Colloid transport in unsaturated porous media

Abstract

This paper explores the significance of the gas‐water interface on colloid sorption and transport. Three types of common saturation conditions were simulated in packed sand columns: (1) a completely water‐saturated condition, (2) gas bubbles trapped by capillary forces as a nonwetting residual phase (15% gas), and (3) gas present as a continuous phase (46% gas), or in other words, a vadose zone situation. Different saturations provided different interfacial conditions. Two types of polystyrene latex particles (0.2 μm), hydrophilic and hydrophobic, were used in each of the three saturations. Relative surface hydrophobicity of latex particles was characterized by contact angle measurements. Each experiment was repeated five times. A total of 30 columns with good reproducible packing and gas content gave reproducible particle breakthrough curves. The retention of both hydrophilic and hydrophobic colloids increased with gas content of the porous medium. Colloids preferentially sorbed onto the gas‐water interface relative to the matrix surface. The degree of sorption increased with the increase of colloid surface hydrophobicity. The results validate and quantify direct microscopic scale visualization observations made in two‐dimensional pore network glass micromodels (Wan and Wilson, 1994). These findings suggest an additional mechanism for filtration and for particulate transport in the subsurface environment, whenever more than one fluid phase is present. The results also give clear evidence that the presence of inadvertently trapped residual gas could help explain the discrepancy between water‐saturated laboratory column experiments and related theory.