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Published in Vadose Zone Journal 3:395-402 (2004)
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

SPECIAL SECTION: COLLOIDS AND COLLOID-FACILITATED TRANSPORT OF CONTAMINANTS IN SOILS

Comparison of Hanford Colloids and Kaolinite Transport in Porous Media

Jie Zhuanga, Yan Jin*,a and Markus Fluryb

a Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19717
b Department of Crop and Soil Sciences, Center of Multiphase Environmental Research, Washington State University, Pullman, WA 99164
* Corresponding author (yjin{at}udel.edu).

Received 17 July 2003.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Understanding colloid transport at the Hanford site in Washington State is critical in assessing migration of radionuclides because colloid transport is a potential means for facilitated off-site migration of radioactive wastes. In this study, eight saturated column experiments were conducted to investigate transport of Hanford colloids and a model colloid (kaolinite) through two types of porous media (Hanford sediments characteristic of 2:1 clay minerals and silica Accusand). Experiments were conducted at a pH value of 10 to mimic the conditions at the Hanford site. The Hanford colloids used were obtained by reacting Hanford sediments with simulated tank waste solutions. The results show that factors influencing transport of Hanford colloids and kaolinite include flow velocity, solution ionic strength, medium type, and colloid properties. Hanford colloids exhibited higher deposition rates than kaolinite in both Hanford sediments and Accusand. Likewise, Hanford sediments retained more colloids than did the silica Accusand. Comparison of transport behaviors of the two colloids through two sands supports the assumption that chemical heterogeneity is important in controlling particle–particle and particle–collector interactions in colloid retention and transport.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
COLLOIDS ARE UBIQUITOUSLY present in subsurface formations and are formed in situ through geochemical alterations of primary minerals (McCarthy and Zachara, 1989). Colloid transport and its potential to enhance subsurface contaminant transport have been well documented (Mills et al., 1991; Ryan and Elimelech, 1996; Kretzschmar et al., 1999; Ryan et al., 2000; McGechan and Lewis, 2002). Colloid-facilitated transport has been recognized as an important mechanism controlling migration of strongly sorbing contaminants in subsurface environments (Ramsay, 1988; Grolimund et al., 1996; Saiers and Hornberger, 1996; Zhuang et al., 2003).

Colloid deposition kinetics in natural and model porous media have been studied as a function of colloid size, colloid type, surface properties, flow velocity, water content, pore size, and solution chemistry (Goldenberg et al., 1989; Elimelech and O'Melia, 1990a, 1990b; McDowell-Boyer, 1992; Wan and Tokunaga, 1997; James and Chrysikopoulos, 2000; Gamerdinger and Kaplan, 2001; Lenhart and Saiers, 2002). Although great advancements have been made in the study of colloid reaction and transport in porous media, both theoretically and experimentally, our understanding of colloid–soil interactions and our ability to predict transport of colloids in natural subsurface media are limited. In many of the studies conducted to date, model colloids (latex microspheres, silica and pure mineral colloids) and model porous media have been used to study colloid or colloid-facilitated contaminant transport (Kretzschmar et al., 1999). These colloids, however, are not necessarily good surrogates for colloids that are found in the natural environment (Grolimund et al., 1998) because the surface properties of model colloids are different from colloids that are heterogeneous in composition and properties.

The objective of this study was to examine how transport of Hanford colloids, which are heterogeneous in nature, is influenced by flow velocity, solution ionic strength, and matrix properties. We also included kaolinite in the study to provide a systematic comparison on transport behavior between chemically heterogeneous Hanford colloids and a relatively homogenous mineral colloid kaolinite.


   THEORY
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Calculation of Colloid Attachment Efficiency ({alpha}) and Travel Distance (LT)
Colloid deposition in porous media can be described by the following equation (Yao et al., 1971; O'Melia, 1990):

[1]
where C is colloid concentration (mg L–1), dg the diameter of collectors (sand grains) (m), {epsilon} the porosity of porous medium (m3 m–3), {eta} the collector efficiency, and {alpha} the attachment efficiency, both of which are dimensionless.

