The removal efficiency and mechanism of microplastic enhancement by positive modification dissolved air flotation

Microplastics (MPs) are widely distributed in freshwater and have the following characteristics: small size, strong hydrophobicity, difficult degradation, and easy adsorption of toxic substances. These characteristics pose a potential threat to the environment and human health. In this paper, three common MPs in freshwater were removed by conventional dissolved air flotation (DAF) and positive modification DAF (Posi-DAF). The results showed that the optimal removal efficiency of MPs by conventional DAF was 32%-38% at 0.4-0.5 MPa. This indicated that the adhesion between microbubbles (MBs) and MPs was not ideal when the hydrophilic/hydrophobic interaction was the dominant mechanism. However, the removal efficiency of MPs was increased by 13.6%-33.7% compared with that of conventional DAF after two surface modifiers were used on MBs. This result indicated that in addition to the hydrophilic/hydrophobic interaction, both charge attraction and sweeping had positive effects. PRACTITIONER POINTS: The removal efficiency and mechanism of MPs by posi-DAF were investigated. The characteristics, particle number, and adhesion of microplastics were measured. Conventional DAF was dominated by hydrophilic/hydrophobic interactions. CTAB Posi-DAF was dominated by charge attraction. PDADMAC Posi-DAF was dominated by charge attraction and sweeping.