Retention of mineral colloids in unsaturated porous media as related to their surface properties

We investigated the transport of mineral colloids through variably saturated Hanford sediments. Column experiments were conducted under steady-state water flow conditions with effective water saturations ranging from 0.56 to 1.0. Four types of colloidal particles were used: native Hanford colloids, Hanford colloids reacted with waste tank solutions, pure kaolinite, and pure Na-montmorillonite. Colloid transport was described by the mobile–immobile region model accounting for first-order colloid removal from the mobile region. Under saturated conditions, no colloids were removed from the liquid phase during transport, while under unsaturated conditions colloids were removed from the mobile water region. Colloid removal increased with decreasing system saturation. Under constant chemical conditions, colloids captured within the column could quantitatively be recovered in the column outflow by re-saturating the column after each unsaturated-flow breakthrough. Through microscopic observations in a glass micromodel containing suspended air bubbles, we found that colloids did not adhere to the liquid–gas interface. Using the extended DLVO theory, free energies of Lifshitz–van der Waals, electrostatic and Lewis acid/base interactions between colloids, sediments and the liquid–gas interface were calculated based on their independently determined surface thermodynamic properties. Experimental results and surface thermodynamic calculations support the hypothesis that colloids were retained near thin water films of thickness comparable to the colloid diameters.