Microalgae biomass harvesting by bioflocculation-interpretation by classical DLVO theory

• Bioflocculation is efficient, economic and environmental friendly for microalgae harvesting.
• A conceivable mechanism of microalgae bioflocculation by γ-PGA broth is presented.
• Algae interaction energy was found dependent on zeta potential.

Poly-γ-glutamic acid (γ-PGA) broth was found to be a good bioflocculant of microalgae. However, the mechanism governing this bioflocculation process is not fully understood. In this study, Zeta potential measurement, microscopy examination and classical DLVO theory (named after Derjaguin, Landau, Verwey and Overbeek) analysis were used to explore the flocculability of microalgae induced by bacterial γ-PGA broth bioflocculant. Microalgae flocculation could be significantly improved by modifying ionic strength of the microalgae suspension, lowering pH value and bioflocculant addition due to the stronger attraction interactions between microalgal cells. In the present study, both the pH reduction with the bioflocculant induced better the flocculation process compared to the modification of ionic strength of the microalgae suspension in the presence of the bioflocculant. The DLVO theory indicated that when the bioflocculant was introduced, the total interaction energy decreased sharply, resulting in higher flocculation efficiency (>96 %) at a separation distance of 5 nm and shorter settling time (from 2 h to 10 min) compared with that obtained only by reducing the initial pH. The microalgae interaction energy was found to be dependent on the Zeta potential. This study provided a detailed interpretation of conceivable mechanism of microalgae bioflocculation by γ-PGA broth.