Soil Science Society of America Journal

Soil Sci. Soc. Am. J. doi:10.2136/sssaj2015.01.0005 Received 4 Jan. 2015. Accepted 27 Mar. 2015. *Corresponding author (amostafa@zu.edu.eg; aboaoab@gmail.com). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Clay Upward Movement in Sand Columns under Partially Saturated Conditions

Pedology

Almost all the studies on clay movement have been focused on downward movement rather than upward except one study that focused on upward movement. Therefore, this study was conducted to assess the impact of different clay suspension concentrations on the upward movement of clay particles. In this experiment, 24 transparent glass tubes with dimensions of 50-cm height and 3.7-cm internal diameter were packed with quartz sand. All of the sand columns were installed vertically in plastic containers. Two experiments were conducted: one represented a continuous wetting (CW) system and the other represented a wetting and drying system (WD). Each experiment had 12 sand columns distributed in four groups with three replicates. A stock of clay suspension was prepared from a 3-kg clayey textured soil sample obtained from a Vertisol in the Delta of the Nile River. Four clay suspension concentrations (2, 4, 6, and 8 g L−1) were prepared and provided to the sand columns from the bottom by filling the plastic containers to a 13-cm height. Each group of plastic containers was filled with one of the four clay suspensions (2, 4, 6, and 8 g L−1). The results revealed that clay particles moved 43 cm upward in all of the clay suspension concentrations and precipitated on the walls of the pores and at the capillary fringe (air–water interface). Clay contents in all of the layers in the WD cycles were higher than those in the CW system. The middle layers of the sand columns showed low clay content compared with the lower and upper layers.

Abbreviations: CW, continuous wetting; WD, wetting and drying.

The soil fabric is composed of solids, liquids, and gases. Solid particles that are larger than 2 mm in diameter create the skeleton of the soil; however, fine particles that are smaller than 2 mm in diameter, e.g., clay, colloids, or nanoparticles, form soil plasma. Soil plasma is all of the clay-sized materials (organic and inorganic) that are capable of moving within soil pores (Kretzschmar and Schäfer, 2005; Schaetzl and Anderson, 2005). In the past few decades, mobilization and transport of colloids have been extensively investigated under either saturated or partially saturated conditions in field- and laboratory-based experiments (Ryan and Elimelech, 1996; Zachara et al., 2002; Syngouna and Chrysikopoulos, 2015). However, all of the previous researchers have considered only the downward movement of clay particles. Consequently, they always supplied the treatments from the top of their sand or porous media columns (Dixit, 1978; Worrall et al., 1999; Majdalani et al., 2008; Quenard et al., 2011).

The transport of colloids in soil pores is affected by different factors such as soil physical properties, the intervals between rainfall events or irrigation, rainfall intensity, soil water evaporation, drainage water, redistribution of water within

Mostafa A. Ibrahim* Elsayed A. Elnaka

Soil Science Dep. College of Agriculture Zagazig Univ. Zagazig, 44511 Egypt

C. Lee BurrasAgronomy Dep. 1126 G Agronomy Hall Iowa State Univ. Ames, IA 50011

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the soil profile, and the air–water–solid interface (Kjaergaard et al., 2004; Jacobsen et al., 1997; Zhuang et al., 2007; Wang and Keller, 2009). Capillary forces have a great impact on colloid movement in soils in terms of detachment and transportation, especially at the air–water–solid interface (Corapcioglu and Choi, 1996). Colloidal movement in soils is important in terms of the environment and soil formation. For example, contaminants and pathogens may be adsorbed on the surface of colloids and be transported with them through soil pores, polluting the groundwater, which passively affects human health (Ryan et al., 1998; Redman et al., 2001). Moreover, movement of nanoparticles within soil profiles (eluviation and illuviation) has an important role in soil genesis (Buol and Hole, 1961). For example, translocation of clay particles from the upper horizons to the underneath horizons has an important role in forming special horizons such as albic, argillic, and spodic (Soil Survey Staff, 1999). Lamellae can be formed as a result of clay movement either downward or upward (Soil Survey Staff, 1999; Ibrahim and Burras, 2012).

