99Tc retention on pyrite and alumina: the effect of Fe2+

For the safety case of a nuclear waste repository, Tc is treated very conservatively, assuming no retention by the geotechnical and geological barriers. Tc(VII), pertechnetate (TcO4-), the most dominant species in oxidizing environment is considered to be inert, hardly interacting with minerals and highly mobile. In contrast, the reduced form Tc(IV) is mainly found as solid, TcO2, and its mobility is limited.
The presence of reductants in the near-field of a nuclear waste repository, e.g. Fe2+ is expected due to canister corrosion. Therefore, most of recent studies consider Tc reductive immobilization by mineral containing reductant moieties, such as magnetite (FeIIFe2IIIO4) or mackinawite (FeS) [1,2] or pre-sorbed reductants on mineral phases, like Fe2+ on corundum (a-Al2O3), diaspore (a-AlOOH), goethite (a-FeOOH), and hematite (a-Fe2O3)[3,4].
In this work, we compare the Tc immobilization by two systems: i) pyrite (FeS2) and ii) nano particular alumina in presence of Fe2+ (ternary system).
Pyrite is capable to remove almost 100% of Tc(VII) from solution within 7 days at pH = 6.5. In a further step, we study the effect of ionic strength on the Tc immobilization under different NaCl concentrations, as the retention mechanism could be affected by the change of Tc(IV) solubility, due to different degrees of salinity [4].
Alumina is capable to retain 6.5% of Tc in the absence of reducing Fe2+. However, in the ternary system Tc retention is 100% for pH > 6.5. In this case, the improvement on the Tc reduction is not only due to Fe2+ presence, but also to the surface properties of alumina, triggering heterogeneous reduction of Tc by high Fe2+ surface coverage or possible LDH formation.

This work has been developed in the frame of VESPA II project (02E11607B), supported by the German Federal Ministry of Economy and Energy (BMWi).