Synergistic enhancement of chitosan-diatomite composite in microalgae flocculation: Mechanistic insights into extracellular polymeric substances-mediated harvesting.

Chitosan is a promising bioflocculant for harvesting microalgae, its practical implementation is constrained by high dosage demands (typically >1 g/L) and suboptimal settling kinetics. And the microalgae harvesting process is susceptible to the significant influence of extracellular polymeric substances (EPS), the mechanism of which is still unclear. This study synthesized amino-functionalized chitosan-diatomite composites (APTES-CTS/DTE) and revealed the action mechanism of EPS in the flocculation of microalgae. The optimized APTES-CTS/DTE (1:10 ratio) achieved a 96.26 % harvesting efficiency for Chlorella sorokiniana at a substantially dosage (700 mg/L) under acidic conditions (pH = 3), with rapid separation within 20 min. Systematic characterization of EPS revealed protein-mediated interfacial interactions: post-harvest analyses revealed a decrease in tryptophan-like protein content along with an increase in surface hydrophobicity, indicating preferential EPS-protein engagement. Studies combining E-DLVO analysis and floc characterization demonstrated action mechanisms: (i) Lewis acid-base dominated interfacial adhesion facilitating microalgae-flocculant-EPS ternary complexation; (ii) synergistic charge neutralization with polymer bridging. ATPES-CTS/DTE can quickly and efficiently harvest microalgae. At the same time, it does not affect the integrity of the biomass and maintains a high biomass component recovery efficiency. This study provided fundamental insights into EPS-mediated flocculation dynamics and a suitable framework for developing next-generation flocculants with process intensification capabilities.