Application of Layered Double Hydroxides for 99Tc remediation

The present study investigates possible use of Layered Double Hydroxides (LDH) for Tc(VII) remediation. Mg/Al- and Mg/Fe-LDH were obtained by a hydrothermal route and thermally activated at 450°C, which was shown to significantly improve the Tc(VII) removal efficiency. Based on XRD investigation of Tc-LDH phases, the Tc(VII) uptake follows the restoring of an LDH structure. X-ray absorption spectroscopy demonstrates that Tc ions interact solely via the Tc-O bond, leaving no evidences of farther atomic interactions with, e.g., layers of LDH. The presence of competing anions, like NO3-, or CO32- in the solution decreases Tc(VII) uptake by LDH. Presently investigated thermally activated Mg0.67/Al0.33-LDH revealed a maximum uptake capacity of up to 1.27 mol/kg (or 20 wt.%), which is higher than that of the Mg0.75/Fe0.25-LDH (0.9 mol/kg). In agreement with these findings, theoretical simulations predicted incorporation energies for Mg0.67/Al0.33-LDH and Mg0.75/Fe0.25-LDH of -128 kJ/mol and -110 kJ/mol, respectively. Investigation of Tc-LDH in different leaching media demonstrated a rather high Tc(VII) stability in LDH in contact with diluted solutions containing Cl- and OH-, however, in a high saline solution, like Q-brine a rather fast release of TcO4- occurs due to anionic exchange with Cl-.