Published in Vadose Zone Journal 4:175-183 (2005)
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
ORIGINAL RESEARCH
Suitability of Fiberglass Wicks to Sample Colloids from Vadose Zone Pore Water
Szabolcs Czigánya,
Markus Flurya,*,
James B. Harsha,
Barbara C. Williamsb and
Jason M. Shirab
a Department of Crop and Soil Sciences, Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164
b Department of Biological and Agricultural Engineering, University of Idaho, Moscow, ID 83844
* Corresponding author (flury{at}mail.wsu.edu)
Received 11 May 2004.
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ABSTRACT
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Fiberglass wicks are frequently used to sample pore water and determine water fluxes in soils. In this study we evaluated the performance of fiberglass wicks to sample colloids. Different colloids were used for the wick testing: feldspathoids, ferrihydrite, montmorillonite, kaolinite, and a mixture of mineral colloids extracted from a coarse calcareous sediment. The colloids were dispersed in either a buffered Na2CO3–NaHCO3 solution (ionic strength 6.7 mM, pH 10) or deionized water. Colloid breakthrough curves through 77-cm-long fiberglass wicks were determined for three different flow rates. Flow rate, pH, and colloid type affected colloid breakthrough. Colloid recovery in the effluent was higher at pH 10 than at pH 7, and increased with increasing flow rate. The mixture of mineral colloids extracted from sediment moved almost conservatively through the wicks; the colloid recoveries ranged from 88 to about 100% for pH 7 and 10, respectively. Ferrihydrite at pH 10 moved conservatively, with recoveries ranging from 95 to about 100%. All other colloids, however, showed lower mass recoveries. At pH 10, colloid recovery ranged from 55% for montmorillonite to about 100% for ferrihydrite and the mixture of mineral colloids, whereas at pH 7, the recovery ranged from <5% for kaolinite and ferrihydrite to approximately 100% for the mixture of mineral colloids. These results suggest that for certain conditions and colloid types, fiberglass wicks can be an acceptable tool for colloid sampling in the vadose zone. However, under many conditions studied here colloids were significantly retained inside the wicks, and consequently, the use of wicks for colloid sampling in the vadose zone must be considered with caution.
Abbreviations: SEM, scanning electron microscopy
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INTRODUCTION
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DIFFERENT TECHNIQUES are used to sample pore water in the vadose zone, such as free-drainage lysimeters, suction cups or porous plates, and fiberglass wicks or rockwool samplers. Each of these devices has advantages and disadvantages. Free-drainage lysimeters only operate when the soil becomes saturated, and thereby cause hydrodynamic artifacts (Abdou and Flury, 2004). These artifacts are eliminated when using a porous material on which a suction is pulled to remove the pore water from the soil. Different types of porous materials have been used for that purpose, including ceramic materials, stainless steel, Teflon, or fiberglass.
Fiberglass wicks are attractive because no external vacuum device is needed to extract pore water. They were introduced as pore water samplers by Brown et al. (1986) and have been used since in numerous studies (Gee and Campbell, 1990; Boll et al., 1992; Brandi-Dohrn et al., 1996; Louie et al., 2000; Brahy et al., 2002; Cox et al., 2002). The volume of the soil sampled by the wicks can be closely estimated (Boll et al., 1991). Wick lysimeters are relatively inexpensive and easy to maintain. The length of the hanging wick determines the suction exerted on the soil above, and it needs to be matched with the soil type and the particular experiment (Rimmer et al., 1995). Wick lysimeters can collect pore water samples from soils with a wide range of structure (Holder et al., 1991).
Fiberglass wicks need to be cleaned before their use for sampling, as they may contain organic residues (Knutson et al., 1993). Knutson et al. (1993) recommended combustion at 400°C for 3 h to remove impurities, unless the wicks contain more than 3.5% (w/w) impurities, for which higher temperature and longer combustion time may be needed. Additional treatments, such as an acid wash in 10-mM HNO3, may be required if wicks are used to determine pore water composition (Goyne et al., 2000). Brahy and Delvaux (2001) suggested that, besides acid treatment, the wicks should be soaked in deionized water until the electrical conductivity of the water falls below 2 µS cm–1.
