Surface analysis of soilmaterial by X-ray photoelectron spectroscopy

The surface composition of particles present in the fine earth (< 2 mm) of 50 soil horizons differing in composition and pedogenetic origin (13 soil profiles) was analysed by X-ray photoelectron spectroscopy (XPS) to assess the capability and limitations of this technique and to gain better knowledge of the soil samples. The surfaces were systematically enriched in carbon, sometimes up to 1000 times, indicating that the soil particle surfaces are coated with organic substances, even in horizons where the bulk organic content is less than 0.1 g kg(-1). The distribution of carbon in the various oxidation states was 0.569 +/- 0.008 C-[0], 0.275 +/- 0.004 C[+1], 0.089 +/- 0.003 C[+2] and 0.066 +/- 0.002 C[+3] for most horizons (mean +/- standard error, 69 data). Only Andosol surface horizons systematically had surface organic matter in a more oxidized state. After correcting the results for the presence of organic coatings, we found that Si was generally depleted and Al enriched at the surface of soil particles, while Fe was either depleted or enriched depending on the sample considered. However, the coating of the coarser soil particles by the finer ones and their differential composition explained this observation and limits the interest of XPS for characterizing the surface enrichment of inorganic elements in crude soil samples. These limitations should be considered when interpreting XPS results in future work. Nevertheless, XPS can analyse the adsorbed organic matter and its functional composition of carbon without the need for any chemical or physical extraction that might alter the structure and composition of the organic molecules