Published in Soil Sci. Soc. Am. J. 68:789-794 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-2—SOIL CHEMISTRY
Critical Coagulation Concentration of Paddy Soil Clays in Sodium–Ferrous Iron Electrolyte
Atinut Saejiewa,
Olivier Grunbergera,
Somsri Aruninb,
Fabienne Favred,
Daniel Tessierc and
Pascal Boivin*,d
a Institut de Recherche pour le Développement-UR067–BP 5045 Montpellier Cedex 1, France
b Land Development Dep., Paholyothin Rd., Chatuchak, Bangkok 10900, Thailand
c Institut National de la Recherche Agronomique, Route de St. Cyr, 78026 Versailles Cedex, France
d Lab. of Soil Science, Swiss Federal Inst. for Technology, EPFL-ENAC-ISTE, 1015 Lausanne, Switzerland
* Corresponding author (pascal.boivin{at}epfl.ch).
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ABSTRACT
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Sodium affected rice (Oryza sativa L.)-cropped fields are very common. Due to their high exchangeable Na percentage, clay dispersion is one of the major risks for these soils when they are flooded. During flooding, Fe2+ may become a major cation due to reductive dissolution of Fe oxides, but the flocculation effect of Fe2+ is not known. In this paper, the effect of Fe2+ on clay flocculation is studied by establishing critical coagulation concentration (CCC) curves of clays extracted from a paddy soil of Northeast Thailand. The effect of Na-Fe2+ and Na-Ca electrolytes on the CCC values is compared for sodium adsorption ratio (SAR) values ranging from 0 to 40 and total electrolyte concentration (TEC) ranging from 0.5 to 10 mmol L–1. The extracted clay is a mixture of kaolinite and smectite but only the smectite and some poorly ordered kaolinite could be dispersed. The CCC values largely reflected the behavior of smectite, in agreement with previous studies. The CCC values were equal for electrolytes with Fe2+ or Ca2+ cations, suggesting that Fe2+ strongly adsorbs on smectite exchange sites and behaves similarly to Ca2+. The Fe2+ concentration needed to flocculate the clays was at maximum 0.6 mmol L–1 for the most dispersive electrolytes, which is a common concentration in flooded rice fields. Reductive dissolution of Fe can protect sodic soils from clay dispersion upon flooded rice cropping.
Abbreviations: CCC, critical coagulation concentration • CEC, cation-exchange capacity • EC, electrical conductivity • EGME, ethylene glycol monoethyl ether • SAR, sodium adsorption ratio • TEC, total electrolyte concentration • XRD, X-ray diffraction
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INTRODUCTION
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SODIUM AFFECTED SOILS may lose their fertility due to clay dispersion, structure collapse, or reduction in hydraulic conductivity (e.g., Sumner, 1993) and ultimately clay protonation (McBride, 1994). In rice fields, flooding with fresh water may lower the ionic strength of the soil solution to critical levels where clay dispersion can occur. There are extended areas of Na affected rice fields over the world, like in West Africa (Boivin, 1997; Ceuppens and Wopereis, 1999; Ndiaye et Guindo, 1999), Australia (e.g., Chartres, 1993; Naidu et al., 1993), and Asia (Qadir et al., 1998; Arunin, 1984; Wongsomsak, 1986).
Clay dispersion depends in part on the thickness of the electrical double layer, which is related to clay mineralogy, type of exchangeable ions and concentration of the electrolyte (Sumner, 1993). The thickness of the double layer increases (i) with decreasing exchangeable ion charge, (ii) with increasing hydrated ion size, and (iii) with decreasing concentration of the solution. The concentration for which Van der Waals attractive forces become higher than electrostatic repulsive forces is the flocculation concentration or CCC. The flocculation– dispersion behavior of clays is usually studied using Na-Ca electrolytes having various levels in electrolyte concentration and SAR, where SAR is defined by the equation
where Na+, Ca2+, and Mg2+ represent the concentrations in mmolc L–1 of the respective cations (e.g., Arora and Coleman, 1979; Goldberg and Forster, 1990; Abu-Sharar, 1988; Curtin et al., 1994). The threshold level for SAR and electrolyte concentration on clay flocculation is determined on suspensions of extracted clays and expressed as the CCC, the minimum electrolyte concentration necessary to flocculate a given colloidal dispersion in a specified time (van Olphen, 1977). Goldberg and Glaubig (1987) and Goldberg et al (1991) studied the influence of pH on clay flocculation for pure clay minerals and clay mixtures. Critical coagulation concentration increased with pH particularly for pH values higher than 7 and kaolinite was found to be more sensitive to pH than montmorillonite. Iron oxides as well as organic matter were found to decrease CCC. Kaolinite–montmorillonite mixtures behaved like montmorillonite.
