Plutonium(III) complexation by humic substances studied by X-ray absorption fine structure spectroscopy

We determined structural parameters for the near-neighbor surrounding of plutonium(III) in complexes with humic and fulvic acids at pH 1 and for the purpose of comparison also for the plutonium(III) aquo ion by means of X-ray absorption fine structure (XAFS) spectroscopy. It could be shown that in the complexes with humic substances as well as in the plutonium(III) aquo ion sample the trivalent oxidation state of plutonium was stable within the time of the experiment. In the humate and fulvate complexes, the plutonium(III) is surrounded by about eight oxygen atoms with an average Pu–O distance of 2.48 ± 0.02 Å. The structural parameters determined for plutonium(III)–humate and –fulvate complexes were compared to structural parameters of plutonium(III) and plutonium(IV) aquo ions.