Effect of manganese oxide minerals and complexes on gold mobilization and speciation

Abstract

(Bio)geochemical processes mediate the dispersion and re-concentration of gold (Au) in Earth surface environments. The fundamental mechanisms underlying the mobilization of Au in surface environments and the chemical speciation of mobile Au remain largely unexplored. For more than half a century authors have argued that highly reactive manganese (Mn) oxides are involved in oxidizing, and hence mobilizing, Au in waters, soils and sediments. However, no systematic study has assessed the mechanisms of Au oxidation under conditions occurring in natural environments. Therefore, this paper explores the links between Mn-oxides and the oxidation/mobilization of Au under acidic to circum-neutral conditions. Using a newly developed high performance liquid chromatography–inductively coupled plasma-mass spectrometry (HPLC–ICP-MS) method, a range of Au(I/III)-complexes occurring in aqueous solution were directly quantified. We show that the oxidation of Au(I) to Au(III) with birnessite occurs in acidic environments, and that the rapid release of Au(III) sorbed (on Mn- or Fe-minerals) via influx of neutral waters transfers Au(III) into solution, making it mobile and transportable. This provides a viable pathway for explaining the occurrence of mobile Au(III)-complexes under neutral to alkaline conditions, as was reported from a Western Australian salt lake environment. In addition, we show that the [(bpy)2MnIII(μ-O)2MnIV(bpy)2]3− complex (bpy=2,2′-bipyridyl), a useful model compound for the interaction of Au with bioorganic Mn in natural systems, mediates the formation of soluble Au(III)-complexes from metallic Au and from Au(I)-complexes under oxic and anoxic conditions. In conclusion, this study shows that reactive Mn-oxide minerals are important drivers of Au mobilization, and further suggests that natural organometallic compounds play an important role for the mobilization of Au in Earth surface environments.