Impact of surface charge density on colloid deposition in unsaturated porous media

This research investigated the impact of surface charge density on colloid deposition in water unsaturated porous media using column experiments. The same-sized carboxyl functionalized microspheres with variable surface charge densities (0.56–1.95 μC/cm2) were used as the model colloids in this research. Column experiments were conducted under steady-state water flow conditions with effective water saturation ranging from 0.2 to 1.0. Colloid transport was described by the advection-dispersion equation under water saturated conditions and the mobile–immobile two-region transport model under water unsaturated conditions. Under water saturated conditions, no colloids were retained in the porous media. Under water unsaturated conditions, colloid retention was a function of both colloid surface charge density and water saturation, i.e., colloid retention increased with increasing surface charge density; colloid retention decreased with increasing water saturation. In a separate experiment, we conducted bubble column experiments to provide evidence that the colloids used in this research did not attach to the liquid–gas interface. Under steady-state water flow conditions, colloid retention in the porous media was believed to be retained at the liquid–solid–gas three-phase interface, or more precisely the liquid–gas meniscus and the solid interface, where the water film thickness approached the size of the colloidal particles. The repulsive electrostatic interactions between the colloids and the liquid–gas interface aided colloids to overcome the repulsive electrostatic interaction barrier with the sediments, leading the colloids to attachment.