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

12.1 Neptunium carbon compounds and complexes 211This is a sparse set when compared with the species and thermodynamicdata proposed [92GRE/FUG] for uranium(VI) carbonate and mixed hydroxidecarbonatecomplexes: UO 2 CO 3 (aq), UO 2 (CO 3 ) 2−2 ,UO 2(CO 3 ) 4−3 , (UO 2) 3 (CO 3 ) 6−6 ,(UO 2 ) 2 CO 3 (OH) − 3 , (UO 2) 3 O(OH) 2 HCO + 3 ) and solids (UO 2CO 3 (s),Na 4 UO 2 (CO 3 ) 3 (s).Using the complexes NpO 2 (CO 3 ) 4−3and (NpO 2 ) 3 (CO 3 ) 6−6as a starting point, severalother published studies were interpreted or reinterpreted in the present review:• Solubility products for K 4 NpO 2 (CO 3 ) 3 (s) and (NH 4 ) 4 NpO 2 (CO 3 ) 3 (s) were estimatedfrom the work of Gorbenko-Germanov and Klimov [66GOR/KLI] andfrom solubility measurements of Moskvin (despite the completely erroneous interpretationof the experimental results in the original paper [71MOS2], see AppendixA).• Although there are really insufficient solubility data available, the work of Vitorgeand Capdevila [98VIT/CAP] was used to estimate the solubility product ofNpO 2 CO 3 (s) and the formation constants of the lower mononuclear complexes,NpO 2 (CO 3 ) 2−2and NpO 2 CO 3 (aq).• As for uranium [92GRE/FUG], Maya [84MAY] showed evidence of formationof at least one polynuclear mixed Np(VI) hydroxide-carbonate complex. Unfortunately,in this potentiometric study, the total concentration of Np(VI) waskept practically constant. Thus, the data are insufficient to unambiguously definethe stoichiometry of any polynuclear complexes (see the discussion of [84MAY]in Appendix A). Maya proposed the formation of (NpO 2 ) 2 CO 3 (OH) − 3whichseems quite reasonable when compared with species found in the uranium system;but, for the reasons explained below and in Appendix A, no accurate quantitativeinterpretation could be extracted from this work.Other published work [74SIM/VOL, 81WES/SUL, 93LI/KAT] contain data that,after reinterpretation (Appendix A), are consistent with the species and values for formationconstants of the carbonate complexes selected in the present review. However, itwas not possible to use the results of these studies to deduce quantitative informationrelated to the minor species in the Np(VI)-CO 2−3system.The Np(VI) and U(VI) carbonate complexes are formed under similar conditions.At high ionic strength (3 M NaClO 4 ) a regular progression was observed in the U,Np, Pu series for the formation and stepwise complexation constants, and for theMO 2 CO 3 (s) solubility products (M=U, Np, Pu) [98VIT/CAP]. The limiting and thetrinuclear Np(VI) complexes are a little less stable than the corresponding uraniumones, while the solubility products are probably of the same order of magnitude for uraniumand neptunium. Consequently solubility in carbonate-bicarbonate (3 M NaClO 4 )media is lower for Np(VI) than for U(VI). In such solutions, the trinuclear Np(VI)complex can occur at 25 ◦ only in supersaturated solutions. However, for kinetic reasons(the solid NpO 2 CO 3 (s) is much slower to precipitate from carbonate solutionsthan UO 2 CO 3 (s) and PuO 2 CO 3 (s) under similar conditions), the trinuclear specieswas probably present in several reported studies, and it is actually the easiest trinuclearspecies to study in the U, Np, Pu series.