Photoeffects on U(VI) sorption onto TiO2 studied by in situ ATR FT-IR spectroscopy.

Uranium(VI), is known to be the most stable oxidation state of uranium in oxygenated waters, with the uranyl ion UO22+ forming soluble hydroxo and carbonate complexes [1, 2]. Thermodynamic unstable U(IV) complexes might be formed in oxygenated waters by photoreduction of U(VI) supplied by solar radiation. Although U(VI) photoinduced reduction is reported to occur in acidic solutions, it is prevented at neutral conditions because of structural arrangements resulting from hydrolysis reactions. However, sorbed U(VI) species at the water-mineral interface on particles with semiconducting properties, e.g. TiO2 have shown different redox behavior than solution species [3-5].

TiO2 is the material mostly applied for the investigation of photocatalysis, due to its exceptional optical and electronic properties, chemical stability and low cost. Anatase was found photocatalytically more active than rutile [6]. The mixed sample of 80% antase and 20% rutile Degussa P-25 was investigated for U(VI) photoreduction using laser fluorescence spectroscopy [3, 4]. ATR FT-IR spectroscopy, which was applied to photoreactions of oxalic acid, has not been applied for U(VI), yet.

We investigated the effects of UV-visible light on the sorption processes of U(VI) onto TiO2 by application of in situ ATR FT-IR spectroscopy. Thus, changes of the excitation wavelength and the TiO2 phase, namely anatase, rutile, and a mixture of both (P-25), were considered. Furthermore, the results were compared to photoeffects observed for U(VI) sorption onto ZnO.

From the obtained spectra different photoinduced effects on U(VI) sorption onto different TiO2 phases are derived. Photocatalysis is clearly suggested for the P-25 sample. In contrast, pure anatase and rutile did not show spectral differences for the sorption mechanisms under dark and light conditions. Furthermore, the photoreaction was found to be limited to incident light with wavelengths below 580 nm. The comparison between TiO2 and ZnO evidence a higher photoactivity for TiO2.

1.Guillaumont, R.; Fanghänel, T.; Fuger, J.; Grenthe, I.; Neck, V.; Palmer, D. A.; Rand, M. H., Update on the Chemical Thermodynamics of U, Np, Pu, Am and Tc. Elsevier: Amsterdam, 2003.
2.Müller, K.; Brendler, V.; Foerstendorf, H., Inorganic Chemistry 2008, 47, (21), 10127-10134.
3.Eliet, V.; Bidoglio, G., Environmental Science & Technology 1998, 32, (20), 3155-3161.
4.Selli, E.; Eliet, V.; Spini, M. R.; Bidoglio, G., Environmental Science & Technology 2000, 34, (17), 3742-3748.
5.Amadelli, R.; Maldotti, A.; Sostero, S.; Carassiti, V., Journal of the Chemical Society-Faraday Transactions 1991, 87, (19), 3267-3273.
6.Litter, M. I., Applied Catalysis B-Environmental 1999, 23, (2-3), 89-114.