Np(V/VI) redox chemistry in cementitious systems: XAFS investigations on the speciation under anoxic and oxidizing conditions

The use of cementitious materials is foreseen to immobilize long-lived intermediate level wastes that may contain significant amounts of 237Np. Predicting the release of Np from a cement-based repository requires an adequate understanding of its interaction with the main sorbing components of cement. Although Np(IV) will prevail under repository conditions after depletion of O2, Np(V) is expected to control the chemistry of Np in the early stage after repository closure as well as in the presence of oxidizing waste forms (i.e. high content of NO3-). Moreover, little is known of the stability of Np(VI) under hyperalkaline conditions, resulting in a rather ill-defined Np(V/VI) redox chemistry in cementitious environments. XANES studies on Np(V)-doped calcium silicate hydrates (C-S-H phases) and hardened cement paste (HCP) samples prepared under anoxic and under oxidizing conditions (provided by 5 × 10−3 M NaClO) clearly showed the predominance of Np(V) and Np(VI), respectively. The presence of two different neptunyl moieties was confirmed by EXAFS, revealing significantly shorter Np-Oax and Np-Oeq distances obtained for the samples with NaClO (1.79–1.85 Å and 2.24–2.25 Å) compared to those prepared under anoxic conditions (1.88–1.89 Å and 2.39 Å). This indicates oxidation of Np(V) to Np(VI) in the presence of NaClO. The short Np–Oeq distances along with structural properties of the Si-shells (number of neighboring Si atoms, bond distances) suggest that an incorporation mechanism is responsible for Np(V/VI) uptake by C-S-H phases. The coordinative environments of both Np redox states in cement paste were found to be similar to those in C-S-H phases, indicating C-S-H as the uptake-controlling phase. In contrast to Np(V), the molecular environment of Np(VI) in C-S-H phases depends not only on the Ca:Si ratio of these phases but also on pH, presumably reflecting differences between the hydrolysis scheme of the two Np redox states under these pH conditions. EXAFS data further confirmed the predominance of aqueous Np(VI) species (as NpVIO2(OH)42-) in hyperalkaline and oxidizing tetramethylammonium hydroxide (TMA-OH) solutions. Under anoxic conditions and in the presence of CO32- (as an impurity of TMA-OH), predominance of a mixed Np(V)-OH-CO3 species was indicated by EXAFS. These Np(V/VI) aqueous species are not considered in the current NEA thermodynamic selection and, therefore, deserve further attention.