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

8.1 Aqueous neptunium hydroxide complexes 109quite scattered, and the decrease in mobility at low pH values raises questions aboutthe sensitivity of the method (see Appendix A). Although the paper-electrophoresisstudy of Nagasaki et al. [88NAG/TAN] is in qualitative agreement with those of Röschet al. and Lierse, Treiber and Kim, the reported value for the first hydroxide complexationconstant, log 10 K 1 ≈ 6, is greater than even the limiting values from otherstudies [48KRA/NEL, 80SCH/GOR]. The values from the study of Nagasaki et al.[88NAG/TAN] are rejected in the present review.Recently Pan and Campbell [95PAN/CAM] carried out solubility measurementsfor Np 2 O 5 (cr). The total concentrations of neptunium species as a function of pH donot parallel those from solubility measurements [92ITA/NAK, 92NEC/KIM] over hydratedNpO 2 OH(am) except for pH values below 8. The solid was carefully characterisedbefore and after the experiments. The equilibration time for the Np 2 O 5 (cr) study[95PAN/CAM] was considerably longer than for the other studies. This raises the questionas to whether equilibrium was established in those earlier studies (especially nearpH 11), or whether the long equilibration times resulted in radiolytic oxidation. Thereis also a possibility that some carbonate was present in the solutions of Pan and Campbell(see Appendix A). Although the data are not reported in a form suitable for determininghydrolysis constants, the even longer-term study of Silber et al.[94SIL/NIT]shows trends similar to those reported by Pan and Campbell [95PAN/CAM](thenatureof the solid controlling the solubility in the experiments of Silber et al.[94SIL/NIT]isnot clear).The differences between the results of the long-term studies and the fairly extensivedata from other studies of the solubility of NpO 2 OH(am) cannot be resolved at thistime. In the absence of other studies corroborating the hydrolysis behaviour suggestedby Pan and Campbell, in the present review we have selected hydrolysis constantsfor NpO + 2 based only on the studies of the solubility of NpO 2OH(am). Because it isan amorphous solid, NpO 2 OH(am) is a very difficult solid to characterise. Also, thevalue estimated in this review for f G ◦ m (NpO 2(OH) 2 (cr)) is such, that under oxidisingconditions, NpO 2 OH(am) may not be the stable neptunium solid in contact withaqueous solutions (even though it may be in equilibrium with Np(V) solution species).This suggests that values for the hydrolysis constants of Np(V), as derived fromsolubility measurements over uncharacterised NpO 2 OH(am), must be accepted onlywith considerable caution. Solubility measurements over mixed solid phases such asNpO 2 OH(am) (or NaNpO 2 (OH) 2 (s)) and NaNpO 2 CO 3 (s) (or Na 3 NpO 2 (CO 3 ) 2 (s)) orsuch as Ca(OH) 2 (cr) and CaCO 3 (cr) might prove useful. Carbonate complexation maywell account for the high solubilities reported by Nakayama et al. [88NAK/ARI] forpH values above 11 (cf. Figure 8.3, Section 8.2.4.2). However, the study of Ewart et al.[86EWA/HOW] showed no indication of hydrolysis of Np(V) even in very basic solutions.This study was carried out in a water that had been equilibrated with aged concreteand had a carbonate concentration of 0.03 mM. Hence, it is somewhat puzzlingthat carbonate complexation did not cause a levelling out or increase in the concentrationof neptunium for pH values above 11. One possible cause of the low values is thatthe bulk of neptunium may have been coprecipitated with calcium-containing solids inthe more basic solutions. Also, when Lierse, Treiber and Kim [85LIE/TRE] measuredsolubilities in experiments using filters with markedly different pore sizes, they found