The collector efficiency ({eta}) describes the approach of colloids to the collector surface. It can be determined from (Rajagopalan and Tien, 1976; Logan et al., 1995)

[2]
where As is Happel's porosity-dependent parameter, accounting for the influence of neighboring collectors on the flow, and NPe, NLo, NR, and NG are dimensionless parameters accounting for colloid-collector collisions due to diffusion, London–van der Waals interactions, interception, and sedimentation, respectively. As can be calculated by As = 2(1 – {gamma}5)/(2 – 3{gamma} + 3{gamma}5 – 2{gamma}6), where {gamma} = (1 – {epsilon}) 3. The diffusion term is given by the Peclet number, NPe = qdg/Dp, where q is the water flux (m s–1) defined as q = v{epsilon} with v being the pore water velocity, and Dp is the aqueous diffusion coefficient of colloidal particles. The diffusion coefficient Dp can be calculated from the Stokes–Einstein equation Dp = BzT/(3{pi}µdc), where Bz is the Boltzmann constant (1.38 x 10–23 J K–1), T is the absolute temperature (K), dc is the diameter (m) of colloidal particles, and µ is the dynamic viscosity (1.025 x 10–3 kg (m s)–1 at 20°C) The interception and sedimentation terms are defined as NR = dc/dg, and NG = g({rho}c – {rho}f)/(18µdc2q), respectively, where {rho}c is the colloid density (kg m–3), {rho}f is the fluid density (kg m–3), and g is acceleration due to gravity. The term accounting for London–van der Waals interactions is given as NLo = 4H/, where H is the Hamaker constant. For both Hanford sand and Accusand, we used the same Hamaker constant of 1.6 x 10–21 J, which is the measured Hamaker constant (in water) for quartz (Ackler et al., 1996), because H does not vary very much for different materials and we do not know the value for Hanford sand.

The attachment efficiency ({alpha}) describes the attachment of colloids to collector surfaces by accounting for electrostatic interactions between colloids and the porous medium. It is the ratio of the number of colloids approaching the collector surface to the number of colloids attached on that surface, and can be calculated by (Yao et al., 1971)

[3]
where Ka is the deposition rate, defined as Ka = –(v/L)ln(Ce/C0), where Ce represents the stable effluent concentration of the colloid (g m–3), C0 the influent colloid concentration (g m–3), and L the column length (m).

Integrating Eq. [1] with the boundary conditions of C = C0 at x = 0 and C = CL at x = L yields (Yao et al., 1971)

[4]

This equation can be rearranged in terms of column length L and can be used to calculate how far a certain fraction of colloids will move in a porous medium. For this study, we define the distance (LT) at which 99.9% of colloids (CL/C0 = 0.001) are removed as the maximum travel distance for the colloids.

Calculation of Fractional Surface Coverage ({theta})
The dynamics of colloid deposition in porous media can be illustrated by the temporal change of fractional surface coverage of sediment grains. For irreversible colloid deposition on spherical collectors, the dimensionless collector surface coverage ({theta}) can be estimated as a function of time from experimental results of colloid breakthrough from the porous media according to (Song and Elimelech, 1993a):

[5]
where {rho} is the specific density of the colloids (taken as 2650 kg m–3), t is time, and we have assumed that the colloidal particles in our experiments are spherical.

It should be noted that filtration theory in its present form is only applicable to ideal systems where colloids are spherical and monodisperse, which is not the case in our experimental systems, especially with the Hanford colloids. Therefore, the calculated parameters presented in this paper allow only qualitative comparisons between the experiments and should not be used for prediction. Absolute values calculated in this study should be considered with caution.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Porous Materials and Colloids
Two types of porous materials, Hanford sediments and Accusand, were used in this study. Coarse Hanford sediments, representative of the material underlying the Hanford waste tanks, were obtained from the Submarine Pit (218-E-12B) at the Hanford Site in Washington, USA. The sediments came from the same site as the ones used by Zhuang et al. (2003), but from a different layer of the Hanford formation. The sediments had the same mineralogical composition as the ones used by Zhuang et al. (2003), but a different particle size distribution (Table 1). Detailed characterization of the sediments is given in Serne et al. (2002). The sediments were dry sieved and the fraction between 0.053 and 2-mm grains (particle size distribution by weight: 0.65% of 0.053–0.3 mm, 4.58% of 0.3–0.5 mm, and 94.77% of 0.5–2.0 mm) was used in the experiments. Accusand, a typical silica sand, was purchased from Unimin Corporation (Le Sueur, MN). Its particle size distribution was 9% 0.1 to 0.25 mm, 69.8% 0.25 to 0.5 mm, and 21.2% 0.5 to 1.0 mm. The two sands were rinsed with deionized water, until there were no particles suspended in the liquid phase as verified by turbidity measurement, and dried at 60°C.


