Cation exchange capacity methodology I: An efficient model for the detection of incorrect cation exchange capacity and exchangeable cation results

doi:10.1016/j.clay.2005.12.006

Applied Clay Science

Volume 34, Issues 1–4, October 2006, Pages 31-37

https://doi.org/10.1016/j.clay.2005.12.006 Get rights and content

Abstract

In this study a model is proposed enabling the detection of incorrect cation exchange capacity (CEC) and exchangeable cation values. Numerous CEC and exchangeable cation analyses of clayey sediments, soils and bentonites were performed using triethanolamine-buffered barium chloride, ammonium acetate, silver thiourea and other exchange techniques. As long as these naturally clayey materials only contain adsorbents like clay minerals, organic substances and a group of mainly detrital minerals like quartz, feldspar and mica, results obtained with most procedures are correct. Problems arise when materials contain secondary phases like soluble Ca-carbonates and -sulphates. During the CEC-experiments, these phases interact with the exchange solution. According to expectations, results of exchangeable calcium values are incorrect but CEC is also affected [Deller, B., 1981. Determination of exchangeable acidity, carbonate ions and change of buffer in triethanolamine-buffered solutions percolated through soil samples containing carbonates. Commun. Soil Sci. Plant Analysis 12, 161–177.]. Using the proposed Carbonate and Sulphate Field Model (CSF model) an evaluation of the accuracy of results is possible.

Introduction

The idea of this study is to provide soil scientists and clay researchers with a tool allowing them to decide whether experimentally determined CEC and exchangeable cation results of a given set of samples are correct. The paper focuses on the development of a model that allows the recognition of incorrect results. It is simple to use and because of its precision it provides the user with clear information about the accuracy of experimentally determined data.

The cation exchange capacity is a fundamental property of clay minerals. “The CEC is defined as a measure of the ability of a clay or a soil to adsorb cations in such a form that they can be readily desorbed by competing ions” (Bache, 1976). CEC is reversible and it is a sum parameter comparable to soil-pH.

In common CEC procedures the negative charge of a material is balanced with an index-cation. After that the CEC is determined by measuring the difference between the initial and the remaining content of the index-cation (e.g. silver thiourea method, Chhabra et al., 1975). A further possibility is to re-exchange the index-cation chosen with an appropriate salt and to determine the amount of the released index-cations by radioactive counting, visible spectroscopy or by a measurement using atomic absorption.

Knowledge of CEC and cation distribution of the exchanger surfaces can be used as a powerful tool for the characterisation of clay minerals, clays and soils, especially for quantification of the clay minerals, origin and genesis of soils, clayey sediments and clay deposits, quality control of clay raw material and improvement of clay mineral properties for industrial use. Exact CEC and exchangeable cation data is often indispensable for a better understanding of the system under consideration.

The correct determination of the CEC is difficult due to a number of methodical problems. With the exception of some special conditions such as an acidic pH value of ca. < 5 or in saline and alkaline (electrolyte-rich) clays, the correct value of the CEC is more or less independent of pressure, temperature, solution/solid ratio, electrolyte composition and concentration (Bache, 1976). The proposed model requires that only samples of this type are chosen. High charge smectites, vermiculites, and zeolites show a different cation exchange behaviour. High charge smectites and vermiculites have such a high layer charge density that the interlayer cations cannot be readily desorbed quantitatively. In the case of vermiculites the rate of cation transportation between the 2 : 1 layers is slowed down by the high layer charge density (Walker, 1959, Graf von Reichenbach, 1966, Malcolm and Kennedy, 1969). The complete cation exchange therefore is only possible at increased temperature or during long term experiments. Zeolites on the other hand show a pronounced selectivity for certain cations, here the determination of an overall CEC for unspecified cations as demanded in the definition is not possible.

