Technetium Retention and Remobilization Potential after Prolonged Reaction in Fe(III)- and Sulfate-Reducing Model Aquifer Systems

⁹⁹Tc is a long-lived radioactive fission product whose subsurface mobility is governed by redox conditions. Under oxic conditions, soluble Tc(VII)O₄^- is mobile, whereas under reducing conditions, poorly soluble Tc(IV) phases limit transport. Microcosm studies have frequently reported TcO₂-like solids and, less consistently, Tc(IV)-sulfides. The stability of Tc(IV)-sulfides under environmentally relevant conditions remains unclear. Here, we used flowing sediment columns representative of the Sellafield subsurface to examine Tc speciation and stability over ∼1 year. Under reducing conditions, >90% of added TcO₄^- (400 μg) was retained under both Fe(III)- and sulfate-reducing conditions. X-ray absorption spectroscopy showed TcO₂-like phases dominated in Fe(III)-reducing columns, while Tc(IV)-sulfides dominated after sustained sulfate reduction. Sequential extractions indicated that Tc in sulfidic sediments was more recalcitrant (≤23% released by weak acids) than in Fe(III)-reducing systems (∼60% released). With oxic groundwater pumping, effluent Tc sourced from the sediments rose rapidly. Over 160 days, the sulfidic columns remobilized ∼25% of their Tc inventory compared to ∼50% in Fe(III)-reducing columns. The Tc(IV)-sulfides also gradually oxidized to form TcO₂ phases. While Tc(IV)-sulfides may enhance Tc retention under reducing conditions, TcO₂ phases more likely govern ⁹⁹Tc mobility during long-term redox cycling. Our findings provide new constraints for modeling Tc fate at contaminated sites and in radioactive waste disposal.