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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Sampling Silica and Ferrihydrite Colloids with Fiberglass Wicks under Unsaturated Conditions

^a Department of Environmental Science, University of Idaho, Moscow, ID 83844
^b Current address: Washington State Department of Ecology, Olympia, WA 98504
^c Department of Biological and Agricultural Engineering, University of Idaho, Moscow, ID 83844-0904
^d Department of Crop and Soil Sciences, Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164
^e Current address: Department of Physical Geography, Geographical Institute, University of Pécs, Hungary
^f Soils and Land Resources Division, University of Idaho, Moscow, ID 83844-2339

^* Corresponding author (barbwill{at}uidaho.edu)

Received for publication April 25, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The suitability of passive capillary samplers (PCAPS) for collection of representative colloid samples under partially saturated conditions was evaluated by investigating the transport of negatively and positively charged colloids in fiberglass wicks. A synthetic pore water solution was used to suspend silica microspheres (330 nm in diameter) and ferrihydrite (172 nm in diameter) for transport experiments on fiberglass wicks. Breakthrough curves were collected for three unsaturated flow rates with silica microspheres and one unsaturated flow rate with ferrihydrite colloids. A moisture characteristic curve, relating tensiometer measurements of matric potential to moisture content, was developed for the fiberglass wick. Results indicate that retention of the silica and the ferrihydrite on the wick occurred; that is, the wicks did not facilitate quantitative sampling of the colloids. For silica microspheres, 90% of the colloids were transmitted through the wicks. For ferrihydrite, 80 to 90% of the colloids were transmitted. The mechanisms responsible for the retention of the colloids on the fiberglass wicks appeared to be physicochemical attachment and not thin-film, triple-phase entrapment, or mechanical straining. Visualization of pathways by iron staining indicates that flow is preferential at the center of twisted bundles of filaments. Although axial preferential flow in PCAPS may enhance their hydraulic suitability for sampling mobile colloids, we conclude that without specific preparation to reduce attachment or retention, fiberglass wicks should only be used for qualitative sampling of pore water colloids.

Abbreviations: AWS, air–water–solid • PCAPS, passive capillary sampler • SEM–EDS, scanning electron microscopy–energy dispersive spectrometry

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
THE VADOSE ZONE is an important source of colloids and contaminants (Sprague et al., 2000). Transport of colloids is collectively influenced by the stability of colloidal material (tendency to stay suspended), the hydrodynamics of porous media, and the tendency for retention to occur in the soil or rock medium. Colloid stability is affected by elemental composition (Mayer and Jarrell, 1996), aqueous phase pH, ionic strength (Saiers and Lenhart, 2003), and electrolyte speciation (Liu et al., 1995; McCarthy et al., 2002). At least two mechanisms for retention occur: attachment and straining. Attachment is usually assumed to be the primary mechanism for retention in porous media. Attachment can be due to electrostatic, van der Waals, hydrogen bonding, and hydrophobic interactions. The mechanism of straining may be due to mechanical trapping when pore throats are too small to permit passage (McDowell-Boyer et al., 1986; Bradford et al., 2002), or, at lower moisture contents in the vadose zone, may result from thin film entrapment or entrapment at air–water–solid (AWS) interfaces (Wan and Tokunaga, 1997; Jin et al., 2000; Lenhart and Saiers, 2002; Crist et al., 2004). Hydrodynamic properties of the porous medium such as the saturated hydraulic conductivity (K_s), unsaturated hydraulic conductivity [K()], pore water velocity (v), and pore geometry govern the residence time of the pore water solution and pathway that a given colloid may take.

