1. Introduction
Gold is a unique noble metal with high conductivity, ductility, and resistance to corrosion. It is utilized globally in the finance, jewelry, dentistry, aerospace, and electronics industries [
1]. Gold extraction from primary resources is an energy and chemically intensive process that is driven by size reduction (i.e., crushing and grinding) followed by chemical leaching with a suitable lixiviant. The grade of gold in deposits has decreased significantly in the last few decades, meaning a larger volume of ore must be processed to maintain constant production [
2]. With this and the advent of new technologies many new mines being developed to process ‘refractory’ ores, or ores with significant concentrations of sulfur and organic carbon which may impact the conventional crushing—cyanidation process. These ores must often be subjected to pre-treatment to remove or alter the chemical state of these materials. This may include fine grinding to increase the rate of the pre-treatment or expose fine gold particles within the mineral grains [
3]. With these considerations combined with the demand for more environmentally sustainable processing, the mining industry has been forced to consider alternatives to the cyanidation process which has been the primary method for gold leaching since the late 1800′s.
Cyanide is currently used in over 75% of global gold mining operations where strict controls and procedures make it a safe process despite the extremely high toxicity of the chemical [
4]. The largest risk comes during transportation of cyanide, and as such many jurisdictions have banned its movement, making the process effectively banned for use in mining. In addition, the prevalence of lower grade and more refractory ores have contributed to higher cyanide consumption during leaching, producing hazardous tailings that must be treated further to be stored safely [
5]. There have been several alternative gold lixiviants proposed as technically viable at the bench scale including thiosulfate, thiocyanate, thiourea, chlorination, and bioleaching using various biogenic lixiviants [
5,
6]. Thiocyanate and thiourea have not been implemented at mining sites to date despite high initial leaching rates and acidic operating conditions due to high reagent consumption when applied to many ores and significant toxicities [
7,
8].
Thiosulfate is favored above other alternative lixiviants like thiourea and thiocyanate due to its low toxicity, acceptable leaching rate, and stability in the presence of many components of gold ores [
9]. The leaching reaction of gold by thiosulfate with oxygen as an oxidant is shown in Equation (1) [
10]. Copper is often added as a catalyst to increase the leaching rate [
11].
The thiosulfate leaching system is not without its shortfalls, and to date has only been applied at scale in one commercial operation, Goldstrike, operated by Nevada Gold Mines Ltd., Elko, NV, USA, where calcium thiosulfate leaching of pressure oxidized concentrate was applied [
12]. Despite issues with high reagent consumption and gold recovery from solution it is the most promising of the alternatives with respect to environmental and human health concerns [
5]. Aside from these concerns, in jurisdictions including Germany, Czechia, and U.S. state Montana cyanide use for mining is fully banned [
13]. Therefore, in these locations any return on gold mined as a byproduct of other metals must be extracted by other means.
Biogenic lixiviants pose an interesting option when comparing lixiviants. While they consist of the same leaching agents the option to produce them using microorganisms and a suitable substrate could be a method to reduce the cost and risk of bringing large quantities to the leaching plant from other locations [
14]. The largest potential benefits of applying biogenic lixiviants for gold mining are a lower cost of production at scale, less toxic chemicals, less energy intensive chemical production, and scalability for smaller e-waste recycling operations [
15,
16]. Biogenic cyanide, thiosulfate, iodine, organic acids, and amino acids have been reported in the literature to leach or improve the leaching rate of gold from ores [
6,
17,
18]. There are still many knowledge gaps in the suitability of biohydrometallurgy techniques for gold mining, despite their commercially accepted application to copper extraction, recycling, and pre-treatment of gold ores [
19,
20,
21].
Many microorganisms produce thiosulfate as biogenic products of metabolism or as a consequence of side reactions involving other biogenic products, such as polysulfide and sulfite [
22].
Methylophaga sulfidovorans is a species of particular interest due to its ability to produce thiosulfate stoichiometrically as a product of chemolithoheterotrophic growth on organic and inorganic sulfur compounds [
23]. Chemolithoheterotrophy is a mixed metabolic mode that some microorganisms employ to grow on organic carbon sources while deriving extra energy for cellular activities from the oxidation or reduction of inorganic compounds to conserve energy [
24]. In the case of
M. sulfidovorans the primary growth substrates are methanol, dimethyl sulfide (DMS), methylamine, and dimethylamine. These primary substrates are catabolized by
M. sulfidovorans, and carbon atoms are assimilated into biomass by way of the ribulose monophosphate pathway [
23]. The bacteria can also oxidize H
2S during growth on the organic substrates in a process that does not contribute directly to cell proliferation [
18,
25]. It is assumed that the oxidation of inorganic substates is for increased energy production [
24].
