Surface Chemical Characterisation of Pyrite Exposed to Acidithiobacillus ferrooxidans and Associated Extracellular Polymeric Substances

Abstract

A. ferrooxidans and their metabolic products have previously been explored as a viable alternative depressant of pyrite for froth flotation; however, the mechanism by which separation is achieved is not completely understood. Scanning electron microscopy (SEM), photoemission electron microscopy (PEEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and captive bubble contact angle measurements have been used to examine the surface physicochemical properties of pyrite upon exposure to A. ferrooxidans grown in HH medium at pH 1.8. C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra collected from PEEM images indicate hydrophilic lipids, fatty acids and biopolymers are formed at the mineral surface during early exposure. After 168 h, the spectra indicate a shift towards protein and DNA, corresponding to an increase in cell population and biofilm formation on the surface, as observed by SEM. The Fe Ledge NEXAFS show gradual oxidation of the mineral surface from Fe(II) sulfide to Fe(III) oxyhydroxides. The oxidation of the iron species at the pyrite surface is accelerated in the presence of A. ferrooxidans and extracellular polymeric substances (EPS) as compared to HH medium controls. The surface chemical changes induced by the interaction with A. ferrooxidans show a significant decrease in surface hydrophobicity within the first 2 h of exposure. The implications of these findings are the potential use of EPS produced during early attachment of A. ferrooxidans, as a depressant for bioflotation.

Review of Potential Microbial Effects on Flotation

“…This enabled the selective separation of sphalerite, galena and quartz from pyrite and chalcopyrite after prior treatment with bacterial proteins. In the study of La Vars et al [26], EPS was detected within 2 h of contact with Acidithiobacillus ferrooxidans on pyrite surface rendering the surface hydrophilic. Acidithiobacillus ferrooxidans produced polysaccharides and fatty acid compounds in the initial adhesion stage, and only later hydrophobic protein-and nucleic acid-rich EPS.…”

“…The quantity of microorganisms needs to be high for efficient bioflotation to compete with conventional flotation chemicals [22]. La Vars et al [26] showed that mineral surface is modified already by low cell coverage.…”

“…Acidithiobacillus ferrooxidans produced polysaccharides and fatty acid compounds in the initial adhesion stage, and only later hydrophobic protein-and nucleic acid-rich EPS. This observation means that the excreted compounds can change over time depending on the stage of EPS formation [26].…”

“…The bioleaching microorganism Leptospirillum ferrooxidans was preferentially attached to pyrite over chalcopyrite resulting in biofilm formation on pyrite surface, but not on chalcopyrite surface [34]. Leptospirillum ferrooxidans was shown to have a higher affinity to chalcopyrite than to pyrite, which resulted in higher depression of chalcopyrite in flotation and better settling behavior [26]. In contrast, Paenibacillus polymyxa enforced a higher depression of pyrite than chalcopyrite, which resulted from a higher proportion of polysaccharides produced on the cell surface in pyrite adapted cells compared to chalcopyrite adapted cells, which also rendered the pyrite-adapted cells less hydrophobic [47].…”

Editorial for Special Issue “Geomicrobiology and Biogeochemistry of Precious Metals”

“…This study, therefore, also points to the need for a better integration of biogeochemical cycles of precious metals with other large biogeochemical cycles, e.g., those of carbon, nitrogen and sulfur. In their application-driven study, La Vars et al [14] achieve this by assessing Acidithiobacillus ferrooxidans and its associated extracellular polymeric substances (EPS). This bacterium forms biofilms on the surface of gold-bearing sulfide mineral pyrite and the resulting EPS may be a viable alternative depressant of pyrite for froth flotation.…”