Abstract

Adsorption of aqueous Au species by goethite, smectite and kaolinite has been investigated as a function of pH at 25 °C. Adsorption isotherms indicate Au is mainly adsorbed as anionic complexes with adsorption edges centred at pH 8.5 for goethite, 7.5 for smectite (Swy-1) and 8.0 for kaolinite (KGa-1). The possibility that colloidal Au species interact with the mineral surfaces cannot be excluded. A zone of slow adsorption of Au(III) complexes exists for goethite at pH <6.5 and kaolinite at pH <8, with complete adsorption from solutions containing <5 μM Au and 1 g/l solids requiring over 100 hours of reaction time. Adsorption by goethite in thepH range 7–9 is more rapid and may be largely attributed to adsorption of Au(OH)3.(H2O)o. The loss of Au from solution at pH values above adsorption edges may be due to adsorption of uncharged Au species or development of colloidal gold. Aqueous phase equilibrium modelling indicates Auo (monomer or colloid) to be the dominant phase at pH 4–10 but its rate of formation in 0.1 M NaCl is slow. The uptake of Au by goethite has been modelled as an inner-sphere surface complexation using the diffuse layer model. The model of best fit was obtained using a single binding site and setting AuCl4, AuCl3(OH) and Au(OH)3.(H2O)o as the main Au(III) sorbing species with logKint values for the formation of the adsorbed species of 9, 35 and 17 respectively.

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