Flocculation of Chlorella vulgaris-induced algal blooms: critical conditions and mechanisms

Algal blooms have posed great threats to livestocks and human health. Although flocculation is effective, its efficiency may hinder the direct application for algal blooms. In this study, critical (optimal) conditions and mechanisms for AlCl3, FeCl3, poly-aluminum chloride (PAC), chitosan, and polydimethyldiallylammonium chloride (PDADMAC)-induced flocculation of Chlorella vulgaris (C. vulgaris) were studied. Results identified the critical conditions which can cause flocculation efficiencies over 90% in 45 min for the five flocculants. Specifically, 4 similar to 10-mg/L doses of PDADMAC were proved to be appropriate for the treatment of C. vulgaris-induced algal blooms at pH 6.0 similar to 12.0. To probe the underlying mechanisms, functional groups involved in flocculation, zeta potential, and species distribution were analyzed during flocculation. FT-IR results indicated that N-H stretching in amine and C-H deformation in aliphatics were involved in algal flocculation with FeCl3, and C-H deformation played an important role with PDADMAC, PAC, and chitosan. For AlCl3, zeta potential and species distribution results suggested that charge neutralization and adsorption bridging were responsible for algal flocculation at pH 6 similar to 8. However, adsorption bridging and sweeping effects were the main mechanisms at pH >3 for FeCl3. The flocculation mechanisms for the rest of the three polymers were charge neutralization, adsorption bridging, and sweeping. Meanwhile, all the flocculation processes followed second-order kinetics. Strong linkages were found between the rate constant, fractal dimension, and flocculation efficiency (P < 0.05). The results of critical flocculation conditions and mechanisms indicated that PDADMAC was an excellent flocculant for C. vulgaris removing and recycling, especially in water bloom treatment.

Automated summary

This study investigated the critical conditions and mechanisms for flocculation of Chlorella vulgaris induced by five different flocculants. The results identified the optimal conditions for achieving efficient flocculation and revealed the underlying mechanisms involved. PDADMAC was found to be an excellent flocculant for removing and recycling Chlorella vulgaris, particularly in water bloom treatment.