Field evidence for uranium nanocrystallization and its implications for uranium transport

High-resolution transmission and scanning electron microscopies (HRTEM and SEM) both equipped with energy dispersive X-ray spectrometers (EDS) have been used to determine the chemical form of U(VI) in rocks and to examine U uptake mechanisms at low U concentrations (10− 8-10− 7 mol/L) in the downgradient of the Koongarra U ore deposit in Australia. Uranium was concentrated in Fe-(hydr)oxide nodules and veins that consisted mainly of goethite and hematite. Nanocrystals (20-100 nm in size) of uranyl phosphates such as metatorbernite [Cu(UO2)2(PO4)2 · 8H2O] were scattered between and firmly attached to nanocrystals (2-50 nm) of goethite and hematite. The environment where the nanocrystallization occurred was probably isolated from the Koongarra groundwaters undersaturated with respect to the uranyl phosphates. Chemical analyses by EDS showed that the uranyl-phosphate nanocrystals contribute more to the U uptake than adsorbed U. The presence of the uranyl-phosphate nanocrystals between Fe-(hydr)oxides explains the U distribution previously studied in the macro- and micro-scale in Koongarra. Our HRTEM and SEM observations suggest that during crystallization of goethite and hematite from ferrihydrite, U, P, and Cu or Mg adsorbed onto ferrihydrite are released from ferrihydrite, accumulated on the surface of forming goethite and hematite, and finally form nanocrystals of uranyl phosphates. Thus, uranyl-phosphate nanocrystallization, though initiated by adsorption, is a dominating mechanism of the U uptake and controls long-term U transport at low U concentrations.