Transport and retention of colloidal particles in partially saturated porous media: Effect of ionic strength

We directly observed pore-scale attachment of fluorescent synthetic polystyrene colloids (1.0 mu m diameter) in a partially saturated sand pack (pore space saturation ranging from 0.7 to 0.9) at four solution ionic strengths (0, 1, 100, 200 mmol NaCl). Sequential confocal laser microscope images were analyzed to quantify colloid retention, particularly at air-water meniscus-solid (AW(m)S) interfaces. We concurrently measured effluent colloid concentrations to determine overall matrix retention. Ionic strength had no effect on meniscus contact angles (26.7 +/- 3.7 degrees) or surface tension (63-67 mN/m), both important components of the capillary forces thought to play the primary role in retention at the AW(m)S interfaces. AW(m)S interfaces attachment was greatest at 1 mmol, with the 0 mmol ionic strength reducing attachment by half. Increasing ionic strength to 100 and 200 mmol markedly decreased colloid retention at the AW(m)S interfaces due to observed increased competing attachment at grain surfaces (solid/water interface) that reduced the number of colloids available for AW(m)S interface attachment.