The role of green rust in the migration of radionuclides: An overview of processes that can control mobility of radioactive elements in the environment using as examples Np, Se and Cr

Green rust is an Fe(II)-Fe(III) layered double hydroxide that can be present in groundwater in rock formations where long term nuclear waste repositories are being planned. Green rust (GR) is redox-active, making data for its behaviour a necessity in safety assessment modeling because the mobility of many elements, including radionuclides, is controlled by redox reactions. In this paper, we compare the influence of geometry and charge of three redox active ions on their reaction with colloidal size particles of green rust sodium sulfate (GR_Na,SO₄ ). Pentavalent neptunyl, selenite and chromate all have different geometries and valence states. Ions with shape and charge similar to SO^2-₄, the GR_Na,SO ₄ interlayer anion such as, for example, CrO^2- ₄-, diffuse into the GR interlayer where electrons are readily transferred, and the element may become incorporated into the final oxidised product. More often, the geometry and/or charge of the ion differ significantly from the interlayer anion, such as is the case for SeO^2-₃ and NpO⁺₂. These are reduced at the exterior of the flat hexagonal GR crystals, primarily at the edges. Se(IV) is reduced to Se(0) and precipitates as a separate phase, while Np(V) is reduced to Np(IV) and precipitates possibly as NpO₂ or as a substitute in an iron oxide. Whether it adsorbs to pore walls in the groundwater flowpath, or it travels as a colloid, green rust can have a significant effect on radionuclide mobility in the near and far field of repositories.