Protactinium production

Protactinium is a relatively heavy, silvery-white metal that, when freshly cut, slowly oxidizes in air. AH the isotopes of protactinium and its compounds are extremely radioactive and poisonous. Proctatinium-231, the isotope with the longest half-life, is one of the scarcest and most expensive elements known. It is found in very small quantities as a decay product of uranium mixed with pitchblende, the ore of uranium. Protactiniums odd atomic number (gjPa) supports the observation that elements having odd atomic numbers are scarcer than those with even atomic numbers. 

As mentioned, protactinium is one of the rarest elements in existence. Although protactinium was isolated, studied, and identified in 1934, little is known about its chemical and physical properties since only a small amount of the metal was produced. Its major source is the fission by-product of uranium found in the ore pitchblende, and only about 350 milligrams can be extracted from each ton of high-grade uranium ore. Protactinium can also be produced by the submission of samples of throrium-230 (g Th) to radiation in nuclear reactors or particle accelerators, where one proton and one or more neutrons are added to each thorium atom, thus changing element 90 to element 91. 

The efficiency of the van Arkel-De Boer process (Section II,D) for refining thorium and protactinium metals can be increased by repeating the process to achieve higher purity of product metal. 

X 10 yr) and ends with stable ° Pb, after emission of eight alpha (a) and six beta (jS) particles. The thorium decay series begins with Th (ti/2 = 1.41 X 10 °yr) and ends with stable ° Pb, after emission of six alpha and four beta particles. Two isotopes of radium and Th are important tracer isotopes in the thorium decay chain. The actinium decay series begins with (ti/2 = 7.04 X 10 yr) and ends with stable Pb after emission of seven alpha and four beta particles. The actinium decay series includes important isotopes of actinium and protactinium. These primordial radionuclides, as products of continental weathering, enter the ocean primarily by the discharge of rivers. However, as we shall see, there are notable exceptions to this generality. 

Pa, protactinium, was first identified in 1913 in the decay products of U-238 as the Pa-234 isotope (6.7 h) by Kasimir Fajans and Otto H. Gohring. In 1916, two groups, Otto Hahn and Lisa Meitner, and Frederick Soddy and John A. Cranston, found Pa-231 (10 years) as a decay product of U-235. This isotope is the parent of Ac-227 in the U-235 decay series, hence it was named protactinium (before actinium). Isolation from U extraction sludges yielded over 100 g in 1960. 

Uranium-238 emits an alpha particle to become an isotope of thorium. This unstable element emits a beta particle to become the element now known as Protactinium (Pa), which then emits another beta particle to become an isotope of uranium. This chain proceeds through another isotope of thorium, through radium, radon, polonium, bismuth, thallium and lead. The final product is lead-206. The series that starts with thorium-232 ends with lead-208. Soddy was able to isolate the different lead isotopes in high enough purity to demonstrate using chemical techniques that the atomic weights of two samples of lead with identical chemical and spectroscopic properties had different atomic weights. The final picture of these elements reveals that there are several isotopes for each of them. 

Thorium(III) and protactinium(III) complexes are unknown, and relatively few uranium(III), neptunium(III) and plutonium(III) compounds have been described. This is mainly because of the ease of oxidation to the +4 state in all three cases, accentuated for plutonium(III) by the oxidizing nature of the a-radiolysis products formed in solutions. 

The only examples of compounds of this type are the borohydrides, MIV(BH4)4 (MIV = Th-Pu) and MIV(MeBH3)4 (MIV = Th, U, Np). These compounds are conveniently prepared by reaction, for example, of the metal tetrafluoride with A1(BH4)3 in a sealed tube, followed by vacuum sublimation of the product.159 The neptunium and plutonium compounds are liquids at room temperature and are more volatile than the thorium, protactinium or uranium analogues. 

Thorium phthalocyanine is prepared by heating the metal (previously etched with HC1) and o-phthalonitrile, 1 25, at 270 to 300°C for 5 hr. The dark blue product is cooled to room temperature, washed with benzene, and purified twice by sublimation at 520°C and 10 4Torr. The protactinium-233 produced by w-irradiation of pure thorium phthalocyanine is separated in high purity in the residue after repeated sublimation of the thorium phthalocyanine. The thorium-231 produced by (w,2w) reaction in the thorium phthalocyanine is found to be enriched in the residue after sublimation, indicating decomposition of the phthalocyanine by irradiation. Uranyl phthalocyanine is prepared by heating a mixture of uranyl acetate and phthalonitrile at 230 to 240°C. 

