The effect of water chemistry on homoaggregations of various nanoparticles: Specific role of Cl− ions

Abstract

Colloidal behavior of various nanoparticles (NPs) could be quite different under the influence of complicated water chemistry. Homoaggregations of four types of nanoparticles (NPs) were investigated with a focus on the effects of electrolyte ions, pH, and natural organic matter (NOM). Results show that critical coagulation concentrations (CCCs) of NaCl and CaCl2 for anatase TiO2, rutile TiO2, or CeO2 NPs were all inversely proportional to the valence of cations, following the Schulze–Hardy Rule, and homoaggregations of the three NPs were similarly inhibited by the presence of NOM within pH 3–9. Although the homoaggregation of Ag NPs was also increased as the concentration or valence of cations increased, the relationship between CCC and valence of the cations was far away from the Schulze–Hardy Rule; moreover, NOM only slightly increased CCC of CaCl2 and surprisingly decreased CCC of NaCl for Ag NPs. Excessively-adsorbed Cl− on the formed Ag–AgCl colloidal nucleus was detected, which could increase the electronegativity of Ag NPs and thus limit the aggregation effect of chlorides and the dispersion effect of NOM as well. These results are expected to increase our knowledge on the colloidal behavior and fate of NPs in aquatic environments.