Pore Scale Simulation of Colloid Deposition

Abstract

Mobile subsurface colloids have received considerable attention because the migration of colloids and colloid-contaminant complexes through the solid matrix substantially increase the risk of groundwater pollution. Typically defined as suspended particulate matter with diameter less than 10μm, colloids include both organic and inorganic materials such as microorganisms, humic substances, clay minerals and metal oxides. Accurate prediction of the fate of colloids is important to predict colloid facilitated transport of pollutants, and the transport of biocolloids such as viruses and bacteria. In colloid transport studies colloid deposition, that is, the capture of colloids by grain surfaces, is considered as the primary mechanism controlling the transport of colloids in groundwater (Ryan & Elimelech; 1996). The role of electrostatic and hydrodynamic forces in controlling colloid deposition behavior of colloids has been afforded detailed investigation in the field of colloid science to gain more understanding about colloid-surface interaction processes. The study of deposition rates of colloids onto model collectors has provided substantial information on the electrostatic and hydrodynamic forces involved in the transport of colloids (Elimelech et al., 1995; Tien & Ramarao, 2007). Most of these studies have focused on colloid transport under saturated conditions (Yao et al., 1971; Rajagopalan & Tien, 1976; Ryan & Elimelech, 1996; Keller & Auset, 2007). However, there is not much information available on colloid behavior under unsaturated conditions due to the complexity of the conditions involved (DeNovio et al., 2004; Keller & Sirivithayapakorn, 2004; Auset & Keller; 2004; Crist et al., 2005; Zevi et al., 2005; Keller & Auset, 2007). Most of the experimental and modeling studies on colloid transport under unsaturated conditions have focused primarily on colloid concentration in drainage water with very little emphasis on the precise mechanisms retaining the colloids in the pores (Corapcioglu & Choi, 1996; Lenhart & Saiers, 2002; DeNovio et al., 2004). Generally, the approaches used to simulate colloid transport can be classified into two types, Lagrangian or Eulerian. The Lagrangian approach focuses on the movement of distinct particles and tracks particle position in a moving fluid (Rajagopalan & Tien, 1976; Ryan and Elimelech, 1996). In contrast, the Eulerian approach considers the concentration distribution of particles in a porous media (Yao et al., 1971; Tufenkji & Elimelech, 2004). The Eulerian approach has advantages over the Lagrangian approach, in that it does not require high computational performance, and it is easy to incorporate Brownian motion (Ryan and Elimelech, 1996; Nelson & Ginn, 2005).