Investigating microalgae cell-microsphere interactions during microalgae harvesting by ballasted dissolved air flotation through XDLVO theory

Abstract

Ballasted dissolved air flotation (BDAF) method is an innovative and efficient technology for harvesting microalgae. However, this flotation process is not yet fully understood. In this study, surface properties of Scenedesmus obliquus (S. obliquus), Chlorella vulgaris (C. vulgaris) and microspheres were characterized by Zeta potential and contact angle measurements, and then the attachment behavior of the microalgae cells on microspheres was investigated by thermodynamic and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory approaches. The calculated results revealed that the van der Waals (vdW) interaction played an important role in determining the net attraction between the microalgae cell and the microsphere, while the Lewis acid-base (AB) interaction was always repulsion. Microalgae harvesting efficiency could be significantly improved by adding ferric chloride and lowering the pH value because the electrostatic (ES) interaction changed from repulsion to neutralization, and even to attraction, while the vdW and the AB interactions remained almost unchanged. The XDLVO analysis indicated that there was an effective attachment with a secondary minimum of −0.548 × 103 kT for S. obliquus-microsphere system and −0.744 × 103 kT for C. vulgaris-microsphere system. The deeper the well depth of secondary minimum, the better the attachment of microalgae cells to microspheres, which may result in a better harvesting efficiency. Finally, a suitable XDLVO prediction model was proposed to describe the relationship between microalgae cell-microsphere interactions and harvesting efficiency of BDAF.