The Transformation of Uranyl Oxide Hydrates:  The Effect of Dehydration on Synthetic Metaschoepite and Its Alteration to Becquerelite

• Published in: Environmental science & technology 1999-10, Vol.33 (20), p.3552-3557
• Main Authors: Sowder, A. G, Clark, S. B, Fjeld, R. A
• Format: Article
• Language: English
• Subjects: 05 NUCLEAR FUELS; Applied sciences; BECQUERELITE; Chemistry; DEHYDRATION; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental monitoring; ENVIRONMENTAL TRANSPORT; Exact sciences and technology; Groundwater; HYDRATES; OXIDES; PHASE STUDIES; Pollution; Pollution, environment geology; Radioactive wastes; SCHOEPITE; Uranium; URANYL COMPOUNDS; Wastes; WEATHERING
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es9901516

The U(VI) solid phases schoepite, metaschoepite, and dehydrated schoepite are important reservoirs of mobile uranium in the environment. These simple uranyl oxide hydrates result from weathering of uranium minerals and the corrosion of anthropogenic uranium solids. We have studied the role of hydrational water among these phases and in subsequent tranformation to other secondary metal-U(VI) oxide hydrates. Synthetic metaschoepite (MS, UO3·2.0H2O), its dehydrated phases, and its secondary alteration products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and high-resolution thermogravimetric analysis (HRTGA). Drying MS at 105 °C resulted in the formation of a dehydrated phase (UO3·0.9H2O) that was structurally distinct from natural dehydrated schoepite (DS, UO3·0.75H2O) reported by others. Unlike natural DS, our dehydrated material was easily rehydrated, although crystallinity of the rehydrated phase was reduced. The rates of transformation of synthetic MS and dehydrated MS in the presence of Ca2+ to form becquerelite (Ca[(UO2)6O4(OH)6]·8H2O) were determined. Alteration rates were significantly faster when the starting material had been dehydrated. These results are explained in the context of structural aspects of U(VI) solid phases, and the possible impact of hydration on long-term stability of U(VI) oxide hydrates in environmental systems is discussed.