NIR Spectroscopic Study on the Influence of Phenolic OH Groups on the Neptunium(V) Humate Complex Formation

We investigated the influence of phenolic OH groups on the Np(V) complexation by humic acids at pH 7 and pH 8. The studies were performed by near-infrared (NIR) absorption spectroscopy applying unmodified and chemically modified humic acids with blocked phenolic/acidic OH groups. The experimental data were evaluated with the metal ion charge neutralization model. For all humic acids under investigation comparable complexation constants were determined. However, compared to the original humic acids, modified humic acids with blocked phenolic/acidic OH groups show significant lower loading capacities for Np(V) under the applied experimental. This result indicates a lower amount of maximal available humic acid ligand sites for the complexation of Np(V). Thus, it can be concluded that humic acid phenolic/acidic OH groups contribute to the interaction between humic acid and Np(V) under the studied conditions.