Flocculation properties of clays in Na-Ca electrolytes have been widely studied, but the Ca concentration can be very low in many flooded soils. On the other hand, it is well known that flooding induces reductive dissolution of Fe oxides, and thus leads to increasing Fe2+ concentrations in the soil solution (Ponnamperuma, 1972). The flocculation–dispersion properties of clays in Na-Fe2+ electrolytes have not been evaluated. Many authors state that ferrous Fe is preferentially adsorbed on clays in flooded soils. The preferential sorption of ferrous Fe is the first step of ferrolysis theory (Brinkman, 1970), which is widely accepted as the major soil forming process for temporarily waterlogged soils (McBride, 1994; McDaniel et al., 2001; Singh et al., 1998; Hardy, 1993). It applies to temporarily waterlogged and leached soils and proceeds in two steps as summarized in Van Breemen (1988). In the first step, when the soil becomes reduced, ferrous ions displace other exchangeable cations and saturate the exchange sites of the clay minerals. In the second step, Fe2+ is oxidized and precipitates as Fe3+ insoluble oxides, and the protons produced in the reaction take the place of adsorbed Fe2+. Then, the unstable H+ clay converts to a weathered clay with appreciable amounts of exchangeable Al. Repeated redox and leaching cycles lead to acidic soils, with an upper sandy horizon containing weathered clays with a very low cation-exchange concentration (CEC) and the formation of small quartz and chlorite particles. Preferential sorption of ferrous Fe and displacement of other exchangeable cations is also assumed when interpreting the increase in concentration of major cations in soil solution during flooding (Narteh and Sharawat, 1999). Reported concentration values of Fe2+ in soil solution of flooded paddy fields range from 0.7 to 20 mmolc L–1 (Ponnamperuma, 1972; Génon et al., 1994; Boivin et al., 2002). Ferrous ion size is close to Ca2+, thus a similar behavior with respect to adsorption on clay surfaces is expected. However, clay selectivity coefficients for Fe2+ are poorly documented (Kamei et al., 1999; Tournassat and Charlet, 2002).
The aim of this study was to evaluate the CCC properties of clays extracted from Na affected paddy soils of northeast Thailand in Na-Fe2+ electrolytes and to compare them with CCC measured in Na-Ca electrolytes.
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MATERIALS AND METHODS
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Soil
The soil used in the current study is a fine loamy, mixed, active, typic Natraqualf (Soil Survey Division Staff, 1998) from the KulaRonghai Thailand soil series. It was collected during the dry season in a rainfed lowland rice field in Northeast Thailand. It is representative of Na-affected soils in this area. The soil structure is weak fine subangular blocky in the sandy upper layer (0–36 cm) and weak coarse columnar in the loamy second layer (36–122 cm). Three kilograms of soil were collected in the second layer, at 50- to 80-cm depth. The soil belongs to a typical soil sequence with Roi Et soils (Brinkman, 1977) in the upper part, and the clay minerals of the sampled horizon were found to be representative of the clay minerals of the soil sequence (Saejiew, 2003).
The soil was air dried and passed through a 2-mm sieve. Chemical, physical, and mineral properties were determined as follows: pH in a 1:2.5 soil water suspension, electrical conductivity (EC) and soluble ions in paste extract, CEC and exchangeable cations extracted with cobalt hexamine (Afnor, 1994), texture by sedimentation, specific surface of clay with ethylene glycol monoethyl ether (EGME) (Djeran-Maigre et al., 1998), and clay mineralogy by X-ray diffraction (XRD) analysis. Oriented samples of Ca saturated clay, Ca saturated, and heated to 200°C clay and an ethylene glycol saturated clay were analyzed by XRD using a SIEMENS D 5000 apparatus (Siemens, Germany) with Cobalt-K
radiation.
Clay Suspension Preparation
Calcium-clay and Na-clay (<2 µm) suspensions were obtained by sedimentation after organic matter destruction with H2O2 in the soil sample. The soil was dispersed in deionized water and decanted at a depth and a calculated settling time of clay particles (<2 µm) using Stokes' law. The resulting clay suspension was then flocculated with 1 mol L–1 CaCl2 or 1 mol L–1 NaCl. The excess Cl was removed by washing this clay suspension with deionized water (Ca-clay) or ethanol (Na-clay) until excess chloride in washed solution is absent (test with silver nitrate) (Robert and Tessier, 1974). Clay concentration was adjusted to 10 g L–1 (1% w/v) of clay with deionized water after weighting dried clay at 105°C.
Critical Coagulation Concentration Determination
A modified method of the Goldberg and Forster (1990) method was used to determine the CCC for the Ca and Na-saturated clays.
