The effect of cations on the aggregation of commercial ZnO nanoparticle suspension

Nanoscale ZnO materials have been largely used in many products due to their distinct properties. However, ZnO nanoparticles (NPs) are hazardous to human health and the ecosystem. The characteristics and the stability of ZnO NPs are relevant to their fate in the environment and their potential toxicities. In this study, a stable commercial ZnO NP suspension was chosen to investigate its aggregation under various salt additions. Different concentrations of NaCl, KCl and CaCl2 were chosen to represent various environmental conditions. Under pH 8-9, the surface charge of commercial ZnO NPs was negative. The behavior of the stabilized ZnO NPs in water was affected by ionic combinations and ionic strength; that is, divalent cations were more effective than monovalent ones in promoting aggregation formation. The attachment efficiencies of ZnO aggregates were calculated based upon the aggregation kinetics. The critical coagulation concentration values for this commercial ZnO NPs were higher than previous reported for ZnO NPs, indicating this ZnO NP could be stable in the aquatic environment and might have increased hazardous potentials. Based upon the Derjaguin-Landau-Verwey-Overbeek theory, interactions between ZnO NPs in the presence of different ions were evaluated to illustrate the aggregation mechanism. Our results indicated that critical ionic type and concentration promote the aggregation of stable ZnO NPs. These understandings also can facilitate the design of the precipitation treatment to remove NPs from water.