Cation exchange capacity and soil pore system play key roles in water vapour sorption

Understanding the controlling factors of soil water vapour sorption at different water activities (a_w) is essential for accurate estimation of soil properties, such as cation exchange capacity (CEC) and specific surface area (SSA). The objective of this study was to identify the role of CEC and soil pore system in water vapour sorption and sorption hysteresis over a range of a_w values. The CEC, SSA, pore volume, and water and nitrogen adsorption/desorption isotherms for eight soils with different clay contents and mineralogies were measured. Irrespective of a_w and sorption direction, there was a significant correlation between water content and CEC, and the correlation varied with a_w. The water content change was mainly related to CEC and SSA_H2O for a_w < ∼0.6, and to SSA_N2 and pore volume for a_w > ∼0.6. Similar to the forced closure phenomenon of the soil nitrogen desorption branch at a relative pressure (P/P0) of ∼ 0.45, the soil water vapour desorption branch also exhibited a sudden drop in the range of 0.2 < a_w < 0.4. The water vapour sorption hysteresis for a_w < ∼0.75 was mainly due to the difference in cation hydration during the adsorption and desorption process, and the hysteresis phenomena for a_w > ∼0.75 was attributed to the different sizes of narrow pore necks and the connected pores.