Removal of TcO₄^- from Representative Nuclear Waste Streams with Layered Potassium Metal Sulfide Materials

Many efforts have focused on the sequestration and immobilization of ⁹⁹Tc because the radionuclide is highly mobile in oxidizing environments and presents serious health risks due to its radiotoxicity and long half-life (t_1/2 = 213 000 a). One of the more common methods for Tc removal from solution and immobilization in solids is based on reducing Tc from highly soluble Tc^(VII) to sparingly soluble Tc^(IV). Here, we report results obtained with two potassium metal sulfides (KMS-2 and KMS-2-SS) that are capable of reducing Tc^(VII) to Tc^(IV). Batch sorption experiments were performed in both oxic and anoxic conditions for 15 d in both deionized water (DIW) and a highly caustic (pH ∼ 13.6), high ionic strength (8.0 mol L^-1), low-activity waste (LAW) stream simulant solution. Tc removal for both materials in DIW is improved in anoxic conditions compared to oxic conditions as a result of a higher solution pH. In DIW and anoxic conditions, KMS-2 is capable of removing ∼45% of Tc, and KMS-2-SS is capable of removing ∼90% of Tc. Both materials perform even better in the LAW simulant and remove more than 90% of available Tc after 15 d of contact in anoxic conditions. Postreaction solids analyses indicate that Tc^(VII) is reduced to Tc^(IV) and that Tc^(IV) is bonded to S atoms in a Tc₂S₇ complex. Examination of the materials after Tc removal by X-ray diffraction shows that the initially crystalline KMS-2 materials lose much of their initial long-range order. We suggest a Tc removal mechanism wherein the TcO₄^- enters the interlayer of the KMS-2 materials where it is reduced by sulfide, which results in a distorted crystalline structure and a solid-state Tc₂S₇ complex.