Wei Zhang - Colloid Transport and Retention in Unsaturated Porous Media: Effect of Colloid Input Concentration

Version 1

      Publication Details (including relevant citation   information): Zhang, Wei, Morales, Verónica L., Cakmak,   M. Ekrem, Salvucci, Anthony E., Geohring, Larry D., Hay, Anthony   G., Parlange, Jean-Yves, Steenhuis, Tammo S., Environ. Sci.   Technol., 2010, 44 (13), pp   4965-4972

      Abstract: Colloids play an important role in   facilitating transport of adsorbed contaminants in soils. Recent   studies showed that under saturated conditions colloid retention   was a function of its concentration. It is unknown if this is the   case under unsaturated conditions. In this study, the effect of   colloid concentration on colloid retention was investigated in   unsaturated columns by increasing concentrations of colloid   influents with varying ionic strength. Colloid retention was   observed in situ by bright field microscopy and quantified by   measuring colloid breakthrough curves. In our unsaturated   experiments, greater input concentrations resulted in increased   colloid retention at ionic strength above 0.1 mM, but not in   deionized water (i.e., 0 mM ionic strength). Bright field   microscope images showed that colloid retention mainly occurred   at the solid?water interface and wedge-shaped air?water?solid   interfaces, whereas the retention at the grain?grain contacts was   minor. Some colloids at the air?water?solid interfaces were   rotating and oscillating and thus trapped. Computational   hydrodynamic simulation confirmed that the wedge-shaped   air?water?solid interface could form a ?hydrodynamic trap? by   retaining colloids in its low velocity vortices. Direct   visualization also revealed that colloids once retained acted as   new retention sites for other suspended colloids at ionic   strength greater than 0.1 mM and thereby could explain the   greater retention with increased input concentrations.   Derjaguin?Landau?Verwey?Overbeek (DLVO) energy calculations   support this concept. Finally, the results of unsaturated   experiments were in agreement with limited saturated experiments   under otherwise the same conditions.

      Address (URL): http://dx.doi.org/10.1021/es100272f