Hydration of the pertechnetate anion. DFT study

Abstract

The long-lived radionuclide 99Tc (t1/2 = 2.11 × 105 years) is one of abundant and problematic fission products in the nuclear fuel cycle with the yieldof greater than 6 % for thermal neutronfissionof uranium-235. During the reprocessing of the spent nuclear fuel (SNF) in PUREX process, 99Tc is stabilized as pertechnetate ion TcO4-. Owing to long half-life of 99Tc and high environmental mobility, TcO4- is rather environmentally hazardous radioactive pollutant. Preferentially it is to be removed at the head-stage operations when the used fuel rods are dissolved in highly concentrated nitric acid. The lack of reliable knowledge on the structure and thermodynamics of aqua complexes that the pertechnetate anion forms in aqueous solutions hampers the development of technologies for 99TcO4- separation from SNF. To fill this gap, in this paper we used DFT simulation of clusters [TcO4(H2O)n]- and {[TcO4(H2O)n-m]-[H3O]+m} (n = 1–50, m = 1–4). 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 not more than 7 water molecules. Only three 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−1) than the energy of the hydrogen bond in the water dimer (≈ 20 kJ mol−1). The hydration shell of HTcO4 preserves the same asymmetric structure in acidic solution up to acid concentration of 3 – 4 mol L–1. This suggests that during extraction in two-phase system the pertechnetate anion will predominantly concentrates at the water/organic interface, displacing more hydrophilic anions from it. When acid concentration in solution is above 4 mol L–1, water molecules are attached to the technetium atom with formation of ortho-pertechnetic acids O2Tc(OH)3 and OTc(OH)4.