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

320 17. Plutonium oxygen and hydrogen compounds and complexes0.1 M Pu(VI) in a 1.0 M ionic strength medium (0.26 M ClO − 4, adjusted with NaCl,apparently room temperature measurements), formation constants were calculated forthe 2:2 and 7:4 species. No evidence for a 1:1 hydrolysis species was found in theserather concentrated solutions.Only the potentiometric study of Cassol et al. [72CAS/MAG] provides evidencethat PuO 2 OH + and (PuO 2 ) 2 (OH) 2+2coexist, although the solubility study of Kim etal.[84KIM/BER] was interpreted as being consistent with that conclusion.It appears the higher polymeric species deduced from the potentiometricstudies, (PuO 2 ) 3 (OH) + 5and (PuO 2 ) 4 (OH) + 7, are metastable if they exist at all[93PAS/KIM2], although other polymers may exist. The existence of both PuO 2 OH +and (PuO 2 ) 2 (OH) 2+2seems reasonable, although even the dimer may be metastable[95PAS/KIM]. If log10 ∗β1 and log10 ∗β2,2 have values near −5.5and−7.5, respectively,much of the spectroscopic and potentiometric data can be rationalised. One of theexceptions to this would be the solubility data of Kim et al.[84KIM/BER].The results of Pashalidis et al. [95PAS/KIM] lead to a formation constant forPuO 2 (OH) 2 (aq) of log10 ∗β◦2,1 =−(13.15 ± 0.18) at 22◦ C (Appendix A). This wouldmean the species is more stable than predicted in other work (e.g.[93PAS/KIM2]), andwould be consistent with the polymeric Pu(VI) hydrolysis species being metastableexcept under very specific conditions.Woods et al. [78WOO/MIT] reported Pu(VI) solutions, 10 −3 to 10 −4 M, 0.15 Min NaClO 4 were stable at a pH value of 11.4 for as long as two months. However, thenature of the hydrolysis species formed at high pH (13.3) was not established in theRaman study of Madic and Begun [84MAD/BEG]. From NIR spectroscopy, Tananaev[89TAN] suggested there was evidence for the existence of PuO 2 (OH) 2−4(in1MLiOH) and PuO 2 (OH) − 3in less basic solutions, however, no values were proposedfor the hydrolysis constants. Moskvin and Zaitseva [62MOS/ZAI], Perez-Bustamante[65PER], Kim et al.[84KIM/BER] and Musante and Porthault all reported evidence forincreased solubility of plutonium(VI) in basic solutions. None of these authors triedto interpret their data by including polymeric anionic species in their models. Suchspecies have been proposed for the U(VI) system [92GRE/FUG]. With the possibleexception of the last two studies, no useful thermodynamic information was reportedin these papers (see Appendix A).This review recommendslog10 ∗ β◦ 1 =−(5.5 ± 0.5)(essentially identical to the value of Pashalidis et al.[95PAS/KIM]) andlog10 ∗ β◦ 2,2 =−(7.5+0.5 −1.0 )For the neutral monomeric hydrolysis species, PuO 2 (OH) 2 (aq), the 22 ◦ Cvaluefrom Pashalidis et al. [95PAS/KIM] is accepted at 25 ◦ C, but with increased (and unsymmetrical)uncertainties,log10 ∗ β◦ 2,1 =−(13.2+0.5 −1.5 )