Gamma-Irradiation-Enhanced Reduction of 99Tc(VII) and Re(VII) by Natural Pyrite

Reductive immobilization of 99Tc(VII) to sparingly soluble TcO2-type phases is a key process limiting 99Tc transport in the environment. However, the reactivity of natural pyrite toward 99Tc(VII) under conditions relevant to nuclear waste repositories remains poorly understood. Here, we investigate the interactions of 99Tc(VII) and its nonradioactive congener Re(VII) with natural pyrite under γ-irradiation (0-100 kGy; 0-100 Gy/min). In the absence of γ-irradiation, natural pyrite exhibited limited reactivity toward 99Tc(VII) at circumneutral to alkaline pH over the investigated time scales. Under γ-irradiation, however, pyrite-mediated reductive sequestration of both Tc(VII) and Re(VII) was substantially enhanced, producing a TcO2/TcOx-TcSx mixture and ReO2 as dominant reduction products. Mechanistic analyses indicate that radiolytically generated hydrated electrons (eaq-) and hydrogen radicals (·H) directly reduce TcO4- and ReO4-, while pyrite and dissolved Fe2+ scavenge hydroxyl radicals (·OH) and sustain a reducing interfacial environment. Under acidic conditions, protonation of eaq- to ·H limits Tc(VII)/Re(VII) reduction efficiency. At higher pH, OH- suppresses intermediate disproportionation, promoting efficient stepwise reduction via consecutive one-electron transfer from eaq-. These findings demonstrate that γ-irradiation fundamentally alters the redox reactivity of natural pyrite and promotes radionuclide immobilization under nuclear waste repository conditions, providing critical constraints for predicting long-term radionuclide transport in deep geological disposal systems.