M. Ekrem Cakmak

Colloid retention at the meniscus-wall contact line in an open microchannel

Colloid retention mechanisms in partially saturated porous media are currently being researched with an array of visualization techniques. These visualization techniques have refined our understanding of colloid movement and retention at the pore scale beyond what can be obtained from breakthrough experiments. One of the remaining questions is what mechanisms are responsible for colloid immobilization at the triple point where air, water, and soil grain meet. The objective of this study was to investigate how colloids are transported to the air-water-solid (AWS) contact line in an open triangular microchannel, and then retained as a function of meniscus contact angle with the wall and solution ionic strength. Colloid flow path, meniscus shape and meniscus-wall contact angle, and colloid retention at the AWS contact line were visualized and quantified with a confocal microscope. Experimental results demonstrated that colloid retention at the AWS contact line was significant when the meniscus-wall contact angle was less than 16°, but was minimal for the meniscus-wall contact angles exceeding 20°. Tracking of individual colloids and computational hydrodynamic simulation both revealed that for small contact angles (e.g., 12.5°), counter flow and flow vortices formed near the AWS contact line, but not for large contact angles (e.g., 28°). This counter flow helped deliver the colloids to the wall surface just below the contact line. In accordance with DLVO and hydrodynamic torque calculations, colloid movement may be stopped when the colloid reached the secondary minimum at the wall near the contact line. However, contradictory to the prediction of the torque analysis, colloid retention at the AWS contact line decreased with increasing ionic strength for contact angles of 10-20°, indicating that the air-water interface was involved through both counter flow and capillary force. We hypothesized that capillary force pushed the colloid through the primary energy barrier to the primary minimum to become immobilized, when small fluctuations in water level stretched the meniscus over the colloid. For large meniscus-wall contact angles counter flow was not observed, resulting in less colloid retention, because a smaller number of colloids were transported to the contact line.

Biocolloid retention in partially saturated soils

Unsaturated soils are considered excellent filters for preventing the transport of pathogenic biocolloids to groundwater, but little is known about the actual mechanisms of biocolloid retention. To obtain a better understanding of these processes, a number of visualization experiments were performed and analyzed.

The impact of biofilm-forming potential and tafi production on transport of environmental Salmonella through unsaturated porous media

Understanding the factors influencing the transport of microbial pathogens, such as Salmonella and Escherichia coli, through porous media is critical to protecting drinking water supplies. The production of biofilms, along with individual biofilm-associated components, such as tafi, is believed to hinder transport of microorganisms through soil. This study investigated the relationship between biofilm formation and tafi production and the transport of environmental Salmonella through porous media. Thirty-two Salmonella isolates were initially assayed for their ability to form biofilms, from which a subset of these was selected to represent a range of high and low biofilm-formation potential and tafi formation capabilities. These were subsequently examined in unsaturated sand columns for transport characteristics. No obvious correlation was observed between Salmonella phenotypes and column retention. The results indicated that while transport of well-characterized laboratory E. coli strains can often be hindered by the presence of tafi and the potential to form biofilms, the presence of tafi did not retard the transport of the Salmonella strains.

Pore Scale Simulation of Colloid Deposition

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).