29, 76), as observed in the series chloride < nitrate < perchlorate < pertechnetate, which inversely follows the anion hydration energy order AG/CI ) > AG/NO3) > AG(,(CI04) > AG/TcO/. 

---