Crossflow membrane filtration of interacting nanoparticle suspensions

A model describing the structure, resistance, and flux decline associated with interacting nanoparticle cake layers formed during crossflow membrane filtration is used to interpret data from bench scale experiments using silica nanoparticles. Model predictions of permeate flux decline agree well with experimental flux data for all conditions tested and provide new insight into the delicate balance of interaction forces (van der Waals, acid–base, electrostatic, and permeate drag) that govern nanoparticle cake structures over a wide range of experimental conditions. At high flux and ionic strength, the porosity of nanoparticle cake layers was controlled by short-range acid–base and permeate drag forces, whereas at low ionic strength and flux, nanoparticle cake layer porosity was controlled by from electrostatic double layer and permeate drag forces. Use of an interfacial hydrodynamic drag correction suggests that colloidal cake structures may have a stronger than expected dependence on particle size, where smaller nanoparticles generally produce more porous cake layers. In addition, the model explains how microfiltration membranes can experience greater flux decline than ultrafiltration membranes without invoking pore blocking mechanisms.