Interaction of Neptunium(IV) with Humic Substances Studied by XAFS Spectroscopy

We studied the complexation of neptunium(IV) with various natural and synthetic humic and fulvic acids at pH 1 using X-ray absorption fine structure (XAFS) spectroscopy to observe the local structure around the neptunium ion. The results were compared to the interaction of neptunium(IV) with Bio-Rex70, a cation exchange resin having solely carboxylic groups. For both neptunium humate complexes and neptunium Bio-Rex70 complexes, 11 oxygen atoms were found at a mean distance of 2.36 Å. This verified that the carboxylic groups of humic substances are responsible for binding neptunium(IV) in the acidic pH range. The humic acid carboxylate groups are predominantly monodentately coordinated to neptunium(IV) ions.