A sorption isotherm model for soil incorporating external and internal surface adsorption, and capillarity

Soil water sorption isotherm is governed by soil-water interactions of adsorption and capillarity, depending on particle surface properties and pore size distribution, respectively. Recent studies have separated soil particle surfaces into two categories in terms of distinct adsorption behaviors, i.e., external and internal surfaces, identifying three independent sorption processes with varying energy levels, namely external adsorption, internal adsorption, and capillarity. Here, an isotherm model is developed to describe water sorption on soils in the full relative humidity (RH) range by incorporating the three physical processes. The internal and external adsorptions are represented by an augmented-Brunauer-Emmett-Teller (A-BET) equation, and capillarity is described by Kosugi's model. Two scaling factors are introduced to expand A-BET equation's application RH range from 0%-40% to 0%-100%--one for the reduction of available adsorption area in restricted pores and another for the disparity between heats of adsorption and liquefaction. Experimental validation demonstrates that the proposed model excellently matches the isotherm data for a wide array of soils in the full RH range. Further, the proposed model performs well in capturing soil water retention data in the full matric potential range, providing a seamless linkage between soil sorption isotherm and water retention behaviors.