Quantitively Analyzing the Variation of Micrometer-Sized Microplastic during Water Treatment with the Flow Cytometry-Fluorescent Beads Method

Microplastic pollution has aroused surging concerns, while methods for quantitively analyzing the removal of microplastic in water treatment are insufficient. Herein, a flow cytometry (FCM)-fluorescent beads (FB) method was developed for evaluating the removal of micrometer-sized microplastic in water treatment. Commercial fluorescently labeled polystyrene beads (Φ = 3 μm) were spiked into water to mimic microplastic, and FCM was used for quantitive determination. The coefficient of determination (R2) of the amount of spiked microplastics and detected signals in pure water, surface water, and wastewater treatment plant effluent surpassed 0.99. For the removal of microplastics by different coagulants (50 mg/L) at pH 7.0, the removal ratio of microplastics by FeCl3 and polyaluminum chloride was significantly higher than that by polyferric sulfate, alum, Al2(SO4)3, and FeSO4. Over 85% of the polystyrene beads (1.5 × 105 pieces/L) were removed by 50 mg/L of FeCl3 or polyaluminum chloride, and the removal effect was correlated with the size of the floc formed in the hydrolyzation of the coagulants. Filtration was less effective for the removal of microplastics, and the removal efficiency was below 66% when using six kinds of conventional filter materials. Ferrate oxidation was more effective for microplastic removal, as over 93.7% of polystyrene beads (1.5 × 105 pieces/L) were removed by 5 mg/L of ferrate (as Fe) at pH 7.0. The FCM-FB method can be a paradigm for studying the removal of micrometer-sized microplastics in water treatment procedures.