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

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 77K (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.