Role of water temperature in the fate and transport of zinc oxide nanoparticles in aquatic environment

Abstract

The influence of water temperature on the aggregation and dissolution kinetics of zinc oxide nanoparticles (ZnO NPs) (mean diameter ∼40 nm) was investigated. Samples of 100 mg.L-1 ZnO NPs were incubated at 15, 25 and 35 °C, similar to the surface temperature of cold freshwater, temperate estuarine, and tropical/sub-tropical coastal marine ecosystems, respectively. The natural organic matter (NOM) content, pH, electrolyte type, and ionic strength (IS), were adjusted on the basis of the water chemistries of typical aqueous systems. Specifically, the time-dependent hydrodynamic diameters (HDDs) and sedimentation plots were obtained over the first 3 h and after 24 h using time-resolved dynamic light scattering (TR-DLS) and UV-visible spectroscopy, respectively. The settling distance was further modeled for the aggregates with various HDDs according to the Stokes' sedimentation equation. The dissolution kinetics was studied over the first 12 h and after 48 h in term of percentage of released zinc ions. The results showed that the HDD increased at elevated temperatures, termed as temperature-induced aggregation, while dissolution was reduced. The aggregation at higher temperatures further hindered the dissolution due to the decrease in the surface area of the NPs. We express this process as «aggregate-suppressed dissolution». The maximum aggregation was reached in the tropical coastal marine environment with the HDD >3 μm, and the released zinc ion of 9.2% was obtained in the cold freshwater. Based on the results, the aggregation rate of 1.57 nm.s-1 was estimated for the former, and the dissolution rate of 7.44 × 10-5 mol.L -1.h-1 was calculated for the latter, respectively. The predicted values successfully fitted to genuine water samples (