Glycine betaine grafted nanocellulose as an effective and bio-based cationic nanocellulose flocculant for wastewater treatment and microalgal harvesting.

Flocculation is a widely used technology in industry including for wastewater treatment and microalgae harvesting. To increase the sustainability of wastewater treatment, and to avoid contamination of the harvested microalgal biomass, there is a need for bio-based flocculants to replace synthetic polymer flocculants or metal salt coagulants. We developed the first cellulose nanocrystalline flocculant with a grafted cationic point charge, i.e. glycine betaine (i.e. N,N,N-trimethylglycine) grafted cellulose nanocrystals (CNCs) effective for the flocculation of kaolin (a model system for wastewater treatment), the freshwater microalgae Chlorella vulgaris, and the marine microalgae Nannochloropsis oculata. We successfully grafted glycine betaine onto CNCs using a one-pot reaction using a tosyl chloride activated esterification reaction with a degree of substitution ranging from 0.078 ± 0.003 to 0.152 ± 0.002. The degree of substitution is controlled by the reaction conditions. Flocculation of kaolin (0.5 g L-1) required a dose of 2 mg L-1, a comparable dose to commercial polyacrylamide-based flocculants. Flocculation was also successful for freshwater as well as marine microalgae (biomass concentration about 300 mg L-1 dry matter), although the flocculation efficiency of the latter remained below 80%. The dose to induce flocculation (DS = 0.152 ± 0.002) was 20 mg L-1 for the freshwater Chlorella vulgaris and 46 mg L-1 for the marine Nannochloropsis oculata, comparable to other bio-based flocculants such as chitosan or TanFloc.

Harvesting of marine microalgae using cationic cellulose nanocrystals.

Flocculation of marine microalgae is challenging because of the high ionic strength of the culture medium. We investigated cationic cellulose nanocrystals (CNCs) as biobased flocculants for the marine microalgae Nannochloropsis oculata, and compared its performance to chitosan. Cationic CNCs induced flocculation at a low dose of 11 mg L-1, while chitosan required a dose of 35 mg L-1. Our cationic CNCs possess a permanent positive charge, allowing flocculation over a wide pH-range (4 to 10). The CNC maximum flocculation efficiency was 90 %, while chitosan achieved > 95 %, attributed to small flocs remaining in suspension for CNCs. However, centrifugation for 1 min at 180 g or gravity filtration using a 30 µm nylon filter after CNC flocculation resulted in the removal of these small, stable flocs (∼150 µm diam) and > 95% harvesting efficiency. Cationic CNCs can thus serve as a sustainable alternative natural flocculant for harvesting both freshwater and marine microalgae.