The immobilization of gold from gold (III) chloride by a halophilic sulphate-reducing bacterial consortium

Abstract

A consortium containing halophilic, dissimilatory sulphate-reducing bacteria was enriched from Basque Lake #1, located near Ashcroft, British Columbia, Canada to evaluate the role these bacteria have on the immobilization of soluble gold. The consortium immobilized increasing amounts of gold from gold (III) chloride solutions, under saline to hypersaline conditions, over time. Gold (III) chloride was reduced to elemental gold in all experimental systems. Salinity did not affect gold immobilization. Scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold (III) chloride was immobilized as c. 3-10 nm gold colloids and c. 100 nm colloidal aggregates at the fluid-biofilm interface. The precipitation of gold at this organic interface protected cells within the biofilm from the 'toxic effect' of ionic gold. Analysis of these experimental systems using X-ray absorption near-edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes. The immobilization of gold by halophilic sulphate-reducing bacteria highlights a possible role for the biosphere in 'intercepting' mobile gold complexes within natural, hydraulic flow paths. Based on the limited toxicity demonstrated in this experimental model, significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably 'non-toxic' to the biosphere.

Biogeochemical Cycling of Silver in Acidic, Weathering Environments

“…Residual zinc sulfide minerals could have acted as a selective precipitant for dissolved Hg forming an insoluble mercury sulfide [51] and thereby immobilizing dissolved Ag as a separate precipitate (Figure 2). The presence of gold nanoparticles also exemplifies the biomobility of precious metals within gossan since soluble gold complexes are generally unstable under inorganic/chemical weathering conditions [31,32,52]. Crevices within gossan allows groundwater to descend and provides ideal microenvironments for microbial attachment and growth [53][54][55].…”

“…From a biogeochemical perspective, the dissolution and subsequent re-precipitation of gold has been the focus of many studies regarding precious metal mobility within weathering environments since its cycling is closely linked to iron and sulfur cycles [31,32]. However, in contemporaneous to gold biogeochemical cycling, little is known about the "fate" of Ag in regard to its kinetic mobility within an acidic weathering environment.…”

Challenges to global mineral resource security and options for future supply

“…Even for the major industrial metals and their main deposit classes, such as porphyry copper deposits, which have been the subject of decades of study, researchers are continuing to enhance the knowledge base and to question some of the fundamental controls on their formation, particularly for giant systems (Sillitoe 2010a). There are likely to be continued developments in our understanding of the role of biological processes and bacteria in ore deposit formation and the importance of 'economic geomicrobiology' (Southam & Saunders 2005;Shuster et al 2013). This knowledge will have applications in mineral exploration, mineral processing, tailings management and site remediation (Zammit et al 2012;Kalmar 2014).…”

Ore deposits in an evolving Earth: an introduction

“…The recognition that certain microorganisms may be associated with gold in solution suggests the development of bioindicators for gold that rely on detection of these specific microorganisms, and of biosensors employing gold-binding proteins from bacteria that allow rapid field-based analysis of gold at very low levels (Gwynne 2013). Shuster et al (2013) contribute to this rapidly expanding field of research by demonstrating that sulphate-reducing bacteria can immobilize gold from saline and hypersaline solutions where chloride occurs in excess. This has implications for the development of geochemical haloes around weathering gold deposits in semi-arid regions, where evaporation of groundwaters leads to high salinities and the development of goldenriched calcretes (e.g.…”

“…Lintern et al 2012). Shuster et al (2013) show that the gold is precipitated as nano-particles, which have potential applications in optoelectronics for imaging technologies, catalysis and drug delivery (Gwynne 2013). Thus it may be that microbiological processing of gold ores might have the additional advantage of producing gold nanoparticles in a form that is directly useful for high-tech industries.…”