Advanced 129Xe NMR techniques for examing soil meso- and micro porosity and its role in hosting soil organic matter
Zusammenfassung der Projektergebnisse
Soil is widely recognized as one of the most complex systems in the universe. As a consequence, there is a need of appropriate methodologies and instrumentation to characterize its porous structure. With this project we intended to promote innovative technologies of the 129 Xe NMR spectroscopy of adsorbed xenon for elucidating soil meso- and micropores for better understanding their role in the processes of soil organic matter (SOM) stabilization. We have tested the applicability of the laser-polarised or hyperpolarised (HP) 129Xe NMR to soil materials. This approach was expected to significantly increase sensitivity of the measurements. In parallel, conventional (i.e. using thermally-polarised xenon, TP) studies were carried out on a number of different soil mateirals. The soil materials tested originated from the following soil types: allophanic and non-allophanic Andosols, Chernozem, fire-affected Arenosol and Podzol. Briefly, in soil porous components, xenon gas behaved as an efficient probe for interrogating their structures through: i) higher sensitivity for probing micropores within polymeric organic structures e.g. charcoals, as compared to common adsorption methods; ii) possibility to use elevated pressures of the adsorbate for enhancing pore accessibility; iii) evaluating not only the pore size range but also adsorption enthalpies from the observed 129Xe shifts that reflect the nature of Xe - pore surface interactions. The SOM sorption within the model mineral (hydr)oxides (Al2O3, AlOOH, allophane) was shown to occur inhomogeneously (patchy) as the determined Xe adsorption enthalpies corresponded to co-existing “empty” pores and those coated with organic species. In both synthetic and natural charcoals (derived from the Arenosol and non-allophanic Andosol), the observed differences between the HP- and TP 129Xe NMR patterns were explained by the slow xenon diffusion through the highly constricted pore system, and the estimated width of those constricted pore openings was of the order of one, or two diameters of the Xe atom. Similar “bottle neck” effects were also considered in the natural soil particle size fractions (Andosol) based on the increased pore access observed for Xe adsorption performed at elevated pressures (2-4 bar). In the particle size fractions of our studied soils, the fine porosity could only be observed after the removal of SOM. Unusually high microporosity found in the non-allophanic Andosols was attributed to the “multi-domain” structure of soil particles i.e. particles formed by agglomerated nano-sized crystallites. Those crystallites were assumed to be the polymeric Aln(OH)m clusters formed by hydrolysis reactions of Al3+ species after the destroying of Al-humus complexes by the H2O2-oxidation. For the first time, we evidenced micropores in the particular organic matter (POM) fractions thus highlighting the advanced potential of the 129Xe NMR spectroscopy in interrogating the soil polymeric organic structures. Combined 13C NMR and 129Xe NMR spectroscopic analysis of the fire-affected Arenosol, characterized by a high content of charred residues, confirmed the abundance of the charred species interacting with clay minerals. To summarize, we demonstrated that the 129Xe NMR spectroscopy of adsorbed xenon can successfully be used in a combination with methods more traditional for soil science (solid-state 13C NMR, X-ray diffraction, N2 adsorption, Mössbauer spectroscopy) to achieve an extended understanding of microstructural properties of soil pores and their role in carbon stabilization.
Projektbezogene Publikationen (Auswahl)
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New applications of 129Xe nmr spectroscopy for studying meso- and micropores in soil constituents. International Symposium on Xenon NMR of Materials (XEMAT), Ruka, Finland, 7-10 June, 2009
S. Filimonova, A. Nossov, H. Knicker, A. Gédéon, I. Kögel-Knabner
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New applications of 129Xe nmr spectroscopy of adsorbed xenon for studying meso- and micropores in soil components. Jahrestagung der Deutschen Bodenkundlichen Gesellschaft, Bonn, Germany, 7-11 September 2009
S. Filimonova, A. Nossov, H. Knicker, A. Dümig, I. Kögel-Knabner
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129Xe NMR spectroscopic study of meso- and microporosity in soil components. European Congress on Magnetic Resonance, Firenze, Italy, 5-9 July 2010
S. Filimonova, A. Nossov, A. Dümig, H. Knicker, A. Gédéon, I. Kögel-Knabner
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2010. Elucidation of mesoand microporosity in soil components with 129Xe NMR spectroscopy of adsorbed xenon. Geophysical Research Abstracts, Vol.12, EGU2010-6166, EGU Assembly, Wien, Austria, 3-7 May 2010
S. Filimonova, A. Nossov, H. Knicker, I. Kögel-Knabner
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2010. Evaluating pore structures of soil components with a combination of “conventional” and hyperpolarised 129Xe NMR studies. Geoderma
Svetlana Filimonova, Andrei Nossov, Alexander Dümig, Antoine Gédéon, Ingrid Kögel- Knabner and Heike Knicker