If an air–water interface passes by an adhered clay nanoparticle on a solid surface (e.g., a pore wall), strong capillary forces could form between the particle and the air–water interface that may detach the particle from the solid surface and move it elsewhere (Noordmans et al., 1997; Gomez-Suarez et al., 1999). Furthermore, repeatedly moving the air–water interface across adhered particles may cause more particle movement (Gomez-Suarez et al., 1999). A moving air–water interface could detach a considerable amount of colloidal particles from solid surfaces and transport them to other positions (Sharma et al., 2008). In a laboratory experiment, Keller and Auset (2007) were able to capture the process of colloidal particle attachment and transport in a porous medium using microscopic visualization. Evaporation could enhance the accumulation of colloidal particles at the contact line between evaporation and the surface of particle deposition (Ghosh et al., 2007).

In soils, wetting and drying cycles may generate transient flow, which could promote rapid colloidal particle mobilization (El-Farhan et al., 2000). Almost all of the research work on clay movement to date has considered downward translocation, but an experiment conducted by Ibrahim and Burras (2012) demonstrated upward movement of clay-sized particles. The aims of this work were to reinvestigate the upward movement of clay particles and to assess the effect of different concentrations of clay suspensions on the amount of clay that can move upward.

MATERIALS AND METHODSSand Preparation

To conduct the experiment, approximately 50 kg of silicate sand was obtained from Elkhatarah Sand Quarry, Sharkia, Egypt (30°37¢55² N, 31°48¢35² E). The sand was first air dried and sieved with a 2-mm sieve to discard particles >2 mm. To remove all of the silt and clay fractions, all of the sand was washed approximately 10 times with distilled water (DW) using a 0.250-mm sieve. All of the sand particles <0.25 mm

in diameter were received in a bucket during the first washing step and washed again with DW to remove all of the silt and clay particles by the decanting process following Stoke’s law. The sand was pretreated with H2O2 to ensure the removal of any organic compounds and with 10% HCl to remove any traces of carbonates. Afterward, the sand was washed again with DW to remove any remaining chemicals. The sand was dried at room temperature. A subsample of the sand (1 kg) was oven dried at 105°C and passed through a set of sieves to determine the sand fractions. The sand content was 3.7, 29.8, 49.7, 9.5, 5.7, and 1.6% in the 2- to 1-, 1- to 0.5-, 0.5- to 0.25-, 0.25- to 0.21-, 0.21- to 0.13-, and 0.13- to 0.05-mm fractions, respectively.

Sand Column SetupTransparent thin plastic sheets were sealed as tubes to fit

inside transparent glass tubes with dimensions of 50-cm height and 3.7-cm internal diameter. Twenty-four tubes were installed in 2-L plastic containers and supported vertically with wooden racks (Fig. 1). All of the tubes were packed with air-dry sand to a height of 50 cm. The bulk density of the sand was 1.81 g cm−3 with a total porosity of 31.7%. The lower end of each tube was fractured to allow the clay suspension to have free access to the tube, and then rested in the bottom of the suspension container. The surface of each suspension at the beginning of the experiment was adjusted to a height of 13 cm above the immersed end of the tube.

Clay Suspension PreparationA soil sample with a clayey texture was collected from a

Vertisol formed in a lacustrine parent material at Ghazala Village, Sharkia, Egypt. The particle size analysis of the soil sample was determined by the pipette method (Pansu and Gautheyrou, 2006). The soil sample had 63.5% clay, 11.4% sand, and 25.1% silt, and the pH was 8.4 measured in a 1:1 soil/water suspension using an Orion pH meter (Thermo Scientific). The electrical conductivity (EC) was 1.7 dS m−1, which was measured in a 1:2.5 soil/water suspension using an EC meter (Thermo Scientific). Approximately 3 kg of the soil sample was pretreated to remove all of the aggregating agents (e.g., carbonates were removed by HCl, organic matter was removed by H2O2, and dissolved salts were removed by washing the sample with DW). Sodium hexametaphosphate solution was used to complete dispersion of the soil particles (Pansu and Gautheyrou, 2006). The clay fraction was separated from the dispersed soil sample using Stoke’s law and collected in a 20-L glass container. The density of the initial clay suspension was 53 g L−1, its pH was 7.5, and the EC was 0.07 dS m−1. A subsample (500 mL) of the initial clay suspension was used to determine the clay fractions such as coarse (0.2–2.0 mm), medium (0.02–0.2 mm), and fine (<0.02 mm). Four replicates of 100 mL each were put in centrifuge tubes, shaken mechanically using a vortex mixer for 5 min, and centrifuged for 20 min at the appropriate speed ( Jackson, 1985, p. 100–166). The initial clay suspension had 40.9% coarse clay, 38.4% medium clay, and 20.7% fine clay. Four sets of clay concentrations were prepared to conduct the experiment. These

