The interactions of transition metals with natural systems play an important role in the mobility and the bioavailability of these metals in soils. In this study, the adsorption of copper(II) onto natural soil particles was studied as a function of pH and metal concentration. The retention capacity of soil particles was determined at pH 6.2 to be equal to 6.7 mg of copper/g of solid. The Langmuir and Freundlich isotherm equations were then used to describe the partitioning behavior of the system at different pH values. A combination of EPR, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) spectroscopies was used to probe the Cu atomic environment at the soil particles/aqueous interface. The spectroscopic study revealed that copper(II) ions are held in inner-sphere surface complexes. It also revealed that Cu was in an octahedral coordination with first-shell oxygen atoms. A weak tetragonal distortion was pointed out due to the Jahn-Teller effect, with a mean Cu-Oequatorial bond distance of 1.96 A and a Cu-Oaxial bond distance of 2.06 A. A detailed analysis of the spectroscopic data suggested that Cu(II) was bonded to organic matter coated onto the mineral fraction of soil particles.

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Knowledge of the structure and composition of the solid phases of a soil is needed to understand the retention mechanisms of transition metals or organic pollutants on the molecular scale. With this aim, the characterization of a soil sample from the Champagne-Ardenne region was carried out using various non-destructive methods such as X-ray diffraction (XRD), transmission electron microscopy-energy dispersive X-ray spectrometry (TEM-EDX), Mössbauer and electron paramagnetic resonance (EPR) spectroscopies, X-ray photoelectron spectroscopy (XPS), and protometric studies. The combination of these techniques allowed us to obtain a precise knowledge of the solid structure and composition. We showed that soil particles consist of quartz, alumino-silicate and goethite which are partially covered by organic matter. XPS spectra showed that organic matter is preferentially coated onto an alumino-silicate surface. EPR spectroscopy revealed the presence of unexpected vanadyl moieties in the organic matter. Acid-base titrations provided information on available surface sites (0.25 mmol × g -1 , i.e. 2.25 sites per nm 2 ), and the point of zero charge pH zpc (3.9).

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