Aggregation and dissolution of aluminium oxide and copper oxide nanoparticles in natural aqueous matrixes

Abstract

Aggregation and dissolution kinetics of aluminium oxide nanoparticles (nAl2O3) and copper oxide nanoparticles (nCuO) in deionised water (DIW) and freshwater sourced from two river systems were studied with the objective to understand the influencing factors. Dynamic light scattering and inductively coupled plasma mass spectrometer were used to study aggregation and dissolution, respectively. In DIW, humic acid was observed to have a concentration dependent stabilization effect on ENPs. Increasing the ionic strength destabilised the ENPs. The pH influenced aggregation with maximum aggregation observed at the isoelectric point. ENPs were stable in freshwater systems with HDD < 350 nm at 100 µg/L. Aggregation of both ENPs was concentration dependent. The ENPs exhibited higher stability in freshwater with low, rather than high, concentrations of both natural organic matter (NOM) and electrolytes. Dissolution was higher in Elands river than in Bloubank river water. ENPs had a high tendency for dissolution at low concentrations. NOM impeded dissolution of ENPs by providing a protective coating via steric and electrostatic interaction. Released ions may have formed precipitates and chelate compounds with ligands present in freshwater especially for nCuO where low dissolution was apparent. These findings provide insights on aggregation and dissolution of ENPs in freshwater systems as influenced by source-specific water chemistry. Therefore, it is not possible to make generalized statement on the outcome of ENPs transformation in aquatic systems.