Abstract
The behavior of uranium in environments, ranging from those of natural systems responsible for the formation of uranium deposits to those of nuclear reactors providing 11% of the world’s electricity, is governed by processes involving high-temperature aqueous solutions. It has been well documented that uranium is mobile in aqueous solutions in its oxidized, U6+ state, whereas in its reduced, U4+ state, uranium has been assumed to be immobile. Here, we present experimental evidence from high temperature (>100 °C) acidic brines that invalidates this assumption. Our experiments have identified a new uranium chloride species (UCl4°) that is more stable under reducing than oxidized conditions. These results indicate that uranium is mobile under reducing conditions and necessitate a re-evaluation of the mobility of uranium, particularly in ore deposit models involving this metal. Regardless of the scenario considered, reducing conditions can no longer be considered a guarantee of uranium immobility.
Highlights
The behavior of uranium in environments, ranging from those of natural systems responsible for the formation of uranium deposits to those of nuclear reactors providing 11% of the world’s electricity, is governed by processes involving high-temperature aqueous solutions
The accepted model for the formation of unconformity-type uranium deposits involves the interaction of an oxidized basinal brine, which transports the uranium as U6+, with a reducing, graphite-bearing metapelite that leads to the precipitation of uraninite or the U4+-bearing phase, pitchblende[2,3]
Evidence collected from magnetite group iron oxide copper–gold (IOCG) deposits, which are another important source of uranium, suggests that, whereas this paradigm satisfies observations made at low-to-moderate temperatures, e.g., for unconformitytype uranium deposits, it likely fails for temperatures greater than 250 °C4
Summary
The behavior of uranium in environments, ranging from those of natural systems responsible for the formation of uranium deposits to those of nuclear reactors providing 11% of the world’s electricity, is governed by processes involving high-temperature aqueous solutions. A lack of thermodynamic data for uranium species at elevated temperatures is a key factor preventing accurate predictions of uranium behavior during such an event[8,9] These and other conclusions about the behavior of uranium in the presence of aqueous fluids, whether it be for nuclear industry applications or models of uranium ore formation, routinely draw upon the dogma that “uranium is immobile in the reduced state”. Studies of uranium behavior in reduced fluids have either not defined the exact oxygen fugacity of the fluid (i.e., the accurate redox state of the solution) or neglected to consider the impact of common ligands in the fluid such as chloride, fluoride, sulfate, and carbonate[13,14,15] Such ligands may enhance the solubility of metals due to the formation of metal complexes (e.g., UO2CO3°)[16]. Both oxygen fugacity and ligand concentration must be considered in order to accurately evaluate uranium mobility
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