Dissolution behavior of ZnO nanoparticles at environmentally relevant low concentrations in surface waters: Equilibrium and kinetics.

Environmentally relevant fate parameters are essential in accurate prediction of nanomaterial's exposure. This study investigates the dissolution kinetics and equilibrium of ZnO nanoparticles (ZnONPs) using environmentally relevant low concentrations (50-200 μg/L) of ZnONPs in river water and lake water samples, and a seawater-influenced river water. We found that ZnONPs at an initial concentration of 50 μg/L completely dissolved independent of water matrices, while at 100 and 200 μg/L the dissolution level of ZnONPs was strongly dependent on the water chemistry. Carbonate alkalinity was found to control the dissolution levels, and can react with dissolved Zn ion to form secondary solid product hydrozincite. An analysis of our kinetic data and comprehensive literature results reveals that the dissolution kinetic coefficients largely increased with decreased initial ZnONP concentrations especially in environmental water matrices. The result highlights the importance to measure and derive representative dissolution parameters of nanomaterials using environmentally relevant concentrations.