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Spéciation des ions phosphate et uranyle à l'interface corindon colloïdal/solution : étude expérimentale et analyses spectroscopiques in-situ

Abstract : Several field studies suggested that the (co)sorption of phosphate and uranyl at the iron or aluminum (hydr)oxides surface controls the retention of uranium in soils, in conditions where uranyl phosphate precipitation is hindered (acidic pH and/or low U concentrations). This thesis deals with the sorption mechanisms of phosphate and uranyl on colloidal corundum, and the influence of parameters such as reaction time, pH and uranyl and phosphate concentrations. Up to day, it is the only study aiming to identify in situ the surface species formed by phosphate and uranyl at the corundum-solution interface, during the sorption process and corundum dissolution. For the study of the speciation at the interface several techniques has been used. These include zetametry, attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR). The ATR FTIR spectroscopy allows to probe the surface of layers composed of colloids, deposited on ATR crystals, and put into contact of an aqueous solution. It has been showed the characterization of this layer by CPM microscopy allows to make reproducible IR analyses. The surface modification of the corundum in contact with solutions at different pH has been followed by ATR FTIR spectroscopy. At pH 3.3, corundum surface hydration leads to a fast reorganization of the interfacial structure which seems to be a surface bayeritisation, or possibly boehmitisation. IR analyzes shows a fast boehmite formation at pH 4.5, by dissolution/reprecipitation, and suggests the massive formation of a pseudo boehmite more or less amorphous at neutral pH. Phosphate and uranyl sorption has thus been mainly studied at a pH of 3.3, where the corundum modification is limited to a surface hydration. Phosphate sorption on corundum is a chemisorption since it moves the isoelectric point values of corundum (8.2) to lower pH. The different methods used show that phosphate are adsorbed at acidic pH (3.3) by a combination of surface complexation and surface precipitation, with relative contribution depending on the phosphate surface coverage rate and on the contact time. At high coverage rate, aluminum phosphate surface precipitation is observed, characterized by an absorption band at 1137 cm-1, which can be attributed to the PO elongation vibrations, which absorbance increases with contact time and phosphate concentration. During this precipitation, absorption bands also appear at characteristic O-H deformation vibrations positions revealing a reorganization of the hydroxyl groups due to he diminution of the potential and the surface charges. The study thus shows that the corundum/phosphated solution interface change over long periods, the corundum dissolution leading to the formation of surface precipitates and to the reorganization of surface hydroxyl. The sorption mechanisms of uranyl (1 - 10 μM) in presence of phosphate have been studied for corundum suspensions at pH 3.3. The zeta potential variations in function of the surface coverage rate in uranyl and phosphate indicates the formation of surface species composed of both phosphate and uranyl, implying several mechanisms. The apparition of strongly correlated IR bands at ~1107, ~1024 cm-1 and 971 cm-1, observable during the addition of uranyl at the corundum/phosphated solutions systems and attributable to the P-O elongation vibration, confirms the formation of phosphated uranyl surface species. IR spectroscopy shows also that the quantitative sorption of uranyl, observable at high phosphate coverage rate, happens during the transformation of the corundum surface into aluminum phosphate. This suggests the incorporation of U in the surface precipitate structure and/or the uranyl phosphate surface precipitation. Although this study has been mainly performed at acidic pH, it is of interest in the comprehension of the phosphate retention mechanisms (as pollutant) in soils. It permits to highlight for the first time, by in situ techniques, the formation of aluminum phosphate precipitate on aluminum oxide. It shows also clearly the formation of phosphated uranyl surface species at acidic pH. It suggests that the incorporation of U in the aluminum phosphate surface precipitate network and/or the formation of uranyl phosphate surface precipitates controls the long time retention of U at the state of traces in soils i.e. in conditions where the soils solutions are under saturated according to uranyl phosphate.
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https://pastel.archives-ouvertes.fr/pastel-00544360
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Submitted on : Tuesday, December 7, 2010 - 5:23:23 PM
Last modification on : Wednesday, October 14, 2020 - 3:52:24 AM
Long-term archiving on: : Tuesday, March 8, 2011 - 4:57:13 AM

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  • HAL Id : pastel-00544360, version 1

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Éric Halter. Spéciation des ions phosphate et uranyle à l'interface corindon colloïdal/solution : étude expérimentale et analyses spectroscopiques in-situ. Sciences de l'environnement. École Nationale Supérieure des Mines de Paris, 2010. Français. ⟨NNT : 2010ENMP0006⟩. ⟨pastel-00544360⟩

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