Implications of Cation Exchange On Clay Release and Colloid-Facilitated Transport in Porous Media.

Column experiments were conducted to study chemical factors that influence the release of clay (kaolinite and quartz minerals) from saturated Ottawa sand of different sizes (710, 360, and 240 µm).  A relatively minor enhancement of clay release occurred when the pH was increased (5.8 to 10) or the ionic strength (IS) was decreased to deionized (DI) water.  In contrast, clay release was dramatically enhanced when monovalent Na⁺ was exchanged for multivalent cations (e.g., Ca²⁺ and Mg²⁺), and then the solution IS was reduced to DI water.  The amount of clay release was demonstrated to be directly dependent on the Na⁺ concentration of the exchanging solution and on the initial clay content of the sand (0.026-0.054%) that increased with decreasing sand size.  Exchange of Na⁺ for multivalent cations was demonstrated to decrease the adhesive force as a result of an increase in the magnitude of the clay zeta potential, whereas decreasing the IS reduced the adhesive force as a result of expansion of the double layer thickness and increasing the magnitude of the surface potential.  Column results and scanning electron microscope (SEM) images clearly demonstrated that the clay was reversibly retained on the sand, despite predictions of irreversible interaction in the primary minimum.   One plausible explanation was that cation exchange increased the separation distance between the clay-solid interface as a result of non-DLVO forces.   A cleaning procedure was subsequently developed to remove clay via cation exchange and IS reduction, and SEM images demonstrated the effectiveness of this approach.   The transport of Cu²⁺ was then shown to be dramatically enhanced by an order of magnitude in peak concentration by adsorption on clays that were released following cation exchange and IS reduction.