The transformation of uranyl oxide hydrates: The effect of dehydration on synthetic metaschoepite and its alteration to becquerelite

The U(VI) solid phases schoepite, metaschoepite, and dehydrated schoepite a re important reservoirs of mobile uranium in the environment. These simple uranyl oxide hydrates result from weathering of uranium minerals and the co rrosion of anthropogenic uranium solids. We have studied the role of hydrat ional water among these phases and in subsequent tranformation to other sec ondary metal-U(VI) oxide hydrates. Synthetic metaschoepite (MS, UO3. 2.0H(2 )O), its dehydrated phases, and its secondary alteration products were char acterized using X-ray powder diffraction (XRD), scanning electron microscop y (SEM), X-ray energy dispersive spectroscopy(EDS), and high-resolution the rmogravimetric analysis (HRTGA). Drying MS at 105 degrees C resulted in the formation of a dehydrated phase (UO3. 0.9H(2)O) that was structurally dist inct from natural dehydrated schoepite (DS, UO3. 0.75H(2)O) reported by oth ers. Unlike natural DS, our dehydrated material was easily rehydrated, alth ough crystallinity of the rehydrated phase was reduced. The rates of transf ormation of synthetic MS and dehydrated MS in the presence of Ca2+ to form becquerelite (Ca[(UO2)(6)O-4(OH)(6)]. 8H(2)O) were determined. Alteration r ates were significantly faster when the starting material had been dehydrat ed. These results are explained in the context of structural aspects of U(V I) solid phases, and the possible impact of hydration on long-term stabilit y of U(VI) oxide hydrates in environmental systems is discussed.