title: Technetium Incorporation into Goethite (α-FeOOH): An Atomic-Scale Investigation.
abstract: 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 alternative 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 undercoordinated bonds surrounding Fe(III)-vacancies in the goethite structure.
authors.affiliation: Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, United States.
authors.forename: Frances N
authors.initials: FN
authors.lastname: Smith
authors.affiliation: Fontana Corrosion Center, Materials Science and Engineering, The Ohio State University , Columbus, Ohio 43210, United States.|Strategic Research and Innovation, DNV GL, Dublin, Ohio 43017, United States.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Taylor
authors.affiliation: Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, United States.
authors.forename: Wooyong
authors.initials: W
authors.lastname: Um
authors.affiliation: United States Department of Energy, Office of River Protection, P.O. Box 450, Richland, Washington 99352, United States.
authors.forename: Albert A
authors.initials: AA
authors.lastname: Kruger
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry; D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry*; D008962:Models, Theoretical; D010084:Oxidation-Reduction; D011522:Protons; D011789:Quantum Theory; D011850:Radioactive Waste; D013667:Technetium / Q000737:chemistry*; D013696:Temperature
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D008903:Minerals; D011522:Protons; D011850:Radioactive Waste; C094886:goethite; C021024:ferric hydroxide; D013667:Technetium