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Technetium Incorporation into Goethite (α-FeOOH): An Atomic-Scale Investigation

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journal contribution
posted on 17.11.2015, 00:00 by Frances N. Smith, Christopher D. Taylor, Wooyong Um, Albert A. Kruger
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.

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