Hollow and solid silica nanoparticles for environmental applications: characterization and transport in porous media

In recent years, SiO2 nanoparticles (NPs) have been extensively investigated for their potential in a wide range of environmental applications, including contaminant adsorption, environmental catalysis or as carriers for encapsulated pesticides. In this study, a combination of experimental methods, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS) and disc centrifuge was used to characterize two sets of hollow and solid silica NPs. NPs transport in sand-packed columns was then studied at various electrolyte concentrations to assess their mobility in typical geochemical conditions. Due to the similarities in their physical properties, solid SiO2 NPs were used as templates for mesoporous silica particles, that represent the real competitor to hollow spheres for applications as carriers or adsorbents. Classical theoretical foundations of colloids science, namely Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, single collector contact efficiency and macro-scale transport equations were used for the interpretation of experimental data. Analysis of breakthrough curves (BTC) revealed high mobility features for both sets of particles, that were consistently able to reach C/C0 values higher than 0.6. NPs retention within the porous medium increased with increasing ionic strength: in the same chemistry conditions, solid particles showed higher mobility. These results are in good accordance with single collector efficiency calculations and DLVO interaction energies. Modelling based on classical filtration theory’s (CFT) clean bed hypothesis was not able to approximate experimental BTCs for both hollow and solid particles. Reversible particle-solid phase interactions were found to be better suited to model experimental BTCs and estimated kinetic rates were consistent with data found in literature. Further confirmation regarding SiO2 particles’ high mobility was given by a percentage of mass recovery always exceeding 80%, even in the most favourable attachment conditions. In conclusion, the density difference was proven to be of minimal influence on macro-scale transport, and higher retention levels showed by hollow particles were imputable to surface properties. There is strong evidence that both sets of SiO2 NPs could behave as vectors for adsorbed contaminants but conclusive evaluations in this regard should be based on transport tests performed on loaded particles.