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

A. Discussion of selected references 721when precipitated from oversaturated solutions (0.1 mM< [CO 2−3 ] < 1mM)whereNa 3 NpO 2 (CO 3 ) 2 (s) solid precipitates slowly.This publication also contains further experimental details related to some ofthe data for Np(VI) complexation as published by Grenthe, Riglet and Vitorge[86GRE/RIG]. In addition, lattice parameters of the compounds PuO 2 CO 3 (cr) andNpO 2 CO 3 (cr) are reported. These solids have the same structure as rutherfordine,UO 2 CO 3 (cr). The lattice parameters reported for NpO 2 CO 3 (cr) are reasonablyconsistent with those published by Thevenin, Jove and Madic [86THE/JOV] (bothX-ray diffraction studies were done using the same apparatus in the same laboratory,but the samples were prepared independently).b) PlutoniumThe paper summarises a variety of carbonate complexation experiments, only two ofwhich concern Pu(VI); (1) the stability of (PuO 2 ) 3 (CO 3 ) 6−6and (2) the existence andstability of (PuO 2 ) 2 UO 2 (CO 3 ) 6−6. The experimental procedures and results are morecompletely described in [86GRE/RIG].[86LIE/KIM]This report contains much of the experimental and calculation detail that was omittedfrom the papers published by this group. In particular, it clarifies two points withrespect to the Pu(VI) hydrolysis study by Kim et al.[84KIM/BER].First, it indicates that in the data analysis log 10 K s,0 was fixed with a value of −23.0,and that a fixed value of log 10 β 1 =−8.3 was also selected. These values were basedon best estimates from the literature, and were not determined experimentally in anyother way. The other four constants, β 2,2 , β 2,1 , β 5,3 and β 3,1 were then derived fromthe solubility data.Second, it suggests the radiolysis from the 238 Pu may have resulted in reactionswith the plastic container. The net result of these reactions that may have led to theplutonium remaining in the +6 oxidation state over the long equilibration period.The value of log 10 β 3,1 = 10 ± 2.1 for the formation of the PuO 2 (CO 3 ) 5−3was calculatedusing the Gibbs energy changes calculated from the standard potential of thePu(V)/Pu(VI) couple, the Gibbs energy of formation of PuO 2 (CO 3 ) 4−3 ,andthereversibleformal potential of the Pu(VI)/Pu(V) couple in 1 M sodium carbonate solution.The temperature was probably ∼20 ◦ C. A recalculation of this value, and extrapolationto zero ionic strength using the same SIT parameters as in [92CAP] results in a valueof log 10 β3 ◦ = 4.2 ± 0.9.The results of a series of long-term (up to 650 days) studies on the solubility ofPuO 2 (s) were rationalized in terms of the sequential formation of the mono- throughthe pentacarbonato complexes of Pu(IV). The oxidation state was not verified in thecourse of this study, and the identity and stability of Pu(IV) hydrolysis products weretaken as given from another study in this report. The tabulated values are those for0.3 and1MNaHCO 3 /Na 2 CO 3 solutions at 25 ◦ C. The interpretation of these solubilitystudies was complicated by the effects of radiolysis on the solution and solid that