Wicks have been tested extensively in terms of water and solute collection efficiency (Boll et al., 1991; Steenhuis et al., 1995; Zhu et al., 2002). Experimental and numerical studies indicate that wicks are useful in determining water fluxes in the vadose zone (Louie et al., 2000; Gee et al., 2002, 2003). Solute transport characteristics of wicks were assessed with anions and organic dyes, and it is generally reported that dispersion and retardation is much smaller in wicks than in soils (Boll et al., 1992; Poletika et al., 1992; Knutson and Selker, 1996). Only a few studies are available on colloid transport in fiberglass wicks. The results of these studies are inconsistent in terms of the usefulness of wicks for colloid sampling. Poletika et al. (1992) reported that only 28.8 to 52% of an MS-2 virus was recovered in wick outflow. Biddle et al. (1995) found that colloids of particle diameters between 0.45 and 2 µm were not retained in the wicks, and colloid mineral composition in the effluent did not differ from that of the bulk soil. It remains to be shown whether fiberglass wicks are useful to sample colloids.
The objective of this study was to systematically test fiberglass wicks for their suitability to sample vadose zone colloids. We conducted transport experiments with five types of colloids: kaolinite, montmorillonite, ferrihydrite, feldspathoids, and colloids extracted from a calcareous sediment. Colloids were directly applied to the top of the wicks, and breakthrough curves determined in the outflow. Solution pH was either 7 or 10, and three different flow rates were used.
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MATERIALS AND METHODS
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Colloidal Material and Fractionation
We used different types of colloids in this study: colloids extracted from sediments (mixture of minerals) and mineralogically homogeneous colloids (montmorillonite, kaolinite, and ferrihydrite). The sediments used as colloidal source material were gravelly-sandy, very coarse Hanford sediments obtained from the submarine pit (218-E-12B) at the Hanford Site, WA in January 2000. Serne et al. (2002) provided a detailed description of the Hanford formation sediments. The mineralogy of the sediments is dominated by quartz, phyllosilicates (primarily micas, illites, and smectites), and feldspars. Plastic buckets were used to collect and store the sediments. The sediments were air-dried and sieved through a 2-mm square screen. This material served as the source for the "native colloids." An aliquot of the sediments was treated with an alkaline solution, which caused quartz and kaolinite to dissolve and the feldspathoids, cancrinite and sodalite, to precipitate (Zhao et al., 2004). These treated sediments served as a source for the "modified colloids." Pure clay mineral standards, Na-montmorillonite (SWy-2) and Na-kaolinite (KGa-1), were obtained from the Clay Minerals Repository (Columbia, MO). Two-line ferrihydrite was synthesized according to Schwertmann and Cornell (2000)(p. 105–111), whereby 10 mmol L–1 Si was used in the synthesis procedure to stabilize the mineral and prevent mineral transformations. The ferrihydrite was stored in suspension at pH 2 in a Nalgene bottle until it was used. We used X-ray diffraction (Philips XRG 3100, Philips Analytical Inc., Mahwah, NJ) to verify whether the ferrihydrite was mineralogically stable during the experiments. The shape of ferrihydrite particles was examined by Transmission Electron Microscopy (JEOL 1200EX, JEOL, Peabody, MA) and found to be roughly spherical.
To fractionate the colloidal fraction, about 1 kg of Hanford sediments or 200 to 250 mg of clay mineral standards were dispersed in 1-L glass cylinders. We used two different solutions to disperse the solids: (i) deionized water and (ii) a 1.67-mM Na2CO3/NaHCO3 solution (pH 10). Dispersions were stirred with a rod, sonicated for 10 min, and briefly shaken end-over-end by hand. Colloids, operationally defined as material with an equivalent diameter of <2 µm, were fractionated by decantation based on Stokes' sedimentation law. The mineralogy of the fractionated "native colloids" was dominated by chlorite, smectite, vermiculite, kaolinite, and quartz (Cherrey et al., 2003), and the "modified colloids" were dominated by cancrinite, sodalite, chlorite, smectite, and vermiculite. Ferrihydrite was not fractionated because its particle size was much less than 2 µm.
