Xe NMR spectroscopy of adsorbed xenon as an approach for the characterisation of soil meso- and microporosity

¹²⁹Xe nuclear magnetic resonance (NMR) spectroscopy of adsorbed xenon was applied for the characterisation of soil meso- and microporosity. Model systems (Ca-montmorillonite, quartz sand) and three soil types (Luvisol Alh, Bt and Cv horizons; Gleysol Go horizon; Podzol Bvs horizon) were studied. For Ca-montmorillonite, the average intercrystallite pore size has been evaluated. In natural soils, ¹²⁹Xe resonance parameters were shown to be affected by different factors: pore heterogeneity, influence of organic functional groups, possible presence of paramagnetic compounds, occurrence of xenon exchange between inter- and intraparticle void spaces. The effect of those factors on the pattern of ¹²⁹Xe NMR spectra was tested. In the three soils studied, no micropores within the mineral phase available for xenon adsorption were found. The most probable reason is that such pores are occupied by small molecules of the soil organic matter (SOM). Variable extent of accessibility of mesopores within the mineral phase of the various soils was revealed. It was highest in the Podzol. Here, xenon exchange between different adsorption zones (i.e., pores of differing size, e.g., internal and external void spaces) was slow on an NMR time scale that allowed to detect separate signals, each characterising xenon behaviour in the respective adsorption zone. The pore system of the soil organic matter was shown to be not accessible for xenon, as it is accepted for N₂ and other nonpolar adsorbates. Based on analysis of the spectra, a model for the possible mutual location of organic matter and iron compounds in natural soils was suggested. According to this model, a certain part of organic matter species can form the layers above iron species, thus masking them and preventing ¹²⁹Xe NMR spectra from significant low-field shifts and signal broadening.