Technetium Uptake by Ferrous Hydroxide and Ferrous Hydroxychloride: Influence of Host Phase Coordination Environments

Abstract

The safe disposal of high-level radioactive waste (HLW) in deep geological repositories requires understanding how radionuclides interact with Fe(II)-bearing secondary phases formed during steel canister corrosion. This study investigates the redox and coordination behavior of 99Tc(VII), a long-lived and radiotoxic fission product, upon reaction with ferrous hydroxide and ferrous hydroxychloride under anoxic, alkaline conditions. Batch uptake experiments showed that both Fe(II)-bearing phases rapidly and effectively immobilize 99Tc, independent of ionic strength, and no compelling localized accumulation of Tc precipitate could be detected by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDXS). Tc K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that technetium prevails as Tc(IV), implying that Fe(II) oxidation occurred concomitantly with pertechnetate reduction. Furthermore, extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that Tc occurs primarily as dimers or as a mixture of monomeric and dimeric species, and ruled out significant TcO2·nH2O formation. The number of detected next-nearest Fe atoms pointed to Fe and Tc octahedra sharing edges at the solid/solution interface, forming either Tc surface-sorbed species or Tc surface-incorporated species. In the latter species, Tc substitutes for structural Fe at the mineral surface, the local excess charge resulting from the substitution of a divalent by a tetravalent cation being possibly balanced by anions present at the solution side. Overall, findings suggest that during the long-term evolution of a nuclear waste repository system, the formation of Fe(OH)2 and β-Fe2(OH)3Cl as canister corrosion products represents a sink capable of efficiently immobilizing 99Tc, preventing its spread in the far-field.