Hydrolysis of Plutonium(VI) at Variable Temperatures (283–343 K)

Abstract

The hydrolysis of Pu(VI) was studied at variable temperatures (283-343 K) by potentiometry, microcalorimetry, and spectrophotometry. Three hydrolysis reactions, mPuO(2)(2+) + nH(2)O=(PuO(2))(m)(OH)(n) (2m-n)( +) + nH(+)), in which (n,m)=(1,1), (2,2), and (5,3), were invoked to describe the potentiometric and calorimetric data. The equilibrium constants (*β(n,m)) were determined by potentiometry at 283, 298, 313, 328, and 343 K. As the temperature was increased from 283 to 343 K, *β(1,1), *β(2,2), and *β(5,3), increased by 1, 1.5, and 4 orders of magnitude, respectively. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature increases. Measurements by microcalorimetry indicate that the three hydrolysis reactions are all endothermic at 298.15 K, with enthalpies of (35.0±3.4) kJ mol(-1), (65.4±1.0) kJ mol(-1), and (127.7±1.7) kJ mol(-1) for ΔH(1,1), ΔH(2,2), and ΔH(5,3), respectively. The hydrolysis constants at infinite dilution have been obtained with the Specific Ion Interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the DQUANT equation, and the Ryzhenko-Bryzgalin model, were evaluated with the data from this work.

Thermodynamic and Structural Trends in Hexavalent Actinyl Cations: Complexation of Dipicolinic Acid with NpO22+ and PuO22+ in Comparison with UO22+

“…[37,38] The stock solutions of 237 Np VI and 242 Pu VI in perchloric acid were prepared using the method of HClO 4 fuming as described previously. [39] Absorption spectra were collected to confirm the oxidation states of Np VI and Pu VI in the stock solutions. The concentrations of Np VI and Pu VI were determined, respectively, by the absorbance at 1122 nm (e = 43 m À1 cm À1 for NpO 2 2 + ) [29] and 830.5 nm (e = 550 m À1 cm À1 for PuO 2 2 + ).…”

“…The stock solution of U VI in perchloric acid was prepared and standardized as described elsewhere 37. 38 The stock solutions of 237 Np VI and 242 Pu VI in perchloric acid were prepared using the method of HClO 4 fuming as described previously 39. Absorption spectra were collected to confirm the oxidation states of Np VI and Pu VI in the stock solutions.…”

Used Fuel Disposal in Crystalline Rocks: Status and FY14 Progress.

“…This will also enhance Pu 4+ hydrolysis. Endothermic hydrolysis behavior of aqueous Pu and other actinide species has been reported in numerous studies (Altmaier et al, 2013;Rao et al, 2004;Rao et al, 2011;Runde et al, 2002). Dash (25 C) and solid lines (80 C) are fitted curves using 1 st order kinetic model.…”

“…Thus, understanding Pu behavior at elevated temperatures is necessary. The paucity of thermodynamic and kinetic data limits the efficacy of current transport model predictions (Altmaier et al, 2013;Rao et al, 2011).…”

Plutonium complexes in water: new approach to ab initio modeling

“…[21], the average number of ligands is 3.3 only (R Pu-OH2 = 2.47 Å); Madic et al [22] found the five water molecules (R Pu-Oyl = 1.81 Å and R Pu-OH2 = 2.47 Å), and according to EXAFS measurements of Panak et al [23], the number of equatorial ligands was 3.8 (R Pu-Oyl = 1.82 Å, R Pu-OH2 = 2.48 Å). In DFT calculations of Hay et al [24], the obtained R Pu-Oyl distance (1.81 Å) 6+ complexes, though the existence of these species in aqueous solutions was experimentally confirmed [1][2][3][4][31][32][33][34][35]. To the best of our knowledge, only one work [25] Thus, in all previous simulations, the two approaches for constructing of Pu Z+ aqueous complexes were used: (1) the more or less constrained geometry optimization of the clusters composed of plutonium (or plytonyl) ion with the fixed first solvation shell and some constant number of water molecules modeling the second solvation shell; (2) MD modeling of periodic supercells based on pair-potentials with fitting parameters.…”