PNNL

Abstract

Spectrophotometry was used to investigate hydrolysis and complex formation of neptunium(V) with silicate ions at pH > 8 and [Np (V)] of (5-10)×10-5 mole/l. Data in the technical literature on Np(V) hydrolysis show large divergence: the magnitude of lg Khydr. varies from -8 to -11.7. In the present work, it is shown that in the absence of dissolved carbon dioxide, appreciable neptunium(V) hydrolysis does not occur until about ?? 9.4 to 9.5. Thus it follows that lg Khydr. = –11 is of similar magnitude to the value of lg Khydr. recommended by the thermodynamic database for Np and Pu (TDB-NEA, 2001). The findings on hydrolysis have been used to calculate equilibria in the presence of silicate ions. Np(V) forms complexes with anionic orthosilicates and, obviously, with other silicate ions. Interaction begins at pH > 8.5 and silicate concentration of 0.004 to 0.005 mole/l (or at pH > 8 and [Si] = 0.02 mole/l). Interaction is observed by decrease in the intensity of the NpO2+ aquo-ion absorption band at 981 nanometers in the presence of silicate as compared with solutions containing no silicate. No new intense absorption bands arise. In the pH range 8 to 9.5, the interaction is adequately described by the reaction NpO2+ + ?Si(OH)3- ? NpO2OSi(OH)3 At higher ??, ligation can also occur with doubly charged ions, including doubly charged polymeric ions. The complex stability constant of NpO2OSi(OH)3 is found to be lg ß1 = 2.1 ± 0.3. Thus, interaction of Np(V) with silicate ions is rather weak and cannot play an important role in natural conditions. Carbonate ions in solution with ?? > 8.5, occurring by equilibration with carbon dioxide from air, are much stronger ligands for Np(V) such that complexation of Np(V) with silicate in the presence of ??32- is practically impossible.