Fate of nanoparticles in aqueous environments: Stability, physicochemical interactions, and separation

The persistence and stability of nanoparticles (NPs) in aqueous environments can lead to their long-term accumulation, increased mobility, and potential adverse effects on ecosystems and human health. This review emphasizes the importance of evaluating bare (non-coated) and surface-coated NPs in diverse aqueous conditions, addressing the limitations of earlier studies that primarily focused on idealized systems or individual NPs types. Special attention has been paid to the stability of metals, metal oxides, and semiconductor NPs in synthetic and real aqueous environments. The stability of NPs is highly dependent on the environment and is controlled by parameters such as pH, ionic strength, natural organic matter (NOM), and biomolecules. However, under certain conditions, these factors can lead to the formation of aggregates several micrometers in size. NOM and biomolecules may stabilize NPs through steric and electrostatic interactions. In contrast, weak or incomplete NOM adsorption can enhance aggregation through charge neutralization, ligand exchange, or bridge-like flocculation. Surface coatings, including polymers, citrate, fatty acids, dextrin, alginate, and quaternary ammonium compounds, further influence aggregation and dissolution, leading to dynamic and condition-dependent behaviors. NPs can be removed from aqueous environments by exploiting their reduced colloidal stability, using coagulation/flocculation, advanced oxidation process (AOP), or magnetic separation. Coagulation/flocculation is widely used and achieves removal rates of over 80 %, although reduced efficiency in saline or organic-rich waters and sludge removal pose challenges. AOP, particularly ozonation, destabilizes NPs by altering surface chemistry and accelerating aggregation, but is energy-intensive and may produce harmful byproducts. Magnetic separation provides rapid and selective removal (>99 %) of magnetic or functionalized NPs, but is limited to specific materials and requires specialized equipment. Despite progress, key knowledge gaps persist in NPs behavior within complex environments, requiring further research to optimize and scale sustainable, efficient removal methods.