Extracellular polymeric substances trigger microalgae flocculation in a combined and green flocculation-flotation...

Microalgae are considered a promising biomass feedstock, but a key challenge hindering their industrial adoption is the lack of cost-effective and efficient harvesting technologies. In current study, a combined flocculation-flotation harvesting method was developed using the bio-flocculant cationic guar gum and the fatty acid-based collector sodium hydroxysulfonate stearate (OA-1). The Plackett‑Burman experimental design identified flocculant dosage, collector dosage, and aeration rate as the key factors influencing the coupled harvesting process. After parameter optimization, a high harvesting efficiency of 91.2% and a biomass enrichment factor of 27.8 were achieved. Compared with single flocculation, the synergistic flocculation‑flotation process significantly enhanced microalgae harvesting through the supportive and stabilizing effect of the OA‑1 foam layer on the flocs. This integrated approach increased the median floc size by 209.2% (7.19 μm to 22.23 μm), improved compactness by 5.7% (1.66 to 1.76), and modified the surface hydrophobicity of the recovered biomass, thereby reducing the moisture content of the flocs by 9.6% (59.3% to 53.6%). The analysis of extracellular polymeric substance physicochemical properties in microalgae confirms its critical role during biomass harvesting. Integrated proteomic profiling further identifies key functional proteins and their active sites involved in the flocculation process. The EDLVO theory overcomed the limitations of classical electrostatic models and successfully elucidated the interaction between OA‑1 (negative charge) and microalgal flocs (negative charge). OA‑1 lowered the energy barrier between microalgal flocs by 73.1% (1981 kT to 533 kT), thereby promoting further aggregation. Furthermore, it quantitatively described the energy-distance profiles for algal cell-cell and floc-collector systems. These findings provide a combined flocculation-flotation process for efficient microalgae harvesting and deeper insights into the synergistic effect of flocculant-collector on microalgae.