Chitosan-β-cyclodextrin composite flocculants dual-drive for efficient microalgae harvesting: Structural evidences, dehydration process, E-DLVO model behavior and biomass utilization

Flocculation is one of the promising methods for harvesting microalgae. In this study, a series of grafted cationic chitosan-cyclodextrin composite flocculants (CTS/beta-CDs) with different structural properties were prepared from chitosan and cyclodextrin. The results showed that the harvesting efficiency of CTS/beta-CD-1 for C. vulgaris reached 99.91%. The chain type of polymeric flocculants with moderate graft chain lengths (L = 10-20) and tighter branched chain density distribution (D = 1.0-1.8) was favorable for C. vulgaris harvesting. The addition of CTS/ beta-CDs caused the discrete C. vulgaris cells to be flocculated and aggregated, and the reorganization of particles resulted in larger particles after flocculation, improving the dewatering performance of microalgae flocs. CTS/ beta-CDs had no significant inhibitory effect on the extraction of lipids, proteins and carbohydrates, and did not affect the extraction and quality of fatty acid methyl esters, and the re-cultivation experiments of the harvested water showed that chitosan-based flocculant was able to effectively reduce the cost of microalgae harvesting. Electrostatic interactions and bridging were the main harvesting mechanisms between CTS/beta-CD-1 and C vulgaris. E-DLVO modeling suggested that flocculation is mainly a result of electrostatic and acid-base interactions when the distance between particles is small. At higher pH, the harvesting efficiency of CTS/beta-CD-1 with C. vulgaris decreased. The study contributes to a better understanding of the structure-activity relationship between the structural characteristics of flocculants and the harvesting performance.