Adsorption and desorption of uranium to goethite was rapid, occurring on time- scales shorter than a minute. Rapid sorption rates of uranium on goethite were
independent of the method of investigation: adsorption following uranium addition, dilution-induced desorption, fluoride-induced desorption, and near-equilibrium surface- solution isotope exchange. Rapid rates of adsorption and desorption of uranium to montmorillonite were also observed.
Uranium sorption on goethite at pH 6 and relatively high ionic strength (0.1 M) was modeled with the Langmuir isotherm, using a maximum sorbed uranium concentration of 114 µmol g-1. Sorption was generally reversible though some irreversibility was
observed with aged uranium-goethite suspensions. Sorption on montmorillonite was consistent with a Langmuir isotherm at pH 6 and high ionic strength but, at lower pH and ionic strength, the data were better fit with a Freundlich isotherm.
Metastable uranium sorption on goethite was observed, in which the dissolved uranium concentration was controlled by sorption even when supersaturated with respect to a uranium-containing precipitate. The metastable sorption-control of dissolved
uranium concentrations in excess of a solubility limit persisted for as long as 30 days. Ultimately, metastable sorption was followed by the formation of a precipitated phase,
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which subsequently controlled the dissolved uranium concentration. The uranium- containing precipitate was identified as a schoepite-like uranyl oxide hydrate phase by X- ray diffraction, and its spatial relationship to goethite was studied with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM showed an increasingly heterogeneous uranium spatial distribution with both increasing contact time and increasing uranium content. Uranium-containing precipitates and goethite particles were observed in close proximity with TEM. Discrete uranium-containing particles were observed for high uranium contents and long contact times, and low-crystallinity uranium clusters on goethite particles were observed for a shorter contact time.
8.1.2 Dissolution and Transformation of Uranyl Minerals
The rates of dissolution of schoepite, soddyite, and hydrogen uranyl phosphate were quantified in flow-through reactors. The maximum soddyite dissolution rate (0.70 µmol U m-2 h-1) is somewhat faster than the maximum rate for hydrogen uranyl
phosphate (0.46 µmol m-2 h-1) when normalized to surface area. Schoepite dissolution is much faster, nearing equilibrium within one hour in batch experiments; a lower limit for the rate (1.58 µmol m-2 h-1) is provided by flow-through experiments.
In batch dissolution experiments, dissolution of a single solid phase to equilibrium with the starting solid was the exception and dissolution followed by precipitation of a secondary phase was the rule. In both schoepite and soddyite systems with as little as 10 mM sodium, a sodium uranyl oxide hydrate (Na(UO2)O(OH)(s)) formed as a secondary phase and, in schoepite suspensions with as little as 1 mM cesium, a cesium uranyl oxide hydrate (Cs3[(UO2)12O7(OH)13]·3H2O(s)) formed. The layered structure of schoepite and
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the affinity of schoepite for alkali and alkaline earth cations favor secondary phase formation through the solid-state transformation of the original solid. The transformation mechanism in systems with soddyite, a phase with a compact framework structure, probably proceeds by release of uranium from soddyite to solution and the subsequent precipitation of uranium from solution as Na(UO2)O(OH)(s). The dissolution of hydrogen uranyl phosphate (which remained after the attempted synthesis of autunite) involved the release of calcium from either sorption sites or a minor autunite phase and the concurrent uptake of phosphate from solution.
The sequential formation of solids of increasing stability is consistent with the Ostwald Step Rule, which postulates that the most soluble solids precipitate first because of their lower interfacial surface energies. The equilibrium solubility of the fine particles studied, particularly in schoepite suspensions, may also be affected by the interfacial surface energy. The growth of particles over the course of experiments (enhanced in the presence of sodium or cesium) coincided with a decrease in the dissolved uranium concentration.
Characterization of systems undergoing dissolution and transformation was only fully accomplished through the integration of analytical techniques offering information at a variety of spatial scales. The best example of this integrated analytical approach is the identification of Na(UO2)O(OH)(s) formation in soddyite suspensions. X-ray
diffraction initially suggested the formation of a crystalline phase distinct from soddyite, SEM images showed the formation over time of needle-like crystals different than the blocky soddyite crystals, and Raman spectra confirmed XRD identification of the solid as
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Na(UO2)O(OH)(s). Raman spectra also proved particularly diagnostic in identifying changes in the hydrogen uranyl phosphate solid, which were not observed with XRD.
Sufficient experiments were conducted to allow for the determination of the equilibrium solubilities of the synthetic phases and of some secondary products. The schoepite solubility product of 105.02 – 105.54 is within the range of published values. Soddyite solubility is consistent with a previously published solubility product of 105.74. The hydrogen uranyl phosphate solid has a solubility product of 10-26.77, which does not agree with published values for both uranyl phosphate and hydrogen uranyl phosphate solids. The sodium and cesium uranyl oxide hydrates Na(UO2)O(OH)(s) and
Cs3[(UO2)12O7(OH)13]·3H2O(s) formed in schoepite experiments have solubility products of 107.65-109.00 and 1068.3 respectively.
8.2 Environmental Implications
8.2.1 Mobility of Uranium at Contaminated Sites
The primary hypothesis of the project, that the rate of uranium release to solution is governed by the speciation of uranium in the solid phase, has been verified. The first distinction is between sorbed and precipitated phases, with uranium release from sorbed phases operating on shorter time-scales. Rapid release from sorbed phases is independent of desorption mechanism or sorption site type; however, in natural environments the occlusion of uranium during mineral diagenesis may lead to slow desorption governed by physical (i.e., diffusive) mass transfer processes. When uranium-containing minerals are present, they will control dissolved uranium concentrations and uranium mobility, and it