The impact of microorganisms on the surface of pyrite: Implications for bioflotation

Author: Sian La Vars

La Vars, Sian, 2018 The impact of microorganisms on the surface of pyrite: Implications for bioflotation, Flinders University, College of Science and Engineering

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Sulfide minerals are of significance in the mineral processing industry as major sources of base metals; however, their similar physicochemical properties make them challenging to separate. Bioflotation is the process by which microorganisms modify the mineral hydrophobicity, enabling selective separation of sulfide minerals. This has several advantages over traditional froth flotation, with the microorganisms being environmentally friendly and non-pathogenic.

To determine if it is the extracellular polymeric substances (EPS), or the combination of EPS and microorganisms that alter the hydrophobicity of the gangue mineral pyrite, the microbial strains Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, and Acidianus brierleyi were investigated. Cell growth and solution conditions were examined, and propagation of cells on the surface of pyrite was investigated. The physical and chemical properties of the mineral surface was studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), contact angle, photoemission electron microscopy (PEEM), scanning transmission X-ray microscopy (STXM), near edge adsorption fine structure (NEXAFS) and time of flight – secondary ion mass spectroscopy (ToF-SIMS).

This study found both A. ferrooxidans and L. ferrooxidans displayed attachment at very early stages of exposure, with no obvious preference for surface defects. The most significant difference in hydrophobicity between pyrite exposed to bacteria and the control occurs at the earliest exposure. L. ferrooxidans and A. ferrooxidans showed decreases in contact angle of 36° and 25°, respectively.

The decrease in hydrophobicity observed in pyrite exposed to mesophiles coincides with the presence of polysaccharide and fatty acid-type structures. It is proposed that these compounds assist with initial cell adhesion to pyrite, before cells produce hydrophobic proteins as colonies begin to spread on the surface. This study suggests that the nature of bacterial excretions changes over the course of exposure, with significant difference in EPS production between strains, which suggests the point of cell harvest and EPS extraction may impact mineral separation efficiency.

It was found that the presence of yeast extract in the growth medium of A. brierleyi made it impossible to determine with accuracy the action of EPS on the surface, and potentially impeded reactions on the surface. Investigation of the impact of A. brierleyi on pyrite in the absence of yeast extract found a decrease in contact angle of 24° compared to the control at 2 h.

It was found that A. brierleyi produced significantly less EPS than the other strains investigated, and that this strain is unlikely to alter the surface through direct attack, with little to no cell attachment observed on the pyrite surface.

These studies have shown that EPS production by cells at early stages of attachment creates significant improvement in the wettability of pyrite, without the need for extensive biofilm formation. This has important implications for the depression of pyrite for bioflotation, which would benefit from short periods of exposure and low cell density requirements. The analytical techniques listed previously were successfully applied to the investigation of physical properties and chemical species on the pyrite surface, and enabled to identification of polysaccharide and lipid-type compounds as being responsible for the decreased hydrophobicity of the mineral.

Keywords: Pyrite, A. ferrooxidans, L. ferrooxidans, A. brierleyi, bioflotation, NEXAFS, ToF-SIMS, contact angle, SEM, AFM

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Sarah Harmer