The dual role of alginate extracellular polymeric substances in cleaner flotation of marine microplastics: Modulating microplastic hydrophilicity and microbubble stability

The growing threat of marine microplastics (MPs) demands effective remediation strategies. This study proposed microbubble flotation (MBF) as a promising solution, utilizing the microbubbles with small size and high stability to enhance the capture efficiency of marine MPs. However, the application of MBF is constrained by the limited comprehension of the interfacial interactions among MPs, microbubbles, and marine cations or microbes. Traditional froth flotation for algae-incubated MPs resulted in removal efficiencies of 0.07% (2–2.5 mm) and 22.39% (48–72 μm), while that of MBF achieved 95.26% and 63.82%, respectively. This decrease in removal efficiencies after algae incubation was caused by the enhanced hydrophilicity of MPs. After 28 days of incubation with algae, the contact angle of MPs decreased from 97.88° to 21.62°, and C=O and O–C=O functional groups were formed while C–O group increased from 2.39% to 10.84%. Although algae-secreted extracellular polymeric substances (EPS) adhered to MPs surface and formed hydrophilic membranes, weakening the hydrophobic interaction between MPs and microbubbles, cations and soluble EPS promoted removal efficiency by reducing microbubble size and enhancing microbubble stability. The positive effects of cations on MPs flotation followed the order: Mg 2+ > Na + > K + > Ca 2+ . Moreover, molecular dynamics simulations further confirmed that fucose-containing sulfated polysaccharide molecules intertwined and sealed the gas–liquid interface via hydrogen bonds, thereby enhancing bubble stability and reducing the radius from 25 Å to 16 Å. These findings offer valuable insights into the environmental interfacial interactions during MPs flotation in marine environments, laying the groundwork for the application of MBF in marine MPs remediation.