Insight into the effect of aqueous species on microplastics removal by froth flotation: Kinetics and mechanism

Froth flotation is expected to be a potential method to remove microplastics (MPs) from the natural environment, while hydrophilization induced by aqueous environments may affect the process of flotation removal. This study applied a novel flotation technology to remove polyvinyl chloride (PVC) and acrylonitrile butadiene styrene (ABS) MPs from water. The effects of MPs properties, flotation conditions, and aqueous species on flotation removal were investigated, and the first-order model was used to fit the flotation kinetics. Flotation removed over 98.5% of MPs under sufficient terpineol dosage and airflow supply, and it was unaffected by pH, temperature, MPs size, and MPs concentration. Instead, common aqueous species, including Na+, K+, Ca2+, Mg2+, Al3+, and humic acid, inhibited the flotation of ABS MPs, and only Al3+ affected the removal of PVC MPs. Therefore, 97% of PVC MPs floated in froth flotation, while only 58.7% of ABS MPs were removed from seawater. The adsorption isotherm confirmed the monolayer adsorption of Na+, K+, Ca2+, Mg2+, and humic acid on MPs surfaces, while the adsorption of Al3+ was heterogeneous. ABS MPs showed a greater affinity for these substances due to functional groups, lower surface potential, and higher surface area. These substances might accumulate in a water film through electrostatic or non-covalent interactions on MPs surfaces, resulting in the hydrophilization. Froth flotation has significant application potential due to its affordable costs and handy operation. This work may be of great significance in removing MPs from the aquatic environment.