Microbubble-microplastic interactions in batch air flotation

Presence of microplastics in waste streams and the environment is a challenge of much recent concern. Bubbles are used for solid–liquid separation in froth flotation and dissolved air flotation (DAF). Bubble-particle interactions are key for understanding flotation removal efficiency of particulates. Limited studies provide in-situ characterization of particle size distribution, shape and concentration before and after flotation. The use of microbubbles to specifically remove microplastics has not been extensively investigated. This study presents a batch flotation method to assess the removal of spherical polyethylene (PE) and non-spherical PE, polypropylene (PP), polyvinyl chloride (PVC), and polymethyl methacrylate (PMMA) microplastic of different densities at 10–600 µm size range with 50–110 µm sized microbubbles. In-situ image analysis allowed measuring particle shape, size and concentration in solution prior to and after flotation as well as capturing particle-bubble interactions at the micro-scale. Besides determining flotation performance for different microplastic sizes, shapes and types, the effect of surfactant concentration and ionic strength was investigated and discussed in relation to particle collection efficiency. This study provides important quantitative results on bubble-particle interaction that allow selective removal of microplastic from solution and presents a straightforward direct in-situ visualization method for tracking and characterizing micrometer-sized particles in solid–liquid-gas multiphase media.