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 uptake Tc(VII) from the solution follows the restoring of a LDH structure. X-ray absorption spectroscopy demonstrates that Tc ions interact solely via the Tc[sbnd]O bond, leaving no evidences of farther atomic interactions with, e.g., layers of LDH. The presence of competing anions, like NO₃ ⁻, or CO₃ ^2– in the solution decreases Tc(VII) uptake by LDH. Maximum uptake capacity of thermally activated Mg_0.67/Al_0.33-LDH and Mg_0.75/Fe_0.25–LDH were derived from fitting the experimental data into the modified Langmuir equation and correspondingly comprise 0.227 mol/kg and 0.213 mol/kg. In agreement with these findings, theoretical simulations predicted incorporation energies for Mg_0.67/Al_0.33–LDH and Mg_0.75/Fe_0.25–LDH of −128 kJ/mol and − 110 kJ/mol, respectively. Investigation of stability of Tc–LDH in different aqueous solutions demonstrated a rather low release Tc(VII) 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 anion exchange with Cl⁻.