LOW TEMPERATURE MOBILITY OF URANIUM IN SURFACE AND NEAR SURFACE AQUEOUS CRUSTAL ENVIRONMENTS

In surface and near surface low temperature aqueous environments the behavior of uranium is strongly controlled by the oxidation state, pH, and chemical composition of the solution because of its: 1) variable oxidation state (uranous and uranyl); 2) tendency to hydrolyze; 3) ability to form a large number of aqueous complexes (40) with a number of major and minor anions (hydroxide, carbonate, chloride, fluoride, sulfate, phosphate); 4) tendency to adsorb onto organic material and various oxyhydroxides; and 5) ability to form a large number of primary and secondary minerals (145). The latter may contain arsenic, selenium, vanadium or phosphate as essential components. Most likely, these systems will be chemically open, therefore changes in pH and/or oxidation state will result in a competition for uranium between solids, resulting in either precipitation or adsorption, and the aqueous phase, resulting in various soluble species. Uranium most commonly is mobile in oxidized solutions as an uranyl species, and immobile under reducing conditions as uranous solids. However, even under oxidizing conditions in the presence of As, Se, P, or V, uranium may be immobilized by the precipitation of various secondary minerals. Therefore, any interpretation of uranium occurrences must consider the possibility of episodic and/or cyclical changes in solution chemistry and aquifer composition, resulting in varying degrees of depletion and progressive enrichment of uranium in space and time, where systems may not be evaluated as a single event, assuming simple water-rock equilibrium thermodynamics.