A novel high-value utilization method of re-frying oil in microalgae buoy bead flotation and mechanism analysis

Abstract

To reduce the cost and energy consumption during the harvesting of microalgae, and break the barrier of commercialization of microalgal biodiesel, a novel method for the flotation of microalgae that was based on re-frying oil emulsion (RFOE) stabilized by Sorbitan oleate (Span 80, CAS:1338-43-8) was proposed. Commercially used C. vulgaris and S. obliquus were evaluated using RFOE and multi-objective optimization was used to determine the optimal harvesting efficiency and enrichment ratio. The optimal harvesting efficiencies of C. vulgaris and S. obliquus were 90 % and 97 %, with the enrichment ratios of 2.07 % and 1.43 %, respectively. Based on the XDLVO (extended Derjaguin-Laudau-Verwey-Overbeek) theory, the interaction mechanism between the RFOE and the microalgae was expounded. It may be due to the presence of aluminum sulfate, which acts as an intermediate medium to bind microalgae to emulsion and uses the low density of the emulsion to induce the flotation of algae to the upper layer of liquid surface. Life cycle assessments (LCA) was conducted for the whole harvesting process and the results showed that the economic cost of harvesting microalgae by RFOE was about $1.37/m3, whereas the carbon emissions were about 0.066 kg CO2/m3, which conforms to the carbon reduction policy and has the potential for commercial applications.