Transport of Colloid-Size Oil Droplets in Saturated and Unsaturated Sand Columns

Oil in wastewater poses significant treatment and subsequent environmental challenges. Accumulation of oils in soil leads to hydrophobicity, and the transport of colloidal-sized oil droplets may facilitate the co-transport of oil soluble contaminants such as pesticides or pharmaceutical materials. In order to determine transport characteristics of colloidal-sized, edible oil droplets, short-pulse column breakthrough experiments were conducted. Oil droplets (mean diameter 0.7 μm, ζ-potential -34±1, density 0.92 g cm^-3)were injected simultaneously with latex microspheres (0.02, 0.2 and 1.0μm, ζ-potentials -16±1, -30±2, and -49±1,respectively, density 1.055 g cm-³) and bromide in saturated and unsaturated quartz sand (ζ-potential -63±2 mV). Breakthrough of oil droplets was consistently detected first and recovery of oil droplets from the column was 20% greater than similarly sized microspheres in the saturated column, and 16% greater in high(0.18±0.01) volumetric water content (VWC) unsaturated columns. Higher variability was observed in the lower VWC (0.14±0.01)column experiments, and oil droplet recovery was just slightly greater than similarly sized microspheres and statistically higher only compared to the 0.02 μm microspheres. The research demonstrated that oil droplets are able to be transported as colloids in both saturated and unsaturated porous media. An important finding was that transport of oil droplets exceeded that of microspheres within the same size range and similar electrostatic properties. Classical filtration theory indicates that oil droplets in the diameter range of about 0.5 to 2 μm will exhibit reduced deposition due to buoyancy. However, current improvements to the theory do not accommodate prediction of buoyant particle transport and the theory needs review. High recovery of oil droplets in unsaturated porous media is a novel finding. As expected, straining appeared to be an important removal mechanism in unsaturated conditions. Future research should be conducted with various density colloids and solutions to determine the effect of buoyancy on the transport characteristics of colloids.