(125e) Colloid Retention in Variably Saturated Porous Media: Investigating the Effects of Hydrodynamic and Physiochemical Forces Using A Capillary Triangular Tube Model

Transport of colloidal particles under unsaturated porous media is much more complex and poorly understood than in fully-saturated systems. Filtration theory does not consider the influence of water saturation on colloid attachment to the solid-water interface (SWI) or air-water interface (AWI), and neglects the influence of pore space geometry and hydrodynamic forces that act on colloids in the vicinity of an interface. To investigate the extent to which varying water saturation levels influence colloid retention, a model has been developed in which the pore space was idealized as capillary tubes of triangular cross-sections and the Navier-Stokes equation was solved to obtain the fluid velocity field. The simulations indicate a coupled effect of hydrodynamic and physicochemical forces on colloid retention to the SWI at various saturation levels, chemical, and hydrodynamic conditions. Our analyses indicate that colloid retention (under unfavorable attachment conditions) occurs at the corners of the triangular capillary tubes and also at the solid-air-water interface where the water velocity is small enough for adhesive forces to dominate over hydrodynamic forces. The accessible surface area of the capillary pores on which attachment could occur increased with solution ionic strength and decreased with decreasing flow velocity and water saturation level. Results from computational simulations that solved the advection-diffusion equation in these capillaries reveal that the colloid mass transfer rate increased with decreasing saturation level. Finally, results suggest that further quantitative evaluation of colloid transport through porous media will require expansion of such non-traditional approaches to account for physicochemical and hydrodynamic conditions, as well as pore structure and geometry of the system.