M. sulfidovorans is a marine strain that was isolated from salt marsh sediments in the Netherlands. It grows near the interface of the oxic and anoxic layers of mixed microbial mats where many reduced sulfur compounds coexist as products of other bacterial processes [
23].
The substrates chosen for this study on the potential sources for Bio-TS and the uses of methylotrophic sulfur oxidizers in the mining industry were DMS ((CH
3)
2S), sodium sulfide (Na
2S), and elemental sulfur (S
0). Two of these reagents were specified in an earlier study using
M. sulfidovorans and elemental sulfur is a potential waste product encountered in gold mining operations [
10,
25]. The possibility of chemolithoheterotrophic growth with these sulfur substrates and methanol deserves further investigation.
When considering the sustainability of gold mining and mining operations in general, the use of fresh water for processing is of similar impact to the control of chemical contaminants in wastewater with respect to acquiring operational permits [
26]. Seawater or desalinated water has been used in some mining operations in arid regions and usually comes with more difficult process and equipment design from increases to corrosion of equipment and ionic strength of process water [
27]. The most common processing step to use seawater was flotation, however given that
M. sulfidovorans is a marine bacterium the potential for partial substitution of seawater in Bio-TS production is of interest and should be investigated. The high water usage in arid mining areas has impacts to surrounding communities that could potentially be lessened by partial substitution of groundwater with seawater [
28].
The purpose of this research is to investigate the potential of applying Bio-TS produced by M. sulfidovorans to gold leaching process options using different potential substrates. The application of biogenic thiosulfate for metallurgical purposes is a poorly understood topic, therefore comparing the feed materials and the outcome of leaching was studied to see if it holds potential for further development. Despite decades of research thiosulfate has failed to be accepted in the gold mining industry, and therefore other methods to improve it must be explored to see if they might improve the case for alternative lixiviants. Biogenic thiosulfate was applied to solubilize gold from oxide ore and gold powder. The viability of each substrate was compared with respect to biogenic thiosulfate concentration after growth, tolerance of the bacteria to increasing concentrations of the substrates, toxicity, ease of handing, and cost. Another point of novelty in the study is the mixing of seawater with bacterial growth mediums to simulate how leaching with biogenic lixiviants may reduce freshwater draws in mining areas.
4. Conclusions
Bio-TS was produced by Methylophaga sulfidovorans during chemoheterotrophic growth on methanol and Na2S or S0, or direct growth on dimethyl sulfide. The highest concentrations of Bio-TS produced were 1891, 200, and 91 mg/L when the sulfur substrate was Na2S, S0, and DMS, respectively. The highest conversion on a molar basis was when using Na2S at 90% when low concentrations were added to methanol grown cultures. The highest rates of conversion were reached after 7 days of growth for all substrates. The Bio-TS was used to solubilize gold powder up to concentrations of 3.5, 1.0, and 0.34 mg/L when the sulfur substrate was Na2S, S0, and DMS, respectively. Ore leaching experiments with an oxide gold ore assayed at 4.02 g/t followed the same trend with average gold extraction efficiency of 60.5, 23, and 12% when the sulfur substrate was Na2S, S0, and DMS, respectively.
Sodium sulfide in bacterial medium was determined to be the most effective method of producing Bio-TS for gold leaching. The system was determined to be effective when up to 70% seawater was used as growth medium, presenting potential for decreased water use in a process that is freshwater intensive in the current form. Elemental sulfur was determined to be the most environmentally safe and cost-effective reagent but low conversion to Bio-TS makes it technically inferior to Na2S. Intermediate bioprocessing using sulfate reducing bacteria is recommended for further study to combine the high technical efficiency of H2S as a substrate with the low cost and easy handing of elemental sulfur. However, before biogenic thiosulfate can become competitive on an economic basis a method to concentrate the aqueous Bio-TS must be developed.