Towards a better understanding of CeO2 manufactured nanoparticles adsorption onto sand grains used in drinking water treatment plants

Increasing production of CeO2 nanoparticles (NPs) will inevitably lead to their higher emissions into natural water bodies. As these emissions pose a significant treat in terms of environmental impacts and possible human health risks, it is necessary to carefully evaluate processes responsible for their mobility, retention and bioavailability. Adsorption is considered as one of the most important processes controlling the fate and transport of nanomaterials in the environment and industrial filtration units. However, mechanisms and physical or chemical parameters influencing these processes are still poorly understood, especially in the case of porous materials used in water purification. In this study, mechanisms responsible for deposition and attachment of CeO2 nanoparticles (NPs) onto quartz sand used as a filter medium in the drinking water treatment plant of Geneva (Switzerland) are examined. For that purpose, a combination of theoretical kinetics and thermodynamic models and a variety of complementary experimental techniques are used. Batch experiments were performed using ultrapure water (pH of 3.0; reference case) and filtered raw Geneva Lake water (pH of 8.6). Size and surface charge of CeO2 NPs dispersed in the supernatant after mixing with sand were determined using dynamic light scattering and laser Doppler velocimetry whereas residual CeO2 NPs concentrations were measured by UV–vis spectroscopy. CeO2 NPs were found to adsorb onto sand grains surfaces under acidic conditions due to electrostatic attractive interactions. The kinetics of adsorption followed well both pseudo-first-order and pseudo-second-order model. The adsorption process was successfully described by Langmuir isotherm which indicated the formation of a monolayer, with a maximal adsorption capacity of 0.85 ± 0.20 mg g-1 at grain surfaces. Further scanning electron microscopy (SEM) images showed individual CeO2 NPs or small aggregates attached to the sand surface, thus confirming the adsorption mechanisms and corresponding model. The results obtained from batch studies conducted using Geneva Lake water showed that aggregation, caused by the presence of natural organic matter (NOM) and multivalent ions, and further aggregate sedimentation are the main processes responsible for CeO2 NPs elimination under environmental conditions. Moreover, as CeO2 NPs and sand grains both carried negative charges, electrostatic repulsion was a main factor limiting the adsorption. The SEM images confirmed these observations and revealed only few aggregates in the vicinity of sand surface irregularities.