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

12.1 Neptunium carbon compounds and complexes 27312.1.2.2.2 Solid alkali metal neptunium(V) carbonate hydratesA summary of the experimental information on neptunium(V) solubility in carbonate/bicarbonateaqueous solutions, and results of reanalysis of these data as carried outas part of the present review are provided in Tables 12.4 and 12.5.The first publication discussing neptunium chemistry in carbonate media was a paperby Nigon et al.[54NIG/PEN] in which the preparation of a hydrated KNpO 2 CO 3 (s)solid was described. Keenan and Kruse [64KEE/KRU] later characterised a compoundwith the same stoichiometry. Keenan [65KEE] claimed that he prepared aCsAmO 2 CO 3 (s) compound, but not the corresponding Np(V) that should be easierto prepare (see Appendix A). Gorbenko-Germanov and Zenkova [66GOR/ZEN] precipitatedM-Np(V)-CO 3 solid compounds (M = K, Cs), with different stoichiometries,including K 3 NpO 2 (CO 3 ) 2 (s), but equilibrium was probably not obtained, and thehydrated solid phases probably were changing during the course of the experiments(see Appendix A). Moskvin [71MOS2] prepared NH 4 NpO 2 CO 3 (s) hydrated compounds,and Kharitonov and Moskvin [73KHA/MOS] reported an infrared spectrum,but the compounds were not characterised. The monoammonium solid was transformedinto another (more stable) solid, possibly hydrated (NH 4 ) 3 NpO 2 (CO 3 ) 2 (s),when the aqueous (NH 4 ) 2 CO 3 concentration was greater than 1.5 M (transformationmay even have occurred in 0.6 M (NH 4 ) 2 CO 3 , see Appendix A). Volkov et al.later proposed a structure for this type of alkali metal carbonate compound (Np(V),Pu(V) and Am(V)), in which alkali metal cations could be exchanged for the NH + 4or possibly divalent cations. However, as discussed in Appendix A, questions remainabout some of the compounds and structures proposed in this series of papers (e.g.,[74VIS/VOL, 81VOL/VIS]).Compounds for which published information is available, but for which no thermodynamicdata can be derived include:• MNpO 2 CO 3 (s) compounds for M + = Li + , K + , Rb + , Tl + and NH + 4 ,[74VOL/KAP, 81VOL/VIS3];• M 0.5 NpO 2 (CO 3 ) 2 (s) hydrated compounds for M 2+ = Mg 2+ , Ca 2+ , Sr 2+ andBa 2+ [81VOL/VIS3] that were claimed to form from NaNpO 2 CO 3 (s) in concentratedMCl 2 or M(NO 3 ) 2 aqueous solutions;• the hydrated compound Na 0.6 NpO 2 (CO 3 ) 0.8 that was proposed [79VOL/VIS];but which might not be stable in contact with aqueous solutions[84VIT, 98VIT/CAP].Simakin [77SIM] was the first author to use this qualitative information to measureand correctly interpret a solid/liquid equilibrium in the Np(V) carbonate system. Hestudied the equilibriumNa 3 NpO 2 (CO 3 ) 2 (s) + CO 2−3Å NpO 2 (CO 3 ) 5−3+ 3Na + (12.37)In this review the value of the equilibrium constant K s,3 (12.37) was calculated andselected (see Appendix A). Earlier (but indirect) evidence that solubility measurements