Hydration of the pertechnetate anion. DFT study

Ustynyuk, Y.A.; Gloriozov, I.P.; Zhokhova, N.I.; German, K.E.; Kalmykov, S.N.

Авторы: Ustynyuk Yuri A., Gloriozov Igor P., Zhokhova Nelly I., German Konstantin E., Kalmykov Stepan N.
Журнал: Journal of Molecular Liquids
Год издания: 2021
Издательство: Elsevier BV
Местоположение издательства: Netherlands
Номер статьи: 117404
DOI: 10.1016/j.molliq.2021.117404
Аннотация: The long-lived radionuclide 99Tc (t1/2 = 2.13 × 105 years, Emax = 300 keV) is one of abundant and problematic fission products in the nuclear fuel cycle. During the reprocessing of spent nuclear fuel (SNF) in the front-head of PUREX process, 99Tc is dissolved in nitric acid and stabilizes as the pertechnetate anion TcO4-. Due to its long half-life, potential radiation hazard, and high environmental mobility TcO4- is rather environmentally hazardous radioactive pollutants. Tc poses important red-ox catalytic properties and would act readily as a catalyst of reductant oxidation with the solvent at the second stage if it is not removed at the first stage when uranium and plutonium are extracted with TBP after the SNF rods are dissolved in highly concentrated nitric acid. Thermodynamics of Tc is given important attention by nuclear community and 3 updated databases were prepared by international NEA-TDB teams in 1999, 2005 and 2020 [1 – Rard, 2 – Xavier Gaona, Bernd Grambow]. But still the behavior of Tc stays often complicated and problematic [3 - Melentiev, Mashkin, German] indicating that some points of Tc speciation and behavior are confusing/not completely understood. The lack of reliable knowledge on the structure and thermodynamics of week aqua complexes that pertechnetate forms in aqueous solutions hampers the development of technologies for 99TcO4- excretion from SNF. In this paper we used DFT simulation of clusters [ТсО4(Н2О)n]- and {[ТсО4(Н2О)n-m]-[H3O]+m} (n = 1-50, m = 1 - 4) to fill this gap. We have shown that unlike NO3-, SO42, and other hydrophilic anions, the pertechnetate anion does not form a dense hydration shell in aqueous solutions. Its first hydration shell contains no more than 7 water molecules. Only three out of four oxygen atoms of the anion in this shell form hydrogen bonds with water molecules. The energy of hydrogen bonds [O3TcO… H-O-H] is significantly lower (10-15 kJ/mol) than the energy of the hydrogen bond in the water dimer (≈ 20 kJ/mol). The hydration shell of HTcO4 retains the same asymmetric structure in acidic solution up to an acid concentration of 3 – 4 mol/L. This suggests that during extraction in two phase system the pertechnetate anion will be predominantly located at the water/organic interface, displacing more hydrophilic anions from it. When the acid concentration in the solution is above 4 mol/L, water molecules are attached to the technetium atom with the formation of ortho-pertechnetic acids O2Tc(OH)3 and OTc(OH)4.
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Hydration of the pertechnetate anion. DFT studyстатья

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