Colloid Mobilization and Transport during Capillary Fringe Fluctuations

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  • Colloid Mobilization and Transport during Capillary FringeFluctuationsSurachet Aramrak,,, Markus Flury,*, James B. Harsh, and Richard L. Zollars

    Department of Crop and Soil Sciences, The Gene and Linda Voiland School of Chemical Engineering and Bioengineering,Washington State University, Pullman, Washington 99164, United StatesDepartment of Crop and Soil Sciences, Washington State University, Puyallup, Washington 98371, United States

    *S Supporting Information

    ABSTRACT: Capillary fringe fluctuations due to changing water tables lead to displacementof airwater interfaces in soils and sediments. These moving airwater interfaces can mobilizecolloids. We visualized colloids interacting with moving airwater interfaces during capillaryfringe fluctuations by confocal microscopy. We simulated capillary fringe fluctuations in a glass-bead-filled column. We studied four specific conditions: (1) colloids suspended in the aqueousphase, (2) colloids attached to the glass beads in an initially wet porous medium, (3) colloidsattached to the glass beads in an initially dry porous medium, and (4) colloids suspended inthe aqueous phase with the presence of a static air bubble. Confocal images confirmed that thecapillary fringe fluctuations affect colloid transport behavior. Hydrophilic negatively chargedcolloids initially suspended in the aqueous phase were deposited at the solidwater interfaceafter a drainage passage, but then were removed by subsequent capillary fringe fluctuations.The colloids that were initially attached to the wet or dry glass bead surface were detached bymoving airwater interfaces in the capillary fringe. Hydrophilic negatively charged colloids didnot attach to static air-bubbles, but hydrophobic negatively charged and hydrophilic positivelycharged colloids did. Our results demonstrate that capillary fringe fluctuations are an effective means for colloid mobilization.

    INTRODUCTIONColloids can promote the transport of radionuclides,14 heavymetals,5 pesticides,6,7 phosphorus,810 and animal hormones andveterinary antibiotics.11,12 In addition, viruses, bacteria, protozoa,and spores are all colloids, and their transport in subsurfacemedia is controlled by colloidal mechanisms.13 Many studies ofcolloid and colloid-facilitated contaminant transport have beenconducted under both saturated conditions14 and unsaturatedconditions.15,16 The airwater interface and the airwatersolidinterface line, that is, the line where the airwater interfaceintersects the solid phase, have been reported to be retentionsites for colloids.1719 However, only a few authors have focusedon the capillary fringe.2023

    The capillary fringe is the zone just above the water table,which is still water-saturated, but has negative capillarypressure.24 The capillary fringe acts as a transition regionbetween vertical unsaturated flow in the vadose zone andhorizontal saturated flow in groundwater.25 As a mixing zone,the capillary fringe is expected to affect the transport of colloidsthat move from the subsurface to groundwater. Knowing themechanisms of colloid behavior in the capillary fringe increasesour fundamental understanding of the capillary fringe system.Fluctuations of the groundwater table lead to moving air

    water interfaces as well as to entrapment of air bubbles in thecapillary fringe.25,26 Moving airwater interfaces play a majorrole in colloid mobilization and transport. Colloids depositedon both initially wet solid surfaces2733 and air-dried surfaces34

    can be removed by moving airwater interfaces. Surface tension

    forces exerted on the particles,27,28,3539 velocities and numbers ofairwater interfaces,29,33 surface properties of particles (i.e., chargeand wettability),30,34 particle size,31 advancing and receding airwater interfaces,32,33 and particle shape40 have all been found to berelevant for particle detachment by moving airwater interfaces. Inprevious experiments, we investigated colloid detachment bymoving airwater interfaces in a single channel,33,40 and here, weexpand on colloid detachment in a porous medium.Our objective was to determine the effect of capillary fringe

    fluctuations on the behavior of colloids. We hypothesized that amoving airwater interface due to a fluctuating capillary fringecan scour the colloids from the medium surface and carry themalong, but only if a colloidairwater contact line is formed. Wefurther hypothesized that trapped air bubbles can capture andimmobilize colloids, but the colloids are being released when thebubbles dissolve or flush out. We tested these hypothesesexperimentally by using a glass bead-filled column and fluorescentcolloids in combination with confocal microscopy.

