Actinyl Ion Crystal Chemistry and Its Impact on Structural Topologies and Environmental Fate

The modern study of the actinide elements began more than 70 years ago, yet much remains to be explored about how these radioactive elements behave in complicated systems like the soils at Department of Energy sites, the forests near Chernobyl or the ocean waters off Fukushima. The fundamental study of actinide chemistry provides a basis for understanding the mechanisms that control actinide migration in the environment. Here two major themes are presented in which one explores the structural properties of U(VI) uranyl germanates as they relate to U(VI) uranyl silicates and the emergence of cation-cation interactions as a structural feature, and the other offers a glimpse at the behavior of Np(V) and U(VI) during the growth of rock- forming minerals for the purpose of understanding the inorganic controls of crystal growth on environmental remediation. Hydrothermal synthesis and single crystal X-ray diffraction were employed in the study of U(VI) uranyl germanates. For the study of Np(V) incorporation into rock-forming minerals, a variety of room temperature syntheses were conducted before a simple synthesis in aqueous solution was devised. Characterization methods included ICP-MS in solid and solution modes and XPS. This research demonstrated (1) the structural differences between U(VI) uranyl germanates and silicates by introducing new (VI) uranyl germanate compounds with uncommon structural features, like cation-cation interactions and chains of GeO5 tetrahedra; and (2) the potential for structural incorporation to play a role in neptunium mobility in the subsurface by showing that calcite has a higher affinity for neptunium than gypsum during synthetic growth.