Does Water Content or Flow Rate Control Colloid Transport in Unsaturated Porous Media?

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<ul><li><p>Does Water Content or Flow Rate Control Colloid Transport inUnsaturated Porous Media?Thorsten Knappenberger,*, Markus Flury, Earl D. Mattson, and James B. Harsh</p><p>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</p><p>*S Supporting Information</p><p>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 &lt; 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.</p><p> 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</p><p>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</p><p>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</p><p>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</p><p>Compared with the saturated groundwater zone, much less isknown about colloid transport in the unsaturated vadose zone.1</p><p>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</p><p>Colloids can be captured at the airwater interface18,19 andmove through a porous medium with an infiltration front.20</p><p>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</p><p>It has been proposed in the literature16,2325 that both watercontent as well as water flow rate are important drivers for</p><p>Received: October 21, 2013Revised: February 15, 2014Accepted: March 3, 2014Published: March 3, 2014</p><p>Article</p><p>pubs.acs.org/est</p><p> 2014 American Chemical Society 3791 dx.doi.org/10.1021/es404705d | Environ. Sci. Technol. 2014, 48, 37913799</p><p>pubs.acs.org/est</p></li><li><p>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</p><p>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.</p><p> EXPERIMENTAL METHODSGeneral Approach. We investigated how colloid transport</p><p>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</p><p>unsaturated porous media, steady-state water flow is describedby the DarcyBuckingham law:</p><p>= </p><p>+</p><p>q K z z( )w m</p><p>m g</p><p>(1)</p><p>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</p><p>= </p><p>q K r r( )w m</p><p>m 2</p><p>(2)</p><p>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</p><p>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,</p><p>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 &amp; 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.</p><p>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).</p><p>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-immobile water fractions. We calculated the critical accelerationbeyond which colloid behavior will be affected by centrifuga-tion:28</p><p> =</p><p>a</p><p>kTd r36</p><p>critc3p (3)</p><p>where k is the Boltzmann constant, T is the absolutetemperature, dc is the colloid diameter, rp is the average poreradius, and is the density difference of colloids and liquid.For our polystyrene colloids (density = 1.05 g/cm3, diameter =220 nm) and porous medium (rp = 52.2 m) the criticalacceleration is 173g. Colloid behavior should therefore not beaffected by centrifugal accelerations up to 173g.</p><p>Solution Chemistry and Sequence of Liquids. Colloidswere suspended at a concentration of 1012 particles/L in a100 mM NaCl solution buffered at pH 10 with 1.67 mMNaHCO3 and 1.67 mM Na2CO3. According to DLVOcalculations, colloids would attach to sand particles in asecondary energy minimum (see Supporting Information,Section S2 and Figure S4). We measured the hydrodynamicdiameter of the colloids over time to ensure that the colloidsuspension was stable (Supporting Information Figure S5).The column was first flushed with two pore volumes of</p><p>deionized water (E-pure, Brandstaedt, IA, electrical conductiv-ity &lt; 5.5 106 S/m), followed by the nitrate tracerbreakthrough of four pore volumes. Afterward, the columnwas flushed with two pore volumes of the pH 10, 100 mM</p><p>Environmental Science &amp; Technology Article</p><p>dx.doi.org/10.1021/es404705d | Environ. Sci. Technol. 2014, 48, 379137993792</p></li><li><p>NaCl solution without colloids to condition the column for thefollowing colloid breakthrough. A seven pore volume pulse ofcolloid suspension was then introduced into the column,followed by elution with five pore volumes of colloid-free NaClsolution. Finally, 10 pore volumes of deionized water wereintroduced to release colloids attached in the secondary energyminimum (see Supporting Information, Table S2 for asummary of the sequence). This sequence was used for eachdifferent water content described below. After each sequence,the sand was removed, sonicated, washed, and then repackedinto the column.Column Transport Experiments. Experiments were</p><p>carried out under normal gravity and under centrifugalacceleration to vary water content and flow rates. For theexperiments under centrifugation, we used the geocentrifugefacility at the Idaho National Laboratory31 (50 g-tonne ActidynSystemes model C61-3, France). The centrifuge has a radius of2 m and accepts a pay load of 500 kg and accelerations up to130g with platform dimensions of 70 cm length, 50 cm depth,and 60 cm height.The gravity experiments were essentially the same as under</p><p>centrifugation, except that we used a peristaltic pump (IPC 4,Ismatec, Glattbrugg-Zurich, Switzerland) under gravity and apiston pump (Encynova, model 2-4, Broomfield, CO) undercentrifugation. To relate acceleration, water content, and flowrate, we first developed calibration curves by setting thecentrifuge to different accelerations (2, 10, 20, 30, and 40g) andapplying different flow rates. We then determined thecorresponding pore water velocities for the different accel-erations based on the imposed flow rate and the measuredwater content (Figure 1a). On the basis of these measurements,we selected appropriate accelerations to obtain a series ofdistinct water contents and flow rates for the colloid transportexperiments.The selected flow rates, water contents, and water saturations</p><p>for all experiments are summarized in Table 1 and Figure 1b.Water saturation was calculated as Se = ( r)/(s r),where is the volumetric water content, r is the residual watercontent, and s is the saturated water content. Under gravity,we made a series of experiments at water contents of 0.38 and0.30 cm3/cm3 with corresponding pore water velocities of 10.5and 6.2 cm/min, respectively. The 0.38 cm3/cm3 is thesaturated water content of the medium. Under centrifugalacceleration, we made two series of experiments at constantpore water velocities of v 5.0 and v 10.0 cm/min, each withthree different water contents.UVvis Spectrophotometry and Data Processing.</p><p>Nitrate and colloids in the column outflow were measuredreal-time with a in-line flow cell connected to a UVvisspectrophotometer (USB-4000, Ocean Optics, Dunedin, FL).Nitrate breakthrough was measured at 230 nm and thesubsequent colloid breakthrough at 240 nm wavelengths.Calibration curves were developed from dilutions of concen-trated stock solutions (see Supporting Information for details).The nitrate and colloid breakthrough curves were smoothed</p><p>with a SavitzkyGolay32 filter to remove instrumental noise.The nitrate breakthrough curves were analyzed with CXTFIT33</p><p>to determine mobile-immobile water fractions and to check forchanges of dispersion at different accelerations.</p><p> THEORETICAL CONSIDERATIONSInterconnection of Pendular Rings. Under unsaturated</p><p>flow, the flow pathways for colloids are restricted by the</p><p>presence of air. At higher water saturation, continuous flowpathways exist, but as the saturation decreases, these pathwaysdisconnect and water is mostly located in the angular porespace formed by neighboring soil grains. For porous mediamade of spherical grains, the water at low saturation formspendular rings.34,35</p><p>The water saturation (and matric potential) at whichpendular...</p></li></ul>

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