does water content or flow rate control colloid transport in unsaturated porous media?

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  • Does Water Content or Flow Rate Control Colloid Transport inUnsaturated Porous Media?Thorsten Knappenberger,*, Markus Flury, Earl D. Mattson, and James B. Harsh

    Department of Crop and Soil Sciences, Washington State University, Puyallup, Washington 98371, United StatesIdaho National Laboratory, Idaho Falls, Idaho 83415, United StatesDepartment of Crop and Soil Sciences, Washington State University, Pullman, Washington 99164, United States

    *S Supporting Information

    ABSTRACT: Mobile colloids can play an important role incontaminant transport in soils: many contaminants exist incolloidal form, and colloids can facilitate transport of otherwiseimmobile contaminants. In unsaturated soils, colloid transportis, among other factors, affected by water content and flow rate.Our objective was to determine whether water content or flowrate is more important for colloid transport. We passednegatively charged polystyrene colloids (220 nm diameter)through unsaturated sand-filled columns under steady-stateflow at different water contents (effective water saturations Seranging from 0.1 to 1.0, with Se = ( r)/(s r)) and flowrates (pore water velocities v of 5 and 10 cm/min). Watercontent was the dominant factor in our experiments. Colloidtransport decreased with decreasing water content, and below a critical water content (Se < 0.1), colloid transport was inhibited,and colloids were strained in water films. Pendular ring and water film thickness calculations indicated that colloids can moveonly when pendular rings are interconnected. The flow rate affected retention of colloids in the secondary energy minimum, withless colloids being trapped when the flow rate increased. These results confirm the importance of both water content and flowrate for colloid transport in unsaturated porous media and highlight the dominant role of water content.

    INTRODUCTIONSubsurface colloids can enhance the movement of stronglysorbing contaminants, a phenomenon called colloid-facilitatedcontaminant transport.1 In the presence of mobile subsurfacecolloids, some contaminants may move faster and farther,thereby bypassing the filter and buffer capacity of soils andsediments. Many contaminants can sorb onto colloids insuspension; this increases their mobile-phase concentrationsbeyond thermodynamic solubilities.2 Colloid-facilitated trans-port has been reported in several studies for heavy metals,3,4

    radionuclides,5,6 pesticides,7,8 hormones,9 and other contami-nants.10,11 Failure to account for colloid-facilitated solutetransport will underestimate the transport potential for thesecontaminants.As colloids can enhance the transport of contaminants

    through soils, it is important to measure and understand colloidmobilization, deposition, and movement. Experimental andtheoretical results reveal that colloid mobilization anddeposition rates are sensitive to several physical and chemicalfactors, including water content, flow rate, porewater ionicstrength, and colloid size and composition.12 Colloids arefiltered from the bulk fluid to mineral grains by Browniandiffusion, interception, and sedimentation.13 The transportrates due to these three mechanisms can be calculated forwater-saturated media as functions of physical factors of the

    porous medium-water-colloid system, including colloid diam-eter and density, grain size, and flow velocity.1315 Underunfavorable attachment conditions, a repulsive energy barrierexists between mineral grains and colloids. Colloids may notovercome this energy barrier for attachment to mineral grainsbut can be immobilized by a secondary energy minimum.12

    Compared with the saturated groundwater zone, much less isknown about colloid transport in the unsaturated vadose zone.1

    The amounts of colloids transported are usually less underunsaturated flow than under saturated flow.16,17 The interactionof colloids with the airwater interface has been invoked as adominant process in colloid retention in the vadose zone.1

    Colloids can be captured at the airwater interface18,19 andmove through a porous medium with an infiltration front.20

    When colloids are attached to the airwater interface, thecapillary forces acting on the colloids are so strong that theattachment of colloids to the airwater interface can beconsidered irreversible.2123

    It has been proposed in the literature16,2325 that both watercontent as well as water flow rate are important drivers for

    Received: October 21, 2013Revised: February 15, 2014Accepted: March 3, 2014Published: March 3, 2014

