Heavy Metal Ion Adsorbents Formed by the Grafting of a Thiol Functionality to Mesoporous Silica Molecular Sieves:  Factors Affecting Hg(II) Uptake

A new approach to heavy metal ion adsorbents based on the covalent grafting of 3-mercaptopropylsilyl groups to the framework pore walls of mesoporous silica molecular sieves has been investigated with regard to hydroxyl group densities, channel dimensions, and morphologies. Two types of silicas were examined, namely, MCM-41 with an initially anionic silicate framework and HMS with an electrically neutral framework. The MCM-41 derivative was obtained through electrostatic S+X-I+ assembly, where S+ was a quaternary ammonium ion surfactant, X- was a halide, and I+ was the silica precursor derived from tetraethyl orthosilicate (TEOS) in acidic solution. HMS silicas were assembled by a S°I° pathway using alkylamines as surfactants (S°) and TEOS as the neutral silica source (I°). Prior to thiol functionalization of the mesostructures by a one-step grafting procedure, S+ was removed from MCM-41 by calcination, whereas for HMS, S° was removed by solvent extraction. The grafting process was much more effective for the functionalization of HMS than for MCM-41 owing to a higher surface concentration of surface hydroxyl groups. Consequently, the functionalized HMS derivative was able to bind quantitatively more Hg(II) ions from aqueous solution compared to MCM-41. The Hg(II) adsorption capacities for HMS were interpreted in terms of the size and accessibility of the framework pore structure.