The colloid suspensions were diluted to a particle concentration of about 50 mg L–1 and used for the wick experiments described below. Colloid suspensions were sonicated for 30 min before the start of the wick experiments. Colloids were used within at most 7 d after fractionation. Average hydrodynamic diameter and electrophoretic mobility of the colloids were measured by dynamic light scattering (ZetaSizer 3000HSa, Malvern Instruments Ltd., Malvern, UK) in a weak electrolyte solution (1.67-mM NaHCO3/1.67 mM Na2CO3, pH 10).
Wick Treatment and Characterization
Fiberglass wicks of 12.5-mm diameter were obtained from Pepperell Braiding Co. (catalog no. 1381, Pepperell, MA). We treated the wick with a procedure described by Goyne et al. (2000). The wicks were first cut into 77-cm-long pieces, weighed, and rinsed extensively with deionized water and combusted in a kiln at 400°C for 4 h. The wicks were weighed after combustion to determine the weight loss. Then, they were soaked in deionized water (EC = 3.8 µS cm–1) in a 17-L Tupperware plastic container. Six wicks were treated simultaneously. We measured pH and electrical conductivity daily. Water was replaced with clean deionized water each day until pH and conductivity were constant. It took 5 to 6 d to reach constant pH and electrical conductivity. The equilibrated wicks were then soaked in a 10-mM HNO3 solution in the same container. The solution was replaced daily for 7 d. After the acid treatment, wicks were soaked again in deionized water for 7 d. It took 4 to 6 d to reach a constant pH and EC.
Treated and untreated (as shipped from supplier) wicks were characterized by scanning electron microscopy (SEM) and specific surface area determination. The SEM images were taken on gold-coated samples with a Hitachi S-570 SEM. We also took SEM images of wicks after they were used for colloid transport experiments to check for colloid deposition. For N2 BET surface area determination, about 5 g of wick material was used (ASAP 2010, Micromeritics Inc., Norcross, GA). The specific density of wicks was measured on individual wick braids with a Le Chatelier pycnometer (ASTM, 2000). The bulk density was approximated by weighing oven-dried pieces of wicks and estimating the volume using a circular wick cross section. The porosity was then calculated from these measurements. Quantitative measurements were made with three replicates.
Experimental Setup for Breakthrough Curves
The wick was mounted into an acrylic tube (18.5-mm i.d., 68.5-cm length) topped by a Plexiglas disk 9 cm in diameter (Fig. 1)
. The top 4.5 cm of the wick was unwoven, and the individual braids were spread and tightened to the disk with a Plexiglas ring and binder clips. A steady-state flow rate was established by dripping solution onto the center of the wick from a point dripper. The flow rate was controlled with a peristaltic pump (Ismatec IP4, Glattburg, Switzerland). The top of the wick was covered with a plastic beaker and a wrap to minimize evaporation.
Nitrate and Colloid Breakthrough Curves
Nitrate and colloid breakthrough curves were performed at three different flow rates: 5, 10, and 55 mL h–1. First, the wick was equilibrated with at least 12 pore volumes at a flow rate of 55 mL h–1, and the flow rate was then adjusted to the desired value. A new wick was used for each experiment to avoid contamination between experiments. Consequently, the wick pore volume differed slightly from run to run. We ran some experiments with the same wick to check for "history" effects in the wick. All experiments were performed at a room temperature of 22 ± 1°C.
Nitrate was used as a conservative tracer. A pulse of about three pore volumes of 0.2-mM NaNO3 was injected into the wicks. Effluent NO3– concentrations were determined with a UV-VIS spectrophotometer (Hewlett-Packard HP8452A) at 214-nm wavelength. The pH of the solutions for these experiments was between 6.5 and 7.5.
For the colloid breakthrough experiments, the wicks were first equilibrated for at least 12 pore volumes with the desired solution, either deionized water (pH 
6.5) or a buffered 1.67-mM NaHCO3/1.67-mM Na2CO3 solution (pH 10). The inflow solution was then switched to an approximately 50 mg L–1 colloid suspension for two to four pore volumes. Effluent pH was determined with a pH meter, and colloid concentrations were determined spectrophotometrically at 300-nm wavelength, except the ferrihydrite, which was measured at 214 nm. Before the measurements, individual vials were vigorously shaken by hand to resuspend any sedimented particles. The effluent flow rate was determined by weighing every sixth vial. After each breakthrough experiment, the overall water content of the wicks was determined gravimetrically by drying at 105°C for 24 h. This water content was used to estimate pore volumes for the breakthrough experiments. Some experiments were repeated to test reproducibility of the results.
