Neptunium(V) Complexation by Various Humic Acids in Solution Studied by EXAFS and NIR Spectroscopy

The structure of Np(V) humic acid (HA) complexes at pH 7 was studied by EXAFS. For the first time, the influence of phenolic OH groups on the complexation of HA and Np(V) in the neutral pH range was investigated using a modified natural HA with blocked phenolic OH groups and Bio-Rex70, a cation exchange resin having carboxyl groups as proton exchanging sites. Near-infrared (NIR) spectroscopic measurements verified the formation of Np(V) humates under the applied conditions. Coordination numbers and bond distances were determined for the axial and equatorial coordination shells of neptunium. These were compared to structural parameters of Np(V) hydrates and carboxylates.