Radiochim. Acta 68, 99-104 (1995) © R. Oldenbourg Verlag, München 1995 Spectroscopic Study of the Hydrolysis of PuO| + in Aqueous Solution By I. Pashalidis 1 , J. L Kim 12 , T. Ashida 1 * and I. Grenthe 2 * 1 Institut für Radiochemie, Technische Universität München, 85747 Garching, Germany 2 Kernforschungszentrum Karlsruhe, Institut für Nukleare Entsorgungstechnik, 76021 Karlsruhe, Germany (Received August 9, 1994; revised September 12, 1994) Plutonium(VI) / Hydrolysis / Spectroscopy / Stability constants Abstract The hydrolysis of Pu(VI) is investigated under Ar atmosphere in the pH range between 4.5 and 9.0 at 22 °C in 0.1zyxwvutsrqponmlkjihgfedcbaVUTSRPONLIHCA Μ NaC10 4 solution in contact with an initial solid phase, Pu0 2 C0 3 (s). Under the given conditions, Pu0 2 C0 3 (s) is changed to Pu0 2 (0H) 2 (s). The estimated equilibrium partial pressure for the transformation: Pu0 2 C0 3 (s) + H 2 0(1) - Pu0 2 (0H) 2 (s) + C0 2 (g) can reachlPOC P COl ~ 10" 3 6 atm. In contrast to earlier investigations in oversaturated solution, no polynuclear species formation is observed. Two mononuclear complexes, Pu0 2 0H + in near neu- tral and Pu0 2 (0H) 2 (aq) in weak alkaline solution, are found to be the dominating hydrolysis species of Pu(VI). The stability constants of the two species, evaluated from UV/VIS spectro- scopic measurements, are found to be: log/?,, = 8.10±0.15 and log/?, 2 = 14.25±0.18, respectively. The first equilibrium con- stant is in good agreement with the corresponding value for the formation of U0 2 0H + , log/?,, = 8.4±0.3, indicating a close similarity in hydrolysis between the two actinide ions. 1. Introduction The hydrolysis of the PuO! + aquo-ion is important in determining the solubility and migration behaviour of plutonium in natural aquatic systems. A large number of studies have been made to evaluate stability con- stants of various hydrolysis products [1 — 14], Indepen- dent of experimental methods, there is clear evidence for the formation of polynuclear species [3, 5, 6, 8, 11-14], However, the proposed stoichiometry of these complexes differs from author to author, depending on experimental conditions and on the data analysis used to evaluate experimental results. In the presence of polynuclear species, the reliable determination of the mononuclear hydroxo complexes appears very diffi- cult, since the latter are always minor species under these conditions [5, 11—14]. It is therefore not surpris- ing to find the first hydrolysis constants differing up to about three orders of magnitude from \ogß n = 7.7 [10] to log)?!! = 10.45 [2], and also a disagreement about the actual stoichiometry of the species formed [3, 5, 6, 8, 11-14]. * On leave from Power Reactor and Nuclear Fuel Development Corporation (PNC), GI Section, Tokaimura, Japan (T. Ashida) and Royal Institute of Technology (RIT), Chemistry Dept., Stockholm, Sweden (I. Grenthe). Although the formation of polynuclear species seems to govern the solubility behaviour of hexavalent plutonium in near neutral aqueous solutions, there is some evidence that the species, which are formed when OH" is added to an acidic Pu(VI) solution, are metastable [14, 15]. Because of the importance of mononuclear hydroxo complexes for modelling chemical species in the environment, we have made an experimental study at sufficiently low total concen- tration of plutonium(VI) to ensure that the formation of polynuclear species is negligible. We have started from a well defined solid phase Pu0 2 C0 3 (s) [16], which results in a suitable solubility for our purposes and allows a spectroscopic investigation using UV/ VIS spectroscopy even in near neutral and alkaline aqueous solutions. The C0 2 gas generated from the carbonate is removed by bubbling argon through the aqueous phase containing the solid. Radiometric measurements in addition to spectroscopic speciation facilitate the interpretation of the hydrolysis equilibria and the evaluation of their equilibrium constants. 2. Experimental The experiment was carried out with 242 Pu of relatively high isotopic purity (99.9%). The PuOi + stock solution was prepared by fuming with concentrated HC10 4 in a Pt crucible and subsequently an additional oxidation with ozone, and then purging of the solution with Ar gas. The oxidation state purity of the stock solution was verified by comparison of the total concentration as determined by liquid scintillation counting with that from spectrophotometry. A portion of the stock solu- tion was diluted with 0.1 Μ NaC10 4 , neutralized to pH 7 with NaOH, and then bubbled with C0 2 (100%). In the absence of C0 2 , there was no precipitation over a period of 5 months in the neutralized solution with ca. 10~ 2 molL -1 Pu(VI) [15]. Only by passing C0 2 through the solution, a white precipitate of fine crystal- line Pu0 2 C0 3 (s) could be generated [16]. This precipi- tate was characterized by powder diffraction, IR and UV spectroscopy after separation by ultrafiltration [16]. Solubility experiment was performed in a closed glass vessel under normal or argon atmosphere at 22±1°C. pH was adjusted by addition of 0.1 Μ HC10 4 or 0.1 Μ NaOH, and measured by a ROSS type com- bination electrode with 3 Μ NaC10 4 as a filling solu- tion. The electrode was calibrated regularly against 6 Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 8/4/15 1:24 PM