Elsevier

Geoderma

Volume 216, March 2014, Pages 75-87
Geoderma

The measurement of the specific surface area of soils by gas and polar liquid adsorption methods—Limitations and potentials

https://doi.org/10.1016/j.geoderma.2013.10.015Get rights and content

Highlights

  • Soil specific surface area depends not only on the method but also on pretreatment.

  • Recommendations to improve the quality of the data are presented.

  • Clay mineral type dominates relation between BET surface area and EGME retention.

  • A combination of methods allows for closer characterisation of the soil interface.

Abstract

The specific surface area (SSA) of soils is a basic property and closely related to other physical and chemical properties like e.g. cation exchange capacity, clay content, organic matter content, porosity and hydrodynamic and geotechnical characteristics. Therefore, the SSA of soils has been measured frequently for decades. However, no universal method to determine SSA exists. The existing methods can generally be grouped into two categories, the adsorption of gases and the adsorption of polar liquids or molecules from solution. Depending on the method applied, the SSA of a soil can vary, as by these different methods, different surfaces of the soil are determined. The most frequently used representatives of these two groups for measuring SSA of soils are the physisorption of nitrogen gas at 77 K (BET-N2) for the gas adsorption methods, yielding the external surface area of the mineral particles, and the retention of ethylene glycol monoethyl ether (EGME) for the adsorption of polar liquids, probing the total surface area including interlayers of clay minerals and micropores of organic material. Studies dealing with the determination of SSA of soils are numerous, and it has also been shown that the resulting SSA values differ not only depending on the method but also on the sorbate used and the sample pretreatment. This review shortly presents the principles of these methods and emphasises their limitations and difficulties, when applied to soil samples, like sample pretreatment, (micro-)porosity and attachment of organic material to mineral surfaces. In particular the drying of the samples prior to measurement seems to be crucial for the results obtained. Recommendations are given in order to improve the quality of the data and to facilitate the comparability of SSA data of different studies. It is shown for clayey soil samples that the relationship between BET-N2 and EGME SSA depends predominantly on the type of clay mineral and not on the content of organic material. Thus, from the SSA measurements, an estimation of the dominant clay mineral seems possible. Consequently, a suitable combination of various SSA determination methods together with related techniques can result in a more detailed characterisation of the reactive interface of a soil to the liquid and gaseous phases.

Introduction

Soils consist of a mixture of various organic and inorganic components interacting with each other and thereby forming a heterogeneous interface between the solid and the liquid and gaseous phases of the soil (Totsche et al., 2010, Young and Crawford, 2004). The characterisation of the solid surface is essential to understand the fate and effects of dissolved and dispersed substances entering the soil because the surface provides a multitude of various reactive sites (Totsche et al., 2010). The size of this surface is a fundamental parameter, as a larger surface area can provide more reactive sites and thus more possibilities of a substance to interact with the solid soil constituents during its travel through the soil. The surface area between the solid and the liquid and gaseous phases of a soil is a property of the solid, which is called specific surface area (Chiou et al., 1993). The specific surface area (SSA) is defined as the mass normalised surface area (e.g. Metz et al., 2005) and is a basic soil property. Many physical and chemical soil properties are influenced by or closely related to the SSA like e.g. cation exchange capacity, clay content, organic matter content, porosity and hydrodynamic and geotechnical characteristics (Feller et al., 1992, Petersen et al., 1996, Theng et al., 1999, Yukselen-Aksoy and Kaya, 2010b). Consequently, the SSA of soils has very often been determined during the last decades.

Although it seems that the determination of the SSA of soils is nowadays a more or less routine measurement, still no universal standard method exists. Rather, several methods have been proposed and are used to determine SSA. Generally, these methods can be divided in two groups, the adsorption of gases, i.e. the condensation of molecules on the solid surface, and adsorption of polar liquids including water or adsorption of molecules from solution onto the surface (Chiou et al., 1993, Gregg and Sing, 1967, Santamarina et al., 2002, Tiller and Smith, 1990, Yukselen-Aksoy and Kaya, 2010a). Depending on the type of method applied, the SSA of a soil can vary. Thereby, gas adsorption methods commonly yield lower values of SSA than the adsorption of liquids or molecules from liquids. It is widely accepted that gas adsorption measurements reveal only the external surface area, i.e. the entire surface surrounding the separate soil grains. On the other hand, methods involving liquids determine the total surface area, which includes additionally to the external surface area also the internal surface area, consisting of the surfaces of the interlayers of clays or micropores of organic material that are inaccessible to the gas molecules (Chiou et al., 1993, de Jong, 1999, Santamarina et al., 2002, Tiller and Smith, 1990, Yukselen-Aksoy and Kaya, 2010a). Besides, SSA can also be inferred from known thermodynamic properties, the dissolution rate of soluble minerals, microscopy, diffusiveness of X-ray diffraction patterns (Santamarina et al., 2002) and from atomic force microscopy measurements (Bickmore et al., 2002, Macht et al., 2011, Metz et al., 2005). But for soils, most commonly gas adsorption and adsorption of polar liquids are applied to measure SSA.

