(154d) Silica Gel and Hydroxyapatite Composites with Ormosils for Heavy Metal Removal from Water

Organically modified silanes were used to graft heavy metal binding ligands on silica gel particles and hydroxyapatite (HA)particles. Glycidoxypropyltrimethoxysilane (GPS) was the reactive silane that was hydrolysed in 50% water-ethanol solvent. Epoxy groups were then reacted with ethylenediamine, diethylenetriamine, triethylenetetramine or tetraethylenepentamine ligands by adding excess amounts of the polyamines diluted in ethanol, followed by solvent evaporation and oven drying at 60°C during 24 hours. The amounts of ligands grafted were determined by thermal analysis and also by visible spectrophotometry following ligand hydrolysis at basic pH and copper complexation. In each case, the optimum pH and wavelength was sought and used. Ligand grafted silica gel particles showed rapid heavy metal uptake in batch or flow through experiments with capacities reaching 10^-4 moles/g for copper, zinc, cadmium or lead ions. Columns packed with the modified particles could be easily regenerated by acid washing without loss of activity. The particles can be used for the colorimetric detection of heavy metal pollution or for preconcentration for analytical purposes. The longer chain multiamine ligands did not bind more than the shorter ones. Hydroxylapatite particles were made and sieved into several size ranges. Heavy metal removal by filtration on HA beds proceeded by an ion exchange mechanism, with one mole of calcium released for one mole of divalent metal ion trapped. The smaller particles (less than 1mm) removed heavy metals more than the larger ones (up to 5mm diameters) but impeeded liquid flow. Submillimeter HA particles were reacted with stoichiometric amounts of GPS and aminopropyltrimethoxysilane dispersed in water-ethanol solvent. The paste was spread on a stainless steel slabs, air dried and crosslinked by heating to 60°C. The resulting composite electrodes were immersed during 24 hours into heavy metal containing solutions at 0.1mM concentrations. Heavy metal adsorption occured and was found to be accelerated by electromigration. Applying a 1V potential to the electrodes diminished solution metal concentrations more rapidly than 0.5V. Electromigration was faster in the presence of supporting electrolyte (KCl) but slowed in the presence of strong chelating ligands in solution. Cross sections of the electrodes were examined by wavelength dispersive X ray analysis. It was found that heavy metals concentrated on the external surface of the electrodes when no current was applied. However, in the presence of the electric field, bound metal concentrations were lower but distributed inside the composite material. Detailed investigations revealed that the heavy metals were located both on the surface of embedded HA particles and bound to the polysiloxane networks filling the interparticle voids. We believe that the combination of HA and organically modified silica gels can make new materials with large heavy metal removal capacities, rapid kinematics and high complexation strength.