Fate of uranium in the environment

Uranium is present in the Earth’s crust at an average concentration of 2 mg/kg, similar to As and Mo, but ten times higher than Sb. It is enriched in granitic and phosphate rocks, lignite and monazite sands, and occurs in numerous minerals, uraninite (pitchblende), carnotite, and autunite being most abundant. During mining and ore processing (World production of U 35,000 Mg/a), these relatively insoluble, U(IV) and/or U(VI) containing minerals are converted into highly soluble and mobile U(VI) aqueous species, which may present an immediate risk for the environment, or are converted back to less mobile chemical forms. We will give an overview of processes, which influence the fate of U in this greatly perturbed biogeochemical cycle.
Even at low redox potential and in equilibrium with uraninite, the aqueous solution of soils, the vadose zone and aquifers is dominated by U(VI) species. The extremely complex speciation of U(VI) is influenced by pH, dissolved carbonate and the presence of organic ligands. The mobility of these species is reduced by sorption processes to minerals, insoluble natural organic matter, and biota. On the other hand, the presence of carbonate and organic ligands with strong affinity for uranyl may prevent sorption to surfaces and enhance the mobility. The migration of U is furthermore influenced by the formation of, or sorption to mobile colloids.
A strong immobilization of U presents the reduction to U(IV) and the subsequent precipitation as uraninite or other minerals. This process may proceed as a catalytic reaction on inorganic surfaces. Microorganisms, however, which are abundant even in U waste piles with high heavy metal concentrations, may be more important for U(IV) mineral precipitation. Processes involved are a direct biological reduction of U(VI) or an indirect reduction by changing redox potential and pH of the aqueous solution. Microorganisms can reduce the U mobility also by processes like bioaccumulation and biomineralization. Alternatively, particular groups of microorganisms can mobilize U from the ores by direct and indirect (metabolic-conducted) oxidation of U(IV), a process called bioleaching. Microorganisms can also increase the U mobility by releasing chelating ligands into the environment. Thus, they play one of the key roles in the biogeochemical cycling of U.