Grafting of the 2,8-dithia-5-aza-2,6-pyridinophane macrocycle on SBA-15 mesoporous silica for the removal of Cu and Cd ions from aqueous solutions: synthesis, adsorption, and complex stability studies.

Silica-based mesoporous materials have received growing attention in metal recovery from industrial processes, although, in general, the adsorption of metal ions by silanols is rather poor. Nevertheless, a great improvement of metal ion removal from aqueous solutions can be achieved by grafting metal-chelators on the particles' surface. Combining the metal-chelating properties of organic ligands with the high surface area of mesoporous silica particles makes these hybrid nanostructured materials a new horizon in metal recovery, sensing and controlled storage of metal ions in industrial and mining processes. Here, the 2,8-dithia-5-aza-2,6-pyridinophane (L) macrocycle was grafted on SBA-15 mesoporous silica to obtain the SBA-L mesoporous adsorbent for the removal and controlled recovery of Cd and Cu ions from aqueous solution in a broad pH range (4-11). By grafting about 0.3 mmol g of L on SBA-15 a maximum loading capacity of 20.9 mg g and 31.8 mg g was obtained for Cu and Cd, respectively. The adsorption kinetics can be described with the pseudo-second order model, while the adsorption isotherm (298 K) followed the Langmuir model. The latter, together with potentiometric studies, suggests that the adsorption mechanism is based on metal chelation by the grafted macrocycle. In summary, SBA-L is an effective copper(II) and cadmium(II) chelator for possible applications where metal removal, storage and recovery are of basic importance.