During the last decades, a number of methods for the determination of the CEC have been developed: ammonium acetate (Lewis, 1949), triethanolamine buffered barium chloride (Mehlich, 1948, Bascomb, 1964), radioactive tracers (¹³³Ba, Bache, 1970; ⁸⁵Sr, Francis and Grigal, 1971), nephelometry (Adams and Evans, 1979), methylene blue (Hang and Brindley, 1970), silver thiourea (Chhabra et al., 1975), Cu (II) ethylenediamine (Bergaya and Vayer, 1997), Cu (II) triethylenetetramine (Meier and Kahr, 1999) and several other techniques. The ammonium acetate and the barium chloride methods are used widely and they are very time consuming. A disadvantage of these procedures is that resulting CEC and exchangeable cation values are often distorted to a large extent (Kick, 1956, Deller, 1981). For pure clay fractions (< 2 μm equivalent diameter) problems usually do not occur but this changes when samples with unfavourable mineralogical components (concerning CEC methodology) such as carbonates are examined. When CEC and exchangeable cation values of calcareous clayey materials are determined, these minerals become partially dissolved during the exchange experiments. This is caused by a decrease of the minerals' stability when they interact with electrolyte-rich solutions.

The paper focuses on the development of a model that allows the recognition of flawed CEC and exchangeable cation results. It is simple to use and because of its precision it provides the user with clear information about the correctness of experimentally determined data.

Section snippets

Materials

Thirty-one German clayey sediments are used in this study. The samples were characterised by conventional XRD techniques (Siemens D 500): random powder diffraction, textured air-dried and glycolised clay fractions (5 mg/cm² prepared on glass slides) and quantitative quartz content determined with an internal standard. Semi-quantification of the mineralogical composition was performed considering grain size distribution, CEC, chemical analyses (XRF), inorganic and organic carbon content. For

Cation exchange procedure

The method studied is a batch procedure with triethanolamine-buffered BaCl₂ solution (c = 0.1 M) followed by a re-exchange with aqueous MgCl₂ solution (c = 0.1 M). All analyses were performed in duplicate with additionally at least two different solution/solid ratios. This was done by varying the solid content (2.0 g and 0.5 g, precision = ± 0.0005 g) at fixed amounts of solution. To minimise matrix effects AAS and ICP-measurements were performed with diluted solutions using an exact commercial

Application of the CSF model

In Fig. 3 the Carbonate- and Sulphate Field principle is applied to a series of calcareous and non-calcareous clays. Regarding the results it is obvious that using this simple graph it is possible to evaluate the accuracy of exchangeable calcium data.

The model can also be used for exchangeable sodium, potassium and magnesium, as well as for CEC (Fig. 4).

For exchangeable sodium (Fig. 4 A) and potassium values (Fig. 4 B) no deviation of data points from the y = x-line is observed. Therefore results

Limits of the CSF model

The limit of the application of this model is attained when soluble minerals or substances are dissolved completely during the exchange experiment. Usually dissolution is only partial but this is not always the case. It occurs when the concentration of the soluble mineral is very low or when the dissolution capability of an exchange solution for a certain component is very high.

This limitation of the model is demonstrated in Fig. 5. Here a natural clay was analysed for CEC and exchangeable

Conclusions

The Carbonate- and Sulphate-Field diagram is a useful tool for a fast evaluation of the accuracy of experimental CEC data. It provides the user with helpful information and is easy to use. The limit of the model is reached when soluble phases are only minor constituents or when the exchange solution has a high dissolution capability with respect to the soluble phase. Another restriction is that the correctness of exchangeable cation data of samples with a high electrolyte concentration like

Acknowledgements

The author would like to acknowledge the financial support generously provided by ‘Stipendiatenamt of Aachen Technical University, Germany’ as well as the ‘Deutsche Forschungsgemeinschaft’, project II C 6-Ec 72/3, and Prof. Dr. Wolfram Echle for critical discussions.

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Cation exchange capacity methodology I: An efficient model for the detection of incorrect cation exchange capacity and exchangeable cation results

Abstract

Introduction

Section snippets

Materials

Cation exchange procedure

Application of the CSF model

Limits of the CSF model

Conclusions

Acknowledgements

Applied Clay Science

Applied Clay Science

Determination of the cation-exchange capacity (layer charge) of small quantities of clay minerals by nephelometry

Clays and Clay Minerals

Barium isotope method for measuring cation-exchange capacity of soils and clays

Journal of the Science of Food and Agriculture

The measurement of cation exchange capacity of soils

Journal of the Science of Food and Agriculture

Rapid method for the determination of the cation exchange capacity of calcareous and non-calcareous soils

Journal of the Science of Food and Agriculture

The measurement of the cation exchange capacity and exchangeable cations in soils: a new method

Determination of exchangeable acidity, carbonate ions and change of buffer in triethanolamine-buffered solutions percolated through soil samples containing carbonates

Communications in Soil Science and Plant Analysis