Aggregation kinetics and surface charge of CuO nanoparticles: the influence of pH, ionic strength and humic acids

In this study, the role of pH, ionic strength and humic acids (HAs) on the aggregation kinetics and surface charge of commercial copper oxide (CuO) nanoparticles were examined. Results show that the aggregation of CuO nanoparticles is favoured near pH 10, which was determined as the isoelectric point where the hydrodynamic diameter of the aggregates is the greatest. The aggregation of CuO nanoparticles is also ionic strength dependent. The increase in the ionic strength reduces the zeta potential, which leads to an increase in aggregation until 0.15 M. After this point an increase in ionic strength has no influence on aggregation. In the presence of HA for concentrations below 4 mg C L-1, aggregation was enhanced for acidic to neutral pH, whereas for higher concentrations, at all pH tested, aggregation does not change. The influence of HA on CuO nanoparticles is due to steric and electrostatic interactions. The sedimentation rates of CuO nanoparticles showed a relation between particle diameter and zeta potentials values confirmed by Derjaguin-Landau-Verwey-Overbeek calculations. The results obtained have important implications for predicting the stability and fate of CuO nanoparticles in natural water.