Clay Flocculation Experiments
Solutions with SAR values of 0, 1, 3, 5, 7, 11, 15, 20, 30, and 40 and TEC ranging from 0.2 to 10 mmol L–1 were added to the clay suspensions.
In a first experiment, the solutions were prepared with NaCl and CaCl2 and in a second experiment the solutions were prepared with NaCl and FeCl2. The SAR of the Na-Fe2+ solutions was calculated as:
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All experiments were performed in a glove box under N atmosphere, and both clay suspension and electrolyte were deoxygenated by bubbling N2 gas, to prevent oxidation of ferrous Fe. The atmosphere of the glove box was continuously bubbled in a green FeSO4·7H2O solution that served as an oxygen indicator and trap. No oxidation of Fe in the oxygen trap was observed during the experiment. The experiment was performed in triplicate by mixing 0.5 mL of homogenized Ca-clay or Na-clay suspension and 3.5 mL of electrolyte into spectrophotometer cuvettes. The cuvettes were closed with a cap before horizontal shaking during 30 min and allowed to settle 24 h at room temperature. Clay concentration was determined by measuring the absorbance of the suspension at a 641-nm wavelength on a Secomam 250I spectrophotometer (SECOMAM, Cedex, France). Absorbance values were converted to clay concentration in grams per liter after establishing a standard curve of clay concentration as a function of absorbance. The absorbance of the suspensions was measured immediately after shaking, to account for variability in the initial suspension, and after 24 h to measure the dispersed clay. The pH of the suspensions was measured after the absorbance measurement. When partial flocculation was observed, flocculated and dispersed clays were collected separately and prepared for oriented-clay X-ray determinations. The CCC values were operationally defined for each SAR as the critical electrolyte concentration at which 80% of the clays were flocculated after 24 h (Goldberg and Forster, 1990). When the electrolyte and the clay suspension are mixed, exchanges occur between the clay and the electrolyte cations, thus the equilibrium SAR is not the initial SAR of the electrolyte. Goldberg and Forster (1990) used a more sophisticated method to determine more precise CCC values as they mixed Na- and Ca-clay suspensions with calculated ratios corresponding to the expected SAR. We simplified the method because the objective of the study was to compare the effect of Ca2+ and Fe2+ ions on clay flocculation.
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RESULTS
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Soil Analysis
The general properties of the soil are reported in Table 1. The soil is alkaline and sodic, with a high exchangeable sodium percentage. The CEC of the clay fraction is about 50 cmolc kg–1. The XRD diagrams of extracted clay presented in Fig. 1
show that the <2-µm fraction is mainly composed of smectite and kaolinite.
Flocculation of Clay Suspensions
The pH of the suspensions was close to 6 for all the experiments and showed little variation. Figure 2
shows the clay concentration measured in the suspensions after 24 h, for the Ca-clay and the Na-Fe2+ electrolyte at various SAR and TEC values. The points labeled control on Fig. 2 are concentrations of clay in the suspensions measured immediately after shaking. The control points show a uniform clay concentration of 1.25 g L–1 in the suspensions at the beginning of the experiments. Absorbance measurements after 24 h show a suspended clay concentration close to 0.9 g L–1 for the lowest concentrations (below 0.5 mmol L–1), followed by a sudden decrease in clay concentration with increasing concentration of the electrolytes. The threshold concentration increases with increasing SAR.
Thus, for the lowest TEC a mean difference in suspended clay concentration of 0.35 g L–1, that is, about 30% of the amount of clay introduced, was observed between measurements performed immediately after shaking and measurements performed after 24 h, which means that about 30% of the clays was flocculated after 24 h regardless of the electrolyte concentration and SAR. The corresponding XRD diagrams of flocculated and suspended clays collected for an electrolyte concentration below the threshold flocculation value (Ca-clay with SAR 7 and 1 mmol L–1 TEC) are presented in Fig. 3
. Figure 3a shows well-crystallized kaolinites, and Fig. 3b shows mainly smectites with some poorly ordered kaolinites. According to this result, well crystallized kaolinites account for about 30% of the clay in the sampled horizon. Since they are never dispersed, the flocculation experiment generally accounts for smectite and poorly ordered kaolinites behavior. Similar observations were made in all experiments. The clay scale is therefore presented in percentage of the dispersed clay concentration measured after 24 h at the lowest TEC in the following figures. Figure 4
shows the results of flocculation experiments performed on Ca-clay with Na-Ca electrolytes. Figure 5 shows the results of flocculation experiments performed on Na-clay with Na-Ca and Na-Fe2+ electrolytes respectively. Results obtained on Na-clay are similar to those obtained with Ca-clay, though flocculation occurred at higher electrolyte concentrations.
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Fig. 3. X-ray diffraction diagram of (a) sedimented clay and (b) suspended clay for total electrolyte concentration (TEC) below the critical coagulation concentration (CCC), regardless of the electrolyte.