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four clay suspension concentrations were selected to test the impact of a wide range of clay suspension concentrations on the upward movement of clay particles. Furthermore, we selected the highest concentration to be 8 g L−1 to make sure that the clay could stay in suspension longer and to avoid the coagulation of clay particles, which could take place at higher concentrations. The first concentration was 2 g L−1, its pH was 7.2, and the EC was 0.04 dS m−1. The second concentration was 4 g L−1, its pH was 7.3, and the EC was 0.04 dS m−1. The third concentration was 6 g L−1, its pH was 7.3, and the EC was 0.05 dS m−1. The last clay concentration was 8 g L−1, its pH was 7.4, and the EC was 0.06 dS m−1.

Experiment DesignThe experiment was performed inside a laboratory at

room air temperature (25–28°C) and an evaporation rate of 3 to 5 mm d−1. It began on 7 May 2014. The experiment had two parts. The first part is called the continuous wetting (CW) experiment and consisted of four subgroups of sand columns. The first group was comprised of four concentrations of clay suspensions (2, 4, 6, and 8 g L−1). Each clay concentration had three replicates. All of the sand columns in this group (12 sand columns) were exposed to a constant height of clay suspension during the experimental period (Fig. 1). The clay suspension in the containers under the sand columns was replenished daily by adding more suspension to the container to keep the suspension at a constant height (13 cm above the immersed end of the tubes). The total amount of clay suspension used during the CW experiment for each treatment was approximately 6.8 L.

The second group of sand columns is called the wetting and drying cycle (WD) experiment and also had four concentrations of clay suspensions (2, 4, 6, and 8 g L−1). Each clay concentration

had three replicates. Sand columns in this group (12 sand columns) were exposed to alternate wetting and drying (WD) cycles of the clay suspension to imitate the fluctuation of a groundwater table. The first cycle of wetting lasted 1 mo, during which clay suspension was added in the beginning of the experiment and left without any supplementation until it had dried. Two weeks of drought were allowed before another cycle of wetting with the clay suspension. These sand columns were exposed to two cycles of drought and three cycles of wetting (i.e., the total time of the WD experiment was 3 mo of wetting and 4 wk of drying). The total quantity of clay suspension used in each treatment during the WD experiment was approximately 6 L. Clay suspensions inside all of the plastic containers were agitated every day with a small hose connected to a laboratory rubber pump to prevent clay particles from flocculating at the bottom of the sand columns.

Determination of Clay ContentThe two groups of sand columns, CW and WD, were

harvested on 20 Aug. 2014. Each sand column was carefully laid on a white, clean, and flat surface. The sand column was extracted from the glass tube by pulling the internal cylindrical plastic tube from the glass tube. The highest rise of the clay suspension was determined by watching the highest wet point and measuring the height from the bottom of the sand column to the highest wet point with a measuring tape. Each sand column was divided into seven sections from the bottom to the top (0–13, 13–18, 18–23, 23–28, 28–33, 33–38, and 38–43 cm) (Fig. 2). The first section (0–13 cm) was the bottom part of the sand column immersed in the 2-L plastic containers of clay suspension, and the 38- to 43-cm height was the highest rise of the clay suspension corresponding with the capillary fringe (Fig. 2).

Fig. 1. Sand columns were vertically installed in plastic containers and exposed to clay suspensions at the bottom.

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Each section of each sand column was entirely collected in a 250-mL glass beaker and oven dried at 105°C for 24 h. The sand and clay mixture in each beaker was weighed and transferred to a 1-L glass cylinder; 10 mL of Na hexametaphosphate solution was added to the contents of each cylinder and agitated to determine the clay content in each section of the sand columns using the pipette method (Pansu and Gautheyrou, 2006). Clay suspensions in each cylinder for each section of sand column were photographed using a digital camera. All of the results were statistically analyzed using SAS 9.2 (SAS Institute).

