Surface Characterization of Olivine-Rock by X-ray Photoelectron Spectroscopy (XPS). Leaching and U(VI) Sorption Experime
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EXPERIMENTAL PART Samples and procedure The mineral content of the high-FeO rock was: olivine (65%, fayalite content in olivine: 39-58 mol-%), plagioclase (20%), magnetite (8%), pyroxene (4%) and serpentine (3%) (total FeO content 28%). Crushed samples of particle size between 120-160 microns were used in this study. Leaching experiments were performed with 2 g of olivine in methacrylic vessels. We have used two different solutions; a) initial pH=3, obtained with perchloric acid and b) HCO 3- 0.002 mol Lj, with an initial pH=8.8, to simulate a granitic groundwater. The experiment at acid pH was performed in a nitrogen atmosphere, while experiment at basic pH was performed in both nitrogen atmosphere and in oxidizing conditions equilibrating the leaching solution with air. The temperature was kept constant at
25'C. After 50 days, solid samples were taken and washed with deionized water. The washed samples were dried in a dessicator at room temperature. In the U(VI) sorption experiment, we have used a very high S/V ratio (1.68 10¢' m-) because the detection limit of an element on the solid surface by XPS is 1%. Therefore, 10 ml of a UO22' 2-10-4 mol L1 solution was put in contact with 2.9 g of olivine rock at pH = 4, under an inert atmosphere (N2 ). We used an inert atmosphere in order to keep low the oxygen concentration and in this way to favor the possible reduction of U(VI). Preliminary results had shown that at pH equal to 4, the sorption is high. Solid samples were taken after 2, 27 and 60 days and the same procedure described above was carried out. X-ray Photoelectron Spectroscopy (XPS) The olivines were examined by XPS before and after both leaching and sorption experiments. Photoelectron spectra were recorded on a PHI Perkin Elmer ESCA Multianalyzer 5500 using an Al-K, X-ray source (1486.6 eV) for a pass energy of 23 eV (used analyzing single peaks) the energetic resolution is 0.50 eV for Ag 3d5/2 peak, while for a pass energy of 93 eV (used for the general spectrum) the energetic resolution is 0.80 eV. The error in the determination of the electron energies was ± 0.2, except for that of Fe 2p3/2 binding energy which was ± 0.3 due to the broad character of this peak. RESULTS AND DISCUSSION Unreacted and leached samples In Figure 1, the general spectrum of unreacted olivine rock is shown. The absence of calcium and the low intensity of the aluminum peak indicates that the spectrum can be attributed to the olivine. From general spectrum, it is possible to calculate the relative atomic abundance of elements for unreacted and reacted samples as well as to know the oxidation state of the iron on the solid surface. The distance between Fe 2p3/2 and Si 2p peaks is characteristic of iron oxidation state, for fayalite (Fe(ll) silicate) this distance is 608.5 eV and for nontronite (Fe(IlI) silicate) is 609.9 eV [6]. Analyzing by XPS the surfaces of FeO (s) and Fe20 3 (s), we have determined the distance between Fe 2p3/2 and Fe 2 p1/2 peaks in both solids, this distance is 13.0 322
eV if the iron is Fe(lI) while a d
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