HEAVY-METAL LAN 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 gra fting of 3-mercaptopropylsilyl groups to the framework pore walls of m esoporous silica molecular sieves has been investigated with regard to hydroxyl group densities, channel dimensions, and morphologies. Two t ypes of silicas were examined, namely, MCM-41 with an initially anioni c silicate framework and HMS with an electrically neutral framework. T he MCM-41 derivative was obtained through electrostatic S+X-I+ assembl y, where S+ was a quaternary ammonium ion surfactant, X- was a halide, and If was the silica precursor derived from tetraethyl orthosilicate (TEOS) in acidic solution. HMS silicas were assembled by a S degrees I degrees pathway using alkylamines as surfactants (S degrees) and TEO S as the neutral silica source (I degrees). Prior to thiol functionali zation of the mesostructures by a one-step grafting procedure, S+ was removed from MCM-41 by calcination, whereas for HMS, S degrees was rem oved by solvent extraction. The grafting process was much more effecti ve for the functionalization of HMS than for MCM-41 owing to a higher surface concentration of surface hydroxyl groups. Consequently, the fu nctionalized HMS derivative was able to bind quantitatively more Hg(ll ) ions from aqueous solution compared to MCM-41. The Hg(ll) adsorption capacities for HMS were interpreted in terms of the size and accessib ility of the framework pore structure.