Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Detailed molecular information of the Np(V) sorption species on amorphous and crystalline gibbsite were obtained for the first time by in situ time-resolved Attenuated Total Reflection Fourier-Transform Infrared (ATR FT-IR) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy under environmentally relevant conditions. The results consistently demonstrate the formation of mononuclear inner sphere complexes of the NpO2+ ion irrespective of the prevailing atmospheric condition. In particular, the impact of the presence of atmospheric equivalent added carbonate on the speciation in solution and on the surfaces becomes evident from vibrational data. Whilst the 1:1 aqueous carbonato species (NpO2CO3−) was found to become predominant in the circumneutral pH range, it is most likely that this species is sorbed onto the gibbsite surface as an inner sphere ternary surface complex where the NpO2+ moiety is directly coordinated to the functional groups of the gibbsite’s surface. These findings are corroborated by results obtained from EXAFS spectroscopy. The identification of the Np(V) surface species on gibbsite constitutes a basic finding for a comprehensive description of the dissemination of neptunium in groundwater systems.