THE USE OF XE-129 NMR-SPECTROSCOPY FOR STUDYING SOILS - A PILOT-STUDY

Xenon nuclear magnetic resonance (Xe-129 NMR) spectroscopy is a powerf ul technique to study microporous solids and semi-crystalline polymers . In this pilot study we investigate the use of Xe-129 NMR spectroscop y for studying soils by applying the technique to model systems of car bon black and kaolin powders and to humified sand. In contrast to e.g. zeolites no xenon is sorbed within the sand crystallites at room temp erature and high pressure (10 atm). Xenon is only adsorbed at the crys tallite surface. The exchange of xenon between the adsorbed and gas ph ase is so fast, that Xe-129 NMR spectroscopy cannot distinguish betwee n the two. This results in a single peak in the Xe-129 NMR spectrum wi th a weighted-average chemical shift between the shift of xenon in the adsorbed and the gas state. A model is presented which relates the ch emical shift to the local volume-to-area ratio of the intercrystallite pores at a submillimetre scale. If other heterogeneity types can be n eglected, Xe-129 NMR spectroscopy potentially reveals information abou t pore size distributions in undisrupted soils. (C) 1997 Elsevier Scie nce B.V.