A new model for soil water vapor sorption isotherms considering adsorption and condensation

Abstract

AbstractA new model, which was based on the Do and Do (2000) model, was developed to describe soil water vapor sorption isotherms (SWSIs) including both adsorption and condensation processes. The model performance was evaluated using measured SWSIs of 33 soil samples with a wide range of clay contents (0.08–0.72 g g−1), clay mineralogy (montmorillonite, kaolinite, and mixed clay samples), and organic C contents (0.001–0.070 g g−1). The new model produced satisfactory fits to measured SWSIs, with the mean relative percentage deviation modulus less than 6.2%. For a given clay mineral type, the adsorption sites and condensation capacity increased with increasing clay content. At a given clay content, the number of adsorption sites was in the order of montmorillonitic samples (ML)>mixed clay samples (MX)>kaolinitic samples (KA), and the condensation capacities of ML and MX were lower than that of KA. For soils with low clay content (<0.12 g g−1) and similar clay mineralogy, the adsorption sites and condensation capacities were positively related to organic C. The model parameter S0 had the potential for deriving cation exchange capacity, specific surface area, and clay content. The C/S0 values could potentially be used as a proxy of the dominant clay mineral type in a sample.