Effects of Humic Acid and Solution Chemistry on the Retention and Transport of Cerium Dioxide Nanoparticles in Saturated Porous Media

When released into natural aquatic systems, cerium oxide (CeO2) nanoparticles (NPs) may have toxic effects to the ecosystems and public health; it is thus important to understand their environmental fate and transport. This work studied the effects of humic acid (HA) concentrations (0-10 mg L-1) and solution chemistry (ionic strength (IS) and pH) on the retention and transport of CeO2 NPs in water-saturated porous media under environmental relevant conditions. HA and IS showed remarkable effect on the retention and transport of CeO2 NPs in the porous media. Even at low concentrations (i.e., 5 and 10 mg L-1), HA stabilized CeO2 NPs in the suspensions by introducing both negative surface charge and steric repulsion and thus enhanced their mobility in the porous media. When solution HA concentration increased or ionic strength decreased, mobility of CeO2 NPs in the porous media enhanced dramatically. Solution pH, however, had little influence on the mobility of the CeO2 NPs under the tested experimental conditions, and increasing solution pH only slightly increased the transport of the NPs. Mathematical models were applied to describe the experimental data. Predictions from the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and advection-dispersion-reaction (ADR) model matched the experimental data well.