Browsing by Author "Muller, K"
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- ItemSorption of U(VI) at the TiO2–water interface: An in situ vibrational spectroscopic study(Pergamon-Elsevier Science Ltd, 2012-01-01) Muller, K; Foerstendorf, H; Meusel, T; Brendler, V; Lefevre, G; Comarmond, MJ; Payne, TEMolecular-scale knowledge of sorption reactions at the water-mineral interface is important for predicting U(VI) transport processes in the environment. In this work, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy was used in a comprehensive investigation of the sorption processes of U(VI) onto TiO2. The high sensitivity of the in situ ATR FT-IR technique allows the study of U(VI) concentrations down to the low micromolar range, which is relevant to most environmental scenarios. A set of highly purified and well characterized TiO2 phases differing in their origin, the ratio of the most stable polymorphs (anatase and rutile), in specific surface area, isoelectric points and in particle size distribution was investigated. Irrespective of the composition of the mineral phase, it was shown that U(VI) forms similar surface complexes, which was derived from the antisymmetric stretching mode nu(3)(UO2) showing a characteristic shift to lower wavenumbers compared to the respective aqueous species. The availability of a fast scanning IR device makes it feasible to perform time-resolved experiments of the sorption processes with a time resolution in the sub-minute range. It is shown that during the early stages of the U(VI) uptake, a surface species on the mineral phase is formed, characterized by a significantly red-shifted absorption maximum which is interpreted as a bidendate inner-sphere complex. After prolonged sorption, the IR spectra indicate the formation of a second surface species showing a smaller shift compared to the aqueous species. These findings were verified by a series of spectroscopic experiments performed on a U(VI)-saturated surface, at different U(VI) concentrations, pH values and in the absence of atmospheric-derived carbonate. This work provides new molecular insights into the sorption processes of U(VI) on TiO2. Basic thermodynamic ideas of surface complexation are substantiated by in situ infrared spectroscopy. © 2012, Elsevier Ltd.
- ItemUranium sorption on various forms of Ti02 - influence of surface area, surface charge and impurities(Czech Chemical Society, 2010-04-18) Comarmond, MJ; Payne, TE; Harrison, JJ; Thiruvoth, S; Muller, KTitanium dioxide has properties that make it an excellent substrate for experimental study and theoretical development of adsorption models, including negligible solubility and a near neutral point of zero charge 1. A number of different forms of Ti-oxide have been used in experimental studies, including hydrous Ti-oxide, anatase, rutile and various commercially available samples that contain a mixture of anatase and rutile. The aim of our work is to investigate uranium sorption phenomena and the influence of surface area, surface charge and impurities for a range of thoroughly characterised Ti-oxide surfaces. We have undertaken uranium(VI) sorption studies on a number of commercially available Ti oxides, some of which were aggressively pre-treated to remove inherent impurities. Characterisations performed on the various Ti-oxides comprised a range of chemical and physical methods including XRD, XRF, ATR FT-IR, chemical assays, BET determinations, and electroacoustic measurements. The sorption of U on these Ti oxides was studied by a baTch sorption method and the effect of pH, ionic strength, mass loading, and U concentration on uranium sorption was also investigated for several of these Tioxides. We found that the sorption of uranium (VI) on these Tioxides was extremely strong and much greater than many other common environmental sorbents on a surface area basis. Aggressive pre-treatment of one Ti-oxide significantly altered its isoelectric point, but did not appear to significantly impact its sorption behaviour. Differences in sorption behaviour between the various Ti-oxides were related to the surface area of these materials. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption, and will be useful in assessing data obtained in diverse experimental studies involving Ti oxides.