Effects of water chemistry on the aggregation, disaggregation, and regrowth behaviors of ZnO nanoparticles in aqueous environments

The growing application of engineered nanoparticles has heightened concerns regarding their environmental release; gaining a robust understanding of their transport mechanisms, environmental fate, and behavioral dynamics in natural aquatic systems is therefore of particular importance. This study systematically investigated the aggregation and disaggregation behavior of ZnO nanoparticles in the presence of humic acid (HA) and/or Ca 2+ ions across a range of pH values. Additionally, controlled shear conditions were applied to assess the influence of hydrodynamic shear forces on ZnO nanoparticle stability. The addition of HA to the suspensions resulted in a significant decrease in zeta potential and an increase in the hydrodynamic diameter of ZnO nanoparticles, implying the occurrence of charge neutralization between nanoparticles. High shear forces were sufficient to overcome interparticle attractive forces, thereby disaggregating large particle clusters into smaller entities; in contrast, low shear forces promoted nanoparticle aggregation by enhancing collision frequency and efficiency. Higher strength factors (SF) values observed in reaction systems with elevated Ca 2+ ion concentrations indicated that aggregates formed under such conditions exhibited more compact structures and greater resistance to shear-induced fragmentation. Fluorescence spectra analysis indicated that the compact structure of ZnO nanoparticles formed under high Ca 2+ ion concentrations was most likely due to the strong chemical binding with HA and bridging mechanisms involving divalent cations formed during the aggregation process. The findings of this study provide a mechanistic basis for predicting the environmental fate and transport behavior of ZnO nanoparticles in natural aquatic environments.