A study of the transport characteristics and mechanisms of submicron particles in unsaturated porous media

Particle facilitated transport of contaminants from radioactive waste through both saturated and unsaturated porous media has received increased attention in recent years. This is important because some particles may experience enhanced transport through porous media. This research considered applying deep-bed filtration theory to predict and model the extent of submicron particle transport through unsaturated porous media environments. Two sets of laboratory experiments, using columns packed with soda-lime glass beads, were performed to obtain data on relative equilibrium particle concentrations. Four experiments in each set were performed in unsaturated conditions. The columns were challenged with a continuous flow of 0.23-$\mu$m fluorescent microspheres suspended in a 1.0-mM ionic strength solution of reagent grade NaCl. Relative concentrations were determined using fluorescence microscopy techniques. Besides the unmodified form of deep-bed filtration theory, four modified forms of deep-bed filtration theory were tested to determine their predictive abilities. All modifications were intended to help explain differences between saturated and unsaturated porous media environments. The modifications examined considered the expected increased tortuosity of the unsaturated environment and particle attachment to the gas-water interface. Additionally considered were separate theoretical attachment efficiency factors applicable to both the gas-water interface and the solid-water interface. Particle breakthrough through 0.5 to 0.75-mm diameter glass bead packed unsaturated column systems occurred on average 1.8 $\pm$ 0.2 times faster than expected for particles moving with the average water velocity. Particle breakthrough through 160 to 250-$\mu$m diameter glass bead packed unsaturated column systems occurred on average 1.4 $\pm$ 0.4 times faster than expected for particles moving with the average water velocity. Normalizing theoretical attachment efficiency values for the solid-water interface, by those determined for the saturated condition, had utility when applied to the otherwise unmodified form of deep-bed filtration theory. Accounting for particle attachment to the gas-water interface slightly enhanced prediction capability