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Nanosized activated carbons (NACs) are emerging as a new class of engineered nanomaterials, but their colloidal stability governing the fate and transport in aquatic environments has not yet been evaluated. We have characterized four representative NAC materials and examined their colloidal stability and early-stage aggregation kinetics under various water chemistry conditions. The results showed that these NAC particles had intensity-weighted hydrodynamic diameters (D_h) and number-weighted averaged diameters (D_n) of approximately 200 and 100 nm, respectively, and that their aggregation kinetics exhibited both reaction- and diffusion-limited regimes in the presence of monovalent (NaCl) or divalent (CaCl₂) salt with distinct critical coagulation concentrations (CCC), indicating that their colloidal stability under the tested aqueous conditions was consistent with the classic Derjaguin-Landau-Verwey …