Evaluating the Role of Calcium Ions in the Removal of Nanoplastics ( 607 nm) by Flocculation‐Enhanced Sedimentation using Alginate

The removal of nanoplastics (NPs) from aquatic environments remains a significant challenge due to their persistence and potential ecological risks. Flocculation, together with coagulation and sedimentation, is widely used as a phase-separation technique in industrial water treatment. In this study, alginate (ALG)-enhanced sedimentation of polystyrene (PS) NPs under varying CaCl₂ concentrations was investigated via turbidimetric analysis. The destabilization mechanism was assessed through floc morphology, zeta potential measurements, attenuated total reflectance-Fourier transform infrared spectra, and scanning electron microscope/energy-dispersive X-ray spectroscopy analysis. The rheological properties of ALG solutions in the presence of CaCl₂ were expressed as complex viscosity. To better simulate environmentally relevant conditions, we employed a novel PS-NP dispersion without commercial stabilizers. The results show that ALG effectively destabilizes the system at moderate and high coagulant ionic strengths, with an optimal dosage of 10 ppm ALG. Calcium ions can interact with ALG chains through the formation of intermolecular complexes. At the highest CaCl₂ concentration, changes in the system's rheological properties altered floc morphology and delayed sedimentation. This study highlights the potential of natural bioflocculants such as ALG for removing PS-NPs from calcium-rich waters and reducing reliance on synthetic coagulants.