The use of 129Xe NMR spectroscopy for studying soils. A pilot study

Abstract

Xenon nuclear magnetic resonance ( 129Xe NMR) spectroscopy is a powerful technique to study microporous solids and semi-crystalline polymers. In this pilot study we investigate the use of 129Xe NMR spectroscopy for studying soils by applying the technique to model systems of carbon black and kaolin powders and to humified sand. In contrast to e.g. zeolites no xenon is sorbed within the sand crystallites at room temperature and high pressure (10 atm). Xenon is only adsorbed at the crystallite surface. The exchange of xenon between the adsorbed and gas phase is so fast, that 129Xe NMR spectroscopy cannot distinguish between the two. This results in a single peak in the 129Xe NMR spectrum with a weighted-average chemical shift between the shift of xenon in the adsorbed and the gas state. A model is presented which relates the chemical shift to the local volume-to-area ratio of the intercrystallite pores at a submillimetre scale. If other heterogeneity types can be neglected, 129Xe NMR spectroscopy potentially reveals information about pore size distributions in undisrupted soils.