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 oxygen, Np(V) is expected to control the chemistry of neptunium 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 on the stability of Np(VI) under hyperalkaline conditions, resulting in a rather ill–defined Np(V/VI) redox chemistry in cementitious environments.
Evaluation of XANES data from Np(V)–doped calcium silicate hydrates (C–S–H phases) and hardened cement paste (HCP) under anoxic and under oxidizing conditions (provided by 510–3M NaClO) clearly indicated the predominance Np(V) and Np(VI), respectively. Two different neptunyl moieties were also observed by EXAFS, with significantly shorter Np–Oax and Np–Oeq obtained for samples with NaClO (1.79–1.85 Å and 2.24–2.25 Å) compared to samples under anoxic conditions (1.88–1.89 Å and 2.39 Å), thus confirming the oxidation of Np(V) to Np(VI) in the presence of NaClO. The strong EXAFS features resulting from the neptunyl–oxygen coordination hamper the signal of atoms beyond their coordination sphere in Np(V/VI)–doped C–S–H and HCP samples. Nevertheless, Np–Oeq distances as well as evaluation of the Si–shells provided some evidence for an incorporation mechanism taking place in the C–S–H structure. 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 the pH of the solution, likely reflecting the differences between the hydrolysis scheme of both Np redox states under these pH conditions.
EXAFS data 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 impurity of TMA–OH), the 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.