To obtain more detailed information about the water content distribution in the wicks, we equilibrated clean wicks with colloid-free buffered solution at the three flow rates used for the colloid transport experiments. The wicks were then cut into 11-cm-long pieces, which were oven-dried to determine the water content.
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RESULTS AND DISCUSSION
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Colloid Properties
The hydrodynamic diameters of the colloids are summarized in Table 1. The hydrodynamic diameters of the colloids ranged from 475 to 620 nm, except for the ferrihydrite, which was smaller (175 nm). The electrophoretic mobilities as a function of pH are shown in Fig. 2
. The values depicted in Fig. 2 are in the range typical for soil minerals (Wu, 2001). The kaolinite particles had the most negative electrophoretic mobilities at both pH 7 and 10. At pH 7, the ferrihydrite particles had the least negative electrophoretic mobilities of all colloids. The isoelectric point for ferrihydrite was at pH 6.2. Ferrihydrite remained mineralogically stable during the experiments; the X-ray diffraction patterns indicated no mineral transformations.
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Fig. 2. Electrophoretic mobility of the colloids as a function of pH, measured in a 1.67-mM Na2CO3/NaHCO3 solution. Error bars denote ± one standard deviation.
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Wick Treatment and Characterization
The weight loss of the wicks during combustion ranged from 0.5 to 1.5% by weight, which is in the lower range reported by Knutson et al. (1993). When the wicks were soaked in deionized water after combustion, the pH of the water increased from 5.7 to 9.2 within the first 24 h. After 4 to 6 d the pH remained constant. The pH of the HNO3 solution remained fairly constant during soaking. The SEM micrographs show some impurities on the surface of the nontreated wicks (Fig. 3a)
. The combustion and washing removed these impurities to a large degree, but not completely (Fig. 3b). Some dark spots, likely caused by combustion, could be observed on the treated wicks. The specific surface areas were 0.48 ± 0.02 m2 g–1 for the nontreated wicks and 0.46 ± 0.02 m2 g–1 for the treated wicks. The specific density of the wick material was 2.10 ± 0.08 g cm–3, and the bulk density was 0.29 ± 0.01 g cm–3. The corresponding porosity was 0.86 ± 0.01 cm3 cm–3.
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Fig. 3. Scanning electron micrographs of (a) nontreated wick fiber (as received from supplier), (b) treated wick fiber (combusted and washed), and (c) treated wick fiber covered with kaolinite particles (after kaolinite breakthrough experiment).
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Nitrate Breakthrough Curves
The overall water content of the wicks increased with increasing flow rate. At flow rates of 5, 10, and 55 mL h–1, the overall gravimetric water contents of the wick material were 1.54 ± 0.10, 1.63 ± 0.06, and 1.78 ± 0.06 g g–1, respectively. Exact values varied somewhat between different wick pieces. These water contents were used to estimate approximate pore volumes for the breakthrough experiments. We tested the water balance during the breakthrough experiments gravimetrically. For the 55 mL h–1 flow rate, about 3% of the inflow water was lost by evaporation.
The detailed measurements of the water content distribution showed that the top of the wick was considerably drier than the bottom (Fig. 4)
. No significant differences were observed in the water content distribution among the different flow rates.
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Fig. 4. Distribution of water content as a function of height above the bottom of the wick for different water flow rates. Error bars denote ± one standard deviation (n = 3).
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The NO3 breakthrough curves indicate that NO3 moved like a conservative tracer through the wicks. Nitrate broke through the wicks after about one pore volume and the breakthrough curves had no tailing (Fig. 5)
. The shape of the NO3 breakthroughs was not affected by flow rate. The variability of the breakthrough curves within the same wick at different flow rates was small (Fig. 5a). The variability among different wick pieces at the same flow rates was small as well (Fig. 5b, 5c), indicating that different wick pieces behaved similarly in terms of NO3 transport.
