Influence of the nature of oxygen-containing minerals on their sorption ability toward U(VI)

Sorption of U(VI) on natural and synthetic oxygen-containing minerals was studied. With respect to the sorption ability toward U(VI), natural minerals can be ranked in the order iron-containing > aluminum-containing > silicon-containing, and synthetic minerals, in the order iron-containing ≈ silicon-containing ≫ aluminum-containing. It was demonstrated by the example of Fe(OH)₃, α-FeO(OH), and γ-FeO(OH) that crystalline minerals bind U(VI) at pH values corresponding to natural media (pH > 7.5) better than do amorphous minerals. The ionic strength does not affect the U(VI) sorption owing to formation of strong inner-sphere complexes on the mineral surface. The presence in solution of complexing agents in small amounts can enhance the U(VI) sorption owing to formation of ternary surface complexes, whereas large excess of the ligand decreases the sorption because of the shift of the equilibrium toward formation of soluble complexes migrating in the environment by long distances with water streams.