Precipitation ability to U(IV) and stability of 1,3-dimethyl-2-imidazolidone for selective precipitation of U(VI) in nitric acid media

As a part of the investigation of precipitants with selectivity to U(VI) in nitric acid media, a preliminary study on the precipitation ability of 1,3-dimethyl-2-imidazolidone (DMI) to U(IV), a simulant of Pu(IV), was performed. DMI is a ring compound like N-n-butyl-2-pyrrolidone (NBP) which is one of the pyrrolidone derivatives (NRPs) and a promising precipitant for U(VI). While DMI is known to precipitate U(VI) from 3 mol dm−3 (=M) HNO3, no precipitate was observed in the solution containing 0.15 M U(IV) and 3 M HNO3 by adding DMI at the ratio of [DMI]/[U(IV)] = 5. This indicates that the selectivity of DMI to U(VI) than U(IV) is much higher compared with that of NBP.On the other hand, the stability of DMI under γ-ray irradiation and heating in HNO3 solutions (≤4 M) was also examined to evaluate the applicability of DMI to the practical process, because gradual acid hydrolysis of DMI is inevitable due to the nature of the chemical structure. As a result, it was found that the stability is strongly affected by the concentration of HNO3. Namely, very few DMI in 2 M HNO3 underwent the ring-opening by the irradiation up to 220 kGy and heating at 50 °C up to 5 h, respectively, indicating that these treated samples may still hold the precipitation ability to U(VI). On the contrary, the cleavage of the ring of DMI in 4 M HNO3 was found to proceed easily. From the above results, it was concluded that DMI may be a candidate as a selective precipitant for U(VI) in HNO3 solutions up to ca. 2 M.

► DMI did not precipitate U(IV) species from 3 mol dm−3 HNO3.
► DMI showed higher stability under γ-ray irradiation and heating with decreasing concentration of HNO3.
► DMI may be a candidate as a selective precipitant for U(VI) in HNO3 solutions up to ca. 2 mol dm−3.