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Fig. 5. (a) Flocculation of Na-Clay in Na-Ca electrolyte and (b) flocculation of Na-clay in Na-Fe2+ electrolyte.
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Critical Coagulation Concentration Values for Na-Fe2+ and Na-Ca Solutions
The CCC values of Na-Fe2+ and Na-Ca solutions as a function of SAR are presented in Fig. 6a
(Ca-clay) and 6b (Na-clay). The CCC values are increasing with SAR. The CCC values obtained with Na-Fe2+ and Na-Ca electrolytes are equal for a given SAR value either for Ca and Na-clay.
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Fig. 6. Critical coagulation concentration (CCC) of (a) Ca-clay in Na-Ca and Na-Fe2+ electrolytes and CCC of (b) Na-clay in Na-Ca and Na-Fe2+ electrolytes.
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DISCUSSION
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The experimented clays are mainly composed of kaolinite and smectite. Although the flocculation of kaolinite is more sensitive to pH than smectite (Goldberg and Glaubig, 1987), the effect of pH on kaolinite flocculation is low for pH values lower than 7. This parameter is thus not suspected to have influenced the results.
At the observed pH, the well-crystallized kaolinite flocculated within 24 h regardless of the electrolyte composition as previously demonstrated (Arora and Coleman, 1979; Goldberg and Forster, 1990). The combination of smectite and poorly crystallized kaolinite displayed more typical flocculation behavior, with CCC increasing with increasing SAR values. Goldberg and Glaubig (1987) and Goldberg et al. (1991) observed that the flocculation properties of kaolinite and smectite 50/50 mixtures were similar to smectite. Our observation supports their conclusion, because if kaolinite tends to flocculate separately, the CCC value determined as the electrolyte concentration at which 20% of the clay remains suspended, will largely account for smectite behavior on a 50/50 clay mixing.
The CCC values obtained for Na-clay are higher than for Ca-clay, due to equilibration of exchangeable Na or Ca with the electrolyte. In lowland rice fields of Northeast Thailand, most of the soils are Na saturated. Hence for practical purpose the CCC determined on Na-clay is relevant to field conditions. Due to the low organic matter content of these soils (Mitsuchi et al., 1986), the CCC determined on extracted clay is relevant to soil clays.
The CCC values observed using Na-Fe2+ and Na-Ca electrolytes are quite similar, suggesting that Ca2+ and Fe2+ have quite similar properties with regard to exchange on smectite. This is in agreement with results of Tournassat and Charlet (2002), who reported a selectivity coefficient of 1 between Ca2+ and Fe2+ on the basal faces of smectite particles. Ferrous Fe is thus strongly adsorbed on smectite, which is in agreement with ferrolysis theory.
The concentrations in Fe2+ corresponding to the CCC values observed with Na-clay range between 0.07 (SAR 40) and 0.6 mmol L–1 (SAR 0). Thus, the maximum concentration of Fe2+ needed to flocculate the clays regardless of the electrolyte is 0.6 mmol L–1, and is much lower for most SAR values. Such concentrations are frequently observed in flooded paddy soils. It is thus likely that rice cultivation may prevent or reduce the dispersion of clays in sodic soils. Indeed, organic matter addition is a common practice in the reclamation and leaching of Na-affected soils all over the world. Among other effects, organic matter addition fastens the drop in redox potential upon flooding, which accelerates reductive dissolution of Fe oxides and thus enhances Na desorption, protecting the soil against clay dispersion.
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CONCLUSIONS
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The flocculation effects of Na-Ca and Na-Fe2+ electrolytes were compared at various SAR and TEC with Ca- and Na-Clay extracted from a flooded-rice field. The well-crystallized kaolinite did not disperse even at very low electrolyte concentrations. The flocculation properties thus reflected mostly the smectite behavior. The critical coagulation concentrations were equivalent for Na-Ca and Na-Fe2+ electrolytes. We conclude that Ca2+ and Fe2+ have similar sorption properties on exchange sites of smectite particles, in agreement with Tournassat and Charlet (2002). The Fe2+ concentrations corresponding to the CCC values are frequently observed in flooded paddy soils, suggesting that flooded-rice cultivation may prevent clay dispersion in Na-affected soils, provided that redox potential drops rapidly. This drop in redox potential may be enhanced by organic matter addition, which is a current practice in Na-affected paddy-soils in Asia and Africa.
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ACKNOWLEDGMENTS
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The authors thank L. Roger (INRA–Montpellier) and J. Delarivière (IRD–Dakar) for technical assistance. This research was partly supported by the Swiss National Science Foundation.
Received for publication February 10, 2003.
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ABSTRACT
INTRODUCTION
