The effect of electrolytes on the aggregation kinetics of three different ZnO nanoparticles in water

Nanoscale ZnO particles are receiving increasing attention because they are widely used in commercial products, but they do have potentially hazardous effects. The aggregation behavior of ZnO nanoparticles (NPs) in the environment contributes to the real risk assessment of nano-toxicity, and the real size of the nano-aggregates should be investigated. In this study, the influences of electrolytes on the stabilities of three ZnO NPs were compared: the commercial powder (NP1), the lab synthesized suspension (NP2) and the commercial suspension (NP3). The initial particle size of NP2 and NP3 in water was at a nanoscale whilst NP1 tended to form microscale aggregates. The capping reagents helped to retain their suspension. The stability of ZnO NPs depends on their zeta potential under specific pH value, ionic types and ionic strength. In general, neutralization plays a major role in aggregation. The effect of divalent counter-ions on ZnO NP aggregation was more than that of monovalent ones. The stabilities of NP2 and NP3 were confirmed by the large critical coagulation concentration (CCC) values of these particles. The experimental results also fit the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation of different ZnO NPs is relevant to their basic properties and is influenced by electrolytes, which decreases the possibility of the penetration of NPs into cells to cause toxicity in the environment. An understanding of the basic properties of NPs is crucial for assessing their fate in the environment as well as for setting up usage regulation and treatment strategy.