Transport and abatement of fluorescent silica nanoparticle (SiO₂ NP) in granular filtration: effect of porous media and ionic strength

The extensive production and application of engineered silica nanoparticles (SiO₂ NPs) will inevitably lead to their release into the environment. Granular media filtration, a widely used process in water and wastewater treatment plants, has the potential for NP abatement. In this work, laboratory-scale column experiments were performed to study the transport and retention of SiO₂ NPs on three widely used porous materials, i.e., sand, anthracite, and granular activated carbon (GAC). Synthetic fluorescent core-shell SiO₂ NPs (83 nm) were used to facilitate NP detection. Sand showed very low capacity for SiO₂ filtration as this material had a surface with limited surface area and a high concentration of negative charge. Also, we found that the stability and transport of SiO₂ NP were strongly dependent on the ionic strength of the solution. Increasing ionic strength led to NP agglomeration and facilitated SiO₂ NP retention, while low ionic strength resulted in release of captured NPs from the sand bed. Compared to sand, anthracite and GAC showed higher affinity for SiO₂ NP capture. The superior capacity of GAC was primarily due to its porous structure and high surface area. A process model was developed to simulate NP capture in the packed bed columns and determine fundamental filtration parameters. This model provided an excellent fit to the experimental data. Taken together, the results obtained indicate that GAC is an interesting material for SiO₂ NP filtration.