Chitosan is a well-known sorbent of biological diluted effluents. Uptake capacities and sorption rates are restricted by diffusion mechanisms. A grafting of specific functional groups onto a chitosan backbone allows sorption performances to be improved due to the appearance of new sorbing functions and by an improvement in diffusion properties. Scanning electron microscopy coupled with an X-ray energy dispersed analysis demonstrates that an uranium concentration gradient exists in the chitosan. polymer but disappears in the glutamate glucan polymer. Chitosan displays a surface controlled sorption mechanism, unlike glutamate glucan in which sorption occurs in the mass of the polymer. Microcrystalline chitosan produced for the sorption of uranyl ions shows diffusion constants comparable to those obtained with glutamate glucan by application of two single models of intraparticular diffusion. This simple technique of modification of the structure of the chitosan allows the uptake capacity and sorption kinetics to be increased. Recrystallization causes chitosan to behave in a way intermediate between raw and substituted forms of chitosan.