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

Abstract

[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.

Tracing Water Flow and Colloidal Particles Transfer in an Unsaturated Soil

“…Interactions on the surface of colloidal particles lead to temporary or permanent attachment to the solid phase of porous medium. The attachment of colloidal particles at the air-water interface depends on pH, ionic strength and surface properties of colloidal particles [4] [5]. Constriction (where pores have diameters inferior to colloidal particles) provides sites for depositing, including areas where local water flow is not zero [6].…”

Transport of Nanomaterials in Unsaturated Porous Media

“…Major forces controlling colloid attachment to the air-water interface include electrostatic, van der Waals, hydrophobic, and capillary interactions (25,27,31,32). As has been observed for solid-water interface attachment, electrostatic interaction has also been found to be significant in air-water interface attachment, especially for hydrophilic colloids (31,36,37). Experimental measurements have shown that the air-water interface is negatively charged with surface potential ranging from À15 to À65 mV depending on pH and ionic strength (38,39).…”

“…For example, the capillary energy retaining carboxylate polystyrene latex spheres (d ¼ 0.05-3 mm) at the air-water interface has been calculated to be approximately two to three orders of magnitude higher than the energy barrier obtained from DLVO theory for the same latex colloids (42). Under transient water flow, colloids attached to the air-water interface can be released into pore solution only when the quantity of air-water interface is diminished due to the resaturation of porous media (37,42). If bubbles of air are mobilized by increased water flow, attached colloids may be transported by the migrating air bubbles (29,30,42).…”

“…Experimental measurements have shown that the air-water interface is negatively charged with surface potential ranging from À15 to À65 mV depending on pH and ionic strength (38,39). As such, positively charged colloids usually show greater adsorption to the air-water interface than negatively charged colloids (29,37,40). Hydrophobic interactions between colloids and the air-water interface are believed to be much stronger than DLVO forces (i.e., electrostatic and van der Waals forces), so are able to overcome the energy barrier for the air-water interface attachment, allowing even negatively charged colloids to attach to the air-water interface in some cases (31,32,41,42).…”

Bacteria transport in an unsaturated porous media: incorporation of air–water interface area model into transport modelling

“…static, like the wall of a capillary tube, or dynamic, like the surface of a flowing river) obviously has some bearing on how they should be modelled. Recent experimental work of Torkzaban et al (2006), who have observed the role of AWI on the retention of bacteriophage on porous media using column experiments, reported that virus attached to AWI under unsaturated conditions could be recovered at the effluent by resaturation of the column. Therefore, further study should be focused on how to relate this characteristic of AWI to the transport modelling of colloids/biocolloids in an unsaturated porous media.…”