Complexation of Neptunium(IV) by Various Humic Substances Studied by X-Ray Absorption Spectroscopy

We studied the complexation of neptunium(IV) with natural and synthetic humic and fulvic acids at pH 1 by X-ray absorption spectroscopy. Neptunium LIII-edge X-ray absorption spectra were collected in transmission mode at the Rossendorf Beamline (ROBL) at the ESRF in Grenoble. Furthermore, we studied the interaction of neptunium(IV) with Bio-Rex70, a cation exchange resin having solely carboxylate groups, at pH 1. The structural parameters of the near-neighbor surrounding of neptunium(IV) obtained for the interaction of neptunium(IV) with humic substances and with Bio-Rex70 are compared to obtain information on the nature of functional groups of humic substances responsible for binding neptunium(IV) in the acidic pH range.
The results are also compared with structural parameters obtained for neptunium(IV) hydrates as well as for the interaction of thorium(IV) and hafnium(IV) with humic acid and Bio-Rex70.