chemical thermodynamics of neptunium and plutonium - U.S. ...

Chapter 17Plutonium oxygen and hydrogencompounds and complexes17.1 Aqueous plutonium hydroxide complexes17.1.1 Plutonium(VI) hydroxide complexesEven the earliest studies on this system [44O’C, 49KRA/DAM2] reported experimentalproblems caused by the slow kinetics of some of the hydrolysis reactions in neutral andbasic solutions. This has been confirmed in later potentiometric [75SCH] and spectrophotometric[93OKA/REE] studies, and means results from most of these studiescannot be used to determine equilibrium constants without some reservations. The situationhas been well summarised by Pashalidis et al. [93PAS/KIM2]: “In solutions thatare initially more basic than pH 5 and have total Pu(VI) concentrations 10 −3 mol·dm −3or greater, the time dependent reactions preclude the use of thermodynamic calculationsunless there are inordinately long time intervals between consecutive additions ofbase.”In addition, radiolysis is caused by the α-decay of plutonium in solution. Thisleads to species that cause the reduction of Pu(VI) (initially to Pu(V)) in perchloricacid solutions. The extent of radiolytic reduction depends on the isotope of plutoniumused, the effects decreasing with increasing half lives ( 238 Pu(t 1/2 = 87.74 a);239 Pu(t 1/2 = 24100 a); 242 Pu(t 1/2 = 376000 a) [79SHI/LED]). The extent of radiolyticreduction increases with increasing concentrations of plutonium solution speciesand also depends on the medium. Thus, reasonably concentrated plutonium inaqueous perchlorate solutions near neutral pH cannot be left long enough for equilibriumto be established without the solutions also undergoing autoreduction. Althoughother types of studies exist, the bulk of the information concerning Pu(VI) hydrolysiscomes from solubility, spectroscopic and potentiometric experiments.The stable hydrolytic solid containing Pu(VI) is probably PuO 2 (OH) 2·H 2 O(see Section 17.2.2.1). There are at least four solubility studies in the literature[62MOS/ZAI, 65PER, 73MUS/POR, 84KIM/BER], that report hydrolysis constantsfor Pu(VI). The study of Perez-Bustamante [65PER] was restricted to strongly basicsolutions. Based on the published hydrolysis constants for Pu(VI) (Figure 17.1) andsolubility product, it appears the solubilities of Musante and Porthault must have beensimilar to those of Kim et al. (within about an order of magnitude, although the pH ofthe minimum solubility differs somewhat).317