Oxidative dissolution of TcO₂ by Mn(III) minerals under anaerobic conditions: Implications on technetium-99 remediation

Manganese oxides are prolific minerals with strong oxidizing properties causing them to strongly influence the stability of any redox sensitive metal and metalloid reduced species in the environment. Their potential to oxidize reduced species is disproportionate to their concentration due to regeneration of Mn(III) mineral formed by abiotic processes or by microorganisms even under suboxic conditions. Tc-99 is produced during uranium fission in nuclear power generation and it is a risk driving environmental contaminant because it is found as highly soluble Tc(VII)O₄⁻ under oxidizing conditions. Current remediation efforts have focused on the immobilization of highly soluble Tc(VII) to sparingly soluble Tc(IV) under anoxic conditions. This publication investigates for the first time the impact of Mn(III) minerals on the oxidative dissolution of TcO₂ under anaerobic conditions. α-Mn(III)₂O₃ and γ-Mn(III)OOH were used in batch experiments in contact with TcO₂ under anaerobic conditions at different pH values and solid:liquid ratios and in the presence of cations common in groundwater. Both minerals incurred significant oxidation of TcO₂ and consequently, release of pertechnetate in the aqueous phase at pH 6.5, and negligible oxidation of TcO₂ at pH 8.0. For both minerals, increasing the solid:liquid ratio increased the extent of TcO₂ oxidation. γ-Mn(III)OOH was insensitive to the presence of Ca²⁺ and Mn²⁺ ions, whereas α-Mn(III)₂O₃ sorbed Ca²⁺, which substantially reduced the mineral's oxidative capacity. These experimental findings demonstrate the liability of manganese minerals in oxidizing Tc(IV) insoluble phases to highly soluble Tc(VII) even in the absence of oxygen and highlights the need to be taken into account when it comes to remediation efforts for the abatement of Tc-99 in the environment.