Effects of Physicochemical Properties on Soil Water Vapour Sorption Kinetics

Abstract

Soil water vapour sorption kinetics is of great significance to understanding the soil water cycle and soil water vapour movement in arid areas. However, the differences and influencing factors of soil water vapour sorption kinetics in different adsorption processes are still not completely clear. Thus, this study aimed to investigate the soil water vapour adsorption/desorption rates (R_a/R_d) for various water activity (a_w) levels and to identify the key factors affecting these rates. In this study, we determined the change of R_a and R_d with a_w and the R_a and R_d during the monolayer adsorption (a_w = 0.05–0.02, R_a0 and R_d0), multilayer adsorption (a_w = 0.2–0.6, R_am and R_dm), and condensation (a_w = 0.6–0.93, R_ac and R_dc) processes for eight mineral soils with different clay contents and mineralogies using a fully-automated AquaLab Vapour Sorption Analyser in dynamic dewpoint isotherm (DDI) mode. Across the entire a_w range, the R_a varied from 2.18 × 10⁻⁵ to 1.85 × 10⁻⁴ g g⁻¹ min⁻¹, and the R_d varied from 2.23 × 10⁻⁵ to 3.93 × 10⁻⁴ g g⁻¹ min⁻¹. The adsorption rate was in the order of R_a0 > R_am>R_ac, and the desorption rate was in the order of R_dc>R_dm > R_d0. The ratios of adsorption and desorption rates R_a0/R_d0, R_am/R_dm, and R_ac/R_dc are 2.82, 0.97, and 0.48, respectively. The monolayer adsorption rate exceeded its desorption rate, while multilayer adsorption exhibited comparable kinetics to desorption. Adsorption kinetics during capillary condensation exhibited significant retardation compared to desorption dynamics. Cation exchange capacity (CEC) and total specific surface area (SSA) were significant determinants of adsorption–desorption kinetic parameters (R_a0, R_d0, R_am, R_dm, and R_ac), whereas pore volume (PV) and clay content showed no statistically significant correlation with these kinetic metrics. In contrast, clay content, external SSA, and PV emerged as key factors affecting the R_ac, while CEC and total SSA exhibited negligible influence on this parameter.