Theoretical predictions of complex formation of group-4 elements Zr, Hf, and Rf in H2SO4 solutions
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Valeria Pershina
Summary
A process of step-wise complexation of group-4 elements Zr, Hf, and Rf in H2SO4 solutions has been considered theoretically. Relative values of the free energy change of the M(SO4)2(H2O)4, M(SO4)3(H2O)22− and M(SO4)44− (M = Zr, Hf, and Rf) formation reactions from hydrated and partially hydrolyzed cations have been calculated using the fully relativistic density functional theory method. They proved to be very similar for reactions of Zr and Hf. At low H2SO4 concentrations, Hf should, however, have a slight preference for the complex formation, especially, when hydrolysis takes place, while with increasing H2SO4 concentration and decreasing hydrolysis, the trend should be Zr > Hf. According to the results, Rf should exhibit a weaker tendency to complex formation than Zr and Hf at the entire range of acid concentrations, i.e., for all types of complexes. Thus, the predicted trend is Zr > Hf ≫ Rf. The same trend is also found for the formation of R4M(SO4)4, where R is a monovalent cation. Accordingly, the trend in the Kd (distribution coefficient) values in anion-exchange separations of group-4 elements should be: Zr > Hf ≫ Rf. The obtained trend for Zr and Hf is in agreement with literature data on extraction of these elements in macro-concentrations from sulphuric acid solutions and with recent experiments on extraction of single species by TOA in toluene using the SISAK technique.
© Oldenbourg Wissenschaftsverlag
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