Aggregation kinetics of nanosized activated carbons in aquatic environments

- Nanosized activated carbons (NACs) are manufactured particles with sizes <100 nm.
- NACs obtain negative surface charge in water by deprotonation of functional groups.
- The aggregation behavior of NACs was consistent with prediction by the DLVO theory.
- The Hamaker constants derived for AC materials are 2.1-2.7 × 10−20 J in water.
- NACs may disperse in freshwater but can aggregate in highly saline/acidic water.

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 (Dh) and number-weighted averaged diameters (Dn) 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-Overbeek (DLVO) theory. The fitting of the aggregation kinetics with predictions based on DLVO theory yielded the Hamaker constant of 2.1-2.7 × 10−20 J (ACWC, aqueous medium). The study demonstrated that NACs may be relatively stable under typical freshwater chemistry conditions, suggesting their potential applications as reactive agents for remedy of contaminated water and soil systems where long-time suspension of introduced particles is desired. The observed strong colloidal stability may also indicate high possibility of NACs being nanosized pollutants in natural and engineered environmental systems.

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