A rational approach in the design of selective mesoporous adsorbents

Two MCM-41 derived adsorbents have been tailor-made for the separation of silver and copper ions using the hard-soft, acid-base (HSAB) principle as the design guideline. NH₂-MCM-41 containing "hard" Lewis base adsorption sites (i.e., RNH₂) was prepared for the adsorption of the "hard" Lewis acid, Cu²⁺, and SH-MCM-41 with a grafted "soft" thiolpropyl base was prepared for the selective removal of Ag⁺, a "soft" Lewis acid. Single- and binary-component adsorption studies were conducted at different metal concentrations, solution compositions, and pH values. The experimental results showed that SH-MCM-41 has excellent affinity and capacity for silver adsorption and adsorbed only the silver ions with copper remaining in the solution. The selectivity was not affected by the metal concentration and composition, anion, and pH. Under similar experimental conditions, NH₂-MCM-41 selectively adsorbed copper from the binary solution. The selectivity of NH₂-MCM-41 remained for the copper at different pH values, although the adsorption capacity diminished at lower pH values. The type of anions used affected copper adsorption on NH₂-MCM-41 with an increased copper uptake in the presence of the sulfate ions. A simple Freundlich adsorption model was sufficient to describe metal adsorption on SH-MCM-41 and NH₂-MCM-41, and the LeVan and Vermeulen model was successfully used to predict the adsorption capacity and selectivity for binary-component adsorptions.