Microbial immobilization of technetium-99

Iron-reducing bacteria perform anaerobic respiration by coupling the oxidation of organic molecules to the reduction of Fe(III)-species via dissimilatory iron reduction. This leads to the formation of ferrous minerals, such as vivianite [Fe(II)₃(PO₄)₂], pyrite (Fe(II)S₂), siderite (Fe(II)CO₃) and jahnsite [(CaMn(II))Fe(II)₂Fe(III)₂(PO₄)₄(OH)₂·(H₂O)₈], which, depending on oxygen exposure and the cultivation nutrients, may oxidize and generate magnetite (Fe(II)Fe(III)₂O₄) and/or hematite (Fe(III)₂O₃). The aforementioned secondary Fe(II)-minerals may promote the reduction of radionuclides such as the β-emitter technetium-99 (⁹⁹Tc).
⁹⁹Tc is a long lived fission product (t(1/2) = 2.13 × 10⁵ years) of ²³⁵U and ²³⁹Pu. It can also originate from the decay chain of ⁹⁹₄₂Mo, (Mo-99 → ₄₃Tc-99m + e⁻, 66 h; ₄₃Tc-99m → ₄₃Tc-99 + γ, 6.02 h). In the environment, Tc is prevalent as Tc(VII) or Tc(IV). Due to the common use of Tc-99m in radiodiagnostics, water contamination through the highly hydrosoluble pertechnetate Tc(VII)O₄⁻ must be considered. Nevertheless, Tc can be immobilized by reducing it to the low-soluble oxide Tc(IV)O₂.
This work aims at unravelling the interactions between pivotal anaerobic bacteria (e.g. the iron reducer Desulfitobacterium sp. G1-2) that can be found in bentonite (a clay potentially used as a barrier material for deep geological repositories) and Tc(VII), to achieve its reduction to Tc(IV) and preserve the environmental safeguard.
The authors acknowledge the German Federal Ministry of Education and Research (BMBF) for the financial support of NukSiFutur TecRad young investigator group (02NUK072).