Experimental investigation of virus and clay particles cotransport in partially saturated columns packed with glass beads Vasiliki I. Syngouna a , Constantinos V. Chrysikopoulos b,⇑ a Environmental Engineering Laboratory, Civil Engineering Department, University of Patras, Patras 26500, Greece b School of Environmental Engineering, Technical University of Crete, Chania 73100, Greece article info Article history: Received 23 August 2014 Accepted 26 October 2014 Available online 6 November 2014 Keywords: Unsaturated porous media Viruses Colloids Cotransport Attachment efficiency DLVO Capillary forces abstract Suspended clay particles in groundwater can play a significant role as carriers of viruses, because, depending on the physicochemical conditions, clay particles may facilitate or hinder the mobility of viruses. This experimental study examines the effects of clay colloids on the transport of viruses in variably saturated porous media. All cotransport experiments were conducted in both saturated and partially saturated columns packed with glass beads, using bacteriophages MS2 and UX174 as model viruses, and kaolinite (KGa-1b) and montmorillonite (STx-1b) as model clay colloids. The various exper- imental collision efficiencies were determined using the classical colloid filtration theory. The experimen- tal data indicated that the mass recovery of viruses and clay colloids decreased as the water saturation decreased. Temporal moments of the various breakthrough concentrations collected, suggested that the presence of clays significantly influenced virus transport and irreversible deposition onto glass beads. The mass recovery of both viruses, based on total effluent virus concentrations, was shown to reduce in the presence of suspended clay particles. Furthermore, the transport of suspended virus and clay-virus particles was retarded, compared to the conservative tracer. Under unsaturated conditions both clay par- ticles facilitated the transport of UX174, while hindered the transport of MS2. Moreover, the surface properties of viruses, clays and glass beads were employed for the construction of classical DLVO and capillary potential energy profiles, and the results suggested that capillary forces play a significant role on colloid retention. It was estimated that the capillary potential energy of MS2 is lower than that of UX174, and the capillary potential energy of KGa-1b is lower than that of STx-1b, assuming that the pro- trusion distance through the water film is the same for each pair of particles. Moreover, the capillary potential energy is several orders of magnitude greater than the DLVO potential energy. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Several theoretical and experimental investigations have shown that suspended mobile colloids can either facilitate or hinder the mobility of various contaminants in water saturated porous and fractured media [1–11]. However, colloid facilitated virus transport in unsaturated porous media is substantially different and more complex than that in saturated porous media. In addition to the retention mechanisms governing colloid and virus transport in saturated porous media (e.g., pore straining and attachment onto solid-water interfaces (SWI)), colloids in unsaturated porous media can be retained at air–water interfaces (AWI), in thin water films (film straining), and air–water–solid (AWS) interfaces [12–26]. Furthermore, in unsaturated porous media, colloids and viruses can also be retained in air–water meniscus–solid (AW m S) inter- faces [27,28]. Note that AW m S interfaces are essentially areas where significant colloid attachment occurs and water meniscuses diminish to thin water films. Capillary forces are known to be important for colloid attach- ment at the AWI as well as for film straining [12,14,25,29,30]. Derjaguin–Landau–Verwey–Overbeek (DLVO) interactions cannot always explain the observed colloid deposition in unsaturated por- ous media [17,30], especially at the contact line of the AW m S inter- face. Colloid retention of the AW m S interface is explained more convincingly by capillary force interactions [23,24,31]. Despite of these and other related research efforts, the role of capillary forces on colloid retention in unsaturated porous media is not fully understood and deserves more attention. The objective of this paper was to explore further the specific interactions of simultaneously transported colloids and viruses with the various interfaces (SWI, AWI, and AWS) present in http://dx.doi.org/10.1016/j.jcis.2014.10.066 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: cvc@enveng.tuc.gr (C.V. Chrysikopoulos). Journal of Colloid and Interface Science 440 (2015) 140–150 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis