Chitosan–Silica Composites: Effect of Polymer Loading and Immobilization Strategy on Surface Structure and Ni(II)/Co(II) Adsorption

In this work, the effects of chitosan loading and immobilization strategy on the structure and Ni(II) and Co(II) adsorption properties of chitosan–silica composites were studied. To examine the effect of polymer loading, CS/SiO2 composites containing 5–20 wt.% chitosan were prepared by pH-induced precipitation of chitosan onto pristine SiO2. The effect of the immobilization strategy was investigated using an alternative approach involving preliminary functionalization of SiO2 with APTES followed by chitosan deposition. The obtained materials were characterized by TGA, elemental analysis, FTIR spectroscopy, nitrogen adsorption–desorption analysis, SEM, and XPS. Direct precipitation of chitosan onto SiO2 resulted in the formation of smaller polymer-containing domains on the silica surface compared with the large agglomerates of pristine chitosan, while regions retaining the characteristic morphology of the silica support remained visible. Chitosan modification improved metal-ion adsorption already at a loading of 5 wt.%. Further increasing the chitosan content to 20 wt.% produced a comparatively smaller additional effect. Preliminary APTES functionalization promoted a more uniform surface morphology of CS/SiO2–APTES compared with directly prepared CS/SiO2. These morphological differences were accompanied by differences in the spectral response of the composites following metal adsorption. CS/SiO2–APTES exhibited smaller shifts in the main N 1s XPS component and almost no change in the silanol-related FTIR band, in contrast to the more pronounced changes observed for CS/SiO2. The directly prepared composite also exhibited faster initial metal uptake. Nevertheless, both composites showed relatively similar equilibrium metal uptake values. These findings highlight the importance of considering polymer organization and surface chemistry when designing supported polymer composites for adsorption and other potential applications.