Cationic Cellulose Nanocrystals for Flocculation of Microalgae: Effect of Degree of Substitution and Crystallinity

Flocculation could offer a low-cost and straightforward solution to harvest microalgae in an economic and energy-efficient way. Cationically modified cellulose nanocrystals (CNCs) have been proposed as an alternative to biopolymer-based flocculants such as chitosan. The aim of this study was to fine-tune the cationic modification of CNCs for use as flocculants for harvesting the freshwater microalgae Chlorella vulgaris. CNCs were functionalized with two cationic groups, pyridinium (PYR) or methylimidazolium (MIM), and the degree of substitution (DS) was varied by controlling reaction conditions (stoichiometry, reaction time, and temperature). The DS ranged from 0.08 to 0.34 for PYR modifications and from 0.10 to 0.29 for MIM modifications. All cationic CNCs achieved flocculation efficiencies of >95% at optimal dosage. The required dose to induce flocculation decreased with increasing DS and did not differ between the PYR and MIM modified CNCs. The relation between DS and the flocculant dose points to the importance of electrostatic interactions in the flocculation mechanism. To check the advantages of a rigid flocculant compared to a flexible flocculant, we compared flocculation between rigid cationic CNCs and the flexible molecular polymer chitosan. While CNCs required a slightly higher dose by weight compared to chitosan, CNCs required a lower dose than chitosan when expressed per number of cationic charges added. Moreover, overdosing resulted in dispersion restabilization when chitosan was used but not when cationic CNCs were used, pointing to a different mechanism of flocculation between rigid cationic CNCs and the molecular polymer chitosan. Opposed to flexible flocculants, CNCs cannot reorient all their charges toward the microalgal cell. This results in patches of positive charges onto the surface. Because of the rigidity of CNCs, we propose an electrostatic patch mechanism as the mechanism of flocculation for CNC-based flocculants.