Bioaccumulation of gold by sulfate-reducing bacteria cultured in the presence of gold(I)-thiosulfate complex

Abstract

A sulfate-reducing bacterial (SRB) enrichment, from the Driefontein Consolidated Gold Mine, Witwatersrand Basin, Republic of South Africa, was able to destabilize gold(I)-thiosulfate complex $(\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-})$ and precipitate elemental gold. The precipitation of gold was observed in the presence of active (live) SRB due to the formation and release of hydrogen sulfide as an end-product of metabolism, and occurred by three possible mechanisms involving iron sulfide, localized reducing conditions, and metabolism. The presence of biogenic iron sulfide caused significant removal of gold from solutions by adsorption and reduction processes on the iron sulfide surfaces. The presence of gold nanoparticles within and immediately surrounding the bacterial cell envelope highlights the presence of localized reducing conditions produced by the bacterial electron transport chain via energy generating reactions within the cell. Specifically, the decrease in redox conditions caused by the release of hydrogen sulfide from the bacterial cells destabilized the $\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-}$ solutions. The presence of gold as nanoparticles (<10 nm) inside a sub-population of SRB suggests that the reduction of gold was a part of metabolic process. In late stationary phase or death phase, gold nanoparticles that were initially precipitated inside the bacterial cells, were released from the cells and deposited in the bulk solution as addition of gold nanoparticles that already precipitated in the solution. Ultimately, the formation of micrometer-scale sub-octahedral and octahedral gold and spherical aggregates containing octahedral gold was observed.

Introduction

The accumulation and migration of gold in natural systems has long been the subject of intense debate. Gold is known to form complexes with chloride, $\text{AuCl}_2{}^{-}$ or $\text{AuCl}_4{}^{-}$, (Webster, 1986, Benedetti and Boulegue, 1991, Ran et al., 2002) thiosulfate $(\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-})$ (Goleva et al., 1970, Plyusnin et al., 1981, Mann, 1984, Webster, 1986, Benedetti and Boulegue, 1991), bisulfide and sulfide $(\text{Au(HS)}_2{}^{-}\text{,}{\text{AuS}}^{-}\text{,}{\text{Au}}_2(\text{HS}{)}_2\text{S}{}^{2-})$ (Krauskopf, 1951, Seward, 1973, Webster, 1986), and possibly sulfite $(\text{Au}({\text{SO}}_3)_2{}^{3-})$ (Pitul’ko, 1976). At circumneutral to alkaline pH values, thiosulfate is a product of chemical oxidation of reduced sulfur compounds (Goldhaber, 1983), and is an intermediate sulfur species produced by microbiological reactions (Suzuki, 1999). Since it is also stable in a geochemical field encompassing mildly acid to highly alkaline pH, and moderately oxidizing to reducing conditions (Mineyev, 1976, Goldhaber, 1983, Webster, 1986), it is considered one of the most likely or important gold complexing agents in natural systems (Saunders, 1989, Vlassopoulos and Wood, 1990).

Bioaccumulation of gold by bacteria has been described in a wide range of natural environments, from Australia to Venezuela to Alaska (Bischoff et al., 1992, Watterson, 1992, Bischoff, 1994, Bischoff, 1997). Several laboratory experiments have investigated the interaction of bacteria with gold using the gold(III)-chloride $(\text{AuCl}_4{}^{-})$ complex (e.g., Southam and Beveridge, 1994, Southam and Beveridge, 1996, Kashefi et al., 2001, Karthikeyan and Beveridge, 2002, Nakajima, 2003), however, only one recent study has described the bioaccumulation of gold from gold(I)-thiosulfate complex $(\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-})$ (Lengke and Southam, 2005). This previous study of bioaccumulation of gold by thiosulfate oxidizing bacteria demonstrated that the precipitation of gold from $\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-}$ occurred inside the bacterial cells and was a part of metabolic process once uncomplexed thiosulfate was consumed. When gold nanoparticles inside the cells were released through the cell envelope to the bulk solution, extracellular gold minerals, as micrometer-scale networks of wire gold and octahedral gold, were ultimately formed.

Because of the importance of gold(I)-thiosulfate complex $(\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-})$ in natural systems, an understanding of bioaccumulation of gold from this complex is important. Thus, the goal of this study was to investigate the interaction between gold(I)-thiosulfate complex $(\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-})$ and a bacterial enrichment that is dominated by sulfate-reducing bacteria (SRB). The formation of gold from the interaction of SRB with $\text{Au}({\text{S}}_2{\text{O}}_3)_2{}^{3-}$ has not been investigated. In this study, thiosulfate was used as the terminal electron acceptor instead of sulfate. The enrichment culture of SRB was chosen in this study because these bacteria are broadly distributed on earth and commonly found in mining areas.

Sulfate-reducing bacteria (SRB) typically oxidize organic compounds using sulfate as their terminal electron acceptor. Hydrogen sulfide (H₂S), produced as one of the major end-products of their metabolism, has a strong affinity for metals and readily forms insoluble compounds by the following reactions (Trudinger et al., 1985):

$$2{\text{CH}}_2\text{O}+{\text{SO}}_4{}^{2-}\to {\text{H}}_2\text{S}+2{\text{HCO}}_3{}^{-}$$

$${\text{M}}^{2+}+{\text{H}}_2\text{S}\to {\text{MS}}_{(\text{s})}+2{\text{H}}^{+}$$

Sulfate-reducing bacteria (SRB) are ecologically diverse and tend to be enriched wherever sulfoxyanions, e.g., sulfate or thiosulfate, are present along with a sufficient supply of organic matter to create anaerobic conditions (Trudinger et al., 1985, Peck, 1992, Kleikemper et al., 2002).