Application of Layered Double Hydroxides for 99-Tc immobilization

99Tc is a long-lived (T1/2=2.13·105 y) fission product (FP) of the nuclear fuel cycle (NFC). As a component of nuclear wastes it remains a FP of concern for the safety assessment of a final disposal site. In the heptavalent form Tc(VII)O4-, it is well soluble, poorly sorbed and hence, a highly mobile anion in the environment. A number of strategies have been proposed to reduce 99Tc mobility by irreversible incorporation into the structure of ubiquitous mineral phases . We have recently demonstrated the incorporation of TcO4- in layered double hydroxides (LDH) that were earlier proposed for 79Se and 129I remediation in water treatment technology.
This work is focused on the interaction of 99Tc(VII) with Pyroaurite (PyA) and Hydrotalcite (HTC) - Mg-Fe and Mg-Al LDHs respectively, on simplified CO2- and O2-free conditions. Earlier experiments with Re(VII), considered to be a chemical analogue for 99Tc(VII), have demonstrated little to no uptake of Re(VII). Similar experiments with 99Tc(VII) however have shown a significant increase in 99Tc uptake on LDHs even in the concurrency with CO32- ions. Subsequent leaching experiments have revealed the irreversible character of the 99Tc(VII)-LDH compound. Modern spectroscopic methods are applied in order to understand how 99Tc is accommodated in between the brucite layers of LDHs and whether 99Tc(VII) is reduced to Tc(IV). This information will help to verify, whether LDHs can be used as a host phase for 99Tc long-term and safe storage. Additionally, these data aquired on controlled conditions can be used for modelling of 99Tc geochemical behavior in more complex repository relevant systems.