Ionic liquids with polychloride anions as effective oxidants for the dissolution of UO2
The polychloride ([Cl3]- or/and [Cl5]-) ionic liquids (ILs) were prepared from their imidazolium chloride precursors by the addition of chlorine gas. The highest storage ability of Cl2 has been found in [Bmim]Cl IL among the six imidazolium chlorides [Rmim]Cl (Rmim = Emim, 1-ethyl-3-methylimidazolium; Bmim, 1-butyl-3-methylimidazolium; Pmim, 1-propyl-3-methylimidazolium; Pnmim, 1-pentyl-3-methylimidazolium; Hmim, 1-hexyl-3-methylimidazolium; and Omim, 1-octyl-3-methylimidazolium). The usage of mixed IL of [Bmim][Tf2N] (1-butyl-3-methylimidazolium bistriflimide) and [Bmim]Cl can maintain the good stability of the IL cation during the chlorination, and the imidazolium cation [Bmim]+ can almost keep the same structure after bringing chlorine gas into the mixed IL by 1H NMR spectroscopy. It has been found that the synthesis of polychloride ILs can not only successfully dissolve UO2, but also save the consumption amount of Cl2 and raise the chlorine efficiency. Spectroscopy studies indicate [UO2Cl4]2- is the principal product in the IL reaction solution. The dissolved uranyl species can be easily recovered from the ILs by crystallization in the form of [Bmim]2[UO2Cl4]. Moreover, even if 57.1 wt% of the lanthanide elements, compared with the total amount of dissolved uranium and lanthanide elements, dissolved in the IL mixture, only uranium-containing crystals would be isolated from the system.