Influence of Phosphate and Silica on U(VI) Precipitation from Acidic and Neutralized Wastewaters

Abstract

Uranium speciation and physical-chemical characteristics were studied in solids precipitated from synthetic acidic to circumneutral wastewaters in the presence and absence of dissolved silica and phosphate to examine thermodynamic and kinetic controls on phase formation. Composition of synthetic wastewater was based on disposal sites 216-U-8 and 216-U-12 Cribs at the Hanford site (WA, USA). In the absence of dissolved silica or phosphate, crystalline or amorphous uranyl oxide hydrates, either compreignacite or meta-schoepite, precipitated at pH 5 or 7 after 30 d of reaction, in agreement with thermodynamic calculations. In the presence of 1 mM dissolved silica representative of groundwater concentrations, amorphous phases dominated by compreignacite precipitated rapidly at pH 5 or 7 as a metastable phase and formation of poorly crystalline boltwoodite, the thermodynamically stable uranyl silicate phase, was slow. In the presence of phosphate (3 mM), meta-ankoleite initially precipitated as the primary phase at pH 3, 5, or 7 regardless of the presence of 1 mM dissolved silica. Analysis of precipitates by U LIII-edge extended X-ray absorption fine structure (EXAFS) indicated that "autunite-type" sheets of meta-ankoleite transformed to "phosphuranylite-type" sheets after 30 d of reaction, probably due to Ca substitution in the structure. Low solubility of uranyl phosphate phases limits dissolved U(VI) concentrations but differences in particle size, crystallinity, and precipitate composition vary with pH and base cation concentration, which will influence the thermodynamic and kinetic stability of these phases.

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…Reference compounds included uranyl oxyhydroxides (K-compreignacite, Na-compreignacite, becquerelite, schoepite), uranyl nitrate, and uranyl sorbed on quartz (reference spectra from this work and Kanematsu et al (2014)). Component weights of the reference spectra were constrained between 0 and 1 in linear combination fits but not forced to sum to unity.…”

“…e Becquerelite spectrum obtained from Kanematsu et al (2014); shell-by-shell parameters were re-fit for this work. f Interatomic distances (R) and N for U(VI) sorbed on quartz were based on EXAFS fit results reported in Sylwester et al (2000) and Batuk et al (2011) for sorbed aqueous uranyl on silica and silica colloids in pH 6.46 and 6.5 experiments, respectively.…”

“…a Linear combination fits were performed using K-compreignacite (K-comp. ), Na-compreignacite, becquerelite, schoepite, uranyl nitrate and U(VI) sorbed on quartz (sorbed U(VI)) reference spectra (from this work and Kanematsu et al (2014)). Spectra contributing less than 10% to the fit were discarded.…”

“…These low atomic number interlayer species are weak photoelectron scatterers when present at distances >$3 Å , and thus do not contribute detectable scattering amplitude to the EXAFS spectra. Nonetheless, changes in the concentration, type, and charge of interlayer cations can influence interatomic distances between U(VI) and oxygen atoms in the polyhedral sheets, particularly for cations with a large ionic radius or higher charge (Allen et al, 1997;Kanematsu et al, 2014). Unreacted Kcompreignacite was stoichiometrically deficient in K compared to U (Table 3), which suggests that H + (H 3 O + ) accounts for charge balance.…”

“…In the absence of dissolved phosphate or silicate, U(VI) has been observed to form a variety of crystalline and amorphous oxyhydroxide mineral phases in oxic subsurface environments (Morris et al, 1996;Hunter and Bertsch, 1998) and in synthetic acidic-tocircumneutral wastewaters (Kanematsu et al, 2014). The first oxidized alteration phase from the weathering of synthetic or natural UO 2 (s) is typically a simple oxyhydroxide, either schoepite ((UO 2 ) 8 O 2 (OH) 12 Á12H 2 O) or metaschoepite ((UO 2 ) 4 O(OH) 6 Á5H 2 O) (Finch and Ewing, 1992).…”