RESULTS AND DISCUSSIONContinuous Wetting System

The results of the CW experiment revealed that clay particles moved upward in all of the sand columns for all of the clay suspension concentrations. Clay content tended to decrease from the bottom of the columns to the 23- to 28-cm layer and then increase again to the top of the column. The highest clay content in each column was at the bottom, and the second highest clay content was at the top (Table 1; Fig. 3). These clay content trends in the sand layers agreed with those found by Ibrahim and Burras (2012). Such upward movement of clay particles could be attributed to the capillarity effect in the pores of the sand columns. Further, the highest layer of sand (38–43 cm) in each sand column having the second highest clay content after the bottom layer (0–13 cm) could be due to the effect of the water–air interface at the capillary fringe. When the clay suspension reached its highest point, the capillary fringe, water tended to evaporate, leaving behind clay particles. Evaporation was continuous because the lost water

was being replaced through the upward movement of clay suspension from the container at the bottom of sand columns. As a result, clay particles were accumulating at the evaporating surface—the upper sand layer.

Among the different clay suspension concentrations in the CW experiment, the clay content in each sand layer of the same depth increased for increasing concentrations of clay. Similarly, the color of the clay suspension extracted from the sand layers became darker for the same depth for increasing concentrations of clay suspension (Fig. 3). For example, the clay content of the bottom layer (0–13 cm) was 0.43, 0.49, 0.70, and 0.85% for the 2, 4, 6, and 8 g L−1 clay suspensions, respectively (Table 1). Similarly, the clay content in the highest sand layer was 0.34, 0.42, 0.51, and 0.58% for the 2, 4, 6, and 8 g L−1 clay suspensions, respectively (Table 1). The clay content of the highest sand layer (0.58%) in the 8 g L−1 clay suspension treatment was less than the clay content of the

Fig. 2. Extraction of a sand column from the glass tube and dividing it to seven sections: (A) the transparent plastic cylinder and the sand column are inside the glass tube and the immersed end of the tube points to the right side of the picture; (B) the plastic cylinder with the sand column was extracted from the glass tube by holding the plastic cylinder on the table and gently pulling the glass tube toward the right side of the picture; (C) the plastic cylinder with the sand column inside it was extracted entirely and divided into seven sections using a ruler—the blue line refers to the sand layer, which was immersed in the clay suspension (its height was 13 cm), while the red lines refer to the sand layers; and (D) the plastic cylinder was gently cut with a sharp cutter to enable us to separate each section individually.

Table 1. Clay content in each sand layer in the continuous wetting (CW) experiment with clay suspension concentrations from 2 to 8 g L−1.

Layer

Clay content

2 g L−1 4 g L−1 6 g L−1 8 g L−1

cm ——————————— % —————————————

0–13 0.43 ± 0.05† 0.49 ± 0.04 0.70 ± 0.07 0.85 ± 0.0613–18 0.25 ± 0.03 0.36 ± 0.04 0.46 ± 0.05 0.53 ± 0.0418–23 0.21 ± 0.03 0.25 ± 0.02 0.31 ± 0.03 0.39 ± 0.0423–28 0.13 ± 0.03 0.16 ± 0.03 0.22 ± 0.04 0.24 ± 0.0228–33 0.17 ± 0.02 0.21 ± 0.03 0.24 ± 0.02 0.33 ± 0.0333–38 0.23 ± 0.04 0.27 ± 0.05 0.33 ± 0.06 0.38 ± 0.0738–43 0.34 ± 0.06 0.42 ± 0.05 0.51 ± 0.03 0.58 ± 0.09† Mean ± standard deviation.

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highest sand layer in the study of Ibrahim and Burras (2012) (0.85%), which could be due to the higher clay suspension concentration used in that study.

Wetting and Drying SystemIn the WD experiment, clay particles moved upward and clay

content showed the same trends as in the CW experiment in all of the sand columns. For example, in the 4 g L−1 clay suspension, clay content was highest in the bottom layer (0–13 cm) at 0.64%, lowest in the middle layer (23–28 cm) at 0.22%, and increased to 0.55% in the upper layer (38–43 cm) (Table 2; Fig. 4).This increase in clay content in the sand layers within each sand column could be attributed to the upward movement by capillarity effects. Furthermore, the upper layer had a higher clay content than

Fig. 3. In the continuous wetting experiment, the color of the clay suspension extracted from each sand layer became darker with increasing clay concentration. The three columns with the lightest color represent wetting with distilled water (not shown).

Table 2. Clay content (%) in each sand layer in the wetting and drying (WD) cycle experiment with clay suspension concentrations from 2 to 8 g L−1.

