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Description
Technetium‑99 ($ ^{99}Tc $) is a radioactive fission product of the nuclear fuel cycle, with a long half-life (211,000 years). In oxidizing environments, $ ^{99}Tc $ is primarily present as the pertechnetate ion ($ Tc(VII)O_4 $), which is a highly soluble species. Due to its high mobility and long-term persistence, $ ^{99}Tc $ poses a significant environmental challenge and is treated conservatively in safety assessments for nuclear waste repositories. Under reducing conditions, however, $ ^{99}Tc $ is immobilized as Tc(IV), forming low-solubility phases such as $ TcO_2·nH_2O$ with stronger retention on mineral surfaces.
Fe(II)-bearing minerals play a particularly important role in immobilizing Tc because they are reductive species and provide reactive surfaces and lattice sites for Tc(IV) retention. Fe(II)-minerals are naturally present in the host rock or formed over time from the corrosion of nuclear waste steel canisters. However, transport calculations employed in safety assessments generally neglect solubility-reducing processes associated with Fe(II) phases. A more detailed understanding of Tc(VII) behavior in contact with corrosion products, especially iron(II) carbonates, is therefore essential to reduce overly conservative assumptions in long-term safety assessments.
This study investigates $ ^{99}Tc $ reductive immobilization through interaction with iron carbonate minerals, namely natural ankerite $(Ca(Fe,Mg,Mn)(CO_3)_2)$, and synthetic siderite $(FeCO_3)$. Prior to the experiments, the minerals were characterized using Mössbauer spectroscopy, scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD) to assess their morphology and confirm phase purity. Batch sorption experiments were conducted to evaluate the effects of $ ^{99}Tc $ concentration, carbonate content, pH, and ionic strength under anoxic conditions in a glovebox. For a molecular understanding, $ ^{99}Tc $ retention will be studied using Raman spectroscopy and X-ray absorption spectroscopy. First results show that siderite has a higher affinity (99.8%) and faster kinetics (72 h) for Tc immobilization than ankerite (13.1% after 30 days).