Adsorption of Au(I, III) complexes on Fe, Mn oxides and humic acid

The adsorption of AuCl 4 −, AuCl 2 − and Au(S 2O 3) 2 3− at low Au concentrations relevant to most supergene waters on geothite, birnessite and soil humic acid was investigated at pH 4, pH 2–11.6, in 0.01 and 0.1 M NaNO 3 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(S 2O 3) 2 3− is significantly greater than that of AuCl 4 − on birnessite, but the adsorption of Au(S 2O 3) 2 3− is significantly smaller for geothite and humic acid. The adsorption of AuCl 4 − 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 AuCl 4 − and 1.06–1.10% for Au(S 2O 3) 2 3−, and for goethite is approximately 2.33–6.02% for AuCl 4 − and 0.6–1.05% for Au(S 2O 3) 2 3−. For the pH ranges from 2 to 11.6 and with 0.1 M NaNO 3 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 K part) can be estimated. For the three surfaces, values of log K part for adsorption of AuCl 4 − are in the order: birnessite>goethite>humic acid; but for adsorption of AuCl 2 − 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