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Influence of Bacterial Detachment on the Bioflotation of Malachite

The effects of the bacterial growth phase on the malachite flotation were investigated in a well-controlled Hallimond tube system. Rhodococcus opacus, which is one of representative hydrophobic bacteria, was employed for this study. The test results showed that the bacteria in the stationary phase exhibit two-fold greater floatability than those in mid-exponential phase. To understand the observed flotation behavior, complementary cell characterization tests (e.g., zeta potential and contact angle measurements) and cell attachment tests were conducted. Interestingly, the bacteria at both phases exhibited similar surface properties as well as almost identical amount of cells attached onto the malachite, suggesting that the growth phase dependent flotation behavior cannot be attributed to the variation of cell surface properties and the extent of cell adsorption. On the other hand, cell detachment tests revealed that the amount of cells detached from the malachite surface is greater for the mid-exponential phase than the stationary phase due to the higher fluid drag applied to the cells at the mid-exponential phase, which was explained by the differences in the size and shape of attached bacteria onto the malachite surface. Specifically, the bacteria in the mid-exponential phase had a larger size and formed loosely-packed structures like an end-to end on the malachite surface. These morphological characteristics were found to cause the bacteria of mid-exponential phase to be separated highly sensitive and easy from malachite surface due to the fluid flow. The findings from this study suggest that in the case of bioflotation using a relatively large bacteria size than the collector, it is important to consider the cell detachment by the fluid flow that occurs during a flotation process.