(409c) Deposition Dynamics of Rod-Shaped Colloids during Transport in Porous Media

The transport and deposition of colloids in saturated porous media is essential to various environmental processes, especially to produce safe drinking water. Almost all colloids in the environment possess certain degrees of inhomogeneity on their surfaces and are non-spherical in shape. However, existing theories often simplify natural colloids as “homogeneous spheres”, which has been increasingly demonstrated inadedquate in describing their transport behavior. In this work, we develop a particle transport model that incorporates the anisotropic features of colloids, i.e. non-spherical shape and heterogeneous patches over particle surface. We will present results obtained during our systematic investigation of rod-shaped colloids with surface charge heterogeneity in Happel sphere-in-cell model (i.e., a well-known model to represent porous media in classic colloid filtration theory).

To improve the classic colloid filtration theory, we performed a series of simulations for ellipsoidal particles with different sizes (e.g., 50 nm – 10 µm), different surface charge heterogeneity coverage (e.g., 0-100%) and aspect ratios (e.g., 1 - 8, where 1 defines spherical particles) under a wide range of flow velocities that represent groundwater flow conditions. The deposition dynamics of rods were compared with that of spheres of equivalent volume under identical conditions. Under favorable conditions (the energy barriers to deposition are absent), for large particle sizes, rods exhibited enhanced attachment than spheres; whereas for small particle sizes, rods showed less retention than spheres. This crossover size strongly depends on flow velocity and porosity. The non-spherical shape of rod particles causes them to respond to hydrodynamic fluid drag very differently in comparison with their counterpart spheres. We found that the flow effect due to shape dominates other factors, e.g. Brownian motion effect for small particle sizes, or gravitational effect for large particle sizes; and can explain the above-observed deposition behavior for rods. Under unfavorable conditions (the energy barriers to deposition are present), surface charge heterogeneity coverage not only affects the extent of attachment, but the orientations of the attached particles. Trajectories of adhered colloids illustrated that they tend to orient to maximize the attraction between colloid surface heterogeneities and the collector surface for firm attachment. In addition, rod particles require less heterogeneous patches for attachment to happen, and for the attachment to behave like favorable conditions, compared to spheres. These results demonstrate that particle shape and surface heterogeneity are very important factors in governing particle transport and deposition.