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

In this study, the aggregation kinetics of two sizes of polystyrene nanoplastics: 28 nm (NP28) and 220 nm (NP220), were investigated using time-resolved dynamic light scattering. The stability of the plastics was evaluated in: (i) the presence and absence of three natural organic matters (NOM) - the Suwannee River humic acid (HA) and fulvic acid (FA), and alginate (AL), (ii) varying concentrations of divalent salt and in artificial seawater (ASW) and (iii) natural surface waters (NW) with varying salinities. In the absence of NOM, the critical coagulation concentration (CCC) in CaCl2 was independent of particle size. The addition of HA enhanced aggregation via bridging, regardless of the size of the plastics. In contrast, the interaction of the plastics with alginate was size -dependent where alginate appeared to enhance aggregation by encapsulation for NP28 and by gel bridging for NP220, while the FA had little or no effect. In ASW, alginate stabilized the particle suspensions. Generally, the effects of the three NOM were more pronounced for the NP220 compared to the NP28. While the bare NP220 was more stable in natural seawater than in ASW and CaCl2 (at the CCC), there were few significant differences in the attachment efficiencies of the bare NP28 in these water matrices. Finally, in CaCl2, the interaction of nanoplastic aggregates with a model silicon-based surface was less repulsive in the presence of AL or HA than FA. This study highlights the importance of considering the interplay of different particle sizes and complex water chemistries when assessing the fate of nanoplastics in the environment.