Mechanistic understanding of the aggregation kinetics of nanoplastics in marine environments: Comparing synthetic and natural water matrices

•The critical coagulation concentration of bare nanoplastics in CaCl2 does not depend on the size of the plastics.•NOM has pronounced effects on the stability of larger nanoplastics.•Nanoplastics behave differently in synthetic seawater compared to a natural seawater matrix.•Interaction of nanoplastic aggregates with model silicon-based surfaces are more favorable in alginate and humic acid.