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  		Table 1. Some basic properties of the experimental porous media and colloids.{dagger}

 
The Hanford colloids used in this study were specially prepared to mimic vadose zone conditions after a waste tank leak, and they are considered to represent colloidal material in Hanford sediments after a tank leak occurred (Zhao et al., 2002, 2004). Hanford colloids were obtained by reacting Hanford sediments with a solution simulating that in the waste storage tanks at the Hanford site (Zhao et al., 2002). The procedure is briefly described as follows. First, 1 L of the simulated tank solution (2.8 mol kg–1 NaOH, 0.125 mol kg–1 NaAlO4, and 3.7 mol kg–1 NaNO3) was added to 1 kg of the Hanford sediment of sizes smaller than 2 mm. The mixture was kept at 50°C and shaken periodically for 40 d. Then, colloidal particles (diameter <2 µm) were separated by gravity sedimentation. The colloidal particles were equilibrated with 1 M NaNO3 for 24 h, centrifuged, washed, and dialyzed against deionized water until the conductivity was <0.01 dS m–1. By tank solution treatment, some of the native colloidal particles (e.g., quartz and kaolinite) partially dissolved, and new minerals (e.g., cancrinite and sodalite) formed. About 20 g of colloidal material was obtained from 1 kg of Hanford sediment.

The kaolinite particles (diameter <2 µm) were extracted by gravity sedimentation in deionized water from well-crystalline kaolin (KGa-1, Source Clay Minerals Repository, University of Missouri, Columbia, MO). Some basic properties of the materials are provided in Table 1.

Transport Experiments
A series of saturated column experiments were performed to investigate effects of flow velocity, solution ionic strength, matrix properties, and colloid types on colloid deposition and transport (Table 2). The column system used in the study was similar to that illustrated in Jin et al. (2000). The column was made of acrylate, with an inner diameter of 5.1 cm and height of 10.0 cm. In the experiments, a stainless-steel screen (0.3-mm mesh size) was placed on the bottom plate for mechanical support. Teflon tubing was used throughout the system except for a portion of tygon tubing used in the peristaltic pump. When the column was packed, a deaerated NaNO3 background solution (pH = 10, either 1 mM, buffered with 0.024 mM mixture of Na2CO3 and NaHCO3 or 10 mM, buffered with 0.24 mM mixture of Na2CO3 and NaHCO3) was preintroduced into the column from its bottom to a certain height. Then the sand was slowly poured into the column in 1-cm increments while it was stirred with a plastic rod to ensure packing uniformity and to avoid air entrapment in the column.


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  		Table 2. Experimental conditions and parameters.{dagger}

 
Before each experiment was run, the deaerated NaNO3 background solution was flushed through the column overnight (at least 15 pore volumes) to precondition the columns for the transport experiments. The flushing removed residual colloids present in the columns, established a steady-state flow, and standardized background ionic strength and pH. The background solution was adjusted to pH 10 to mimic the high pH condition in the pore water at the Hanford site and to avoid dissolution of carbonates from the sediments during the experiments. The input solution, composed of the NaNO3 background solution along with the experimental colloid (50 mg L–1) and a Cl– tracer (0.68 mM in NaCl), was then pumped into the column at a constant flow rate. For experiments that involved different ionic strengths, we adjusted the concentrations of NaNO3 and the carbonate buffer. The influent reservoir was stirred during the entire experiment. Colloids were infused for 6 to 10 pore volumes. Then we applied a colloid-free NaNO3 buffer solution until effluent colloid returned to a baseline level that was determined at the beginning of each experiment. During the course of the experiments, effluent samples were collected from the top of the column into 15-mL polypropylene centrifuge tubes in regular time intervals using a fraction collector. The Cl– tracer was analyzed using ion chromatography (Dionex Corporation, Sunnyvale, CA). We determined the colloid concentration from predetermined calibration curves by turbidity measurements at 350 nm (Zhuang et al., 2003) using a UV-VIS spectrophotometer (DU Series 640, Beckman Instruments, Inc., Fullerton, CA). During the experiments room temperature was 22 ± 1°C.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Effect of Flow Velocity
Figure 1 illustrates the effect of flow velocity on the transport of both Hanford colloids and kaolinite through Hanford sediments. Colloid breakthrough occurred at about one pore volume, and reached a plateau at 1.4 to 2 pore volumes (Fig. 1a and 1b). The shape of the breakthrough curves indicates that the initial attachment of colloidal particles on the sand and elution process by colloid-free background solution were insensitive to the change of flow velocity. However, the steady-state breakthrough concentrations (C/C0) of the two colloids at high velocity were higher than at low velocity, suggesting that a hydrodynamic effect occurred for the colloid attachment. This result agrees with previous studies performed with various colloids and porous media (Goldenberg et al., 1989; Kretzschmar et al., 1997; Compere et al., 2001). A possible mechanism is that high velocity decreased thickness of shear interface of immobile–mobile phase on the sand, and helped the colloids remain in streamlines because of fluid-particle phase stresses, hydrodynamic drag, and lift forces. Consequently, high velocity decreased colloid deposition on the sediment surfaces. The absence of tailing of the breakthrough curves indicates an irreversible sorption of the particles on the sand.



