On the formation of uranium(V) species in alkali chloride melts

Abstract

Uranyl(V) species are normally unstable in solutions but are here shown to be stable in high-temperature chloride melts. Reactions leading to the formation of UO 2 Cl 4 3ions were studied, including thermal decomposition and chemical reduction of uranyl(VI) chlorospecies in various alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl, and NaCl-2CsCl) at 550-850 °C. Decomposition of UO 2 Cl 4 2species under reduced pressure, with inert gas bubbling through the melt or using zirconium getter in the atmosphere results in the formation of UO 2 Cl 4 3and UO 2 . Elemental tellurium, palladium, silver, molybdenum, niobium, zirconium, and hydrogen, as well as niobium and zirconium ions were tested as the reducing agents. The outcome of the reaction depends on the reductant used and its electrochemical properties: uranyl(VI) species can be reduced to uranyl(V) and uranium(IV) ions, and to uranium dioxide.

Uranium (III) precipitation in molten chloride by wet argon sparging

“…Uranium tetrachloride is volatile at the working temperature (T vap ¼ 789 C [17]). Due to the use of acidic salt, this volatility is not prevented by complexation [27,28]. The amount of uranium lost by volatilisation is deduced from the weight of the final precipitate.…”

Spectroelectrochemical Identification of a Pentavalent Uranyl Tetrachloro Complex in Room-Temperature Ionic Liquid

“…The redox chemistry of U VI O 2 2+ in molten salts is of great interest not only for its basic chemistry but also for the pyrochemical reprocessing of spent nuclear fuels. In most cases, chloride melts were used, where U VI O 2 2+ is regarded to form a stable tetrachloro complex U VI O 2 Cl 4 2– . The reduction product of U VI O 2 Cl 4 2– is believed to be the corresponding tetrachloro uranyl(V) species, U V O 2 Cl 4 3– , which was derived by the recent extended X-ray absorption fine structure (EXAFS) studies. , The occurrence of this uranium(V) species is also of special interest in uranium(V) chemistry, which attracts attention in recent actinide chemistry because of the limited investigation of its instability .…”

“…Although the melting points of the salts used as media in the former publications are different, the experiments were performed generally at several hundreds to ca. 1000 K. This condition is, however, very extreme and not always easy to handle.…”

Actinide Chemistry in Ionic Liquids

“…This uranium(V) species has been electrochemically generated from the well-established parent uranium(VI) species [UO 2 Cl 4 ] 2– and exhibits an intense metal-to-halogen charge-transfer band centered at 406 nm (ε = 832 M –1 cm –1 ; M = mol dm –3 ) and much weaker absorption bands at 630 and 770 nm (ε = 9.5 and 7.1 M –1 cm –1 , respectively) attributable to the electric-dipole-forbidden transitions of LMCT and f–f in UO 2 + (Figure ). These spectroscopic properties are common to those in high-temperature chloride melts . The above-mentioned [UCl 6 ] − in the acidic [EMI][Cl/AlCl 3 ] is another form of uranium(V) found in ILs (Figure ) .…”