Surface characteristics of microalgae and their effects on harvesting performance by air flotation

Flotation is considered as an effective and energy efficient method for harvesting microalgae. However, the interaction mechanism between bubble-algae cell and cell-cell interfacial boundaries in microalgae flotation is not completely clear. To better understand the effects of surface characteristics on microalgae flotation performance, the hydrophobicity and the zeta potential of two different microalgae species were quantified based on experimental measurements and the extended DLVO (Derjagin-Landau-Verwey-Overbeek) theory. Flotation experiments were then carried out and the effects of surface characteristics on microalgae flotation performance were analyzed. Algae Chlorella vulgaris and Anabaena vasriabilis show naturally hydrophilic and hydrophobic properties, respectively. The addition of a cationic surfactant (C(16)TAB) can modify hydrophilic microalgae cells into hydrophobic and further Chlorella hydrophobicity is enhanced with increased C16TAB concentrations. The zeta potentials of both algae strains are negative in the tested pH range. Compared with Chlorella vulgaris, the magnitude of zeta potential of Anabaena vasriabilis is found larger at the same pH, resulting in a more dispersed distribution in the suspension. In addition, flotation experiments demonstrated that microalgae hydrophobicity and zeta potential have significant impacts on the harvesting efficiency and concentration factor. The hydrophobic attraction is found to play a more important role in determining the harvesting performance than electrostatic repulsion between the interacting surfaces, especially for hydrophobicity algae in the present study. Finally, the highest flotation efficiency and the highest concentration factor could not be concurrently obtained for both algae strains, suggesting that optimized flotation conditions should be selected as a compromise.