Transport of Latex Particles in Variably Saturated Heterogeneous Media.
Jie Han, University of Delaware, Newark, DE 19711, Scott Bradford, 450 W Big Springs Road, USDA-ARS, USDA-ARS, George E. Brown Jr. Salinity Lab., Riverside, CA 92507-4617, and Yan Jin, Dept. of Plant & Soil Sciences, University of Delaware, Newark, DE 19716-2170.
Colloid transport and retention are complex processes in the subsurface environments, especially in unsaturated heterogeneous media. In this study, saturated and unsaturated column experiment were conducted to examine behaviors of sulfate latex particles (19 nm and 420 nm in diameters, respectively) in 300--355 mm sand (treated to have either hydrophilic or hydrophobic surfaces) under flow conditions. We also conducted batch experiments to independently study attachment of colloids on different surfaces. Breakthrough curves were fitted with HYDRUS-1D, taking into account both attachment/detachment and straining processes. Visualization experiments using confocal microscopy were conducted to make direct observations of the migration behavior of fluorescent particles (210 nm and 1000 nm in diameter, respectively) in a flow channel packed with hydrophilic and hydrophobic grains. Results from different experimental methods consistently suggest that attachment is mainly responsible for the removal of small particles while straining plays a more significant role on the retention of larger particles. Comparing to saturated conditions, we observed reduced removal or earlier arrival under unsaturated conditions in heterogeneous column systems, which is attributed to the different water flow patterns under unsaturated conditions between chemically homogeneous and heterogeneity systems. This was further confirmed by the confocal visualization experiments. The visualization experiment setup (confocal microscopy and the packed flow channel) provides a powerful and unique design for studying water flow and colloidal behavior under dynamic conditions.