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The specific surface area of a soil sample is the total surface area contained in a unit mass of soil. Soils with high specific surface areas have high water-holding capacities, more adsorption of contaminants, and greater swell potentials. Specific surface is therefore an important parameter.
Specific surface is closely tied to particle-size distribution. This can be seen with a simple thought experiment. A cube, 1 cm on a side, with a density of 1 g/cm3 has a surface area of 6 cm2/g. If the cube were divided into smaller cubes, 1 mm on a side, the resulting 1000 cubes would have the same mass of material but a surface area ten times that of the single cube or 60 cm2/g. If the cube were divided into 1012 cubes, 1 μm on a side, the surface area would be 6 x 104 cm2/g. Thus, the smaller the particles, the greater the surface area per unit mass of soil.
Various approaches have been used to measure specific surface area, including adsorption of nitrogen and other gases on the soil. The most common method at present uses the adsorption of ethylene glycol monoethyl ether (EGME). This involves saturating prepared soil samples, equilibrating them in a vacuum over a CaCl2-EGME solvate, and weighing to find the point when equilibrium is reached. The specific surface is then determined from the mass of retained EGME in comparison to the amount retained by pure montmorillonite clay, which is assumed to have a surface area of 810 m2/g (Carter et al. 1986). The measurement typically takes around two days to complete.
Modern methods simplify the process
Soil is typically in a hydrated state, and surface area measurements should apply to that state. It would therefore be ideal if water could be used as the probe to determine the specific surface area. Quirk (1955) reviewed such measurements and concluded that water clusters around cation sites and can therefore lead to errors in the measurements. Recent work, however, using more modern methods for measuring the energy state of the water in the soil show promise as simple methods for determining specific surface of soil samples. Campbell and Shiozawa (1992) correlated specific surface of six soils with measurements of the slope of a moisture release curve and found excellent correlation.
Figure 1 shows the data for the six soils, along with an additional point for Ca-montmorillonite. The slope (x-axis value) is equal to the water content of the sample at a water potential of -123 MPa and is the inverse of the slope used by McKeen (1992) to quantify expansive soils, so it is clear that all these properties are closely linked.
A recent paper by Tuller and Or (2005) obtained the following equation relating surface area and the moisture characteristic
Where w is water content (g/g), ρw is the density of water (1000 kg/m3 ), ψ is the water potential (J/kg), S is the specific surface (m2/kg), and k is the Hamaker constant, which they took as -6 x 10-20 J. They used the WP4C to obtain water potentials for samples at low water content. These, along with the measured water contents, were used to estimate surface area for the same samples shown in Figure 1 plus one additional soil. The results are shown in Table 1.
|Hygrometric Surface Area (m2/g)||EMGE Surface Area (m2/g)|
|Ca - montmorillonite||597||760|
The agreement between the two methods is generally good. The low point here, as well as in Figure 1, is the Salkum soil. Its area may have been underestimated by the EGME method due to the pretreatment. The montmorillonite area is also low, but that value was taken from the literature and not remeasured in this study. These results are preliminary but indicate that the WP4C may be a useful instrument for determining specific surface of soils.
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Campbell, G. S., and S. Shiozawa. “Prediction of hydraulic properties of soils using particle-size distribution and bulk density data.” Indirect methods for estimating the hydraulic properties of unsaturated soils. University of California, Riverside (1992): 317-328.
Klute, A., and Ch Dirksen. “Hydraulic conductivity and diffusivity: laboratory methods.” Methods of soil analysis: part 1—physical and mineralogical methods (1986): 687-734. Article link.
McKeen, R. Gordon. “A model for predicting expansive soil behavior.” In Proc., 7th Int. Conf. on Expansive Soils, vol. 1, pp. 1-6. Reston, VA: ASCE, 1992.
Quirk, J. P. “Significance of surface areas calculated from water sorption isotherms by use of the Bet equation.” Soil Science 80, no. 6 (1955): 423-430. Article link.
Tuller, Markus, and Dani Or. “Water films and scaling of soil characteristic curves at low water contents.” Water Resources Research 41, no. 9 (2005). Article link.