Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Technetium contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington, USA. At these sites, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (214 000 years) and the mobility of the oxidized anionic species Tc(VII)O₄ ^-. Under reducing conditions, TcO₄ ^- is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc-oxide (TcO₂·nH₂O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to reoxidation to TcO₄ ^-, which in turn depend on the (biogeo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO₂ or amorphous TcO₂·H₂O is still above the maximum contaminant level (MCL) established by the U.S. EPA (900 pCi/L), and the kinetics of TcO₂ oxidative dissolution can be on the order of days to years. In addition to oxygen, sulfur can form complexes that significantly affect the adsorption, solubility, and reoxidation potential of Tc, especially Tc(IV). The principal technetium sulfides are TcS₂ and Tc₂S₇, but much less is known about the mechanisms of formation, stabilization, and reoxidation of Tc-sulfides. A common assumption is that sulfides are less soluble than their oxyhydrous counterparts. Determination of the molecular structure of Tc₂S₇ in particular has been hampered by the propensity of this phase to precipitate as an amorphous substance. Recent work indicates that the oxidation state of Tc in Tc₂S₇ is Tc(IV), in apparent contradiction to its nominal stoichiometry. Technetium is relatively immobile in reduced sediments and soils, but in many cases the exact sink for Tc has not been identified. Experiments and modeling have demonstrated that both abiotic and biologic mechanisms can exert strong controls on Tc mobility and that Tc binding or uptake into sulfide phases can occur. These and similar investigations also show that extended exposure to oxidizing conditions results in transformation of sulfide-stabilized Tc(IV) to a Tc(IV)O₂-like phase without formation of measurable dissolved TcO₄ ^-, suggesting a solid-state transformation in which Tc(IV)-associated sulfide is preferentially oxidized before the Tc(IV) cation. This transformation of Tc(IV)-sulfides to Tc(IV)-oxides may be the main process that limits remobilization of Tc as Tc(VII)O₄ ^-. The efficacy of the final waste form to retain Tc also strongly depends on the ability of oxidizing species to enter the waste and convert Tc(IV) to Tc(VII). Many waste form designs are reducing (e.g., cementitious waste forms such as salt stone) and, therefore, attempt to restrict access of oxidizing species such that diffusion is the rate-limiting step in remobilization of Tc.