Redox-dependent suphate coordination of neptunium in aqueous solutions

A direct determination of actinide coordination in aqueous solution under controlled redox conditions is an important issue to understand the specific chemical reactions and to predict the behavior in nuclear waste repositories. We developed several techniques to study actinide solutions under dedicated redox conditions at the ESRF. The methods to control the actinide redox state are based on potentiostatic or galvanostatic electrolyses. The solutions can be investigated either ex situ or in situ with a specific spectroelectrochemical cell where the electrochemical process is combined with EXAFS spectroscopy. As example we show the redox reaction of 0.05 M neptunium in presence of 2.0 M sulfate. The neptunium species occur preferred with one monodentate (Np5+) and two bidentate (Np6+) sulfate ligands. Np4+ forms very strong complexes with up to fife sulfate ligands. The EXAFS signal reveals a unique ligand rearrangement during the redox reaction. This provides the explanation why the cyclic voltammogram show irreversible reaction steps.