Interactions of CeO nanoparticles with natural colloids and electrolytes impact their aggregation kinetics and colloidal stability.

Abstract

The aggregation kinetics and colloidal stability of CeO2-NPs in the presence of monovalent or divalent electrolytes, as well as inorganic (kaolin and goethite) and organic (humic acid, HA) colloids were evaluated using time-resolved dynamic light scattering, advanced spectroscopic tools, and theoretical calculations. Critical coagulation concentrations for CeO2-NPs were generally lower in CaCl2 than that in NaCl electrolyte. The negatively charged kaolin accelerated CeO2-NPs aggregation due to electrostatic attraction, whilst opposite phenomenon was observed for the positively charged goethite in NaCl solution. In CaCl2 solution, goethite destabilized CeO2-NPs because of its well-crystal structure and specific adsorption of Ca2+. The presence of 0.1 mg C/L HA decreased the surface charge of CeO2-NPs, resulting in lower critical coagulation concentrations. Increasing the HA concentration from 0.1 to 1 mg C/L improved CeO2-NPs stability, mainly via electrostatic and steric repulsion. The Ca2+-bridging and complexation contributed significantly to CeO2-NPs aggregation. Additional aggregation experiments in seven natural waters revealed that CeO2-NPs remained stable in water types with high contents of organic colloids and low levels of salts, thus having higher transport potential. These findings provided new insights into the interactive influence of naturally occurring colloids and ions on the heteroaggregation behavior and fate of CeO2-NPs.