XAFS study of actinide coordination structure in Np(IV)-fulvates

Metrical parameters of the atoms coordinating tetravalent neptunium complexed with two different naturally occurring fulvic acids at H⁺ concentrations of 10^-1.0 and 10^-1.5 M are determined from analysis of their Np L3 edge EXAFS spectra. The Np(IV)-fulvate complex is observed to have two distinct nearest neighbor oxygen distances, 2.25 A and 2.44 A. The shorter distance is attributed to a hydroxyl ligand; the longer one is an average Np-O distance for coordinating water molecules and carboxyl groups from the fulvate. The observed anomalous increase in loading capacity of humic substances for Np(IV) with increasing pH, compared to that for actinyl ions and trivalent actinides, is due to the presence of Np(OH)_xFA(IV-x). (author)