The measurement of the specific surface area of soils by gas and polar liquid adsorption methods—Limitations and potentials
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.
References (110)
- et al.
Non-linear chlorinated-solvent sorption in four aquitards
J. Contam. Hydrol.
(1996) - et al.
BET measurements: outgassing of minerals
J. Colloid Interface Sci.
(2000) - et al.
The t-curve of multimolecular N2-adsorption
J. Colloid Interface Sci.
(1966) - et al.
129Xe NMR spectroscopy of adsorbed xenon as an approach for the characterisation of soil meso- and microporosity
Geoderma
(2004) - et al.
Soil micro- and mesopores studied by N2 adsorption and 129Xe NMR of adsorbed xenon
Geoderma
(2006) - et al.
Evaluating pore structures of soil components with a combination of “conventional” and hyperpolarised 129Xe NMR studies
Geoderma
(2011) - et al.
N2-BET specific surface area of bentonites
J. Colloid Interface Sci.
(2010) - et al.
Refractory organic carbon in particle-size fractions of arable soils II: organic carbon in relation to mineral surface area and iron oxides in fractions < 6 μm
Org. Geochem.
(2002) - et al.
Immobilisation of molybdate by iron oxides: effects of organic coatings
Geoderma
(2003) - et al.
Specific surface area of clay minerals: comparison between atomic force microscopy measurements and bulk-gas (N2) and -liquid (EGME) adsorption methods
Appl. Clay Sci.
(2011)
The use of 129Xe NMR spectroscopy for studying soils: a pilot study
Geoderma
The surface properties of carbon—I. The effect of activated diffusion in the determination of surface area
Carbon
Extent of coverage of mineral surfaces by organic matter in marine sediments
Geochim. Cosmochim. Acta
Towards the establishment of a reliable proxy for the reactive surface area of smectite
Geochim. Cosmochim. Acta
Surface area and porosity
Soil organic matter as a nanoporous sorbent of organic pollutants
Adv. Colloid Interface Sci.
Application of thermal analysis techniques in soil science
Geoderma
Biological, chemical and thermal indices of soil organic matter stability in four grassland soils
Soil Biol. Biochem.
TEM study of in situ organic matter on continental margins: occurrence and the “monolayer” hypothesis
Mar. Geol.
Micro- and nano-environments of carbon sequestration: multi-element STXM-NEXAFS spectromicroscopy assessment of microbial carbon and mineral associations
Chem. Geol.
Comparative adsorption of argon and nitrogen for the characterisation of hydrophobized surfaces
Colloids Surf. A Physicochem. Eng. Asp.
Surface chemistry of the calcination of gelatinous and crystalline aluminium hydroxides
J. Appl. Chem.
An improved pretreatment for mineralogical analysis of samples containing organic matter
Clays Clay Miner.
Methylene blue dimerization does not interfere in surface-area measurements of kaolinite and soils
Clays Clay Miner.
Surface area of homoionic illite and montmorillonite clay minerals as measured by the sorption of nitrogen and carbon dioxide
Clays Clay Miner.
The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms
J. Am. Chem. Soc.
Influence of allophane and organic matter contents on surface properties of Andosols
Eur. J. Soil Sci.
Quantifying surface areas of clays by atomic force microscopy
Am. Mineral.
Surface area of soils and clays by an equilibrium ethylene glycol method
Soil Sci.
Ethylene glycol retention by soils as a measure of surface area and interlayer swelling
Soil Sci. Soc. Am. J.
Nitrogen adsorption experiments on several clay minerals
Soil Sci.
Adsorption of gases in multimolecular layers
J. Am. Chem. Soc.
On a theory of the van der Waals adsorption of gases
J. Am. Chem. Soc.
A generalized description of aquatic colloidal interactions: the three-colloidal component approach
Environ. Sci. Technol.
Influence of organic materials on the determination of the specific surface areas of soils
J. Soil Sci.
Sorption of hydrophobic organic compounds by soil materials: application of unit equivalent Freundlich coefficients
Environ. Sci. Technol.
Ethylene glycol monoethyl ether for determining surface area of silicate minerals
Soil Sci.
Determination of surface area of fine-grained soils by the ethylene glycol monoethyl ether (EGME) method
Geotech. Test. J.
Adsorption of Methylene Blue on montmorillonite
J. Dispers. Sci. Technol.
Clay-sized organo-mineral complexes in a cultivation chronosequence: revisiting the concept of the ‘primary organo-mineral complex’
Eur. J. Soil Sci.
The surface area of soil organic matter
Environ. Sci. Technol.
Sorption of N2 and EGME vapors on some soils, clays, and mineral oxides and determination of sample surface areas by use of sorption data
Environ. Sci. Technol.
Comparison of various methods for the determination of specific surfaces of subsoils
J. Soil Sci.
A simplified ethylene glycol monoethyl ether procedure for assessment of soil surface area
Soil Sci. Soc. Am. J.
Surface complexation modeling in aqueous geochemistry
Rev. Mineral.
The shapes of capillaries
Comparison of three methods of measuring surface area of soils
Can. J. Soil Sci.
Adsorption of CO2 and N2 on soil organic matter: nature of porosity, surface area, and diffusion mechanisms
Environ. Sci. Technol.
The microporous structure of organic and mineral soil materials
Soil Sci.
On the problems of total specific surface area and cation exchange capacity measurements in organic-rich sedimentary rocks
Clays Clay Miner.
Cited by (79)
Investigating the influence of particle size ranges on the physical, mineralogical, and environmental properties of raw marine sediment
2023, Construction and Building MaterialsEvaluation of the BET and GAB models for interpretation of soil water isotherms: A molecular simulation study
2023, Computers and Geotechnics
- 1
Tel.: + 49 8161 71 3735; fax: + 49 8161 71 4466.