Retention and transport of amphiphilic colloids under unsaturated flow conditions: Effect of particle size and surface property

The purpose of this study was to examine the mechanisms responsible for deposition and transport of amphiphilic colloids with a wide range of particle sizes (20-420 nm) through porous media. A series of saturated and unsaturated column experiments were conducted using amphiphilic latex microspheres and a hydrophilic silica colloid. We found that the amphiphilic latex particles were retained to a greater extent than the hydrophilic silica colloid in unsaturated media. This was attributed to colloidal attachment at the airwater interface due mainly to hydrophobic interactions. We also found that dependence of colloid retention on particle size was nonlinear. There existed a fraction of colloids with greater mobility than other fractions, which we referred to as the most mobile colloids. As particle size increased from 20 to 420 nm, colloid deposition rate first decreased to reach a minimum value at similar to 100 nm then increased, indicating that different retention mechanisms were involved. We showed that conducting saturated transport experiments and analysis using filtration theory may be an effective approach for determining the most mobile colloid size(s) in porous media, perhaps even for unsaturated flow conditions. This study highlights the importance of including size effect and surface properties when predicting concentrations and fluxes of amphiphilic colloids or colloid-bound amphiphilic and hydrophobic contaminants in the subsurface environment.