ADSORPTION OF AU(I, III) COMPLEXES ON FE, MN OXIDES AND HUMIC ACID

Abstract

The adsorption of AuCl4−, AuCl2− and Au(S2O3)23− at low Au concentrations relevant to most supergene waters on goethite, birnessite and soil humic acid was investigated at pH 4, pH 2–11.6, in 0.01 and 0.1 M NaNO3 solutions. At pH 4 and two electrolyte strengths, the adsorption isotherms for the two Au chloride complexes are well described by the Freundlich equation. The Freundlich parameter 1/n reflects the heterogeneity of the birnessite surface and the nonlinearity of Au adsorption isotherm. The adsorption of Au(S2O3)23− is significantly greater than that of AuCl4− on birnessite, but the adsorption of Au(S2O3)23− is significantly smaller for geothite and humic acid. The adsorption of AuCl4− on birnessite and geothite is depressed by increasing electrolyte strength. As birnessite could only adsorb gold anions specifically and goethite could adsorb gold anions by anionic exchange and specific adsorption, the adsorption on goethite is more sensitive to the electrolyte strength. Under these experimental conditions, the Au surface coverage on birnessite is 0.68–0.85% for AuCl4− and 1.06–1.10% for Au(S2O3)23−, and for goethite is approximately 2.33–6.02% for AuCl4− and 0.6–1.05% for Au(S2O3)23−. For the pH ranges from 2 to 11.6 and with 0.1 M NaNO3 as the background electrolyte, Au adsorption decreases with increasing solution pH, which is consistent with the adsorption regularity for anion adsorption on a variable charge surface. For the three surfaces, true solid–liquid distribution coefficients for the Au complexes at these low concentrations that are relevant to most supergene water are significantly negatively correlated with solution pH with the correlation coefficient ranging from −0.941 to −0.996. According to the Kurbatov plot and surface hydroxyl density, the conditional equilibrium constants (log Kpart) can be estimated. For the three surfaces, values of log Kpart for adsorption of AuCl4− are in the order: birnessite>goethite>humic acid; but for adsorption of AuCl2− are goethite>birnessite>humic acid. The effect of the dissolved humic acid on data could be corrected by using a three-phase partition model that accounts for the complexation of the solute by dissolved organic matter in the liquid phase. For low pH (pH<3) solutions, the sorption of AuCl4− on humic acid may be related to reduction of Au(III) by the humic acid. However, adsorption of AuCl4− and AuCl2− on humic acid is similar to that for birnessite and geothite for the higher pH solutions. Hence, birnessite, geothite, and humic acid preferentially adsorb chloro and chloro-hydroxo Au complexes produced from hydrolysis of AuCl4− and AuCl2− hydrolysis. Gold anion surface complexation and Au speciation in solution lead to the decrease in adsorption of Au complexes with increasing solution pH. As birnessite has very strong oxidation and adsorption abilities for monovalent Au complexes such as Au(S2O3)23−, it may play an important role in the deposition and accumulation of the dissolved gold in the supergene environment. Whether and how Au(III) complexes could be transformed to Au(I) complexes or Au∘ is controversial and needs further investigation.