Polymer-Capped Nanoparticle Transport in Granular Media Filtration: Deviation from the Colloidal Filtration Model

The single-collector removal efficiency based on the colloidal filtration model is widely used to quantify deposition of nanoparticles in porous media filtration. The validity of this theory for nanoparticles, especially at filtration rates used in water treatment, was evaluated. Granular media filtration experiments were performed under widely variant physical conditions. Chemical effects were minimized by selecting spherical branched polyethylenimine capped silver nanoparticles as a positively charged nanoparticle to avoid electrostatic repulsion with the negatively charged silica filter media. The model and experimental results agreed well for 50 and 100 nm particles, but 10 nm particles were removed to a lesser extent than the model predicted. An updated Derjaguin-Landau-Verwey-Overbeek calculation was performed for the interaction energy between polymer-capped nanoparticles and the collector surface, under constant potential, constant charge, and mixed assumptions. The effect of particle size on these calculations was dramatic, leading to far less attractive energy for the smallest particles in the mixed case, and even repulsion in the constant charge case. These revised calculations are the primary means to explain the unexpected data