Understanding microplastic flotation through microbubble-microplastic interactions

Abstract

Microplastic pollution significantly impacts aquatic environment, necessitating effective remediation strategies. Understanding microplastic-microbubble attachment mechanisms is crucial for developing efficient flotation-based removal methods. This study investigates the bubble attachment behaviour of two common microplastics; polyethylene (PE) and polystyrene (PS), across sodium chloride (NaCl) concentrations from 1 mM to 1000 mM. Probability of attachment, Gibbs free energy and adhesion force were analysed to understand their interactions. Results indicate that PE exhibits a higher attachment tendency than PS, particularly at low NaCl concentrations, as evidenced by contact angle measurements and Gibbs free energy. This suggests stronger hydrophobic interactions and surface property influences at low NaCl concentrations. However, the Gibbs free energy difference between PE and PS decreases with increasing NaCl concentration. Adhesion force analysis further reveals that PE consistently shows stronger adhesion than PS, with instant attachment observed at 50 mM and above. At NaCl concentrations above 500 mM, both microplastics display similar Gibbs free energy (∼ -58 mJ/m²), highlighting the dominance of ionic strength in bubble-particle attachment. Laboratory-scale flotation tests confirm improved recovery rates, increasing from 86.7 % to 91.5 % for PE, and from 83.8 % to 92.1 % for PS. These findings underscore the role of high NaCl concentrations in enhancing flotation-based microplastic recovery.