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Fig. 5. Nitrate breakthrough curves at pH 7 (a) for different flow rates in the same wick, (b) for 10 mL h–1 flow rate in three different wicks, and (c) for 55 mL h–1 flow rate in three different wicks.
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Colloid Breakthrough Curves
The results of the colloid breakthrough curves are shown in Fig. 6 and 7
. While there was no effect of flow rate on NO3 transport, the flow rate did affect colloid transport. Generally, as flow rate decreased, fewer colloids were recovered in the effluent. The magnitude of the flow rate effect depended on colloid type, as the different colloids have different deposition rate coefficients. For native colloids and ferrihydrite, the flow rate effect was less pronounced than for modified colloids, montmorillonite, and kaolinite.
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Fig. 6. Breakthrough curves of native and modified Hanford colloids at different flow rates and pH. Breakthrough curves at the same flow rate within one plot are repetitions with different wicks.
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For the deionized water experiments, the pH of the effluent was between 6.5 and 7.5, and we denote the pH of these experiments as pH 7. For the Na2CO3/NaHCO3 solution experiments, the pH of the effluent was generally 0.5 to 1.0 pH units less than the pH of the influent, with a smaller pH drop when the flow rate was high. We denote the pH of these experiments as pH 10.
Generally, colloid recovery was less at pH 7 than at pH 10. This can be explained by the increased electrostatic repulsion of colloids in the wicks. The surfaces of the wick silica fibers are negatively charged at the pH values of our experiments (point of zero net proton charge of amorphous silica is pH 3.5 to 3.9 (Langmuir, 1997, p. 351). At pH 7 and 10, all colloids used in our experiments had a net negative surface charge, as indicated by the negative electrophoretic mobility (Fig. 2). For native, modified, and montmorillonite colloids the electrophoretic mobility did not change much from pH 7 to 10, suggesting that the transport of these colloids should not be different between these two pH values. Indeed, the breakthrough curves for these colloids support this hypothesis (Fig. 6, 7a, 7b).
Compared with all colloids used, native colloids and ferrihydrite had the greatest recovery in the effluent at pH 10 (Fig. 6b and 7f). The colloid breakthrough curves were fairly consistent among repetitive runs (Fig. 6a, 6b). The modified colloids had smaller relative effluent concentration than the native colloids, as a result of the larger particle size of the modified colloids and their less negative electrophoretic mobility (Fig. 2). The pure mineral colloids had lower recovery than the native colloids, except for ferrihydrite at pH 10, for which case an almost complete recovery was observed. The observation that native colloids moved with less restriction than the pure minerals through the wick can be attributed to several factors. Compared with montmorillonite, native colloids had a somewhat more negative electrophoretic mobility at pH 7, making the native colloids more mobile. At pH 10, the differences between native colloids and montmorillonite were not as pronounced as at pH 7, but still, the native colloids showed a higher recovery. This is likely due to differences in the mineralogical compositions. Native Hanford colloids are composed of a mixture of aluminosilicates and quartz. Colloidal stability experiments performed in our laboratory showed that montmorillonite was less stable than "native colloids" at pH 7 and 10 (data not shown), suggesting that the montmorillonite is more susceptible to aggregation and filtration inside the wicks.
No kaolinite and ferrihydrite moved through the wicks at pH 7 (Fig. 7c, 7e). The ferrihydrite has little net surface charge at pH 7 and is efficiently removed by aggregation and physicochemical filtration inside the wick. The kaolinite has a pronounced net negative charge at pH 7; however, it also has protonated aluminol groups at this pH (White and Dixon, 2002), making the particles susceptible to filtration inside the wicks. Breakthrough curves of ferrihydrite at pH 10 showed that the iron oxide moved without retention through the wicks. In addition, there was little effect of flow rate on the breakthrough curves; almost complete recovery was observed for all three flow rates (Fig. 7f).
In many cases, colloids were deposited on the wick fibers (Fig. 3c). The mass recovery for the different breakthrough curves ranged from <5% for kaolinite and ferrihydrite at pH 7 to about 100% for native colloids and ferrihydrite at pH 10 (Table 2). The varying mass recoveries suggest that the suitability of wicks for colloid sampling depends on flow rates and colloid types, although for some cases (native colloids and ferrihydrite at pH 10) the wicks were well suited for colloid sampling.
