Structural Investigation of Technetium Dibutylphosphate Species Using X-ray Absorption Fine Structure Spectroscopy

The speciation of Tc after the extraction of Tc(IV) from H₂O and 1 M HNO₃ by dibutylphosphoric acid (HDBP) in dodecane has been studied by X-ray absorption fine structure (XAFS) spectroscopy. Results show the formation of dimeric species with Tc₂O₂ and Tc₂O units, and the formulas [Tc₂O₂(DBP·HDBP)₄] (1) and [Tc₂O(NO₃)₂(DBP)₂(DBP·HDBP)₂] (2) were, respectively, proposed for the species extracted from H₂O and 1 M HNO₃. The interatomic Tc-Tc distances found in the Tc₂O₂ and Tc₂O units [2.55(3) and 3.57(4) Å, respectively] are similar to the ones found in Tc(IV) dinuclear species. It is likely that the speciation of Tc(IV) in dodecane is due to the extraction of a species with a Tc₂O unit for (2) and to the redissolution of a Tc(IV)-DBP solid for (1). The XAFS results for (1) and (2) were compared to that obtained for the extraction of Tc(IV) with TBP/HDBP/dodecane from 0.5 M HNO₃, (3) which highlight the formation of Tc mononuclear nitrate species {i.e., [Tc(NO₃)₃(DBP)] or [Tc(NO₃)₂(DBP·HDBP)]}. These results confirm the importance of the preparation and speciation of the Tc(IV) aqueous solutions prior to extraction and how much this influences and drives the final Tc speciation in organic extraction. These studies outline the complexity of Tc separation chemistry and provide insights into the behavior of Tc during the reprocessing of used nuclear fuel.