Sampling of colloids from pore water, particularly in the vadose zone, is challenging. The negative water potentials that persist in the vadose zone make retrieval of representative samples of colloids difficult. Aqueous samples are collected from the vadose zone with suction samplers (suction lysimeters), zero-tension samplers (zero-tension lysimeters), or passive capillary samplers (PCAPS). PCAPS collect pore water via a wick, the top end of which is unraveled and pressed against the underside of the horizon to be sampled. Installation of PCAPS is similar to that of zero-tension lysimeters. An access trench is excavated along the study site with horizontal tunnels to install the PCAPS. The use of PCAPS in vadose zone sampling has been studied in detail with respect to solute and flux sampling (Holder et al., 1991; Poletika et al., 1992; Boll et al., 1992; Brandi-Dohrn et al., 1996; Goyne et al., 2000; Louie et al., 2000; Gee et al., 2002, 2003) and also with respect to mobile–immobile pore water (Landon et al., 1999). We assumed a priori that passive capillary samplers (PCAPS), as opposed to suction and zero-tension samplers, have the best chance of yielding a representative sample for reasons described below.

Suction samplers have inherent limitations with respect to conservative transport of colloids. First, colloids may be mechanically strained or attached in the porous tip of the sampler. Second, the application of a vacuum may induce an unrepresentative geochemical condition, such as a geochemical gradient, around the suction sampler. If the vacuum pulls high-ionic-strength water from dead-end pores or smaller pore spaces that had not been contributing to flow at the pre-suction gradient, mobile colloids could flocculate. In other settings, the dispersion of colloids from aggregates could result from a decrease of ionic strength as a result of reduced solute residence time. Finally, if the suction imposed is significantly stronger than the ambient soil matrix potential, colloids may be physically sheared from aggregates.

Zero-tension samplers also have potential limitations for sampling of colloids. Zero tension samplers operate only if a zone of saturation has collected above the sampling device. As water percolates through the profile, flow lines are routed about the zone of saturation due to change in boundary conditions (Abdou and Flury, 2004). Routing biases collection of flux, chemical (dissolved) or colloid (suspended) samples.

Because fiberglass wicks alleviate the boundary condition problems described above, we test if they are suitable for colloid sampling. The degree to which fiberglass wick PCAPS attenuate colloid transport was recently studied in a similar investigation concurrent with this one (Czigány et al., 2005). In both the Czigány study and this one, different moisture contents (flow rates) for transport of negatively charged colloids were compared by evaluation of breakthrough curves, and examination of the transport of positively charged ferrihydrite was performed. In addition, the Czigány et al. (2005) investigation specifically tested negatively charged ferrihydrite, natural colloids, feldspathoids, montmorillonite, and kaolinite. Unique contributions added by our investigation here include development of a moisture characteristic curve and collection of tensiometer data along the length of the wick during the experiments to explicitly relate retention to moisture content, transport tests of monodisperse solutions of microspheres, and comparison of experimental results to calculations of thin-film thickness. The fiberglass wick products used in the two studies differed slightly in terms of cross-sectional area, and therefore in the magnitude of K_s. The conclusions of the Czigány et al. (2005) investigation were that flow rate, pH, and colloid type affected colloid breakthrough, and that the utility of using wicks for reliable sampling was therefore limited.

In another investigation of colloid transport through wicks, Biddle et al. (1995) concluded that PCAPS transmit representative samples of mobile colloids. However, in that study a colloidal suspension was collected from the field site with a PCAPS and then tested for retention in a laboratory wick experiment. If straining or attachment of colloid material occurred during the original collection of soil solution, those colloids will not have been present for the subsequent retention test, and attenuation of colloids may, therefore, have been underrepresented in the retention test.

In this study, we investigated the transport of positively and negatively charged colloids in fiberglass wicks at near-saturated and unsaturated conditions. We hypothesized that (i) negatively charged silica microspheres would be conservatively transmitted at flow rates equal to the saturated hydraulic conductivity (K_s) of the fiberglass wick and (ii) that at progressively lower flow rates than the saturated flux that the colloids would be increasingly retained, indicating thin-film straining or triple-phase (AWS) entrapment. These hypotheses were tested with transport experiments conducted at different flow rates and with different colloids (silica and ferrihydrite).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Column Design
A 47-cm-long column of 1.27-cm-thick acrylic tubing was designed to house the 50-cm-long, 9.5-mm-diameter fiberglass wick (no. 1380; Pepperell Braiding, Pepperell, MA). At the top of the acrylic tube a 6- x 6-cm acrylic plate was secured to provide a mounting platform for the fiberglass wicks (Fig. 1). The fiberglass wicks were unraveled and radially extended to the edges of the acrylic plate, then secured with silicone sealer around the edge. A sprinkler head consisting of 12 hypodermic needles was used to apply inflow solution to the wick surface. Aluminum foil enclosed the area around the sprinkler and splayed-out wick to reduce evaporation from the sprinkler and wick top.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Setup for unsaturated wick colloid transport experiments.

