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 restabilisation 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. 2 Opposed to flexible flocculants, CNCs cannot reorient all their charges towards 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.

In light of climate change, there is a growing interest for sustainable energy. Microalgae are a promising resource for biofuel production, although their industrial use is limited by the lack of effective harvesting techniques. Flocculation consists in the aggregation and adhesion of cells into flocs that can be more easily removed from water than individual cells. Although it is an efficient harvesting technique, contamination is a major issue as chemical flocculants are often used. An alternative is to use natural biopolymers flocculants, such as chitosan. Chitosan is a bio-based nontoxic polymer, which has been effectively used to harvest Chlorella vulgaris cells at pH lower than its pKa (6.5). While the flocculation mechanism reported relied on electrostatic interactions between chitosan and the negative cell surface, no molecular evidence has yet confirmed this mechanism. In this study, we performed force spectroscopy AFM experiments to probe the interactions between C. vulgaris cells and chitosan at the molecular scale to decipher its flocculation mechanism. Our results showed that at pH 6, chitosan interacts with C. vulgaris cell wall through biological interactions, rather than electrostatic interactions. These observations were confirmed by comparing the data with cationically modified cellulose nanocrystals, for which the flocculation mechanism, relying on an electrostatic patch mechanism, has already been described for C. vulgaris. Further AFM experiments also showed that a different mechanism was at play at higher pH, based on chitosan precipitation. Thus this AFM-based approach highlights the complexity of chitosan-induced flocculation mechanisms for C. vulgaris.

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 (socalled "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.

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In a conventional solvent extraction system, metal ions are distributed between two immiscible phases, typically an aqueous and an organic phase. In this paper, the proof-of-principle is given for the distribution of metal ions between three immiscible phases, two ionic liquid phases with an aqueous phase in between them. Three-liquid-phase solvent extraction allows separation of a mixture of three metal ions in a single step, whereas at least two steps are required to separate three metals in the case of two-liquid-phase solvent extraction. In the triphasic system, the lower organic phase is comprised of the ionic liquid betainium- or choline bis(trifluoromethylsulfonyl)imide, whereas the upper organic phase is comprised of the ionic liquid trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide. The triphasic system was used for the separation of a mixture of tin(II), yttrium(III), and scandium(III) ions.