Effect of nanobubbles for improvement of water quality in freshwater: Flotation model simulation

Recently, nanobubbles (NBs) in flotation have been discussed as a highly promising technique for separation or remediation of water contaminants in the environmental field. This study performed model simulations using a flotation model (FM) with NBs and very highly hydrophobic small particles, such as sediments of freshwater containing inorganic particles (clay and silt) and plastic debris. This study examined and confirmed that the effect of NBs for remediation of particles depends on the size and number of bubbles, size and density of aggregates, time of aggregate formation, and so on. This study also confirmed the NBs' features, such as the adjustment of their surface charge in electrolyte solutions as well as their affinity for hydrophobic surfaces leading to their stability. The features affect the attachment efficiency, which is estimated by calculating the surface interaction forces of hydrodynamics and surface chemistry. The aggregates formed by the attached NBs obtained very high formation efficiency (X) due to their very high attachment efficiency but very low separation efficiency (Y) due to their very slow rising velocity. In conclusion, the NBs may deteriorate the water quality instead of improving it because the aggregates formed by the NBs stay in the suspension for a long stagnation time. Therefore, further research is required to discover a more reasonable method for applying NBs with MBs in the field of freshwater treatment that causes a minimal adverse effect on the ecosystem and helps in optimal removal efficiency of the hydrophobic fine particles.