Complexation of uranyl ion by tetrahexylmalonamides: an equilibrium modeling and infrared spectroscopic study

We investigated the extraction of uranyl nitrate from aqueous sodium nitrat e with a series of tetrahexylmalonamides. The tetrahexylmalonamides conside red were N,N,N',N'-tetrahexylmalonamide (THMA), N,N,N',N'-tetrahexyl-2-meth ylmalonamide (MeTHMA), and N,N,N',N'-tetrahexyl-2,2-dimethylmalonamide (DiM eTHMA). This series allowed for a systematic determination of the effects o f alkyl substitution of the methylene carbon. Equilibrium modeling of the e xtraction data indicates that at 1 M NaNO3, two extracted species are forme d: UO2(NO3)(2)L-2 and UO2(NO3)(2)L-3. The relative abundance of these two s pecies depends on the nature of the tetrahexylmalonamide ligand. The UO2(NO 3)(2)L-2 species is dominant in the DiMeTHMA system, with very little forma tion of the UO2(NO3)(2)L-3 species. In contrast, the UO2(NO3)(2)L-3 species is more predominant in the MeTHMA case. The case of THMA lies in between. The greater propensity of MeTHMA versus THMA to bind in a 3:1 fashion to ur anyl ion might reflect the greater basicity of the carbonyl oxygens in MeTH MA. The fact that DiMeTHMA binds primarily in 2:1 fashion suggests that ste ric constraints are more important in that ligand. As the nitrate concentra tion is increased, the ligand-to-metal ratios tend to decrease, i.e. the UO 2(NO3)L-2 species tends to predominate, while the UO2(NO3)(2)L-3 species be comes less important. In the case of THMA and MeTHMA, equilibrium modeling suggests the existence of a UO2(NO3)(2)L species at higher nitrate concentr ations. FTIR spectral studies confirm that at least two uranyl-THMA complex es formed, one of which has been identified as UO2(NO3)(2)(THMA) by thermog ravimetric analysis (TGA). The identity of the second species has not been definitively determined, but is most likely UO2(NO3)(2)(THMA)(2). (C) 1999 Published by Elsevier Science S.A. All rights reserved.