The Role of Cationic Coagulant-to-Cell Interaction in Dictating the Flocculation-Aided Sedimentation of Freshwater Microalgae

The coagulant chitosan and poly(diallyldimethylammonium chloride) (PDDA) were proven effective to aid the sedimentation of microalgal cells through electrostatic patch flocculation. A total amount of 30 mg/L chitosan and PDDA can achieve cell separation efficiency of 96.7 ± 0.7 and 98.4 ± 1.0 %, respectively. The chitosan outperformed PDDA and promoted 4.4 times faster rate of cell sedimentation than the self-sedimentation rate of cells. The chitosan employed the mechanism of charge neutralization to form larger flocs, while the PDDA that favored the formation of loops and tails protruding away from cell surface employed the bridging mechanism to form flocs. The rate of cell sedimentation induced by chitosan was the highest at pH 7, 8 (control) and 9 compared to that of PDDA, where the cell flocculation by using chitosan was dominated by charge neutralization mechanism at pH 7 and 8 (control), while the cell flocculation was mainly driven by bridging mechanism at pH 9. This result shows that the chitosan is feasible in the harvesting of freshwater microalgae without the need of pH adjustment. Therefore, the chitosan was proven more reliable and time effective than the PDDA in harvesting the freshwater Chlorella sp. without the need of pH adjustment.