The Chemistry of Protactinium

The speciation of protactinium since its discovery: a nightmare or a path of resilience

This review concerns the speciation of protactinium in aqueous solution under its both oxidation states +IV and +V. Emphasis is placed on experimental data obtained at trace level but also in macroscopic amount leading to the determination of thermodynamic and structural data. Thus, the complexation of Pa(V) with mineral acids and organic acids, mainly polyaminocarboxylic acids (iminodiacetic acid [IDA], nitrilotriacetic acid [NTA], ethylenediaminetetraacetic acid [EDTA] and diethylenetriaminepentaacetic acid [DTPA]) are highlighted and compared. The review also includes the actual knowledge about the Pa(IV) aqueous chemistry pinpointing its spectroscopic features.

On the volatility of protactinium in chlorinating and brominating gas media

A multi-target recoil chamber technique was applied to study online chemical properties of protactinium in chlorinating and brominating gas media using 226Pa (T 1/2 = 1.8 min) decaying by alpha emission (74%) and β+/EC decay (26%). A 58 MeV proton beam passing 15 × 50 μg/cm2 thick 232Th targets enabled production of 226Pa formed in the reaction 232Th(p,7n). Isothermal gas chromatography in quartz columns allowed for the determination of adsorption enthalpies of oxohalides and pure halides of Pa5+ compounds. On the basis of empirical correlations, these adsorption enthalpies (ΔH0 ads) could be converted to sublimation enthalpies (ΔH0 subl). Resulting values for the assumed compounds PaCl5, PaOCl3, PaBr5, and PaOBr3 were 113 ± 15, 329 ± 16, 165 ± 5 and 235 ± 17 kJ/mol, respectively. These values are rather similar to known ΔH0 subl data for group-5 elements Nb, Ta and Db in support of the assumption that Pa is a pseudo-group 5 element.

Protactinium(V) in aqueous solution: a light actinide without actinyl moiety

This review highlights recent data on the complexation of Pa(V) with inorganic (fluoride and sulphate) and organic (oxalate, nitrilotriacetate, diethylenetriaminepentaacetate) ligands in solution. New thermodynamic parameters relative to the complexation of Pa(V) with sulphate are presented. The review also includes gas phase and theoretical studies focused on the interaction of Pa(V) in the dioxo and oxo forms with water.

First structural characterization of Pa(iv) in aqueous solution and quantum chemical investigations of the tetravalent actinides up to Bk(IV): the evidence of a curium break

More than a century after its discovery the structure of the Pa(4+) ion in acidic aqueous solution has been investigated for the first time experimentally and by quantum chemistry. The combined results of EXAFS data and quantum chemically optimized structures suggest that the Pa(4+) aqua ion has an average of nine water molecules in its first hydration sphere at a mean Pa-O distance of 2.43 Å. The data available for the early tetravalent actinide (An) elements from Th(4+) to Bk(4+) show that the An-O bonds have a pronounced electrostatic character, with bond distances following the same monotonic decreasing trend as the An(4+) ionic radii, with a decrease of the hydration number from nine to eight for the heaviest ions Cm(4+) and Bk(4+). Being the first open-shell tetravalent actinide, Pa(4+) features a coordination chemistry very similar to its successors. The electronic configuration of all open-shell systems corresponds to occupation of the valence 5f orbitals, without contribution from the 6d orbitals. Our results thus demonstrate that Pa(iv) resembles its early actinide neighbors.