Functionalized SiO2 with N-, S-containing ligands for Pb(II) and Cd(II) adsorption

Five novel organosilicas SiO₂-L have been prepared by grafting organic ligands (L) bearing thiazolidine (TAMS), thiazole (TIMS), imidazole (DTIMS), mercaptopropyl (DETATS) or pyridine (DETAPS) functional groups. The SiO₂-L materials have been evaluated for their capacity for Pb²⁺ and Cd²⁺ uptake from aqueous solution as a function of the pH. The present data demonstrate that the grafting of the silica may result in significant enhancement of metal uptake by the SiO₂-L adsorbent. Noticeably, multifunctional ligands such as DTIMS and DETAPS, have exhibited the highest metal capacity between the studied materials. Significant metal uptake was observed for both Pb and Cd on those materials independent of pH. The adsorption data have been analyzed based on a surface complexation model (SCM) with FITEQL by considering the grafted ligands as additional surface sites. FT-IR data are provided in support of this approach. Based on the theoretical speciation results, we may conclude that the determining factors for the metal uptake by the modified silica are the following three: (a) the presence or absence of protonable groups on the ligand determines the pH-edge profile, (b) the complexing affinity of the particular ligand towards each metal, and (c) the amount of grafted ligand on the silica surface. The interplay of these factors will determine the final maximum loading capacity of the sorbing material. Both the nature of the ligand as well as its surface concentration are of equal importance for the sorbing capacity of the material.