A study on pyrrolidone derivatives as selective precipitant for uranyl ion in HNO3

We have found out that N-cyclohexyl-2-pyrrolidone (NCP) can selectively precipitate UO22+ ions in HNO3 solutions. In order to investigate factors of such a specific property of NCP, we have examined reactions of NCP with UO22+ ions in HCl, HClO4, or H2SO4, and the precipitation abilities of pyrrolidone derivatives other than NCP for UO22+ in HNO3 solutions. As a result, it was found that UO22+ ions in HCl, HClO4, or H2SO4 are not precipitated by NCP, that N-methyl-2-pyrrolidone (NMP) and N-ethyl-2-pyrrolidone (NEP) with lower hydrophobicity than NCP do not precipitate UO22+, and that hydrophobic N-dodecyl-2-pyrrolidone can precipitate UO22+. Furthermore, we have investigated the crystal structures of UO2(NO3)2(L)2 (L=NMP, NEP) complexes to compare with that of UO2(NO3)2(NCP)2. The bond distance between uranium and carbonyl oxygen of NCP in UO2(NO3)2(NCP)2 was found to be shorter than those in UO2(NO3)2(L)2 (L=NMP, NEP). From these results, it is proposed that the specific property of NCP is ascribed to its relatively high hydrophobicity owing to cyclohexyl group, to its strong coordination ability to UO22+ for forming the symmetrical complex accompanied by two bidentate NO3−, and to the surface of UO2(NO3)2(NCP)2 which is surrounded by the hydrophobic cyclohexyl groups of coordinated NCP.