Enhanced Microplastic Flotation: Unraveling the Role of Bubble-Chain Hydrodynamics via PIV Analysis

Understanding the transport behavior of microplastic particles in flotation systems is critical for enhancing their removal efficiency in wastewater treatment processes. Here, we investigated the particle-lifting effect of a bubble-chain system on polystyrene (PS) particles under continuous rising flow conditions. A custom-built chamber equipped with syringe-driven needle injectors generated successive air bubbles with mean equivalent diameters of 3.21 and 2.81 mm. As the number of bubbles (N) increased from 1 to 10, the nondimensionalized vertical displacement (L_z) of PS particles increased from 5.29 to 7.66 and 4.53 to 6.07 for 3.21 and 2.81-mm bubbles, respectively. Correspondingly, the nondimensionalized total displacement (L_total) increased from 5.58 to 8.31 and 4.78 to 6.52, showing maximum increases of 44.8% and 48.7%, respectively. As the number of bubbles increased, the axial motion of the particles was significantly enhanced, which was attributed to the vertical expansion of the flow field, the increase in vorticity, and the formation of asymmetric flow structures as identified through particle image velocimetry (PIV) analysis. The formation of overlapping vortices within bubble-chain systems substantially broadens the effective hydrodynamic field, facilitating improved flotation of microplastic particles. This extended flow structure provides a quantifiable framework for enhancing the efficiency of dissolved air flotation (DAF) technologies, particularly in applications targeting microplastic separation.