Effects of Brownian force and pore structure on the removal of nanoparticles by porous membranes

• Membrane pore models to simulate Brownian force effects on nanoparticle transport. • Brownian force increases log reduction values (LRV) of nanoparticles at Pe < 10. • The internal membrane LRV increased linearly with the attachment coefficient. • Pore constrictions and tortuosity provided greater LRVs than simpler structures. • Brownian force increased likelihood of particles entering small membrane pores. This study investigated the effects of Brownian force on the transport and retention of nanoparticles within the pores of water filtration membranes using COMSOL Multiphysics®. Simulation results suggest that, at Péclet numbers below 10, the Brownian force may increase the frequency of nanoparticle collisions with pore walls to substantially increase log reduction rates (LRVs). Indeed, the LRVs increased linearly with attachment coefficient values; even at a low attachment coefficient of 0.001, high LRVs over 1.5 were observed under some geometries owing to the high frequency of nanoparticle-wall collisions. The calculated LRVs were inversely correlated with the Péclet numbers, whereas the slope of the inverse correlation was dependent on the membrane pore size. Complex structures, such as pore constriction, tortuosity, and non-uniform pore size, increased the LRV, whereas the overall porosity was not a determinant removal factor within the pores. In the configuration with parallel large and small pores, the Brownian force increased the number of nanoparticles entering the membrane through small pores relative to the numbers estimated based on water flow rates. These results indicate that the Brownian force may be an important particle removal factor within membrane pores under certain conditions; this should be considered when designing and operating membrane filters.