Extraction of tetravalent uranium by N,N,N',N'-tetramethylmalonamide in ionic liquid

•Compared with the traditional organic solvent system, the distribution ratio of U(IV) in ionic liquid was improved to 23.•U(IV) and U(VI) could be selectively separated, and the separation factor could reach 28.•The U(IV) complex extracted into the ionic liquid phase could be very stable and maintained in the air for at least two weeks without being oxidized. In this paper, the extractant N,N,N',N'-tetramethylmalonamide (TMMA) was used to extract tetravalent uranium (abbreviated as U(IV)) from aqueous nitric acid solution into ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]) system. It is found that the distribution coefficient of U(IV) in [C4mim][NTf2] is improved to 23, and the single stage extraction efficiency is more than 95% at 1:1 vol phase ratio, compared with the traditional toluene system. While U(VI) could be hardly extracted from nitric acid solutions at a concentration between 1.0 and 3.0 M, U(IV) and U(VI) could be selectively separated in this system and separation factor (SFIV/VI) could reach 28. According to the slope analysis, each U(IV) ion probably bounds one TMMA molecule. For nitric acid at a concentration between 1.0 and 3.0 M, the extraction process of U(IV) might follow the cation exchange mechanism. It is also found that the U(IV) complex extracted into the ionic liquid phase could be very stable and maintain in the air for at least two weeks without being oxidized.