Protactinium species

It was shown by Trubert and co-workers that the only protactinium species that exist in the aqueous phase were PaOOH ", PaO(OH)2 and PaO(OH)3(aq). Since protactinium forms the PaO ion in solution, its oxidation number is 3. Thus, Eq. (9.2) can be expressed by the following equation ... 

However, the quantity of Pa produced in this manner is much less than the amount (more than 100 g) that has been isolated from the natural source. The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. AH of these, however, are rendered difficult by the extreme tendency of protactinium(V) to form polymeric coUoidal particles composed of ionic species. These caimot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

The most stable oxidation states for protactinium are Pa(V) and Pa(IV). The chemical behavior of Pa(V) closely mimics that of Nb(V) and Ta(V), and experimental data are consistent with a 5f(l) rather than a 6d(l) electron configuration for the Pa(IV) species [37]. The electrochemical literature for Pa is mainly focused on the characteristics of the Pa(V)/Pa(IV) couple and the electrodeposition of Pa metal films from aqueous and nonaqueous electrolyte solutions. In aqueous solutions, only Pa(V) and Pa(IV) ions are known to exist, and the standard potential for the Pa(V)/Pa(IV) redox couple is in the range of —0.1 to -0.32 V [38]. 

These techniques helped to sort out elements and, as it turned out, also isotopes. The nature of radioactive dements was not initially clear. Did they all correspond to distinct chemical elements To researchers in radioactivity, the word element long denoted a radioactive species, be it a proper chemical element or an isotope. Thus Lise Meitner described protactinium not just as a long-lived radioactive element, but also as chemical one . The title of one of Otto Hahn s papers on the same element is also telling Das Protactinium als radioaktives und als chemisches... 

Pentavalent. Protactinium is known to bind sulfate in solutions of H2SO4 however, very limited data are available on the molecular species. Sulfate complexes with the general formula of Np02(S04) " (n = 1 3) have been reported. In most of these complexes, the Np is found in a seven-coordinate pentagonal bipyramid geometry. 

Since protactinium(IV) is readily oxidized in the atmosphere halo-genoprotactinates(IV) must be prepared in oxygen-free solvents or by heating the component halides together in an inert atmosphere. It is reported that solvent extraction studies have established the existence of the fluoro and chloro species PaXl" " and PaX + in aqueous acid solution (81)... 

A few derivatives of oxyanions have also been prepared protactinium (V) forms hexanitrato complexes, Pa(N03)e , by reaction of the hexachloro complex with dinitrogen pentoxide, in contrast to niobium and tantalum which, under similar conditions, yield only tetranitrato complexes, M0(N03)4 (35). Neptunium (V) nitrates, NPO2NO3 and Np0(N03)3 have also been reported 71). Protactinium (V) sulfato-and selenato complex acids, H3PaO( 804)3 and H3Pa0(Se04)3, have been obtained from aqueous solution (13), but no fully sulfated or sele-nated species have been recorded. 

The pentavalent oxidation state is accessible for the early actinides uranium, protactinium, neptunium, and plutonium. Pentavalent species with neutral Group 16 bases can include either adducts of AnXg or complexes incorporating oxo-containing cations, AnO " or An02". ... 

Protactinium. In terms of equilibria with various oxygenbearing species, Pa represents the worst possible situation. The PaO/Pa is predicted to be of the order of 50 (23) with any oxygen present, and the Pa02 pressure is very appreciable. In this respect the system is similar to the uranium metal plus oxygen system, which took more than 20 years of careful work to solve properly. 

Pentavalent protactinium is the most stable oxidation state in aqueous solution. It shows a strong tendency toward ineversible hydrolysis in solution, but it differs from the other pentavalent actinides in that it does not hydrolyze to form an actinyl ion of the form MOj. Instead, the postulated ionic species in noncomplexing solutions ate PaOOH and PaO(OH)j. With strong complexing agents, even in aqueous solutions, Pa(V) can form a nonoxygenated complex, such as PaFj " [A1, L6]. 

The thorium nitrate solution from the dissolver will be about 9 Af in nitric acid. To obtain satisfactory decontamination of thorium from fission-product protactinium, ruthenium, and zirconium-niobium, it was found necessary to remove all of the nitric acid from the solution and make the solution around 0.15 Af acid-deficient in nitrate ion by converting a fraction of the A1(N03)3 to a water-soluble basic nitrate. This also converts the readily hydrolyzed nitrates of these fission products to basic nitrates that are less extractable than the species present in the acid dissolver solution. 

In the scrubbing section of the HA column, fission products were scrubbed from the organic phase leaving the extraction section by the HAS scrub stream. It contained 0.01 Af H3PO4 to complex protactinium and zirconium-niobium and reduce their extraction. It also contained 0.01 Af ferrous sulfamate to reduce plutonium and chromium corrosion product to inextractable species. 

The terrestrial occurrence of Ac, Pa, U, and Th is due to the half-lives of the isotopes 235U, 238U and 232Th which are sufficiently long to have enabled the species to persist since genesis. They are the sources of actinium and protactinium formed in the decay series and found in uranium and thorium ores. The half-lives of the most stable isotopes of the trans-uranium elements are such that any primordial amounts of these elements appear to have disappeared long ago. However, neptunium and plutonium have been isolated in traces from uranium13 minerals in which they are formed continuously by neutron reactions such as... 

Protactinium as 231Pa occurs in pitchblende, but even the-richest-ores contain only about 1 part of Pa in 107. The isolation of protactinium from residues in the extraction of uranium from its minerals is difficult, as indeed is the study of protactinium chemistry generally, owing to the" extreme tendency of the compounds to hydrolyze. In aqueous solution, polymeric ionic species and colloidal particles are formed, and these are carried on precipitates and adsorbed on vessels in solutions other than those containing appreciable amounts of mineral acids or complexing agents or ions such as F , the difficulties are almost insuperable. 

An additional complication can arise in solution if the radioactive species in trace amounts react with trace concentrations of impurities. For example, in an investigation of the properties of pentavalent protactinium, Pa(V), it was foimd that the protactinium was extracted into pure xylene from 1 M HCIO4 solutions. Further experimentation showed that this extraction was due to the presence in the xylene of organic impurities at concentrations below the detectable limit of 0.01 %. Support for this interpretation was provided when the solution was made 10" M in thorium, which was expected to form complexes with the probable impurity, thereby preventing the reaction with the protactinium. In fact, no protactinium was extracted into xylene from this solution. The thorium in this case acts as a hold-back carrier. 

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