    MATERIALS AND METHODSExperimental Approach. We simulated capillary fringe

    fluctuations in a porous medium made of a glass column filledwith glass beads. The behavior of colloids during the capillary

    Received: April 11, 2014Revised: June 3, 2014Accepted: June 4, 2014Published: June 4, 2014

    Article

    pubs.acs.org/est

    2014 American Chemical Society 7272 dx.doi.org/10.1021/es501797y | Environ. Sci. Technol. 2014, 48, 72727279

    pubs.acs.org/est

  • fringe fluctuations were visualized by confocal microscopy. Tovisualize the different phases, we used fluorescent colloids and afluorescent aqueous solution. This allowed us to distinguish thecolloids; the water and air phases, including the airwaterinterfaces; and the glass beads in real time.We selected four scenarios for the capillary fringe experi-

    ments (Figure S2, Supporting Information). The first threescenarios were situations of colloids interacting with a movingairwater interface. These are (a) colloids suspended in theaqueous phase while the aqueous phase imbibes and drainsthe porous medium to simulate capillary fringe fluctuations;(b) colloids initially attached to the glass beads in a wet porousmedium; and (c) colloids initially attached to the glass beads ina dry porous medium. The last scenario (d) mimics a situationwhen moving colloids interact with a static airwater interface(i.e., a trapped air bubble).Capillary Fringe System. We used a small-diameter

    (i.d. 1.5 mm) glass column of 7.5 cm length (see SupportingInformation and Figure S1 for more details). We prepared andcleaned the column as described previously,33,40 and filled thecolumn with glass beads. The glass beads had a diameter of400600 m (EW-36270-51, Cole-Parmer Instrument Co., IL).These glass beads were soaked overnight in 10% HCl, rinsedwith DI water, and air-dried at room temperature before beingfilled into the channel. We verified the size uniformity of the glassbeads by scanning electron microscopy (FEI Quanta 200F, FEICo., Hillsboro, OR). The porosity of the packed column wasdetermined by measuring the weight difference between a drycolumn and a water-saturated column. We then used this weightdifference to calculate the saturated volumetric water content,

    which is equivalent to the porosity of the packed column. Theporosity of our system was 0.55 cm3/cm3. This high porosity wasdue to boundary effects of the glass channel, preventing a closepacking along the channel walls. We focused our confocal viewduring our experiments into the interior of the channel, andtherefore, this boundary effect is not expected to be significant.Both ends of the column were connected to Tygon tubes,

    and one of the tubes was connected to a withdrawing/infusingsyringe pump (KDS 210, KD Scientific, Holliston, MA) so thatwe could introduce and control capillary fringe fluctuations.The other end of the column was left open to an outflowcontainer to allow free passage of air in and out of the channel.The column was then placed horizontally on the platform of alaser scanning confocal microscope (Axiovert 200 M equippedwith LSM 510 META, Carl Zeiss Jena GmbH, Germany). Seethe Supporting Information for more details on the microscopy.

    Colloids and Liquids. We used hydrophilic carboxylate-modified polystyrene colloids (FluoSpheres, Lot No. 28120W,Molecular Probes Inc., Eugene, OR) with a diameter of 1 m.The colloids were spherical, fluorescent with an excitation/emission wavelength of 505/515 (yellow-green), and negativelycharged, coming from the same batch used by Aramrak et al.40

    For one selected experiment (scenario 4 below), we alsoused hydrophobic sulfate-modified (FluoSpheres, Lot No.556340, Molecular Probes Inc., Eugene, OR), and hydrophilicamine-modified colloids (FluoSpheres, Lot No. 1306543,Molecular Probes Inc., Eugene, OR). These additional twocolloids were also yellow-green fluorescent as the carboxylate-modified colloids. The properties of the colloids are listed inTable S1 (Supporting Information).

    Figure 1. Locations and trajectory of carboxylate-modified colloids from scenario 1: (a) colloids in aqueous phase only (initial condition), (b)colloids deposited at the solidwater interface (SWI), (c) colloids attached to a mobile airwater interface (AWI), (d, e) colloids moving with AWI,and (f) transport back to the bulk liquid phase. Yellow and blue arrows indicate the direction of imbibition and drainage fronts, respectively. A whitecurve arrow indicates trajectories of the colloid of interest. Figures show subsequent snapshots during imbibition and drainage.

    Environmental Science & Technology Article

    dx.doi.org/10.1021/es501797y | Environ. Sci. Technol. 2014, 48, 727272797273

  • The liquid phase consisted of an aqueous solution of 1 mMCaCl2 and pH 5.5, that is, the same solution we had usedpreviously,40 but here, we further added 0.09 mM (0.01% w/v)of sulforhodamine B dye (Acid Form, laser grade, Dye Content95%, Lot No. 20223EAV, Sigma-Aldrich Co., MO). Thisdye is fluorescent with excitation/emission wavelengths of565/585 nm (red). The CaCl2 and pH were selected to provideconditions of unfavorable attachment of the colloids to the glassbea