Formation and properties of mineral-organic soil interfaces as revealed by X-ray photoelectron spectroscopy

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Mineral-organic interfaces represent organic matter-enriched transition zones between mineral surfaces and pore spaces. Many soil processes, including adsorption and desorption reactions, occur at these interfaces and are controlled by their physicochemical properties. A decade ago, Kleber et al. (Biogeochemistry 2007, 85, 9-24) proposed the so-called ´multilayer´ model, which assumes the presence of chemically distinct zones within natural organic matter coatings on mineral surfaces. The model assumes that organic coatings are composed of an inner contact zone followed by a zone of hydrophobic interactions and an outer kinetic zone, each zone containing distinct organic matter components (´chemical zonation´). Although the model is currently considered as ´state of knowledge´ in terms of the composition of soil mineral-organic associations, it has never been validated experimentally. Our proposed research will consequently address the chemical composition of organic coatings as a function of environmental conditions. We analyse the morphology of organic coatings on mineral surfaces and their chemical structure using atomic force microscopy and X-ray photoelectron spectroscopy (XPS). XPS allows for (i) the depth-dependent chemical characterisation of organic coatings using step-wise argon cluster ion beam milling and (ii) the chemical analysis of organic coatings in fully hydrated state (cryo-XPS). Additionally, we explore the effect of specific environmental parameters such as pH and solution composition on the formation and composition of organic multilayers. Our experiments also account for the chemical zonation within organic coatings as influenced by sequential sorption and sorptive fractionation processes of natural organic matter as well as reorganization of organic molecules at mineral-organic interfaces. The proposed research will be the first test of the validity of the multilayer model and provides the scientific fundament for an improved understanding of the chemical composition of natural mineral-organic interfaces.