An Investigation of Gold Adsorption from a Binary Mixture with Selective Mesoporous Silica Adsorbents

Abstract

Gold-selective adsorbents were prepared from mesoporous MCM-41 silica by grafting organic amine groups (i.e., RNH₂, R₂NH, and R₃N; R = propyl). NH₂-MCM-41, NRH−MCM-41, and NR₂-MCM-41 displayed strong affinity for gold and at 1 mmol/g loading adsorbed 0.40, 0.33, and 0.20 mmol/g of gold. Copper and nickel were not adsorbed on these adsorbents. Grafting surface chemical moieties introduces heterogeneity on an otherwise uniform MCM-41 pore surface and metal adsorption is best described by the Freundlich adsorption model. A series of binary adsorption equilibrium studies with NH₂-MCM-41 containing 2.2 mmol RNH₂/g shows that NH₂-MCM-41 adsorbs only gold from solutions containing copper and nickel with an adsorption capacity of 0.6 mol of Au/mol of RNH₂ (1.1 mmol of Au/g of NH₂-MCM-41). Copper and nickel were not adsorbed by NH₂-MCM-41 regardless of the solution concentration, composition, and pH (i.e., 2 to 4) in the presence of gold. The LeVan and Vermeulen adsorption model based on a single component Freundlich isotherm and corrected for the anion effect accurately predicted the binary adsorptions. The adsorbed gold was completely recovered by a simple acid wash and the recovered gold solution is 99% pure. The regenerated NH₂-MCM-41 remained 100% selective for gold removal and exhibited the same adsorption capacity even after several uses.