Evaluating the Efficiency of Enhanced Coagulation for Nanoplastics Removal Using Flow Cytometry

Efficient removal and accurate quantification of nanoplastics in conventional water treatment systems remain closely interconnected challenges. Optimizing removal processes requires robust detection techniques, and the lack of reliable quantification methods hinders process development and validation. In this study, we investigated enhanced coagulation-flocculation techniques for removing fluorescent PS-OSO₃ ^- nanoplastics of different sizes and concentrations from water. Removal efficiency was assessed using flow cytometry (FCM) and compared to a turbidity-based assessment. Coagulation-flocculation was achieved with Fe³⁺ concentrations ranging from 2 to 30 mg/L and varying slow mixing speeds of 100, 50, and 25 rpm. The results demonstrate that FCM quantifies nanoplastics more reliably and accurately than turbidity measurements at lower nanoplastic concentrations. Enhanced coagulation was achieved at a slow mixing speed of 25 rpm (<i>G</i> = 14 s^-1). Among the factors studied, particle size emerged as the most significant factor influencing the coagulation-flocculation performance. Additionally, sweep coagulation was predominant at low nanoplastic concentrations, while a combination of sweep coagulation and charge neutralization was observed at higher concentrations. These findings provide critical insights into developing effective nanoplastic removal strategies through interconnected advancements in the detection and treatment optimization of conventional water treatment systems.