Colloid transport through saturated sand columns: Influence of physical and chemical surface properties on deposition

Colloid transport experiments are often conducted in cleaned sand, and the results are then projected onto mechanisms in natural porous media. We conducted tracer experiments to investigate the effect of the sand-cleaning preparations on physical and chemical surface properties and to compare retention mechanisms in natural sand of three levels of cleanliness under saturated conditions. Negatively charged fluorescent microspheres (0.02, 0.1 and 1 μm) together with a soluble tracer (LiBr) were applied at the top of vertical sand columns, while effluent samples were taken at the column outlets. At the end of the experiment, columns were dissected and samples were collected at different depths. For each sand, the grain size distribution and surface chemistry were examined. The electrostatic interaction free energies between grain surface and colloid were calculated based on their thermodynamic surface properties according to the traditional and extended DLVO theory. Colloid retention was much higher in natural sand than in its washed counterparts. A comparison of the physical and chemical surface properties of the differently cleaned sands implied that a dust-like particle fraction present in the natural sand had relatively greater impact on colloid retention than changes in grain surface charge resulting from the cleaning procedures.