Selective adsorption of metal ions by chemically-modified MCM-41

Selective adsorbents were developed for removal and recovery of heavy metals from aqueous medium. As heavy metals interact with surface moieties differently depending on their chemical properties, tailor-made adsorbents were synthesized for metal separation from aqueous solution based on different adsorption mechanisms. This is the first comprehensive study in the design of adsorbent for selective adsorption of metal ions based on a nanostructured material, MCM-41.; Adsorption mechanism studies by FTIR and XPS found that hard-soft acid-base principle, surface complexation and ion-exchange were the main interactions between metal ions and surface functional groups. The adsorption selectivity of SH-MCM-41 obeys the hard-soft acid-base principle. It was suitable for separation of hard and soft metal ions such as Ag+ and Cu 2+. Stability constants could be as the guideline for predicting the adsorption of NH2-MCM-41 which was able to separate metals with different complexing ability. Furthermore, ion-exchange mechanism governs the adsorption of COONa-MCM-41 and it was capable of separating cationic metals from metallic oxyanions. Factors such as functional group loading, amount of adsorbent, solution pH and metal composition of solution could affect both adsorption capacity and selectivity of the adsorbents depending on the adsorption mechanisms.; By identifying the adsorption mechanism, the selective adsorbents was designed for precious metal recovery and toxic metal separation. It was found that both SH-MCM-41 and NH2-MCM-41 were able to remove gold in the presence of other metals and these adsorbents were efficient to adsorb trace amount of gold from the solution. The regenerable NH2-MCM-41 could be re-used for five times without loss of performance. On the other hand, NH2-MCM-41 was used to separate toxic cadmium ions from Ni-Cd solution. It was found that the addition of EDTA improved the adsorption selectivity. NH2-MCM-41 only remove Cd2+ at pH5.0 while the adsorbent removed Ni2+ at pH3.0. This introduces an opportunity to recover both metal ions from spent Ni-Cd battery wastes.