Role of interfacial phenomena in determining adsorption of Bacillus polymyxa onto hematite and quartz

Utility of microorganisms as surface modifiers in flotation and flocculation has generated great interest in recent times. The surface properties of the minerals and the microorganisms such as zeta-potential ($\zeta$-potential) and hydrophobicity play a major role in determining adsorption of the microorganisms to the minerals and hence the efficiency of flocculation and flotation. These properties are also dependent on the solution conditions such as pH and ionic strength. In the present investigation, the role of ionic strength and pH in determining the surface properties and hence adsorption of the bacterium Bacillus polymyxa to hematite and quartz has been studied in detail. Results indicate that increasing the ionic strength changes the $\zeta$-potential of hematite and the bacterium without changing the point-of-zero-charge (PZC). Contact angle measurements showed that hematite was the most hydrophobic when compared with both quartz and the bacterium. Adsorption tests demonstrated that conditioning time, pH and ionic strength are all important in controlling adsorption of B. polymyxa on to both quartz and hematite. Adsorption of bacteria on to hematite was more compared with quartz. Flocculation tests demonstrated that in the presence of bacteria and electrolyte, hematite settled very rapidly in the pH range of 2–8. However, quartz settled much slower under the same indicating that the quartz particles are being dispersed. Thus selective flocculation of hematite can be used effectively in separating hematite from quartz. Interaction energies calculated using the extended DLVO theory were found to be in good agreement with the adsorption and flocculation tests