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

矿物-有机界面是指矿物表面与孔隙空间之间的富有机质过渡带。许多土壤过程，包括吸附和解吸反应，都发生在这些界面上，并受其物理化学性质的控制。十年前，Kleber等人（生物地球化学2007,85,9 -24）提出了所谓的“多层”模型，该模型假设在矿物表面的天然有机质涂层中存在化学上不同的区域。该模型假设有机涂层由内部接触区、疏水相互作用区和外部动力学区组成，每个区域都包含不同的有机质成分（“化学区”）。尽管该模型目前被认为是土壤矿物-有机组合组成方面的“知识状态”，但它从未得到实验验证。因此，我们提出的研究将解决有机涂料的化学成分作为环境条件的函数。利用原子力显微镜和x射线光电子能谱（XPS）分析了矿物表面有机涂层的形貌及其化学结构。XPS允许(i)使用分步氩气簇离子束铣削对有机涂层进行深度依赖的化学表征，（ii）在完全水合状态下对有机涂层进行化学分析（冷冻XPS）。此外，我们还探讨了特定环境参数（如pH和溶液组成）对有机多层膜形成和组成的影响。我们的实验还解释了有机涂层内的化学区域化，这是受天然有机质的顺序吸附和吸附分馏过程以及矿物-有机界面上有机分子的重组的影响。该研究将首次验证多层模型的有效性，并为进一步了解天然矿物-有机界面的化学成分提供科学基础。

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.