While protactinium is present at less than the parts per million level in uranium ores, this is still the major source because it can be extracted from the residues accumulated in large-scale production of uranium. There are extreme technical difficulties because of colloid formation but more than 100 g have been isolated. 

The most common decay process in which the mass number of the decaying nucleus remains constant is /3-particle production. For example, the thorium-234 nuclide produces a )3 particle and is converted to protactinium-234 ... 

The preparation of the metal was first reported by von Grosse (80) who obtained it by bombarding protactinium pentoxide with 35 keV electrons in a high vacuum and by decomposing the pentachloride on a hot wire. No properties were reported for these products and more recently the pure metal has been obtained by reduction of protactinium tetrafluoride with lithium (73) or barium (65,125) vapor at 1300°-1400°C using the double crucible technique and on a larger scale by reduction with barium (106) or 10% magnesium in zinc alloy (107). 

As mentioned above, protactinium(V) oxytribromide is invariably obtained as a by-product during the preparation of the pentabromide. It is also formed (32, 45) when stoichiometric amounts of the penfa-bromide and either oxygen [Eq. (6)] or antimony sesquioxide [Eq. (7)] are heated together in a sealed vessel at 350°C. Although it is found with... 

Protactinium pentaiodide was first prepared by reacting the pentox-ide with aluminum triiodide at 400°C in a vacuum (104). For large scale preparations, however, direct union of the elements or metathesis of the pentaohloride or pentabromide with an excess of silicon tetraiodide are best (40). The latter reactions take place rapidly at about 180°C in vacuo and the black crystalline product is then purified by vacuum sublimation at 400°-450°C. Silicon tetraiodide also reacts with protactinium pent-oxide, but temperatures in excess of 600°C are required with the reaction vessel completely enclosed in the furnace, and the yield is only about 70% (40). 

Although it is known that protactinium(V) precipitates from dilute sulfuric acid on the addition of potassium sulfate, the product, believed to be K3Pa0(S04)3, has not been satisfactorily characterized (19, 28, 105). There is obviously scope for further research on the preparation of sulfato (and selenato) complexes of pentavalent protactinium. In addition, tetravalent protactinium sulfates, selenates, and their complexes have not been studied. 

The fiuorosulfate dihydrate, PaF2S04 2H2O, analogous to the known uranium(IV) compound (1S2), is precipitated on the addition of aqueous hydrofluoric acid to solutions of protactinium(IV) in dilute sulfuric acid (131). Others (71, 84) have reported the formation of a white, insoluble precipitate in hydrofluoric acid, believed to be a tetrafluoride hydrate, but the product has not been completely characterized. 

Attempts to characterize the pentavalent iodate obtained by the addition of iodic acid to a solution of protactinium(V) in sulfuric acid (Muxart et al.) have indicated that the product contains a variable amount of iodate (PailOa between 1 0.75 and 1 2.0) and more than twelve molecules of water. The infrared vibration associated with the iodate was observed as a broad band between 600 and 870 cm . The white solid slowly decomposes with the liberation of iodine. The pentavalent phosphate, which precipitates from sulfuric acid solution on the addition of orthophosphoric acid is reported to have the composition Pa0(0H)P04 H20 (Lecloarec and Muxart). Although infrared studies confirmed the presence of the P04 ion it was not possible to unambiguously identify a band due to the Pa=0 group [cf. H3Pa0(S04)3 (19)]. 

A wide variety of ions may be adsorbed onto the surfaces of biogenic particles. The removal and deposition of particle-reactive elements such as thorium (Buesseler et al., 1992) and protactinium (Kumar et al., 1993) have been shown to correlate with the primary production of particles in the ocean. Additionally, thorium has been shown to complex with colloidal, surface-reactive polysaccharides (Quigley et al, 2002). 

Both protactinium and thorium are produced in the water column and scavenged by particles onto the seafloor. Because protactinium is scavenged less efficiently by marine particles (Anderson et al., 1983), much of the protactinium produced in the Atlantic Ocean is transported to the Southern Ocean by deep waters before it is scavenged onto the seafloor. This produces a low (below production) Pa/ °Th ratio in Atlantic Ocean and a high Pa/ °Th ratio in Atlantic Ocean in the Southern Ocean (Yu et al., 1996). Reconstruction of the Pa/ °Th ratio suggests... 

Its first product is the element actinium. In 1949, the element s name was changed slighdy to its current form, protactinium. 

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