| The aggregation-dispersion and floating-settling behaviors of microplastics in environmental waters are key factors influencing their migration and distribution in different media. In this study, the effects of three typical water constituents—Na+, Ca2+, and humic acid (HA)—on the dispersion stability and floating-settling efficiency of particles were investigated, using polymethyl methacrylate nanoparticles (PMMA NPs) as the model. Additionally, the aggregation and settling behavior of particles in seven types of water samples, including river, lake, seawater, and domestic sewage, were analyzed. The results indicated that the two metal ions reduced the surface ζ-potential of the NPs to below -15 mV through electrostatic adsorption, which increased the aggregation efficiency of the NPs. At the same time, the relative water body density of the NPs was increased, promoting particle settling. The inconsistency of threshold concentrations of metal ions between the aggregation and settling behaviors led to various migration patterns of particles, such as “settle but not aggregate” “settle and aggregate” and “aggregate but not settle”. HA was found to enhance the dispersion stability and suspension efficiency of NPs, particularly due to the complex interactions involving amino acids-like, proteins-like, and other organic macromolecules in HA, which compete with metal ions for adsorption and complexation on the NP surface. |