TRANSPORT OF NANO-PARTICLES IN VARIABLY SATURATED HETEROGENEOUS MEDIA

Migration of colloids can facilitate transport of bacteria, virus, metal, and radionuclide in the subsurface environment. However, mechanisms of colloid transport and retention are complex and not completely understood, especially in heterogeneous media and under unsaturated flow conditions. In this study, saturated and unsaturated column experiments were conducted to examine the behavior of 19 nm sulfate latex particles in 300--355 mm sand (treated to have either hydrophilic or hydrophobic surfaces) with mixed wettability at a constant ionic strength of 100 mM and pH of 7.5. Batch experiments were conducted with the hydrophilic, hydrophobic sands and the mixture of them to determine attachment coefficients of the colloid at the hydrophilic and hydrophobic sand surfaces. Results show that colloid transport was significantly delayed in the unsaturated columns as compared to the saturated columns packed with hydrophilic sand and the mixture of 70% hydrophilic and 30% hydrophobic sand. Breakthrough kept increasing in the mixture of 30% hydrophilic and 70 % hydrophobic sand under unsaturated conditions. Modification of water flow paths and the resulting change of air-water and solid-water interfacial areas as the fraction of hydrophobic sand changed are likely responsible for the observed different behavior. Results from this study also suggest that there was increasing retardation of latex particles breakthrough as the fraction of hydrophobic sand increased. Batch experimental results further indicate that hydrophobic interactions between the latex particles and the hydrophobic sand played an important role in colloid retention in the heterogonous media with mixed wettability.