Abstract
Biomining applies microorganisms to extract valuable metals from usually sulfidic ores. However, acidophilic iron (Fe)-oxidizing bacteria tend to be sensitive to chloride ions which may be present in biomining operations. This study investigates the bioleaching of pyrite (FeS2), as well as the attachment to FeS2 by Sulfobacillus thermosulfidooxidans DSM 9293T in the presence of elevated sodium chloride (NaCl) concentrations. The bacteria were still able to oxidize iron in the presence of up to 0.6M NaCl (35 g/L), and the addition of NaCl in concentrations up to 0.2M (~12 g/L) did not inhibit iron oxidation and growth of S. thermosulfidooxidans in leaching cultures within the first 7 days. However, after approximately 7 days of incubation, ferrous iron (Fe2+) concentrations were gradually increased in leaching assays with NaCl, indicating that iron oxidation activity over time was reduced in those assays. Although the inhibition by 0.1M NaCl (~6 g/L) of bacterial growth and iron oxidation activity was not evident at the beginning of the experiment, over extended leaching duration NaCl was likely to have an inhibitory effect. Thus, after 36 days of the experiment, bioleaching of FeS2 with 0.1M NaCl was reduced significantly in comparison to control assays without NaCl. Pyrite dissolution decreased with the increase of NaCl. Nevertheless, pyrite bioleaching by S. thermosulfidooxidans was still possible at NaCl concentrations as high as 0.4M (~23 g/L NaCl). Besides, cell attachment in the presence of different concentrations of NaCl was investigated. Cells of S. thermosulfidooxidans attached heterogeneously on pyrite surfaces regardless of NaCl concentration. Noticeably, bacteria were able to adhere to pyrite surfaces in the presence of NaCl as high as 0.4M. Although NaCl addition inhibited iron oxidation activity and bioleaching of FeS2, the presence of 0.2M seemed to enhance bacterial attachment of S. thermosulfidooxidans on pyrite surfaces in comparison to attachment without NaCl.
Highlights
Biomining applies sulfur‐ and iron (Fe)-oxidizing microorganisms to extract valuable metals usually from low-grade sulfidic ores and concentrates via bioleaching or biooxidation (Rawlings, 2002)
In tests on pyrite bioleaching with cells of S. thermosulfidooxidans at various concentrations of NaCl, it was shown that the addition of NaCl reduced pyrite dissolution
S. thermosulfidooxidans is more tolerant to NaCl than many other acidophilic iron-oxidizing bacteria used in the mining industry (Zammit et al, 2012; Rea et al, 2015), which make it potentially useful for bioleaching in saline environments
Summary
Biomining applies sulfur‐ and iron (Fe)-oxidizing microorganisms to extract valuable metals usually from low-grade sulfidic ores and concentrates via bioleaching or biooxidation (Rawlings, 2002). Iron-oxidizing bacteria, for instance, catalyze ferrous iron (Fe2+) oxidation, with ferric iron (Fe3+) being regenerated up to ~106 times faster than by chemical autoxidation in acidic environments (Lacey and Lawson, 1970) Thereby, they increase metal dissolution rates considerably. Acidithiobacillus ferrooxidansT, for example, the most widely studied bacterium in bioleaching, tolerates only 6 g/L (~100 mM) sodium chloride (NaCl; Dopson et al, 2017). Several studies have indicated mechanisms of chloride toxicity in acidophilic bacteria, such as osmotic imbalance, cytoplasmic acidification, and oxidative stress induction (Alexander et al, 1987; Suzuki et al, 1999; Slonczewski et al, 2009; Rivera-Araya et al, 2019). Chloride tolerance levels were found to be varied between different domains, genera, or species of acidophilic bacteria (Rea et al, 2015) but commonly, chloride tolerance concentrations are far below seawater levels (Zammit et al, 2012). Rivera-Araya et al (2017) pointed out that L. ferriphilumT is more tolerant to NaCl than A. ferrooxidansT, with minimum inhibitory concentrations of NaCl being 225 mM (~13 g/L) and 150 mM (~9 g/L), respectively
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