Gj. Lumetta et al., Complexation of uranyl ion by tetrahexylmalonamides: an equilibrium modeling and infrared spectroscopic study, INORG CHIM, 293(2), 1999, pp. 195-205
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
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