Extraction of Th(IV) from an HNO3 Solution by Diglycolamide in Ionic Liquids

Extraction of thorium(IV) from a nitric acid solution into various 1-alkyl-3-methylimidazolium hexafluorophosphate ([C(n)mim][PF6], n = 4,6,8) ionic liquids (ILs) was investigated using N,N,N',N'-tetrabutyl-3-oxapentanediamide (TBDA) and N, N'-dimethyl-N,N'-dibutyl-3-oxapentanediamide (MBDA) as the extractants. The extraction efficiency of Th4+ ions in the ionic liquid is higher than that obtained in chloroform under low nitric acid concentration. The extraction mechanism has been studied by slope analysis, UV-vis measurement, and extraction tests. The conventional log-log plot analysis of the extraction equilibrium data reveals that Th4+ ions are extracted as cation complexes characterized by a metal/ligand ratio of 1:2 using MBDA as an extractant. In addition Th4+ ions form 1:2 complexes with TBDA in [bmim]PF6 and [hmim]PF6 and 1:1 complexes in [omim][PF6] and chloroform. The mode of partitioning of thorium ions into [bmim]PF6 and [hmim]PF6 with TBDA appears to proceed predominantly via a cation-exchange mechanism, whereas for [omim][PF6] thorium extraction mechanism is changed from an cation-exchange process to one involving extraction of a neutral thorium-TBDA-nitrato complex as the aqueous nitrate concentration is increased.