Here, we assessed the heteroaggregation of polystyrene (PS) and poly(vinyl chloride) (PVC) nanoplastics with SiO2 as a model of natural colloids. Homoaggregation and heteroaggregation were evaluated as a function of CaCl2 (0-100 mM) and natural organic matter (NOM) (50 mg L-1) at a designated concentration of nanoplastics (200 μg L-1). Critical coagulation concentrations (CCC) of nanoplastics were determined in homoaggregation and heteroaggregation experiments with SiO2 and CaCl2. The attachment efficiency (α) was calculated by quantifying the number of nanoplastics in the presence of CaCl2, NOM, and SiO2 using single-particle inductively coupled plasma mass spectrometry (spICP-MS) and pseudo-first-order kinetics. The calculated α was fed into the SimpleBox4Plastics model to predict the fate of nanoplastics across air, water, soil, and sediment compartments. Nanoplastics exhibited high stability against homoaggregation, while significant heteroaggregation with SiO2 occurred at CaCl2 concentrations above 100 mM. The influence of NOM was also evaluated, showing a reduction in heteroaggregation with SiO2 for both nanoplastic types. Sensitivity analysis indicated that the degradation half-life of the tested nanoplastics had a more significant impact on persistence than did α. The results emphasize the environmental stability of nanoplastics, particularly in freshwater and soil compartments, and the critical role of NOM and emission pathways in determining their fate.

Center for the Environmental Implications of NanoTechnology Duke University Durham North Carolina 27708 United States

Department of Chemical Sciences University of Padua Via Francesco Marzolo 1 35122 Padua Italy

Department of Civil and Environmental Engineering Duke University Durham North Carolina 27708 United States

Department of Environmental and Biological Sciences University of Eastern Finland 80101 Joensuu Finland

Department of Environmental and Biological Sciences University of Eastern Finland Kuopio 70211 Finland

Department of Plankton and Microbial Ecology Leibniz Institute for Freshwater Ecology and Inland Fisheries Stechlin 16775 Berlin Germany

Institute for Nanomaterials Advanced Technologies and Innovation Technical University of Liberec Bendlova 1409 7 460 01 Liberec Czech Republic

Institute of Biochemistry and Biology Potsdam University 14469 Potsdam Germany

National Institute for Public Health and Environment Centre for Sustainability Health and Environment Antonie van Leeuwenhoeklaan 9 3721 MA Bilthoven The Netherlands

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