Assessments of Water Sorption Methods to Determine Soil’s Specific Surface Area

Abstract

A soil’s specific surface area (SSA) is commonly determined by two conventional methods: adsorption isotherms using nitrogen gas with interpretation through the Brunauer-Emmet-Teller (BET) equation, and ethylene glycol monomethyl ether (EGME) adsorption. The SSA so obtained may possess several drawbacks: (1) it does not directly reflect the SSA for soil-water interaction, (2) nitrogen sorption applies only to external mineral surface area and does not account for intracrystalline (expandable mineral interlayer) surface area, and (3) the EGME method is labor- and time-intensive. Recently, several water vapor adsorption SSA methods have emerged with improved capability to quantify SSA and directly probe soil-water interactions. Here the authors systematically assess the paradigms of non-water-based methods and water-based methods for their suitability to quantify soil’s SSA through a wide spectrum of soils. Independent adsorption isotherms using water vapor, EGME, and nitrogen gas for a variety of soils are measured and considered together with existing data from the literature to comparatively assess the methods. Among the water-based sorption methods, the augmented BET (A-BET) method compares consistently well with the non-water-based paradigm (EGME method). The A-BET method is superior to the other SSA methods as it can quantify and separate a soil’s external SSA (particle surface) and internal SSA (intracrystalline surface), thus providing a new dimension to understanding the role of soil-water retention in soil classification, swelling, collapsing, multiphysics flow, and mechanical behavior.