Nanobubble-enabled foam fractionation to remove freshwater microalgae and microcystin

Climate change and water pollution intensify algal blooms in natural lakes and reservoirs, leading to significant water quality challenges. Effectively removing excessive microalgae and algal toxins or micropollutants is essential for improving water quality, preserving aesthetic value, and protecting human health. This study evaluated the use of nanobubble-enabled foam fractionation for the removal of Microcystis aeruginosa and three microcystin congeners (MC-LR, MC-RR, and MC-YR). Foam was generated using air nanobubbles combined with surfactants-cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and chitosan-to enhance foamability and stability. Among the tested surfactants, CTAB exhibited the highest foamability, yielding improved removal performance. When CTAB concentration increased from 40 to 80 mg·L^-1, algal removal efficiency slightly improved from 60.0 ± 5.2 % to 69.0 ± 2.6 %, while the concentration factor declined from 4.45 to 3.37 due to enhanced frothing. Increasing salinity from 10 mM to 100 mM significantly reduced the concentration factor from 3.9 ± 0.1 to 0.9 ± 0.5, indicating that elevated ionic strength impairs foam-based separation of algal biomass. Removal of microcystins was influenced by both the presence of microalgae and surfactant conditions. MC-RR showed higher removal in the absence of microalgae (43.2 ± 1.5 % to 56.8 ± 2.0 %) compared to that (27.0 ± 5.6 % to 28.3 ± 3.3 %) when algae was present, suggesting competition during foam separation. Among the three congeners, the most hydrophobic MC-LR exhibited the highest foam affinity, with a removal efficiency of 77.4 ± 9.2 %. Test results from real lake water suggest that the complex water matrixes (e.g., salinity and dissolved organic matters) slightly affected foaming ability and reduced removal rates of target algal pollutants, especially at lower surfactant doses. These findings provide new insights into integrating nanobubble-enhanced foam fractionation into water treatment systems for mitigating harmful algal blooms (HABs) and microcystins.