Study on the Inhibition Mechanism of Metal Cations on the Microalgal Harvesting Using Foam Flotation

Abstract

Foam flotation is generally recognized as a low cost and efficient technology for the harvesting of microalgae for food feed and fuel production, as well as environmental remediation. However, the inhibition of metal cations on harvesting performance as well as their inhibitory mechanisms have not yet been revealed . This study investigated the effects of metal cations in the medium on the harvesting efficiency and concentration factor during foam flotation harvesting of Scenedesmus acuminatus . The influencing mechanism was revealed by quantifying adsorption of cetyl trimethylammonium bromide (CTAB) on cells, the change in bubble size, and varied water content in the foam layer in the presence of inhibiting cations at different concentrations. Results showed that the harvesting efficiency dropped linearly with the increase of cationic concentrations. At CTAB dose of 20 mg g -1 , the harvesting efficiency were decreased from 98.65% to 56.77% with a decrease of concentration factor from 25.41 to 9.05 in the presence of metal cations. The Na + and Mg 2+ in the medium were the major inhibitors. The inhibited mechanisms were revealed that metal cations significantly impeded the adsorption of CTAB on the cells by competing adsorption site, resulting in low harvesting efficiency. Presence of metal cations also inhibited the bubble coalescence and slowed down drainage velocity in the plateau channel of foam layer, forming foam with higher water content, and thus reducing the concentration factor. A schematic illustration was proposed to better understand the effect mechanism of metal cations on microalgal foam flotation. This study may benefit the process development for overcoming the inhibition of cations on microalgal foam flotation.

Keywords: Microalgae, Scenedesmus acuminatus, foam flotation, competitive adsorption, film drainage.