Dissolution of uranyl microprecipitates in subsurface sediments at Hanford Site, USA

Abstract

The dissolution of uranium was investigated from contaminated sediments obtained at the US. Department of Energy (U.S. DOE) Hanford site. The uranium existed in the sediments as uranyl silicate microprecipitates in fractures, cleavages, and cavities within sediment grains. Uranium dissolution was studied in Na, Na-Ca, and NH 4 electrolytes with pH ranging from 7.0 to 9.5 under ambient CO 2 pressure. The rate and extent of uranium dissolution was influenced by uranyl mineral solubility, carbonate concentration, and mass transfer rate from intraparticle regions. Dissolved uranium concentration reached constant values within a month in electrolytes below pH 8.2, whereas concentrations continued to rise for over 200 d at pH 9.0 and above. The steady-state concentrations were consistent with the solubility of Na-boltwoodite and/or uranophane, which exhibit similar solubility under the experimental conditions. The uranium dissolution rate increased with increasing carbonate concentration, and was initially fast. It decreased with time as solubility equilibrium was attained, or dissolution kinetics or mass transfer rate from intraparticle regions became rate-limiting. Microscopic observations indicated that uranium precipitates were distributed in intragrain microfractures with variable sizes and connectivity to particle surfaces. Laser-induced fluorescence spectroscopic change of the uranyl microprecipitates was negligible during the long-term equilibration, indicating that uranyl speciation was not changed by dissolution. A kinetic model that incorporated mineral dissolution kinetics and grain-scale, fracture-matrix diffusion was developed to describe uranium release rate from the sediment. Model calculations indicated that 50 -95% of the precipitated uranium was associated with fractures that were in close contact with the aqueous phase. The remainder of the uranium was deeply imbedded in particle interiors and exhibited effective diffusivities that were over three orders of magnitude lower than those in the fractures.

Advective Removal of Intraparticle Uranium from Contaminated Vadose Zone Sediments, Hanford, U.S.

“…However, because U(VI) release records a coupled dissolution-diffusion process, it is difficult to separate the effects of the individual processes. In this regard, a recent study derived a much higher dissolution rate constant for synthetic Na-boltwoodite (9) than the one obtained in the batch study (8). Errors associated with using a low k should be amplified for the advective regime relative to the SF events.…”

“…All other aqueous species were measured (Table 2) and used as input in the model to constrain the dissolution of the intragrain U(VI) phase; the only reaction considered was the dissolution of Na-boltwoodite. All solubility calculations used the same database (19) and activity corrections (the Davies equation) as in the batch study (8).…”

“…The reason for this is not readily apparent, but we speculate that the microscopic distribution of precipitated U(VI) may be a factor. Over 95% of U(VI) in sediment 53AB is in domains with high apparent diffusivity, whereas 60% of U(VI) in sediment 61AB is in domains with lower apparent diffusivity (8). Consequently, the diffusion of bulk solution solutes into sediment 61AB should have been subject to greater mass transfer limitations than for sediment 53AB.…”

Spatially Resolved U(VI) Partitioning and Speciation: Implications for Plume Scale Behavior of Contaminant U in the Hanford Vadose Zone

“…Microscopic and spectroscopic studies of selected samples from this borehole identified uranyl silicate (uranophane) as the dominant U species, residing within micro fractures of feldspar grains (9)(10)(11)(12)(13). Column and batch experiments using contaminated sediments from this borehole indicated that slow dissolution kinetics of the U(VI) silicates and intragranular mass transfer are expected to keep rates of U(VI) release into pore waters low (14). Laser fluorescence spectroscopy studies of the borehole samples identified UO 2 (CO 3 ) 3 4-and Ca 2 UO 2 (CO 3 ) 3 as the predominant U species in the plume aqueous phase (3).…”

Uranium release from a natural rock under near-natural oxidizing conditions

“…To access the reactivity of materials for U retention or removal in the laboratory, many operational leaching solutions have been defined (Duff et al 1997, Fredrickson et al 2000, Gadelle et al 2001, Kaplan & Serkiz 2001, Liu et al 2004). All these solutions are more aggressive than natural waters.…”