Short note on the hydrolysis and complexation of neptunium(IV) in HEPES solution

Abstract

The speciation of actinides in environmental or biological media is often difficult to assess because it involves complex media. We would like to report here on the properties of Np(IV) cation in the well known biological HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer medium. HEPES has been targeted because the possibility to use this biological buffer in actinide toxicological studies presents several advantages although the possible effects of concentrated HEPES medium on the hydrolysis of the actinides (in particular at oxidation state +IV) has not been studied yet. A combination of spectrophotometric and EXAFS measurements at the Np L_III edge shows that stable hydrolyzed neptunium(IV) clusters are obtained between pH 2.5 and 4. In a second step, in order to better understand the reactivity of these hydrolysis species formed in HEPES, the effect of a strong chelating ligands such as the hydroxamic acid (HA) or desferrioxamine (DFO) siderophores has been also investigated using spectrophotometry and EXAFS. Upon addition of HA or DFO, the hydrolyzed clusters of Np(IV) are unstable and monomeric complexes are formed and yield Np environments that are very similar to that of crystallized Pu-DFOE complex [Al(H₂O)₆][Pu(DFE)(H₂O)₃]₂(CF₃SO₃)_5·10H₂O reported in the literature.