Pertechnetate in aqueous

Recovery of technetium, present as pertechnetate in aqueous acidic solution, is of utmost importance because of its long half-life of 2.13 x 105 years and its relative mobility in the environment. The close relation between TCO4 and the isoelectronic perrhenate Re04 makes the latter a widely used model for artificially produced technetium, which only possesses radioactive isotopes. 

Potassium thiocyanate reduces pertechnetate in aqueous hydrochloric acid solution and forms thiocyanato complexes of technetium that arc extractable by a solution of 0.1 M 2-hexylpyridinc in benzene. Also Mo(VI), Au(IlI), As(III). Fe(III), Zn(II), and Hg(ll) are extracted under similar conditions, lire equilibrium is attained in about 3... 

All [TcOX4] spedes hydrolyse in aqueous solution and then disproportionate into pertechnetate and Tc(IV) oxide hydrate according to... 

A general mode of access to polyhydric complexes of Tc(V) is reduction of pertechnetate with two equivalents of stannous chloride in aqueous solution of the excess O-donor ligand, e.g. ... 

Some recent interest in the technetium chemistry has been focused on complexes possessing a Tc=N3+ core. Tetrachloronitridotechnetate(VI) complexes can easily be synthesized by the reaction of pertechnetate with sodium azide in concentrated hydrochloric acid [34], Although its square-pyramidal structure resembles that of tetrachlorooxotechnetate(V) complexes, stable character of the nitrido complexes in aqueous solution shows a remarkable contrast to the oxo complexes. However, when a strong acid and a coordinating ligand are absent, the interconversion of di(p-oxo)nitridotechnetium(VI) complexes to the monomeric form occurs in the following complicated manner [35]... 

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 . 

Procedure To the sample which contains 20-300 /xg of pertechnetate in 5-20 ml of solution, are added potassium perchlorate solution (2 ml, 1 mg KCIO per ml) and enough NaCl to make the solution approximately 1 M. The solution is heated and neutralized with ammonia. Pertechnetate is precipitated with aqueous 5 % tetraphenylarsonium chloride reagent. The precipitate is filtered, washed and dried, and a 2-mg portion is mixed with potassium bromide (300 mg). The mixture is pressed to form a clear disc by the usual technique. The infrared spectrum is recorded between 10 and 12 /x. The peak absorption is measured at 11.09 /X by the base-line technique. 

Free permanganic acid exists only in aqueous solution and the oxide Mn20iy decomposes explosively above 0°. The corresponding compounds of technetium and rhenium are, however, stable the perrhenates are more weakly oxidising than the pertechnetates. The formal charge -j 7 is, in fact, dominant in technetium and rhenium. 

Pyrophosphate in aqueous solutions slightly hydrolyzes to monophosphates, causing the formation of free pertechnetate and hydrolized colloidal technetium. 

Harly extractions of pertechnetate, dissolved in 1 N H2SO4, with a 0.1 M solution of tri-/(-octylamine (TOA) in cyclohexane resulted in distribution coefficients of Dic= 110 at 25 °C jl 11]. Recently, Dy was determined as a function of the nitric acid concentration using 0.01 and 0.1 M solutions of tri- -octylamine in benzene. The distribution coefficients increased with increasing acidity until a maximum was reached near 0.1 M 11N03. At higher concentrations of nitric acid, D y. fell rapidly because of competition from the simultaneously extracted nitric acid, which reduces the concentration of free tri-n-octylamine available to extract TcO (Fig. 7.8.A). llic extraction is considerably affected by the temperature. A linear relation is obtained between D y. and the concentration of TOA. The slope of 1.0 indicates an extractant Tc ratio of 1 1. The extraction of pertechnetate from aqueous HKO3 solution may be represented by the equation [ 117] ... 

In addition, several Tc(V) complexes based on the 0=Tc(S)4-core have been prepared. The preparation w as achieved by exchange reaction of Tc(V) gluconate with various dithiols in aqueous cthanolic solution. The oxo-dithiolatotechnetatc(V) complexes were mostly precipitated as the Et4N] salts. The complex compounds turned out to be diamagnetic in solution, and have probably square pyramidal structure [117,118,119]. Stereoisomcric complexes derived from mesa and racemic 2.3-dimer-captosuccinic acid dimethylcster were identified by H NMR [120]. The kinetics of the reaction between pcrtcchnetatc and meso- or racemic dimcrcaptosuccinic acid in hydrochloric acid solution were studied. The reaction was found to be first order in each reactant [121]. Also the reaction of pertechnetate with para-substituted benzene thiols was followed and showed a simple second order kinetics. The reaction rate decreased when the substituent became more electron-withdrawing [122]. 

Current Radiopharmaceutical Synthesis. The aqueous chemistry of technetium is dominated by the oxidizing power of soluble TcO , and the thermodynamic stability of insoluble TcOa. All technetium-99m radiopharmaceuticals, except pertechnetate itself, are prepared by the aqueous reduction of pertechnetate in the presence of a potential ligand to prevent Tc02 deposition (2). The most commonly employed reductant is stannous chloride, although many other reductants can, and have, been used (1,2). 

Technetium is one such constituent of radioactive waste where the need for a chemical means of detection exists, but a sensor does not. Technetium is not found in appreciable quantities in nature however, the isotope c is a byproduct of the thermal nuclear fission of U, U, and Pu at 6.1%, 4.8%, and 5.9% yields, respectively (7), and significant quantities of c exist at many DOE sites. Tc exhibits rather weak P decay (Ebuk = 0.292 keV), but it is of particular concern for two reasons its long half life (2.13 x 10 y) and the high solubility of its most common form in aqueous environmental media, pertechnetate (TcO/) (2). Pertechnetate does not readily adsorb to most minerals, and therefore in aqueous form and under suitable conditions, it may rapidly present itself to subsurface waters (3,4). 

There are three general types of radiopharmaceuticals elemental radionucHdes or simple compounds, radionucHde complexes, and radiolabeled biologically active molecules. Among the first type are radionucHdes in their elemental form such as Kr and Xe or Xe, and simple aqueous radionucHde solutions such as or I-iodide, Tl-thaUous chloride, Rb-mbidium(I) chloride [14391-63-0] Sr-strontium(II) chloride, and Tc-pertechnetate. These radiopharmaceuticals are either used as obtained from the manufacturer in a unit dose, ie, one dose for one patient, or dispensed at the hospital from a stock solution that is obtained as needed from a chromatographic generator provided by the manufacturer. 

Tc(III), Tc(IV) and Tc(V) P-diketonate complexes are stable in acid solution. In fact, when a chloroform solution of TcCl2(acac)2 was shaken with 1 M hydrochloric acid solution, no detectable change in the distribution ratio of the complex - defined as the ratio of the concentration of technetium in the organic phase to that in the aqueous phase - was observed over a 24 h period [26]. However, when the technetium complexes were backextracted into aqueous alkaline solution, decomposition occurred [26-29]. In all the cases studied, spectrophotometric investigation revealed that pertechnetate was formed quantitatively as a final product. 

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-... 

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 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. 

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. 

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). 

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