Some insights into the chemistry of gold adsorption by thiol and amine functionalized mesoporous silica in simulated thiosulfate system

Organic functional groups such as amines and thiols have been widely used to functionalize pore wall surfaces of mesoporous silicas for a variety of applications relying on adsorption of metals. One of the promising applications of such hybrid materials is in hydrometallurgical separation of metals in a leach solution. Toward this aim, mesoporous MSU silica was prepared and functionalized with short-chain thiol and primary amine organic groups. Adsorption of gold from simulated thiosulfate leach solutions was investigated with respect to the adsorption chemistry. Adsorption capacity of thiol-bearing silica was well showing the monolayer behavior approaching an ultimate metal:ligand = 1:1 capacity. Higher capacities were obtained in either alkaline condition (pH ~ 10.5) or at higher oxidation potential (Eh ~ 300 mV) of the solution. Adsorption capacity of amine groups was to some extent proportional to their surface molar concentration, but it was also beyond the monolayer capacity of chemisorption, besides not being explicable by hard–soft acid–base theory. Despite considerable gold recovery by amine functional groups, gold was not detected in XPS analysis unless adsorption was from pure solutions. Partial electrochemical reduction to metallic gold and formation of dispersed tiny nanoparticles during interaction of gold thiosulfate species in solution with amine ligands were proposed.

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