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

A. Discussion of selected references 795The equilibrium constants extracted from the Np(VI) solubility work are consistentwith those for the corresponding U and Pu complexes. Regular trends are observedin speciation predicted by using these values (Figure VI.3 in [98VIT/CAP]). Thecalculations confirm that the trinuclear carbonate species is most stable for uranium[92GRE/FUG] (the formation constants vary according to U > Np > Pu), and thetrimeric species was observed only in supersaturated solutions for plutonium and evenfor neptunium. As a consequence, the trinuclear species of Np(VI) or Pu(VI) are calculatedto be stable only in oversaturated conditions (even though they were observedexperimentally).Some of the numbers given in this report are different from those finally selected inthe present review, typically the result of (small) errors in the molal to molar conversionformula for Np(V), which changes values for Np(IV) and Np(VI) (through the thermodynamiccycles). Nevertheless these changes are usually within the uncertainties.Supplementary materials related to Np(V) complexation with carbonate include thematerial as presented at the Migration’91 Conference, solubility and X-ray diffractionspectra used in [84VIT, 85CÔM, 85KIM, 86GRE/ROB, 90RIG] tabulated with moredetails and figures for sensitivity analysis, and comparison of the X-ray spectra withpublished spectra. There are original experimental data on a spectrophotometric studyof the formation of a mixed hydroxide-carbonato complex (with a tentative interpretation).The report also lists experimental data taken (or scanned) from the publicationsreviewed in this appendix, with graphical comparison of measurements and their calculatedvalues as reinterpreted, linear extrapolations to I = 0 (as in Appendix B of thepresent review) and graphical comparison of all published solubility results. The sametype of information is also provided for Np(VI).[99RAI/HES]This paper was received after the review was essentially complete. Rai et al. measuredNp(IV) solubility in aqueous solutions by equilibration of NpO 2 (am) in sealedtubes. In their quantitative interpretation, the authors selected samples with concentrationranges of (a) 1.78 m K 2 CO 3 + (0.0123 to 1.514) m KOH (series 1); (b) 6.310 −4 to 2.34 m K 2 CO 3 + 0.01 M KOH (series 2); and (c) 0.0123 to 0.537m KHCO 3(series 3). Solutions 0.001 or 0.05 M in Na 2 S 2 O 4 were used to maintain neptunium inthe +4 oxidation state for up to 8 days; after this period Np(V) was detected in someof the samples. The authors selected samples equilibrated for 3 days (series 1, undersaturation),7 or 8 days (series 2, oversaturation) or 8 days (series 3, undersaturation).Samples obtained with other procedures were rejected because of possible Np(IV) oxidation.This also confirmed the scattered solubility measurements of Prapoto et al.[89MOR/PRA, 90PRA/MOR] should be rejected for the same reason.The authors interpreted the solubility results in 1.78 m K 2 CO 3 (series 1) in termsof the equilibriumNpO 2 (am) + 5CO 2−3+ 4H + Å Np(CO 3 ) 6−5+ 2H 2 O (A.83)Figure 1C of [99RAI/HES] shows a slope -4 for a plot of log 10 m Np vs. log 10 m KOHfor data of series 1 where the ionic strength was approximately constant (I = (5.44 ±