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  		Fig. 1. Effect of flow velocity on transport of Hanford colloids and kaolinite through Hanford sand-packed columns in NaNO3 solution (1 mM, pH = 10).

 
The dynamics of surface coverage ({theta}) (calculated using Eq. [5]) and the relationship between {theta} and attachment efficiency ({alpha}) (calculated using Eq. [3]) are determined for the breakthrough experiments. Figures 1c and 1d show that the values of {theta} of both Hanford colloids and kaolinite in the early stage of transport were not affected by flow velocity, indicating that chemical interactions likely dominated the initial sorption process. As shown in Table 2, the colloid Peclet number (NPe = qdg/Dp), which represents influence of hydrodynamic interaction, is about three times larger for the larger kaolinite particles than for Hanford colloids. This suggests that influence of the hydrodynamic interactions is more pronounced for larger particles (Ko et al., 2000).

The {theta}–{alpha} relationship shows how retained particles affect the retention of the subsequent particles approaching the collector surface. Figures 1e and 1f indicate that the retained particles blocked attachment of the subsequent particles. For Hanford colloids, at the same fractional surface coverage, their attachment efficiency of colloids was higher at higher flow velocity. This effect of flow velocity agrees with the experimental results of Kretzschmar et al. (1997), who used latex colloids, and the theoretical predictions by Song and Elimelech (1993b) of colloid deposition rate under unfavorable particle–surface interaction conditions. At higher flow velocity, more colloids can stay in the streamlines of flow. Consequently the number of colloids approaching the collector surface was reduced, as indicated by the calculated collector efficiency ({eta}) (Table 2). In contrast, kaolinite, with larger sizes than the Hanford colloids, behaved differently. The attachment efficiency ({alpha}) of kaolinite decreased as the flow velocity increased during the early stage of transport but was not affected by flow velocity at the plateau stage (Table 2, Fig. 1f). The deposition rate (Ka) increased for both Hanford and kaolinite colloids as the flow rate increased (Table 2). Song and Elimelech (1993b) concluded that at low to moderate flow velocities (10–6 to 10 m s–1) the deposition rate of colloids is controlled by both flow intensity and particle–surface interaction and the effects of fluid convection and colloidal interaction cannot be separated.

Effect of Solution Ionic Strength
Solution ionic strength influences particle–surface and particle–particle electrostatic interactions through a charge screening effect. Transport of Hanford colloids and kaolinite through Hanford sediments at two ionic strengths (1 and 10 mM) is compared in Fig. 2a and 2b . No Hanford colloids broke through the Hanford sand column in 10 mM solution, whereas kaolinite particles exhibited a steady-state breakthrough rate of 0.2 C/C0 under the same experimental conditions. In 1 mM solution, the maximum effluent concentrations reached about 0.6 C/C0 for kaolinite particles and about 0.45 C/C0 for Hanford colloids. Evidently, kaolinite was less filtered than Hanford colloids during the transport, regardless of the larger size of the kaolinite particles. This effect of ionic strength on the transport of both colloids is expected (Elimelich and O'Melia, 1990a). The dynamics of {theta} (Fig. 2c and 2d) indicate that increasing ionic strength did not cause a significant change in colloid deposition in the early transport stage. However, in the plateau stage, high ionic strength increased surface coverage by Hanford colloids, but had a minimum effect on kaolinite coverage on the sand. This implies that electrostatic interactions were more dominant for the attachment of Hanford colloids than kaolinite particles. Figure 2f shows that the attachment efficiency of kaolinite increased as solution ionic strength rose. For Hanford colloids at 10 mM ionic strength, the value of {alpha} was equal to one, since screening of surface charge resulted in attachment for every collision. The strong nonlinear relationship of {theta}–{alpha} suggests the involvement of different retention mechanisms at different transport stages.



