Synthesis and characterization of dialdehyde starch-crosslinked ε-poly-L-lysine for enhanced harvesting of Chlorella...

Microalgae harvesting remains a bottleneck for cost-effective biomass production. This study reports the synthesis and application of a novel bio-based flocculant, dialdehyde starch-crosslinked ε-poly-l-lysine (DS-PLL), designed to enhance the flocculation performance of ε-PLL. DS-PLL was synthesized via a Schiff base reaction, and its successful formation, including increased molecular weight and heterogeneity, was confirmed by Fourier-Transform Infrared Spectroscopy and Gel Permeation Chromatography. Acute aquatic toxicity tests (fish, Daphnia) showed no toxicity up to 100 mg/L. Compared to unmodified ε-PLL, DS-PLL demonstrated markedly improved harvesting efficiency (>90 % within 6 min at ≥ 4 wt%) and significantly faster flocculation kinetics for Chlorella vulgaris, forming substantially larger flocs (5-30 times larger than those formed by unmodified ε-PLL). Kinetic analysis using non-linear regression indicated the nonlinear pseudo-first-order (NLPFO) model best described the process for both flocculants, although DS-PLL exhibited much higher NLPFO rate constants (k₁). Furthermore, DS-PLL had a lower activation energy (3.9 vs 6.8 kJ/mol), signifying greater operational robustness against temperature changes. DS-PLL pre-treatment enabled efficient biomass recovery (∼90 %) via centrifugation at a 10-fold lower relative centrifugal force. Crucially, DS-PLL retained ε-PLL's antimicrobial properties, preventing biomass contamination during storage. With minimal added cost from modification, DS-PLL presents a multifunctional, highly efficient, robust, non-toxic, and potentially economically viable bio-flocculant for microalgae harvesting.