    Article

    pubs.acs.org/est

    2014 American Chemical Society 3791 dx.doi.org/10.1021/es404705d | Environ. Sci. Technol. 2014, 48, 37913799

    pubs.acs.org/est

  • colloid mobilization and transport in unsaturated porous media.However, no conclusive experimental evidence exists on whichfactor is more important. Under gravity alone, water contentand flow rate in unsaturated porous media are not independentand cannot be varied independently: the relationship betweenwater content and flow rate is a characteristic property of theporous medium. However, if the body force (e.g., gravity) canbe changed, then we can change water content and flow rateindependently. This can be done with a centrifuge, throughwhich the body force can be increased.Centrifuges have been used to study colloid transport under

    both saturated2628 and unsaturated flow.27,29 However, nosystematic evaluation of effects of water content versus flow rateon colloid transport has been reported. Such an evaluation willclarify important mechanisms of colloid transport in unsatu-rated porous media. The objective of our study was toexperimentally determine the effects of water content and flowrate on colloid transport in unsaturated porous media. Wehypothesized that the water content will dominate over flowrate in its effect on colloid transport because both configurationand surface area of the airwater interface are expected todrastically change with water content. We used a geocentrifugeto change the body force, so that we could independently varywater contents and flow rates.

    EXPERIMENTAL METHODSGeneral Approach. We investigated how colloid transport

    was affected by water content and flow rate by conductingcolloid transport experiments under unsaturated steady-stateflow in a geocentrifuge. We designed column experiments withconstant pore water velocities but different water contents andobtained a series of column breakthrough curves.Unsaturated Water Flow in a Centrifugal Field. In

    unsaturated porous media, steady-state water flow is describedby the DarcyBuckingham law:

    =

    +

    q K z z( )w m

    m g

    (1)

    where qw is the water flux, K(m) is the unsaturated hydraulicconductivity, m is the matric potential of the medium, g is thegravimetric potential, and z is the depth. Under centrifugalacceleration, eq 1 can be written as30

    =

    q K r r( )w m

    m 2

    (2)

    where is the density of the liquid, is the angular speed, andr is the radius from the center of rotation. In a centrifugal fieldit is possible, by varying the angular speed, to establish differentfluxes qw at a given matric potential m and hence constantwater content. Furthermore, a given flux can be established atdifferent matric potentials m and hence different watercontents. Consequently, in a centrifugal field it is possible tovary flow rates at constant water contents and vice versa.Column Setup. We used a Plexiglas column with an inner

    diameter of 5.1 cm and a length of 15 cm (Figure S1,Supporting Information). As the bottom boundary, we used anylon membrane, mesh size 500 (NM-E #500, GilsonCompany, Inc., Lewis Center, OH) supported by a metal frit.Suction was applied with a vacuum pump and a vacuumchamber. The suction at the bottom of the column wasmeasured with a pressure transducer (26PCCFG6G, 1 bar,

    Honeywell, Morristown, NJ) placed under the metal frit. At adistance of 4 and 11 cm from the bottom, we installedtensiometers and TDR probes to measure the matric potentialand the water content. The suction on the tensiometers wasmeasured with pressure transducers (26PCCFG6G). The TDRprobes were connected to a cable tester (1502C, Tektronixs,Beaverton, OR), and the reflection curves were recorded with adata logger (CR23X, Campbell Scientific, Inc., Logan, UT).Pressure transducers and TDR probes were calibrated undernormal gravity. We designed the TDR probes to fit the columndiameter and used 3D printing techniques to produce theprobe heads. The liquids were introduced into the columnthrough a porous stone (L8405, Hogentogler & Co., Inc.,Columbia, MD) to ensure even distribution over the wholesectional area of the column. The column was designedspecifically for use in a geocentrifuge, so that centrifugal forcewould not affect the column operation.

    Porous Medium. Silica sand (3382-05, Mallinckrodt Baker,Inc., Phillipsburg, NJ), fractioned between 250 and 425 m bywet sieving, was used for the porous medium. The sand waspretreated with 2 M HCl at 90 C temperature for 24 h toremove organic and iron impurities. The sand was packed intothe column in 1 cm depth increments into standing water toensure saturated conditions. The packed sand had a porosity of = 0.38 cm3/cm3. The saturated pore volume in the columnwas 114.6 cm3. We determined the water retention character-istics with the hanging water column method (see SupportingInformation, Section S1 and Figure S2).

    Model Colloids and Tracer. We injected carboxylate-modified polystyrene colloids with a diameter of 220 nm(PC02N/6481, Bangs Laboratories, Inc., Fishers, IN) at the topof the column. Selected properties of the colloids are listed inTable S1 (Supporting Information). Nitrate (1 mM NaNO3)was used as a tracer prior to each colloid transport experimentto check for uniformity of flow and to determine mobile-

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