RETENTION PROPERTIES OF HUMIC SUBSTANCES ONTO AMORPHOUS SILICA - CONSEQUENCES FOR THE SORPTION OF CATIONS

The sorption of humic substances (Aldrich humic acids, fulvic acids fr om a granitic site) onto a amorphous silica has been investigated as w ell as their role on the sorption properties of tri-and hexavalent act inides (Am(III) and U(IV) respectively) onto this sorbent. Retention e xperiments have been performed (by batch procedures) in 0.1 M NaClO4 e lectrolyte solutions in a pH range of 3-10 with humic and fulvic conce ntrations representative of those encountered in natural waters (1-10 ppm). According to the oxidation state of the cation and the humic/ful vic concentration, the actinide sorption behavior is enhanced or decre ased 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 a pplied in order to describe the mechanisms occurring at the oxide-wate r interface; apparent surface complexation constants have been determi ned for each species (cation, organic ligand) and surface reactions wi th the hydroxyl surface groups of silica surface are proposed.