Fate and transport of viruses and colloids in saturated and unsaturated porous media

Abstract

The fundamental mechanisms involved in fate and transport of colloidal particles (viruses and latex microspheres) in saturated and unsaturated porous media were systematically examined. Two different bacteriophages were used as surrogate for pathogenic viruses to investigate the effects of various water contents and solution chemistries in terms of pH and ionic strength (IS) on virus transport. The experiments were complimented by utilizing a transport model that accounts for virus interaction with the solid-water interfaces (SWI) and air-water interfaces (AWI). It was found that under saturated conditions virus retention enhanced with decreasing the pH and increasing the IS. Under unsaturated conditions, viruses exhibited a high affinity to the AWI only when the pH was lower than 7. In contrast with the saturated experiments that a one-site kinetic model was sufficient to fit the breakthrough curves, a two-site kinetic model was needed to produce a good fit to the breakthrough curves obtained from unsaturated columns. To complement the previous work, packed column and mathematical modeling studies were conducted to explore the influence of water saturation, IS, and grain size on the transport of larger colloids (latex microspheres; 1.1 μm) in porous media. Experiments were carried out under chemically unfavorable conditions for colloid attachment to both SWI and AWI. The breakthrough curve and final deposition profile in each experiment indicated that colloid retention was highly dependent on IS, water content, and sand grain size. Experimental and modeling results suggested that straining – the retention of colloids in low velocity regions of porous media such as grain junctions – was the primary mechanism of colloid retention under both saturated and unsaturated conditions. Increasing the solution ionic strength is believed to increase the depth of secondary minimum in the DLVO interaction energy profile and as a result, increase the adhesive force between colloids and the SWI by increasing. These weakly associated colloids can be funneled to small regions of the pore space formed adjacent to grain-grain junctions. For select systems, the IS of the influent was decreased to a low solution IS following the obtaining of the effluent concentration data. In this case, only a small portion of the deposited colloids was recovered in the effluent and the majority was still retained in the sand. These observations suggest that the extent of colloid removal by straining is strongly coupled to solution chemistry. This research established that colloid retention in porous media is a coupled process that strongly depends on solution chemistry, pore structure, and system hydrodynamics. Therefore, modeling colloid transport through porous media will require nontraditional approaches which account for the abovementioned factors.