Wick Properties and Preparation
Clean 9.5-mm fiberglass wicks (no. 1380; Pepperell Braiding) were used in each experiment. The fiberglass wicks contain residues left as a result of the manufacturing process. To remove these residues, the wicks were combusted at 400°C, following the recommendations of Knutson et al. (1993). The combustion process removed 1.5% w/w of material, as determined by mass balance before dissolution of post-combustion residues. It has been observed that the combustion process leaves a residue of ash, salts, and polymers (Knutson et al., 1993). Combusted wicks were mounted into the acrylic tube and rinsed with deionized water. The initial pH of the post-combustion effluent when rinsing the wicks was 9.3 to 10.4, and dropped to 8.3 after 48 h. The initial electrical conductivity (EC) of the rinse effluent was 364 µS cm^–1, which dropped to 25 µS cm^–1 after 20 min. To further remove carbonates and combustion residue, the fiberglass wicks were soaked in a 10 mM HNO₃ solution bath, as described by Goyne et al. (2000). Once the pH stabilized at the initial concentration of nitric acid (typically pH of approximately 2), double-deionized water was substituted for the nitric acid bath solution. The double deionized water was replaced daily, until the supernatant solution electrical conductivity fell below 2 µS cm^–1.

Measurements of wick contact angle (ProcessorTensiometer K100; Krüss, Hamburg, Germany) were performed with double deionized water on out-of-package wicks, combusted-only wicks, and batch soaked wicks using the Washburn method. These contact angle measurements indicated that the out-of-package wicks, combusted-only wicks, and acid-soaked wicks are all hydrophilic with a contact angle of 0°. Wicks were further characterized by scanning electron microscopy– energy dispersive spectrometry (SEM–EDS; Department of Geosciences, University of Idaho), and Brunauer–Emmett–Teller (BET) surface area analysis (ASAP2010; Micromeritics, Norcross, GA).

The fiberglass wicks were assumed to have a negative surface charge based SEM–EDS analysis that identified the wick material as predominantly silica. The surface charge of silica gel has been documented to be negative in the pH range of 3 to 10 (Langmuir, 1997). This assumption was substantiated in Czigány et al. (2005) for ferrihydrite colloids of different surface charges. In Czigány et al. (2005), positively charged ferrihydrite was nearly 100% removed (attached), while negatively charged ferrihydrite moved without retention through the saturated wick.

Wick Hydraulic Properties
Basic properties of the wick such as, bulk density (_b), porosity (), and volumetric water content (_v) were determined. The total volume (V_t) for several samples was measured by multiplying the length of 2-cm sections by the area of the cross-section (d = 9.5 mm). The bulk density (_b = 0.34 g cm^–3) is an average of multiple air-dried samples and agrees with the bulk density given by Knutson and Selker (1994). The bulk density was used to determine the porosity of the fiberglass wicks, assuming the specific density of the fiberglass wick to be 2.6 g cm^–3 (Knutson and Selker, 1994).

The volumetric water contents of the fiberglass wicks at various matric potentials were measured to determine the moisture retention curve for the fiberglass wick material. The wet end of the moisture characteristic curve was developed using a hanging water column (Hillel, 1980). Two-centimeter-long sections of the fiberglass wick were placed vertically on a porous plate. Suction was applied by lowering the hanging water column. The samples came to equilibrium soon after the suction was applied. The samples were weighed and oven-dried for at least 24 h and immediately reweighed to determine the gravimetric moisture content. Gravimetric water contents (_m) were converted to volumetric water contents using the bulk density. The dry end of the moisture characteristic curve was measured with a Dewpoint PotentiaMeter (WP4; Decagon, Pullman, WA).

