The increasing amount of plastic debris in the environment and its disintegration into submicrometric particles is a cause for concern. Due to the colloidal nature of nanoplastics, their environmental fate should be investigated separately from that of microplastics. Abiotic factors greatly influence nanoplastics' stability. This will affect its residence time in the hydrosphere. So, we investigated the behavior of two different nanoplastic models (with different sizes and shapes) regarding ionic strength, pH, and varying concentrations of two natural organic matters: humic acid and sodium alginate. The results demonstrate that both natural organic matters enhanced the aqueous stability of nanoplastics over time at high ionic strengths. Depending on the organic matter's nature, different stabilizing mechanisms were revealed using dynamic light scattering and asymmetrical flow field flow fractionation coupled to static light scattering. Humic acid provides electrostatic repulsion between particles, and some larger humic acid molecules provide a steric hindrance. Sodium alginate sorbs onto and bridges separate particles and small aggregates of nanoplastics. The covered particles are stabilized by steric hindrance. The results highlight the importance of considering natural organic matters' properties when assessing nanoplastics behavior in the environment.

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