Layer

Clay content

2 g L−1 4 g L−1 6 g L−1 8 g L−1

cm ———————————— % ————————————

0–13 0.49 ± 0.05† 0.64 ± 0.04 0.74 ± 0.07 1.07 ± 0.0613–18 0.38 ± 0.04 0.41 ± 0.05 0.56 ± 0.03 0.68 ± 0.0318–23 0.28 ± 0.04 0.32 ± 0.03 0.39 ± 0.02 0.45 ± 0.0423–28 0.19 ± 0.03 0.22 ± 0.05 0.27 ± 0.04 0.35 ± 0.0328–33 0.23 ± 0.02 0.28 ± 0.03 0.33 ± 0.03 0.44 ± 0.0433–38 0.31 ± 0.03 0.36 ± 0.04 0.40 ± 0.05 0.53 ± 0.0638–43 0.42 ± 0.05 0.55 ± 0.03 0.62 ± 0.07 0.67 ± 0.08† Mean ± standard deviation.

Fig. 4. In the wetting and drying cycle experiment, the color of the clay suspension extracted from each sand layer became darker with increasing clay concentration. The three columns with the lightest color represent wetting with distilled water (not shown).

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some of the underlying sand layers because it had the evaporating surface of the air–water interface. Comparing the clay content in each sand layer at the same depth for all of the sand columns representing different clay suspension concentrations, clay content increased with increasing clay suspension concentration (Table 2; Fig. 4). For example, clay content in the upper sand layer was 0.42, 0.55, 0.62, and 0.67% for the 2, 4, 6, and 8 g L−1 clay suspensions, respectively (Table 2).

In the WD experiment, clay particles moved upward during the wetting cycle and precipitated at the capillary fringe, the air–water interface. Because the clay suspension was not replenished during the wetting cycle, its level in the plastic container dropped with time, which lowered the capillary fringe across the sand column, moving the air–water interface downward and increasing the precipitated clay particles in all of the sand layers. During the drying cycle, the clay suspension level in the plastic container was almost 0.0 cm, and clay particles precipitated on the walls of the pores. Rewetting the sand columns by adding clay suspension to the plastic containers caused another upward movement cycle of clay particles until they reached the capillary fringe and precipitated at the air–water interface again. On the way from the bottom to the top of the column, the new clay suspension enhanced the detachment of the previously precipitated clay particles and moved them to higher positions (Gomez-Suarez et al., 1999). Consequently, the clay content in the sand layers with the same clay suspension concentration was significantly higher in the WD experiment than the corresponding sand layer in the CW experiment (Table 3). For example, in the WD experiment, the clay content in the upper layer (38–43 cm) for the 6 g L−1 treatment was highly significant compared with the upper sand layer of the same clay concentration in the CW experiment (Table 3).

In the WD experiment, the clay content in a sand layer of a specific clay suspension concentration at a specific depth could be greater than or equal to the clay content in the same depth of a larger clay suspension concentration in the CW experiment. For example, the clay content of the 6 g L−1 suspension at the upper sand layer (38–43 cm) in the WD

experiment was higher than the clay content at the same layer of the 8 g L−1 clay suspension concentration in the CW experiment (Tables 1, 2, and 3).

Overall, the deposition of clay particles around the sand particles may lead to a shift of a significant portion of the non-capillary pores to capillary pores with time, which in turn could contribute to increasing the movement of suspended clay particles to the upper layers with time. However, such a deposition might clog some pores entirely when the pore size becomes less than the clay particle size.

CONCLUSIONSClay particles can move upward in quartz sand columns

under partially saturated conditions. Increasing the concentration of the clay suspension led to an increased amount of clay that moved upward. Wetting and drying conditions enhanced the upward movement of clay particles compared with continuous wetting conditions. For example, the clay content in a sand layer of a lower clay suspension concentration at a specific layer in the WD cycle experiment could be greater than or equal to the clay content at the corresponding layer of a larger clay suspension concentration in the CW experiment. The high deposition of clay particles at the upper surface layers of sand columns in either the CW or WD systems was the direct result of continuous evaporation and indirectly of capillary action.

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Table 3. Differences in clay content (%) in all of the layers of sand columns in the continuous wetting and wetting and drying cycle experiments with clay suspension concentrations from 2 to 8 g L−1.

Depth

Clay content

Continuous wetting Wetting and drying

2 g L−1 4 g L−1 6 g L−1 8 g L−1 2 g L−1 4 g L−1 6 g L−1 8 g L−1

cm

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