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The immobilization of U(vi) at the solid-water interface is an important process affecting its transportation and migration in the environment, and is predominantly controlled by the sorption behavior of U(vi). In this study, U(vi) sorption on Fe(ii) and Fe(iii) oxyhydroxides prepared by a coprecipitation method was studied under a range of physicochemical conditions, including pH, ionic strength, presence of humic acid (HA) and temperature. The results showed that the sorption of U(vi) on iron oxyhydroxides is chemical, and that the principal rate limitation is due to intraparticle diffusion. The sorption of U(vi) on iron oxyhydroxides is strongly dependent on pH, but only weakly dependent on ionic strength through the entire pH range studied. Under acidic conditions, the presence of HA increases U(vi) sorption to a large degree, but an inhibiting effect on the sorption of U(vi) can be observed under alkaline conditions, due to the formation of soluble U(vi)-HA complexes. The sorption of U(vi) on iron oxyhydroxides is an endothermic process and favors high temperatures. The surface complexation model suggests three dominant monodentate inner-sphere complexes of [triple bond, length as m-dash]Fe(s)OUO2(+) (log K = 1.65), [triple bond, length as m-dash]Fe(w)OUO2OH(0) (log K = -8.00), and [triple bond, length as m-dash]Fe(w)OUO2(CO3)2(3-) (log K = 17.50), contributing to U(vi) sorption on iron oxyhydroxides over the entire observed pH range.

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