Local environment around gold (III) in aqueous chloride solutions: An EXAFS spectroscopy study

Abstract

The local environment around Au (III) in aqueous solutions containing 1 M NaCl was determined as a function of pH and Au concentration using X-ray absorption spectroscopy (XAS) at ambient temperature and pressure. The solution Au concentrations studied were 10 − to 10 −3 M and the pH ranged between 2 and 9.2. No significant changes of Au speciation were detected with increasing Au concentration; however, major speciation changes were caused by variations in pH. At pH = 2, Au is coordinated by four Cl atoms ( mean d [AuCl] = 2.28 –2.29 ± 0.01 A ̊ ), whereas at pH 7.5 and 9.2, Au is coordinated by three Cl and one O (or OH) and by two Cl and two O (or OH), respectively ( mean d[AuCl] = 2.28 ± 0.02 A ̊ ; mean d[AuO or AuOH] = 1.97 ± 0.02 A ̊ ), indicating replacement of Cl by O (or OH) with increasing pH. In all solutions studied, the number of first-neighbors around Au(III) is close to four. XANES analysis suggests the presence of a square-planar geometry for AuX 4 ( X = Cl, O) at all pH values studied. These results are in excellent agreement with those from our previous Raman, resonance Raman, and UV/visible spectroscopy study of gold(III)-chloride solutions (PECK et al., 1991), which found that AuCl 4 −, AuCl 3(OH) −, and AuCl 2(OH) 2 − are the majority species in the pH ranges 2–6, 6–8.5, and 8.5–11, respectively. We did not find evidence for Au(I)Cl 2 − or Au(I)Cl(OH) − complexes in our pH 7.5 and 9.2 solutions, as was recently suggested by Pan and Wood (1991) for acidic gold chloride solutions at temperatures > 100°C, although we can't rule these complexes out as minority species (<10% of the total Au in solution). Our EXAFS results also provide the first direct evidence for Cl second neighbors around AuCl 4 − complexes in the most acidic solutions studied ( pH = 2 and 4.5). These second-neighbor Cl atoms were also detected at low Au concentrations (10 −3 M) and are similar in number and arrangement to those observed in crystalline KAuCL 4·2H 2O (two Cl at a mean d[Au-Cl(2)] = 4.42 ± 0.03 A ̊ ). No evidence was found for second-neighbor Au atoms, which indicates little or no Au polymers or colloidal particles in any of the solutions studied. Our EXAFS results are in broad agreement with earlier predictions of Au speciation based on a variety of chemical measurements. Moreover, they directly confirm that mixed chloro-hydroxo Au (III) complexes are more stable than predicted on the basis of thermodynamically estimated stability constants.