We tested the differences between repeated colloid breakthrough curves performed in the same wick (in-wick variability) and among different wicks (among-wick variability) (Fig. 8)
. We only used the native colloids at pH 10 because the wicks could be cleaned fairly readily by flushing the wicks extensively with the background solution between consecutive breakthrough curves. We did not observe pronounced differences in the subsequent runs.
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Fig. 8. In-wick and among-wick variability for three repeated breakthrough curves using native colloids at pH 10.
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Colloid removal by the wicks can be explained by two phenomena: (i) physicochemical effects (electrostatic and van der Waals interactions) between the particles and the wick and (ii) physical straining. Electrostatic interactions dominated removal when particles with less negative surface charge were transported through the wick. Nonetheless, we observed approximately 100% removal in the case of the kaolinite, which had the most negative charge of all colloids at pH 7. Positive edge charges were likely responsible for strong particle deposition.
Physical straining can play an important role if the water film on the surface of the wicks is sufficiently thin. On the basis of the water content distributions along the wick, we can estimate the water film thicknesses by dividing water contents by specific surface areas. The estimated film thicknesses ranged from 1.6 to 2.0 µm at the top to 5.4 to 6.8 µm at the bottom, depending on the flow rate. Because these film thicknesses are all much larger than the particle diameters, it is unlikely that particles were removed by straining in water films. Because the water contents in the wicks were similar for the different flow rates used, we do not attribute the observed dependence of colloid transport on flow rate to physical straining, but rather to physicochemical interactions.
Alternative Wick Materials
Our experiments indicate that fiberglass wicks can impede the movement of colloids and that the colloid recovery often depended on the flow rate through the wicks. It would therefore be useful to test materials other than glass fibers for their suitability for colloid sampling. Wicks are made of fibers with diameters of dozens of micrometers, and these fibers can be composed of different types of materials. Alternative candidates for wick fibers are polymers, graphite, or ceramic. The most suitable materials for colloid sampling in the vadose zone (in temperate climate and pH conditions where most colloids are net-negatively charged) should be negatively charged to overcome the attractive van der Waals interactions between colloids and wick surfaces. Noncharged polymers are therefore unlikely to be good candidates for wick materials because there will be no electrostatic repulsion between colloids and wick fibers. Negatively charged polymers may be suitable materials. Systematic experimental tests would be required to determine the most suitable material for use in wicks.
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CONCLUSIONS
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Flow rate, pH, and colloid type affected colloid breakthrough. Only two of the five colloids (native colloids and ferrihydrite) showed complete breakthrough through the wicks at pH 10; all other colloids were retained to some degree inside the wicks. Generally, colloid recovery was less at pH 7 than at pH 10. The mechanism of colloid retention in the wicks was due to physicochemical deposition rather than straining in water films because the estimated water film thicknesses were much larger than the colloid diameters.
Colloid recovery in the wick was very variable. Greatest recovery was observed for the mineral mixture extracted from sediments, 88 to about 100% of the colloid mass was recovered in the wick outflow at pH 7 and 10. Almost complete recovery was observed for ferrihydrite at pH 10. In two cases, kaolinite and ferrihydrite at pH 7, no colloids moved through the wicks. For other cases, colloid recovery varied from 50 to about 100%. Feldspathoids (modified colloids) and montmorillonite showed considerable retention inside the wicks. This inconsistency in colloid recovery limits the use of fiberglass wicks for colloid sampling. The results of this study suggest that fiberglass wicks may be suitable for sampling colloids from vadose zone pore water under certain conditions, particularly at high pH; however, the wicks may retain a considerable fraction of colloidal material.
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ACKNOWLEDGMENTS
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This research was supported by the Office of Science (BER), U.S. Department of Energy, Grant no. DE-FG07-99ER62882, and the Inland Northwest Research Alliance. We thank Jorge Jerez for his help synthesizing the ferrihydrite colloids, Glendon Gee (Pacific Northwest National Laboratory) and anonymous reviewers for helpful comments, and the Electron Microscopy Center at Washington State University for use of their facility.
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ABSTRACT
INTRODUCTION
