Incorporation of actinides into iron (oxyhydr)oxides: A long-term environmental barrier to radionuclide migration

Iron (oxyhydr)oxide minerals are ubiquitous in the sub-surface environment (e.g. soils and sediments) and have the potential to act as a long-term barrier to the migration of radionuclides. The long-lived actinides (e.g. ²³⁷Np, 2.14 million year half-life) are particularly challenging, and pose multigenerational technical and policy issues for the remediation of contaminated land and the disposal of radioactive wastes. Over the last decade, increasing evidence indicates that actinides may substitute for Fe in the structure of iron (oxyhydr)oxides. Given the thermodynamic stability of iron (oxyhydr)oxides over the crustal continuum, incorporation may provide long-term immobilization pathways for anthropogenic actinide contaminants. Here, the speciation and coordination adopted by key actinides (i.e. U, Np, Pu and Am) on incorporation into iron (oxyhydr)oxides is reviewed. The stability of these phases under fluctuating environmental conditions is also discussed, to provide constraints on the long-term fate of actinides incorporated into iron (oxyhydr)oxides.