Unravelling the Mechanism of Chitosan-Driven Flocculation of Microalgae in Seawater as a Function of pH

Chitosan is a nontoxic biobased polymer, attractive for the flocculation-based harvesting of microalgae. While it is generally effective to harvest algae in freshwater medium, its performance in seawater has been unpredictable. This study determined the optimal conditions for flocculation of the marine microalgae Nannochloropsis oculata using chitosan. Whereas in freshwater a low pH (<7.5) is required to protonate the amine groups and to activate the chitosan flocculation activity toward charge-neutralization and bridging, flocculation of Nannochloropsis in seawater only occurred at a high pH (>7.5). The dosage of chitosan required for flocculation of Nannochloropsis in seawater (75 mg/L) was higher than the reported dose to flocculate the freshwater microalgae (±10 mg/L) reported in the literature. Experiments carried out in synthetic seawater with modified magnesium concentration indicated that flocculation induced by chitosan at varying pH was not related to precipitation of magnesium hydroxides (so-called "autoflocculation"). Chitosan flocculation at high pH in seawater medium was found to be caused by precipitation of chitosan due to (partial) deprotonation of the amine groups, resulting in a sudden network formation that induces flocculation by a sweeping mechanism. Visual observations and viscosity measurements indeed confirmed the occurrence of precipitation of chitosan at pH > 7.5.