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Title: Technetium incorporation into goethite (α-FeOOH): An atomic-scale investigation

During the processing of low-activity radioactive waste to generate solid waste forms (e.g., glass), technetium-99 (Tc) is of concern because of its volatility. A variety of materials are under consideration to capture Tc from waste streams, including the iron oxyhydroxide, goethite (α-FeOOH), which was experimentally shown to sequester Tc(IV). This material could ultimately be incorporated into glass or other low-temperature waste form matrices. However, questions remain regarding the incorporation mechanism for Tc(IV) in goethite, which has implications for predicting the long-term stability of Tc in waste forms under changing conditions. Here, quantum-mechanical calculations were used to evaluate the energy of five different charge-compensated Tc(IV) incorporation scenarios in goethite. The two most stable incorporation mechanisms involve direct substitution of Tc(IV) onto Fe(III) lattice sites and charge balancing either by removing one nearby H+ (i.e., within 5 Å), or by creating an Fe(III) vacancy when substituting 3 Tc(IV) for 4 Fe(III), with the former being preferred over the latter relative to gas-phase ions. When corrections for hydrated references phases are applied, the Fe(III)-vacancy mechanism becomes more energetically competitive. Calculated incorporation energies and optimized bond-lengths are presented. Proton movement is observed to satisfy under-coordinated bonds surrounding vacancies in the goethite structure.
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Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 0013-936X; 830403000
DOE Contract Number:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Environmental Science and Technology; Journal Volume: 49; Journal Issue: 22
American Chemical Society (ACS)
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
Sponsoring Org:
Country of Publication:
United States
Technetium, iron oxides, iron oxyhydroxides, goethite, α-FeOOH, atomic-scale modeling, quantum mechanics, Hartree-Fock, CRYSTAL14, incorporation energy, incorporation mechanisms, vitrification, waste forms