Highly Effective Nanocellulose-Based Flocculants for Use in Freshwater and Saltwater Conditions

The rigidity of cellulose nanocrystals and the ability to modify tthem at differnet surface planes makes them interesting candidates as flocculants for larger particles or organisms becasue they can flocculate using a patch mechanism and coiling under high salt conditions is avoided. Therefore, we investigate cationically modified CNCs for the flocculation of microalgae. Cellulose nanocrystals (CNCs), extracted from cotton via a standard H₂SO₄ acid hydrolysis, were chemically modified with methyl-imidazolium (MIM) and pyridinium (PYR) grafts at various degrees of substitutions (DS). The extent of grafting was determined by elemental analysis and FTIR and EXPS spectroscopy, while retention of crystallinity and morphology were confirmed by WAXS and AFM respectively.<br/>When testing <i>Chlorella Vulgaris</i> (freshwater microalgae) flocculation, it was found that the flocculation efficiency did not vary significantly with surface graft or with DS. However, the required dose varied virtually linearly with DS and remained independent of graft type (Fig 1(a)). Applying these flocculants to marine microalgae <i>Nannochloropsis oculata</i>, flocculation remained effective and flocs could readily be removed using a large pore filter (Fig 1 (b)).<br/>Adhesion force histograms and rupture distance measurements by securing (1) a microalgal cell on the AFM tip and probing a film of modified CNCs or (2) modified CNCs on the AFM tip and probing microalgal cells in solution, both showed that the interaction is governed by electrostatic interactions. This explains the linear relation between DS and required dose for flocculation. The interaction force for PYR-CNCs with <i>Chlorella vulgaris</i> was only a third of that for MIM-CNCs but that did not seem to affect its flocculation efficacy. Therefore, the strength of interaction does not seem to be a very important parameter so that our results show that increasing DS is more important than optimizing the type of cationic charge. This work was then extended to prepare fully biobased materials, using glycine betaine as the surface grafts on CNCs, giving similar results to pyridinium and methylimidazolium modified CNCs. We also investigate CO₂-sensitive grafts allowing us to add the CNCs to the microlagal growing medium, after which CO2 could be bubbled through the medium redering the CNCs positively charged thereby flocculating the microalgae. Finally, we are currently investigating in more detail the interaction strength between cationically modified CNCs with microalgal cell walls to be able to devise models capable of predicting flocculation performance.<br/>Our work thus shows that CNCs offer great potential as biobased flocculants, even in high-ionic strength conditions. Extending this work to the removal of particulate matter in waste water is currently in progress and shows similar excellent flocculation performance, showing the wide applicability of cationic CNCs as flocculants.