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In crustal fluids uranium transport and deposition is dominated by redox conditions. The concentrations of dissolved U(IV) species never exceed μg l;1 levels, whereas the concentration of U(VI) species reaches tens of μg l;1 depending on pH and ligand concentration(s). As a result, uranium is transported in aqueous fluids of high Eh but deposited when Eh decreases, e.g. in sedimentary environments and engineered barriers that contain sulphide, organic matter, and/or Fe(II)-bearing minerals; bacteria and mineral surfaces are known to catalyse these reduction reactions. Mining activities result in remobilization of uranium as U(VI). In the presence of oxy(hydr)oxides or clay minerals, aqueous uranyl ions are sorbed at intermediate pH. In rivers, uranium transport is dominated by surface-sorbed uranium on colloids and particles. Upon entering the oceans. uranium is deposited in estuarine sediments due to flocculation, or desorbed due to the presence of carbonate ligands in seawater. Uranyl-carbonate complexes can be transported over long distances and thus the uranium residence time in the oceans is greater than that of water. In the presence of appropriate ligands uranium precipitates to form stable minerals and the transport path can be traced by U isotopic ratios of alteration products. The mobility of uranium and its enrichment in the Earth’s crust is magnified by subduction zone processes.

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