The spectroscopy of neptunium (V) as a function of temperature, ionic strength, and complexation is being used to establish the speciation of Np(V) and model its behaviour in the geosphere. Under oxic to sub-oxic conditions in near-neutral pH, Np(V)O2+is the predominant species and is relatively mobile in the environment due to its high solubility and low tendency toward hydrolysis and complexation. Its potential migration in a high-level nuclear waste repository and its importance as an analog for Pu(V) in a TRU waste repository make it important to study the effects of temperature and ionic-strength on its speciation.Herein we report the progress we have made to characterize the absorption spectroscopy of Np(V) as a function of temperature (20-90 oC), ionic strength (0 to 6 M Na/Mg-Cl system), and complexation of key environmental complexants (borate, carbonate and hydrolysis). Both ionic-strength and temperature impact and slightly change the spectrum of the Np(V) aquo species. The model fits of these spectra are used to establish the effects of temperature and ionic-strength on the complexation of Np(V) where new band formation is almost always observed. The effect of borate concentration on the Np(V) spectrum is shown in Figure 1 and shows an isobestic point indicating the formation of a 1:1 complex. Formation constants and associated Pitzer/SIT parameters were determined for chloride, borate and carbonate are reported.

Spectroscopy of Neptunium (V) at variable temperature and ionic strength

247th National Meeting (2014)

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