Bioflotation of malachite using different growth phases of Rhodococcus opacus: Effect of bacterial shape on detachment by shear flow

• Bacteria in different growth phases are studied for the malachite flotation.
• Bacteria in the stationary phase show higher malachite floatability.
• Bacteria in both growth phases have the same surface properties.
• Bacteria in the mid-exponential phase are more easily detached by shear stress.
• Easy detachment is due to the larger bacteria size and the aggregation shape.

We study the effects of the growth phase of Rhodococcus opacus on the floatability of malachite. We find that bacteria in stationary phase show more than two-fold higher floatability than bacteria in mid-exponential phase. To understand the mechanism of this higher floatability for the stationary phase, we examine bacteria surface properties such as zeta potential and contact angle. Surprisingly, all bacteria surface properties are nearly the same; moreover, the amounts of bacteria adsorbed onto malachite are also identical. Despite bacteria's similar surface properties, we discover that the amount of bacteria detached from malachite during the mixing process in flotation is larger for the mid-exponential phase than for the stationary phase. We attribute this to hydrodynamic shear stress due to the fluid flow established in mineral mixing. Furthermore, we observe that bacteria–bacteria interactions are far more significant for the mid-exponential phase, leading to end-to-end aggregation and thus allowing unique loosely-packed structures on the malachite surface to form. Such loose structures of bacteria on the mineral are highly susceptible to fluid flow and can be easily detached from the mineral surface. Our findings suggest that due to the relatively large size of the bacteria, it is crucial to consider the detachment of adhered bacteria on the mineral surface due to fluid flow during mixing, which is easily ignored in typical flotation processes.

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