Adsorption isotope effects of water on mesoporous silica and alumina with implications for the land-vegetation-atmosphere system

Soil water dynamics within a vadose (unsaturated) zone is a key component in the hydrologic cycle, especially in arid regions. In applying the Craig-Gordon evaporation model to obtain isotopic compositions of soil water and the evaporated vapor in land-surface models (LSMs), it has been assumed that the equilibrium isotope fractionation factors between soil water and water vapor, α(²H) and α(¹⁸O), are identical to those between liquid and vapor of bulk water. Isotope effects in water condensation arise from intermolecular hydrogen bonding in the condensed phase and the appearance of hindered rotation/translation. Hydrogen bonding between water molecules and pore surface hydroxyl groups influences adsorption isotope effects. To test whether equilibrium fractionation factors between soil water and water vapor are identical to those between liquid and vapor of bulk water and to evaluate the influence of pore size and chemical composition upon adsorption isotope effects, we extended our previous experiments of a mesoporous silica (15 nm) to two other mesoporous materials, a silica (6 nm) and an alumina (5.8 nm). Our results demonstrated that α(²H) and α(¹⁸O) between adsorbed water and water vapor are 1.057 and 1.0086 for silica (6 nm) and 1.041 and 1.0063 for alumina (5.8 nm), respectively, at saturation pressure (p_o), which are smaller than 1.075 and 1.0089, respectively, between liquid and vapor phases of free water at 30 °C and that the differences exaggerate at low water contents. However, the profiles of α values with relative pressures (p/p_o) for these three materials differ due to the differences in chemical compositions and pore sizes. Empirical formula relating α(²H) and α(¹⁸O) values to the proportions of filled pores (f) are developed for potential applications to natural soils. Our results from triple oxygen isotope analyses demonstrated that the isotope fractionation does not follow a canonical law. For the silica (15 nm), fractionation exponents (¹⁷θ) are 0.5361 ± 0.0018 and 0.5389 ± 0.0016 at p/p_o = 0.72 and 0.77, respectively. For the silica (6 nm), ¹⁷θ values are 0.5330 ± 0.0011 at p/p_o = 0.65 and 0.5278 ± 0.0010 at p/p_o = 0.81. For the alumina (5.8 nm), ¹⁷θ value is 0.5316 ± 0.0015 at p/p_o = 0.78. These values are greater than or equal to that of liquid-vapor equilibrium of bulk water (0.529 ± 0.001).