The measured data were then analyzed with the van Genuchten model (van Genuchten, 1980):

[1]

where _r is the residual water content ; _s is the saturated water content ; is the matric potential (m); and (m^–1) and n (–) are empirical parameters. Nonlinear regression was used to estimate the model parameters. The model was then used to calculate moisture content from tensiometer data obtained during the experiments.

Silica and Ferrihydrite Colloids
Silica beads were obtained from Bangs Laboratories (Fisher, IN). The beads were spherical with a diameter of 330 nm and specific density of 2.0 g cm^–3. Ferrihydrite was synthesized following Schwertmann and Cornell (2000, p. 105–110) with the following modifications: FeCl₂ was substituted for Fe(NO₃), and silica was added as Na₂SiO₃. We included silica in the synthesis because Si increases the stability of colloidal ferrihydrite (Anderson and Benjamin, 1985; Mayer and Jarrell, 1996; Schwertmann and Cornell, 2000). The point of zero charge (PZC) was determined by stabilizing the pH of a dilute suspension of the ferrihydrite in the synthetic pore water solution used for the experiments and then measuring the electrophoretic mobility at specific pH values (3 to 12) on a Zetasizer 3000 HSA (Malvern, Worcestershire, UK). The isoelectric point was found to be at pH of approximately 6.3. The published particle density for ferrihydrite is 4 g cm^–3 (Schwertmann and Cornell, 2000) and the particle diameter was 172 nm, determined by dynamic light scattering (Zetasizer 3000 HSA).

Colloid Transport Experiments
A synthetic pore water solution comprised of 4.45 mM CaCl₂, 1.4 mM MgCl₂, 0.7 mM NaCl, and 0.4 mM KCl in double deionized water was used for all experiments and colloidal suspensions. This solution has an ionic strength of 18 mM, consistent with Soil Survey data for a Palouse soil (fine-silty, mixed, superactive, mesic Pachic Ultic Haploxerolls) (USDA Natural Resources Conservation Service, 2001). Palouse soil is a loess silt loam typical of the Palouse region of eastern Washington, with silt fraction composed primarily of quartz and feldspar. Ionic strength in the outflow was monitored for electrical conductivity (Model 250; Denver Instruments, Denver, CO). The input colloidal concentrations were 100 mg L^–1 for the silica microsphere solutions, and 300 to 600 mg L^–1 for the ferrihydrite. The pH of the silica microsphere solutions was 6.4 to 6.7 and for the ferrihydrite was 4.0. The pH was adjusted using 1 M KOH or 1 M HCl.

Nitrate was used as a conservative tracer to make comparisons with the colloid breakthrough curves. Nitrate was quantified spectrophotometrically with absorbance measurement at 206 nm (Model 1601; Shimadzu, Kyoto, Japan). The synthetic pore water solution was used as the carrier solution and 0.2 mM Ca(NO₃)₂ was added as conservative tracer.

The silica microspheres were used to test the transport of negatively charged colloids moving along a negatively charged wick, recognizing that surface interactions like London–van der Waals interactions, hydration, steric and/or hydrophobic interactions could result in attachment or exclusion of the colloids. In addition, a reduction of moisture content was expected to cause an increased retention of colloids with the fiberglass wick due to filtering (mechanical straining), thin-film straining, or triple-phase (gas, liquid, solid interface) entrapment (Wan and Tokunaga, 1997; Jin et al., 2000; Lenhart and Saiers, 2002; Crist et al., 2004). To observe whether retention was increased with a reduction in moisture content, three flow rates were chosen for the experiments: a near-saturated flow rate of 6.8 mL min^–1, an unsaturated flow rate of 2.5 mL min^–1, and an unsaturated flow rate of 0.56 mL min^–1. The second two flow rates are reasonable percolation rates in a sandy soil (Boll et al., 1992), and in this system correspond to matric potentials of –10 to –30 cm H₂O, respectively, at the top of the fiberglass wick.

