Separating External and Internal Surface Areas of Soil Particles

Specific surface area (SSA) of soil is an intrinsic property governing many geotechnical and geoenvironmental engineering properties. SSA can be physically divided into two categories: external or particle surface area, and internal or intracrystalline surface area. Although both of them are well known to play different roles in physical, chemical, and biological processes, few methods have been developed to quantitatively distinguish between them. Using a recent theoretical advancement of an augmented Brunauer–Emmett–Teller (BET) adsorption equation for soil, the writers develop a procedure based on measured water adsorption isotherms to quantify the external and internal SSAs. Extensive adsorption isotherms of water, ethylene glycol monomethyl ether, and nitrogen of a variety of silty and clayey soils are used to validate the procedure. Practical implications of SSA are also provided by linking the importance of the internal SSA to swelling behavior, and the importance of the external SSA to adsorption of nonpolar materials. The demonstrated procedure to quantify external and internal SSAs should provide a pressing and powerful method to understand physical, chemical, and biological processes in soils.