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

Many efforts have focused on the sequestration and immobilization of Tc-99 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 similar to 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, KIM-2 is capable of removing similar to 45% of Tc, and KIM-2-SS is capable of removing similar to 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 KIM-2 materials where it is reduced by sulfide, which results in a distorted crystalline structure and a solid-state Tc2S7 complex.