Virus movement in soil during saturated and unsaturated flow

[1] We investigated transport of viruses through saturated and unsaturated sand columns. Unsaturated experiments were conducted under conditions of uniform saturation and steady state water flow. The water saturation ranged from 1 to 0.5. Bacteriophages MS2 and fX174 were used as surrogates for pathogenic viruses in these studies. Phosphatebuffered solutions with different pH values (7.5, 6.2, 5.5, and 5) were utilized. Virus transport was modeled assuming first-order kinetic adsorption for interactions to the solidwater interface (SWI) and the air-water interface (AWI). Under saturated conditions, virus retention increased as pH decreased, and a one-site kinetic model produced a good fit to the breakthrough curves. Under unsaturated conditions a two-site kinetic model was needed to fit the breakthrough curves satisfactorily. The second site was attributed to the adsorption of phages to the AWI. According to our results, fX174 exhibits a high affinity to the AWI at pH values below 6.6 (the isoelectric point of fX174). Although it is believed that MS2 is more hydrophobic than fX174, MS2 had a lower affinity to the AWI than fX174, presumably because of the lower isoelectric point of MS2, which is equal to 3.9. Under unsaturated conditions, viruses captured within the column could be recovered in the column outflow by resaturating and immediately draining the column. Draining columns under saturated conditions, however, did not result in any recovery of viruses. Therefore the recovery can be attributed to the release of viruses adsorbed to the AWI. Our results suggest that electrostatic interactions of viruses with the AWI are much more important than hydrophobicity.

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Role of air‐water interfaces on retention of viruses under unsaturated conditions

Torkzaban

Hassanizadeh

Schijven

et al. 2006

“…In general, viruses as well as other colloids have to move through the vadose zone before they can reach groundwater. Overall, the transport mechanisms of colloids in saturated porous media have been studied in great detail in various aspects [e.g., Small , 1974; Lance and Gerba , 1984; McDowellboyer et al , 1986; Bales et al , 1989; Corapcioglu and Jiang , 1993; Sim and Chrysikopoulos , 1996; Chrysikopoulos and Sim , 1996; Rehmann et al , 1999; Bradford et al , 2002; Sirivithayapakorn and Keller , 2003]. However, the mechanisms of colloid transport in the unsaturated porous media needed to be understood in greater detail, since there are some significant differences.…”

Section: Introductionmentioning

confidence: 99%

Transport of colloids in unsaturated porous media: A pore‐scale observation of processes during the dissolution of air‐water interface

Sirivithayapakorn

Keller

2003

115

141

[1] We present results from pore-scale observations of colloid transport in an unsaturated physical micromodel. The experiments were conducted separately using three different sizes of carboxylate polystyrene latex spheres and Bacteriophage MS2 virus. The main focus was to investigate the pore-scale transport processes of colloids as they interact with the air-water interface (AWI) of trapped air bubbles in unsaturated porous media, as well as the release of colloids during imbibition. The colloids travel through the water phase but are attracted to the AWI by either collision or attractive forces and are accumulated at the AWI almost irreversibly, until the dissolution of the air bubble reduces or eliminates the AWI. Once the air bubbles are near the end of the dissolution process, the colloids can be transported by advective liquid flow, as colloidal clusters. The clusters can then attach to other AWI down-gradient or be trapped in pore throats that would have allowed them to pass through individually. We also observed small air bubbles with attached colloids that traveled through the porous medium during the gas dissolution process. We used Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to help explain the observed results. The strength of the force that holds the colloids at the AWI was estimated, assuming that the capillary force is the major force that holds the colloids at the AWI. Our calculations indicate that the forces that hold the colloids at the AWI are larger than the energy barrier between the colloids. Therefore it is quite likely that the clusters of colloids are formed by the colloids attached at the AWI as they move closer at the end of the bubble dissolution process. Coagulation at the AWI may increase the overall filtration for colloids transported through the vadose zone. Just as important, colloids trapped in the AWI might be quite mobile when the air bubbles are released at the end of the dissolution process, resulting in increased breakthrough. These pore-scale mechanisms are likely to play a significant role in the macroscopic transport of colloids in unsaturated porous media.INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport; 5139 Physical Properties of Rocks: Transport properties; KEYWORDS: air-water interface, DLVO, hydrophobic, colloid, unsaturated, micromodel Citation: Sirivithayapakorn, S., and A. Keller, Transport of colloids in unsaturated porous media: A pore -scale observation of processes during the dissolution of air-water interface, Water Resour.

“…[5] For colloid transport in the unsaturated zone, colloids are believed to be present mainly in the water phase, the air-water interface (AWI), and the solid-water interface (SWI). The transport of colloids in the unsaturated zone is thus greatly influenced by water content [Vilker and Burge, 1980;Lance and Gerba, 1984;Powelson et al, 1990;Poletika et al, 1995]. The AWI has high affinity for both hydrophobic and hydrophilic colloids and the sorption of colloids onto the AWI is often considered almost irreversible under unsaturated conditions, due to the capillary forces that hold the colloids at the interface [Ducker et al, 1994;Wan et al, 1994;Wan and Wilson, 1994b;Wan and Tokunaga, 2002;Costanza-Robinson and Brusseau, 2002;Sirivithayapakorn and Keller, 2003].…”

Section: Introductionmentioning

confidence: 99%

Transport of colloids in unsaturated porous media: Explaining large‐scale behavior based on pore‐scale mechanisms

Keller

Sirivithayapakorn

2004

[1] We conducted column-scale experiments to study the transport of colloids (latex particles and bacteriophage MS2) under water-unsaturated conditions. The objective was to draw connections between observations at the pore scale and the results obtained from column-scale experiments. The same system had been previously operated under saturated conditions to determine colloid collision efficiency. Breakthrough of colloids was first evaluated under unsaturated but steady water content conditions, with constant trickling flow. After monitoring the steady breakthrough of the colloids, the column was flushed with water at higher flow rate to increase the water content up to a saturated condition. Colloid breakthrough was monitored during the entire experiment, as water content increased. Colloid removal increases significantly with decreasing initial water saturation, reflecting retention at the air-water interface and straining in thin-water films. Colloid breakthrough occurs earlier than a conservative tracer even under unsaturated conditions, although the colloid concentrations are much lower than the tracer. After flushing at similar flow rates, there is increased colloid retention under unsaturated conditions even as the system approaches water saturation, indicating that additional removal is occurring, due possibly to the formation of colloidal clusters. These results can be explained to a great extent by pore-scale observations of retention and remobilization mechanisms.