Studies dealing with SSA determinations of soils and also sediments are numerous. Subsequently, the objective of this review can therefore not be to give a complete overview of the entire literature available. Instead, it is intended to focus on the application of physisorption of nitrogen gas at 77 K as a representative of gas adsorption measurements and the retention of ethylene glycol monoethyl ether (EGME) as a representative of polar liquid adsorption measurements to soil samples. After a short introduction of the principles of the methods, special emphasis will be put on their inherent limitations and difficulties for soil samples like sample pretreatment, (micro-)porosity and the attachment of organic material to mineral surfaces. Considering these aspects, a suitable combination of several diverse techniques can result in a more detailed characterisation of the soil's complex interface to its liquid and gaseous phases.

Section snippets

Physisorption of N2 at 77 K according to the BET method

The physisorption of gas molecules is based on the attraction of molecules to a surface by dispersion forces, short-range repulsion forces and forces due to permanent dipoles within the adsorbed molecule. Contrary to chemisorption, no transfer of electrons between the adsorbed molecule and the solid takes place (Gregg and Sing, 1967, Sing et al., 1985). Thus, physisorption is able to probe all surface sites accessible to the gas molecules and not only those sites which possess some particular

Retention of ethylene glycol monoethyl ether (EGME)

The second group of SSA determination methods consists of adsorption of polar liquids, including water, or adsorption of molecules from solution onto the sample surface. If the liquid is a solution, an adsorption isotherm of the solute can be obtained and from this, the SSA can be calculated (Gregg and Sing, 1967). Although the SSA determination by adsorption from solution is experimentally much simpler than the gas adsorption method, it is more complicated on the theoretical side. First of

Good laboratory practice for the determination of SSA

The determination of SSA of soils and sediments by gas adsorption or liquid retention methods are today more or less routine methods. As it is obvious from the discussion above, both groups of methods exhibit some drawbacks and pitfalls; but they can mostly be avoided by sticking to some simple rules. In general, these predominantly concern the sample pretreatment because pretreatment conditions can have a considerable effect on the measured SSA, depending on the soil constituents present.

Characterisation of soil samples by comparative SSA determinations

The SSA of a soil is generally considered to be an intrinsic parameter. Therefore, it would be desirable to have a solid SSA value that can directly be compared with others; and accordingly, there seems to be a need for a simple method which yields reliable results (Yukselen and Kaya, 2006). But it was seen that the SSA of a soil depends on the measuring method. Gas adsorption methods like BET-N2 determine the external surface area, whereas adsorption of polar liquids like EGME or adsorption of

The relationship between BET-N2 and EGME SSA depends mainly on clay type and not on organic material

In order to obtain more information about the characteristics of the interface of the sample, the use of methods that probe preferably different types of surfaces seems most promising. Most often, the comparison between BET-N2 and EGME SSA is performed (e.g. Churchman et al., 1991, de Jong, 1999, Macht et al., 2011, Pronk et al., 2013, Tiller and Smith, 1990, Yukselen and Kaya, 2006). Materials studied by these two methods are commonly clays or clayey soils. The SSA values are expected to be

Characterisation of the soil's interface by SSA determinations and other methods

As described before, the combination of several different methods to determine SSA of soils appears promising to obtain further information about the interface of a soil which exists between the solid and the liquid and/or gaseous phases of a soil. Together with sorption experiments with well-known substances acting as probes for certain types of surfaces, the interfaces that a substance encounters during its movement through the soil can be characterised (Pronk et al., 2013, Xiao et al., 2012

Conclusions

The SSA of soils has been studied by different methods for decades. In general, these methods can be grouped into two categories, adsorption of gases on dry surfaces and adsorption of polar liquids or molecules from solution. It is assumed that the gas adsorption measurements, which commonly yield lower SSA values than the liquid adsorption methods, probe only the external surface area of soil constituents. On the other hand, adsorption of liquids records the total surface area, including

Acknowledgements

The author thanks Elfriede Schuhbauer for assisting in the preparation of Fig. 1 and Geertje J. Pronk and two anonymous reviewers for valuable comments and suggestions on the manuscript.

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