Silicate Binding and Precipitation on Iron Oxyhydroxides

Silica-bearing waters in nature often alter the reactivity of mineral surfaces via deposition of Si complexes and solids. In this work, Fourier transform infrared (FTIR) spectroscopy was used to identify hydroxo groups at goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) surfaces that are targeted by ligand exchange reactions with monomeric silicate species. Measurements of samples first reacted in aqueous solutions then dried under <i>N</i>₂(<i>g</i>) enabled resolution of the signature O–H stretching bands of singly (−OH), doubly (μ–OH), and triply coordinated (μ₃–OH) groups. Samples reacted with Si for 3 and 30 d at pH 4 and 7 revealed that −OH groups were preferentially exchanged by silicate and that μ–OH and μ₃–OH groups were not exchanged. Based on knowledge of the disposition of −OH groups on the major crystallographic faces of goethite and lepidocrocite, and the response of these groups to ligand exchange prior oligomerization, our work points to the predominance of rows of mononuclear monodentate silicate species, each separated by at least one −OH group. These species are the attachment sites from which oligomerization and polymerization reactions occur, starting at loadings exceeding ∼1 Si/nm²and corresponding to soluble Si concentrations that can be as low as ∼0.7 mM after 30 d reaction time. Only above such loadings can reaction products grow away from rows of −OH groups and form hydrogen bonds with nonexchangeable μ–OH and μ₃–OH groups. These findings have important repercussions for our understanding of the fate of waterborne silicate ions exposed to minerals

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

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 L_III-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

Uranium speciation in acid waste-weathered sediments: The role of aging and phosphate amendments

Uranium speciation and lability are strongly coupled to mineral transformations in silicate sediments, particularly for sediments subjected to weathering in acidic, high-level radioactive waste, as occurred at the Department of Energy&apos;s Hanford (WA) site. In this study, uncontaminated Hanford sediments were reacted for 365 days with acidic (pH 3), uranium-bearing waste solutions, with and without phosphate in batch experiments, prior to detailed characterizations using electron microscopy, x-ray diffraction and x-ray absorption spectroscopy. In PO4-reactant free systems, uranium speciation was controlled initially by precipitation of compreignacite [K2(UO2)6O4(OH)6·8H2O]- and becquerelite [Ca(UO2)6O4(OH)6·8H2O]-like species. Subsequent further removal of uranium coincided with that of Si and accumulation of boltwoodite, [(K, Na)(UO2)2O4(HSiO4)2•0.5(H2O)]-like species of uranium at 180 and 365 days. When present, PO4 exerted a direct and strong control over U speciation. The detection of meta-ankoleite, [K2(UO2)2O4(PO4)2·6H2O] at all reaction times when U was present emphasizes the importance of dissolved phosphate as a control on U speciation. Here, meta-ankoleite appears well crystallized and when it occurs as the principal product of sediment weathering, its low solubility is expected to limit dissolved U(VI) concentrations in groundwater. Although boltwoodite solubility is also low, it is formed more slowly (and only when PO4 is absent), after initial precipitation of more soluble, less crystalline uranyl hydroxides. In the context of Hanford crib waste our results suggest that with PO4 present, nearly all uranium would have precipitated in the upper soil.11sciescopu