Plutonium(III,IV,VI) Speciation in Gorleben Groundwater Using XAFS

Pu speciation is investigated by X-ray absorption fine structure (XAFS) at the PuL3-edge in Gorleben (GoHy) groundwater for Pu added to the groundwater in three different oxidation states (III, IV, VI). The GoHy groundwater stems from an anaerobic aquifer rich in humic substances (HS) (GoHy-532 with ~60-70 mg/L DOC) and has a neutral pH. Analysis of XANES spectra shows that the samples contain Pu(IV), regardless if Pu(III), Pu(IV) or Pu(VI) is added to the groundwater. The GoHy-532 sample with Pu(VI) added has some Pu(V) and in addition Pu(IV). EXAFS analysis shows Pu-O bond distances to be split into a short and a longer distance. The short Pu-O distance is from bound hydroxy ligands resulting from Pu hydrolysis and/or from bonding to HS phenol groups. A Pu-Pu distance is also found at 3.82-3.85 Å, indicative of hydrolysis and polymeric species formation. Pu(IV) species formed in GoHy-532 are compared to Pu(IV) complexed with fulvic acid (FA) isolated from GoHy-532 at pHc 1.0 and 1.8. Results indicate that the hydrolysis is less pronounced in the natural groundwater samples than in the Pu(IV)-fulvates prepared from a colloid containing Pu(IV) solution. We observe the Gorleben GoHy and FA apparently stabilize small Pu(IV) hydrolysis species and inhibits precipitation of polynuclear hydrolysis products.JRC.E.6-Actinides researc