The ferrihydrite experiments were conducted to study qualitatively the electrostatic interactions of the fiberglass wick with a positively charged colloid. Electrostatic attachment was assumed to dominate. The experiments were performed at a flow rate of 2.5 mL min^–1.

Three repetitions were performed for each flow rate for silica microspheres, and three repetitions were performed for ferrihydrite. Effluent colloid concentration was measured for every other sample to every fourth sample. The remaining sets of one to three samples were combined to measure the effluent pH and ionic strength.

To compare breakthrough curves, pore volumes (T = vt/L), where v is average pore water velocity for each pore reach, and relative concentration (C/C_o) were used to normalize the time and concentration data. Due to the variable water content over the length of the wick, pore water velocity was calculated for each tensiometer location, and then a single average value of pore water velocity was used to calculate an average value of pore volume.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Wick Characterization
The specific surface area for a wick sample was 4752 cm² g^–1 using Brunauer–Emmett–Teller surface area analysis. The SEM–EDS analysis of an acid-soaked fiberglass wick identified the presence of Na, Mg, Al, K, and Ca on the surface (Fig. 2). This corroborates the findings of Goyne et al. (2000) and Brahy and Delvaux (2001) of cations sorbed to the wick surface. The measured and modeled water retention characteristics of the fiberglass wick are shown in Fig. 3. The data show some hysteretic behavior. The van Genuchten relationship fits the data well. Swelling of the wick was observed at near saturated water contents; the saturated volumetric water content was directly measured to be 0.90, although the porosity calculated from bulk density for a dry wick was 0.87.

View larger version (132K): [in this window] [in a new window]   Fig. 2. Scanning electron microscopy (SEM) image of fiberglass wick after cleaning process; notice impurity on one fiber after cleaning, which indicates that the cleaning process is not complete.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Liquid retention curve for fiberglass wick. Symbols are measured data, line is fitted van Genuchten relationship. Main hysteresis loop of retention curve is shown for drying limb only.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Vertical moisture profiles in fiberglass wick.

View larger version (22K): [in this window] [in a new window]   Fig. 5. Breakthrough curves for nitrate and silica microspheres at different flow rates.

The breakthrough of positively charge ferrihydrite colloids at pH 4 was strongly retarded (Fig. 6). From the breakthrough curves it is noted that the partitioning of the ferrihydrite is not the same for the three wicks tested. Variability among ferrihydrite breakthrough curves may be due to surface chemical heterogeneity of the fiberglass wicks and/or variations in solution suspension concentration. We note that the increased retention for a positively charged surface supports the interpretation that attachment is an operative mechanism. We make the general observation, however, that if wicks are used to sample positively charged colloids such as ferrihydrite at this pH in soils, adsorption of positive colloids to the fiberglass wick is strong, but may not be complete. Some fraction of the positively charged colloids can be sampled with wicks, depending on the pH. Czigány et al. (2005) observed no breakthrough of ferrihydrite at pH 7, but conservative behavior at pH 10.

View larger version (19K): [in this window] [in a new window]   Fig. 6. Breakthrough curves for ferrihydrite colloids at pH 4 (three replicates).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Preferential Flow in Wick Center
The weight-averaged moisture contents were not uniform across the cross-section of each bundle of fibers in the wick. This was concluded because orange ferrihydrite staining was more pronounced at the center of each bundle, indicating that the bundle of filaments is more tightly bound near the center than at the periphery. This allows for the center of each filament bundle to hold water at lower flow rates (i.e., more negative pressures). This suggests that at intermediate to low flow rates the radial anisotropy of each twisted bundle of filaments allows for preferential flow through the more tightly bound pore spaces at the center of each bundle (Fig. 7). As moisture content increases, water would spread to the larger pores, moving outward from the center. This suggests that the wick is effectively self-adjusting the portion of area available to flow in response to changing moisture contents (flow rates). It is likely that we did not observe thin-film straining or triple-phase entrapment because as moisture content was lowered, the flux receded to the central core of each bundle. Lower flow rates than those presented here would be needed to ensure that all the pores drain within the twisted bundle of filaments. This is consistent with the finding by Czigány et al. (2005) that at lower moisture contents than those used in this study, a moisture content–dependent straining mechanism does occur.

