Physicochemical approach to alkaline flocculation of Chlorella vulgaris induced by calcium phosphate precipitates

[Display omitted] •Flocculation of C. vulgaris requires positively charged calcium phosphate particles.•Effective flocculation requires sufficient amount of calcium phosphate particles.•C. vulgaris flocculates also with negatively charged calcium phosphate particles.•Negatively charged particles provoke microalgae flocculation only at state of birth. Alkaline flocculation has been studied due to its potential as a low-cost harvesting method for microalgae. However, surface properties (zeta potential, contact angles) as inputs into physicochemical interaction models have not yet been applied systematically. In this work, forced alkaline flocculation of the freshwater microalgae Chlorella vulgaris induced by calcium phosphate precipitates was studied as a model system. Response surface methodology was used to quantify the effect of independent variables (concentration of Ca2+ (0.5–0.5 mM) and PO43− (0.05–0.35 mM), pH (8–12) and ionic strength (1–19 mM)) on the zeta potential (ZP) of microalgae, and the turbidity (T) of inorganic precipitates. Flocculation tests and their modified versions were carried out. The flocculation efficiencies obtained were interpreted with respect to predictions of physicochemical interaction models. It was found that flocculation was possible under conditions where appropriate precipitates were formed in the presence of cells. Under these conditions, flocculation of negatively charged Chlorella vulgaris was induced not only by positively charged, but also by negatively charged calcium phosphate precipitates at an early phase of nucleation. The driving force for interactions between oppositely charged cells and precipitate particles was electrostatic attraction, while the attraction between equally charged entities may have resulted from a negative total balance of apolar (Lifsitz-van der Waals) and polar (acid-base) interactions. Medium components did not interfere with flocculation, while cellular organic matter decreased flocculation efficiency only to a very limited extent.