Adsorption mechanisms of trivalent gold on iron- and aluminum-(oxy)hydroxides. Part 1: X-ray absorption and Raman scattering spectroscopic studies of Au(III) adsorbed on ferrihydrite, goethite, and boehmite

Gold adsorption products on powdered ferrihydrite, goethite, and boehmite samples, prepared by reacting Au(III)-Cl solutions ([Au] = 4.2 × 10⁻⁵-9.0 × 10⁻³ M; [Cl] = 0.017-0.6 M) with these adsorbents at pH values of 4 to 9 and Au adsorption densities ranging from 0.046 to 1.53 μmol/m² were characterized using Au-L_III XAFS spectroscopy. The solutions (before and after uptake) were investigated by Raman scattering to determine speciation and by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) to determine solution composition. We present an analysis of several effects that are observed in the Au L_III-edge XAFS spectra, including X-ray beam-induced photo-reduction, multi-electronic excitations, disorder effects, and multiple scattering, that would complicate interpretation of the spectra if not accounted for. A combination of methods (spectral deconvolution, principal component analysis, spectral inversion, and wavelet analysis) was used to identify and quantify these effects, to characterize the nature of mixed ligands around gold, and to distinguish between multiple-scattering features and features due to next-nearest neighbors in the XAFS spectra.Analysis of the Au-L_III XAFS spectra showed that Au(III) is present as square-planar Au(III)(O,Cl)₄ complexes in the aqueous solutions and on the surfaces of the Al/Fe-(oxy)hydroxide adsorption samples with dominantly O ligands at pH > 6 and mixed O/Cl ligands at lower pH values. The EXAFS-derived Au-O and Au-Cl distances are 2.00(2) and 2.28(2) Å, respectively, and the magnitudes of the Debye-Waller factors and third cumulants from anharmonic analyses indicate very little thermal or positional disorder around Au(III) in the adsorption samples. Iron second neighbors are present around Au in the Au(III)/ferrihydrite and Au(III)/goethite adsorption samples, with Au-Fe distances of 3.1(1) and 3.3(1) Å. In boehmite, two sets of Au-Al distances were detected at 3.0(1) and 3.2(1) Å. A reverse Monte Carlo study of the XAFS spectroscopic data suggests the presence of a continuum of edge-shared AuO₄-FeO₆ distances, which cannot be described correctly by a classical model of these data in which only a mean distance (although severely under-estimated) is derived.