Complex formation of Np(V) with fulvic acid at tracer metal concentration

Apparent formation constants of pentavalent Np with fulvic acid were determined in 0.1 M NaClO4, at 25 °C using a solvent extraction technique with thenoyltrifluoroacetone and phenanthroline in an isoamyl alcohol solution. The metal concentration was set constant to a tracer level of 10-15 M Np(V), relevant for the far field safety assessment of a nuclear waste disposal site. The impact of several solution conditions, namely pH, ionic strength, the initial fulvic acid concentration and the presence of Ca2+, on the complex formation were studied. Similar to the sodium system, the divalent calcium ion effectively prevents complexation of the neptunyl ion with humic substance. Furthermore, the derived apparent formation constants were comparatively discussed with literature values obtained at very similar solution conditions.