Silica / Humic substances / Actinides / Retention / Surface complexation model / Radwaste disposals SummaryThe Sorption of humic substances (Aldrich humic acids, fulvic acids from a granitic site) onto a amorphous silica has been investigated as well as their role on the sorption properties of tri-and hexavalent actinides (Am(III) and U(IV) respectively) onto this sorbent. Retention experiments have been performed (by batch procedures) in 0.1 M NaC104 electrolyte solutions in a pH ränge of 3-10 with humic and fulvic concentrations representative of those encountered in natural waters (1 -10 ppm). According to the oxidation State of the cation and the humic/ fulvic concentration, the actinide sorption behavior is enhanced or decreased compared to the system without humic substances and is related to the complexation strength of this cation towards the organic ligand. The complexation surface model (with no electrostatic term) has been applied in order to describe the mechanisms occurring at the oxide-water interface; apparent surface complexation constants have been determined for each species (cation, organic ligand) and surface reactions with the hydroxyl surface groups of silica surface are proposed.

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