Effect of reductants and stabilizers on ageing of gold nanoparticles at pHÂ 2-12 and application of nano-gold to study non-cyanide leaching in sodium hypochlorite/chloride solutions using UV-Visible spectroscopy

Despite the growing interest in non-cyanide lixiviants for leaching gold from ores/concentrates and recyclable waste material one of the main challenges in testing novel lixiviants is the slow reaction of gold dissolution which generally takes long hours of experimentation and high cost. The increased interest in naturally occurring nanoparticulate gold component in ore deposits and exploration samples also encourages research in this area. Gold nanoparticles in aqueous medium exhibit strong characteristic absorption peaks in the visible region of the electromagnetic spectrum due to the Surface Plasmon Resonance (SPR). The very large surface area to volume ratio of nanoparticles leads to fast dissolution kinetics. These two properties allow chloride leaching of gold to be conveniently studied by the use of UV-Visible spectroscopy of gold nanoparticles produced in the laboratory. However, the method of production also controls the properties of gold nanoparticles. In this study, nine methods were used for the synthesis of gold nanoparticles using different reagents as reductants and stabilizers: d-glucose, trisodium citrate, sodium borohydride, ascorbic acid, heparin, hydrazine sulfate, monosodium glutamate, gelatine, starch (both with and without ethylene glycol), polyvinyl alcohol (PVA), cetyltrimethylammonium bromide (CTAB) and tannic acid. The UV-Visible spectra and the dissolution rates per unit surface area of nanoparticles in NaOCl/NaCl solutions are compared to show similar leaching behaviour between nano-gold and massive gold which encourages further studies with other non-cyanide gold lixiviants.