Reactions at the iron oxide-water interface can control the mobility of uranium in soil and groundwater. Uranium(VI), present as the uranyl ion UO_2~(2+) and its associated complexes, can be highly mobile. U(VI) may interact with mineral surfaces through adsorption, surface precipitation or co-precipitation. Whereas adsorption is the association of dissolved heavy metal species with mineral surfaces through surface complexes, surface precipitation is the precipitation of a pure heavy metal-containing phase on the surface of goethite and, unlike adsorbed species, surface precipitates have long-range three-dimensional order. Co-precipitation of the heavy metal is its precipitation by either the formation of a solid solution with the substrate solid or by encapsulation of pure microcrystalline phases within the substrate matrix.
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