Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits

Technical Report · Thu Dec 01 04:00:00 EST 1977

DOI:https://doi.org/10.2172/6774931· OSTI ID:6774931

Langmuir, D.

Gibbs free energies, enthalpies, and entropies of 42 dissolved uranium species and 30 uranium-bearing solid phases have been critically evaluated from the literature and estimated when necessary for 25/sup 0/C. Application of the data shows that the uranium in natural waters is usually complexed. At typical concentrations of chloride, fluoride, phosphate, and sulfate, uranous (U/sup 4 +/) fluoride complexes are important in anoxic waters below pH 3-4. At intermediate Eh's (between about +.2 and -.1 volts) and pH values 1-7, UO/sup +//sub 2/ ion may predominate. In oxidized water, uranyl (UO/sup 2 +//sub 2/) fluoride complexes and uranyl ion predominate below pH 5; from about pH 4-7.5, UO/sub 2/(HPO/sub 4/)/sub 2//sup 2 -/ is the principal species; while at higher pH's, UO/sub 2/CO/sub 3/ and the di- and tri-carbonate complexes predominate. Uraninite (UO/sub 2/-UO/sub 2/./sub 25/), ..cap alpha..-U/sub 3/O/sub 8/, and schoepite are the stable uranium oxides and hydroxides in water at 25/sup 0/C. Coffinite, USiO/sub 4/(c), is probably stable relative to UO/sub 2/(c) when dissolved silica exceeds about 60 ppM (as SiO/sub 2/). At low Eh's and pH 4-6, the solubilities of stoichiometric crystalline uraninite and coffinite are below roughly 10/sup -4/ ppB. But at intermediate Eh's and neutral to alkaline pH's in the presence of phosphate or carbonate, the formation of uranyl phosphate or carbonate complexes can increase the solubilities of these minerals by several orders of magnitude. The uranyl minerals carnotite, tyuyamunite, autunite, potassium autunite, and uranophane are least soluble at pH's in the range 5-8.5 and, in the case of carnotite and tyuyamunite, have solubilities as low as 0.2 and 1 ppB uranium, respectively. The autunites and uranophane are usually several orders of magnitude more soluble than this, consistent with their natural occurrences. Sorption of uranyl onto natural materials is maximal in the same pH range of 5-8.5.

• Research Organization: Pennsylvania State Univ., University Park (USA)
• OSTI ID: 6774931
• Report Number(s): GJO-1659-3; GJBX-54(78)
• Country of Publication: United States
• Language: English

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ACTINIDE COMPLEXES
AQUEOUS SOLUTIONS
CHEMISTRY
COMPLEXES
DISPERSIONS
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URANIUM COMPLEXES
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URANYL COMPLEXES
WATER