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

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• 作者：Frances N. Smith ; Christopher D. Taylor ; Wooyong Um ; Albert A. Kruger
• 刊名：Environmental Science & Technology
• 出版年：2015
• 出版时间：November 17, 2015
• 年：2015
• 卷：49
• 期：22
• 页码：13699-13707
• 全文大小：469K
• ISSN：1520-5851

文摘

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 (伪-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 脜) 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.