View larger version (28K): [in this window] [in a new window]   Fig. 7. Conceptual depiction of fiberglass wick pore geometry; shading indicates degree of staining by ferrihydrite. Filament diameter measured with scanning electron microscopy–energy dispersive spectrometry (SEM–EDS).

[2]

where is the surface tension of the liquid (g s^–2), is the specific density of the liquid (g m^–3), g is the acceleration due to gravity (m s^–2), and L is the length of the pore side (m). Solving Eq. [2] for emptying of the largest pore space provided by rhombohedral packing geometry of fiberglass filaments with 7-µm diameter (Fig. 7) indicates that a suction greater than –50 cm H₂O of matric potential is needed to drain the pore space. A matric potential of –30 cm H₂O was the highest suction measured in this system; therefore, these pore spaces were never drained. In the Czigány et al. (2005) study, suctions of –50 cm H₂O were probably reached, as described above, which may be why retention increased with decreased moisture contents in that study. When pores snap open to drain, the remaining menisci in the pore corners provide more geometrically confined, wedge-shaped surface areas where corner straining or AWS interface sorption can occur.

A second potentially favorable feature of wicks, in addition to retreat to a smaller wetted cross-section at lower moisture contents, is that the parallel orientation of the filaments to the direction of flow allows for a longitudinal hydraulic connectivity, with fewer pore necks and dead-end pores along the length of the travel path as compared to ceramic or sintered steel sampling devices.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
For sampling 330-nm silica microspheres in a synthetic pore water system with a pH of 6.5 and an ionic strength of 18 mM, approximately 10 pore volumes must pass through a 47-cm-long wick to obtain a C/C_o greater than 90%. For ferrihydrite, sampled in a pore water system with pH 4 and an ionic strength of approximately 19 mM, more than 60 pore volumes must pass through to obtain a C/C_o of 80%. The mechanism of retention is attachment, and not thin-film straining. Based on these findings, the use of PCAPS to measure the concentrations of colloids in vadose zone pore water results in biased (attenuated) output concentrations. Even negatively charged particles are not transported without retention. Transport of colloids along fiberglass wicks is affected by many of the same factors as transport through soil and fractured rock, including attachment, and colloidal stability controls such as pH and ionic strength. These results confirm that although wicks may be used for qualitative identification of colloids in mobile pore water, without specific preparation to reduce attachment or retention, they should not be expected to provide accurate pore water colloid concentration measurements. These findings are consistent with the results of the Czigány et al. (2005) investigation.

Additional evaluation of PCAPS for the purpose of collecting soil-water colloids should include testing other particles with varied size and surface properties [i.e., organic carbon coated ferrihydrite, soil organic matter (SOM), humic acids], and alteration of the pore water solution (i.e., pH, ionic strength, carbonate concentration, and electrolyte species and ratios). Using a different fiber material in the PCAPS system may yield more conservative transport along wicks. To date, fiberglass PCAPS have been used to minimize cost, and no substitute was sought because fiberglass has proven adequate for the sampling of dissolved constituents and measurement of flux. If the conservative transmission of mobile colloids is critical, the use of engineered filaments of an optimized material braided into wicks will likely provide the best results, where charge characteristics, pore geometry, and roughness can be controlled.

	ACKNOWLEDGMENTS

 
The Idaho National Environmental Engineering Laboratory, Grant #00001671005, and the Department of Biological and Agricultural Engineering at the University of Idaho supported this research. We thank Jorge Jerez for synthesizing the ferrihydrite colloids.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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