Formation and properties of mineral-organic soil interfaces as revealed by X-ray photoelectron spectroscopy
Final Report Abstract
Mineral–organic interfaces in soils represent transition zones enriched with organic matter between mineral surfaces and pore spaces. Many soil processes, such as sorption reactions, take place at mineral–organic interfaces and are influenced by their properties. In 2007, Kleber et al. proposed the so-called "multilayer" model, which assumes a zonation of organic coatings on mineral surfaces consisting of an inner contact zone, a zone of hydrophobic interactions, and an outer kinetic zone, each zone containing distinct organic matter components. The aim of the project was to validate this assumption by applying surfacesensitive photoelectron spectroscopy (XPS) in combination with complementary methods. In sequential sorption experiments with the minerals kaolinite and gibbsite and dissolved organic matter (DOM) from a forest floor layer, we studied the efficiency of mineral-associated organic matter (MAOM) formation using XPS and high-resolution mass spectrometry. Repeated addition of identical DOM solutions to the minerals did not lead to a further carbon increase, as highly sorptive organic molecules (especially aromatic, nitrogen-poor, hydrogen-poor and oxygen-rich molecules) reduced the availability of sorption sites ("legacy effect"). We found little evidence of organic–organic interactions or the preferential accumulation of N-rich compounds being involved in MAOM formation. Similar findings were obtained with angle-dependent XPS analyses. Dissolved organic matter from different sources was sorbed onto sapphire and muscovite wafers and then analysed with XPS at different take-off angles. No zonation or chemical gradients could be detected within the uppermost nanometers of the mineral–organic interface. However, the application of XPS is also associated with challenges. Due to the small differences in the binding energy between aliphatic and aromatic carbon, it was not possible to differentiate them, even when using standard substances. The radiation-induced change in organic matter composition needs also be considered in case of long exposure times (e.g., during angle-dependent XPS). Overall, the project results could not experimentally support a chemical zonation of organic coatings on mineral surfaces.
Publications
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The multilayer model of soil mineral–organic interfaces—a review. Journal of Plant Nutrition and Soil Science, 183(1), 27-41.
Gao, Jiajia; Mikutta, Robert; Jansen, Boris; Guggenberger, Georg; Vogel, Cordula & Kalbitz, Karsten
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Legacy Effects of Sorption Determine the Formation Efficiency of Mineral-Associated Soil Organic Matter. Environmental Science & Technology, 56(3), 2044-2053.
Chen, Shuling; Klotzbücher, Thimo; Lechtenfeld, Oliver J.; Hong, Hanlie; Liu, Chongxuan; Kaiser, Klaus; Mikutta, Christian & Mikutta, Robert