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  		Fig. 2. Effect of ionic strength on transport of Hanford colloids and kaolinite through Hanford sand-packed columns in NaNO3 solution (pH = 10).

 
The influence of ionic strength on colloid mobility can be further illustrated by calculating colloid travel distances using Eq. [4]. Table 2 shows that Hanford colloids can travel 0.9 m in 1 mM NaNO3 in Hanford sediments, while kaolinite particles move about 1.4 m before 99.9% of the colloids are captured by the porous media.

Effect of Medium Surface Properties
Breakthrough results of kaolinite and Hanford colloids in two types of sands are plotted in Fig. 3a and 3b . Different column packing and the use of a peristaltic pump that only could be adjusted incrementally caused the slightly different flow rates between the experiments. The differences in flow rates were small, so they were unlikely to have caused any of the effects discussed below.



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  		Fig. 3. Transport of Hanford colloids and kaolinite through Hanford sand and Accusand in NaNO3 solution (1 mM, pH = 10).

 
Both colloids showed a higher breakthrough from Accusand than from Hanford sand. The steady-state effluent concentration increased by approximately 0.1 and 0.2 C/C0 for Hanford colloid and kaolinite, respectively. A second peak is observed for the colloid breakthrough in Accusand, and we attribute this peak to an experimental artifact. The dynamics of {theta}, as depicted in Fig. 3c and 3d, reveal that deposition of both colloids resulted in higher coverages on Hanford sand than on Accusand. In addition, the fractional surface coverages by Hanford colloids on both types of sands were one order of magnitude larger than those of kaolinite. This was likely caused by the more chemical heterogeneous nature of the natural Hanford colloids than the model kaolinite colloids. The slope of the {alpha}–{theta} relationship curves reduced to a constant value at one pore volume (Fig. 3e and 3f). This indicates a critical point of attachment efficiency ({alpha}), at which the decreasing deposition rate stabilized at a constant value. The attachment efficiency of Hanford colloids leveled off at a larger surface coverage compared with kaolinite. Taking the constant attachment efficiency at the flat portion in Fig. 3e and 3f as the particle–particle collision efficiency, we see that Hanford colloids stick more easily than kaolinite particles under the same experimental conditions.

Johnson et al. (1996) suggested that geochemical heterogeneity has a profound impact on colloid transport. Capacity of attachment-favorable sites of a collector determines the amount of colloidal particles directly attached on the matrix, and this further influences how many subsequent colloidal particles can be removed from the liquid flowing through the column. Commonly, heterogeneous surfaces possess more favorable settings than relatively uniform surfaces for colloid attachment (Song and Elimelech, 1993a; Johnson and Elimelech, 1995; Johnson et al., 1996; Ren et al., 2000). Hanford sediments have more heterogeneous surfaces, both physically and chemically, than the relatively uniform silica Accusand (Serne et al., 2002; Zhuang et al., 2003). The different types of minerals contained in Hanford sediments, differently exposed crystallographic faces, and irregular surface coatings or cracks have been reported (McKinley et al., 2001; Zachara et al., 2002). Therefore, Hanford sediments likely provided more settings favorable for colloid deposition than the Accusand.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
Comparison of the transport of Hanford colloids and kaolinite through chemically heterogeneous Hanford sand and chemically homogeneous silica sand generated several interesting results. Increased water flow velocity caused more colloids to break through the Hanford sand and Accusand, and the colloid deposition rate increased as the flow velocity increased. Chemical heterogeneity associated with the Hanford colloids caused stronger particle–particle and particle–matrix interactions than was the case for the more homogeneous kaolinite colloids. Hanford colloids also had higher deposition rates than kaolinite, again due to pronounced chemical heterogeneity. While both hydrodynamic interaction (due to variation of flow velocity) and electrostatic interaction (due to chemical heterogeneity) were involved in the attachment of Hanford colloids, chemical interaction was relatively weak and hydrodynamic interaction probably dominated the attachment of kaolinite particles. Therefore, solution chemistry had more significant effects on the transport of Hanford colloids than kaolinite. The same trend was found on the natural Hanford sand and the model Accusand. This study indicates that several interaction mechanisms might be involved simultaneously during colloid transport, but their relative importance in the overall transport depends on the chemical and physical properties of colloids and transport media as well as the environmental conditions.


   ACKNOWLEDGMENTS
 
This work was supported by the Environmental Management Science Program, U.S. Department of Energy under Grant no. DE-FG07-99ER62882.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 




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