Interaction of U(VI) with pyrite, galena and their mixtures: a theoretical and multi-technique approach

Full text of publication follows: In several works about the underground storage of spend nuclear fuel, it is accepted that small amounts of pyrite (∼1 wt %) will dominate the redox behaviour of soils, which is in direct relationship with the uranium release in groundwater [1]. The present paper is a contribution to a better knowledge of the U(VI) reactivity towards pyrite and galena and completes previous work of Wersin et al. [2] in several points: the thermodynamic approach of the interactions, the monitoring of the reactions progress with time, the use of various U(VI) concentrations and solid/liquid ratios (s/l), the simultaneous presence of both minerals, the characterizations by means of voltammetry and Raman spectroscopy. The Eh-pH diagrams show the pH ranges where U(VI) reacts with pyrite and galena. The study was performed with [U(VI)] ranged between 0.2 and 10 mM using s/l (w:w) from 0.1 to 1%. The experimental results are in agreement with the thermodynamic predictions. The sulphides conditioning by U(VI), monitored by characterizing the liquid (pH-metry, ICP, voltammetry..) and the solid phases (voltammetry, SEM, high resolution XPS, Micro-Raman Spectroscopy..), pointed out several new results: - The Raman spectra of pyrite shows that for the Eg and Ag modes, wavenumbers shift about 5 cm-1 towards the lower values after interaction with U(VI). - Depending on the experimental conditions, the fixation of U3O8 or of a mixture of U3O8 and U(VI) compounds at the sulphide surface was demonstrated. - Galvanic phenomena occur when the minerals mixture reacts with U(VI): galena particles are oxidized whereas U(VI) is reduced at the pyrite particles surface. Because of the formation of insoluble compounds at the minerals surfaces the reaction rate decreases exponentially but the kinetics is faster than when pyrite or galena are alone. - The reaction of 1 mM U(VI) (s/l=1%) is almost complete after 8 days of conditioning with galena and 5 days with the mixture PbS + FeS2. - The maximum amounts of uranium sorbed are 0.4 mmol/g for pyrite and 0.1 mmol/g for galena (whereas that of their 50:50 mixture is about 0.1 mmol/g). [1] F.J. Pearson et al., 'Mont Terri Project - Geochemistry of Water in the Opalinus Clay Formation at the Mont Terri Rock Laboratory', Reports of the Federal Office for Water and Geology, Geology Series, No. 5, Bern 2003. [2] P. Wersin, M. F. Hochella, P. Persson, G. Redden, J.O. Leckie and D.W. Harris, Geochim. Cosmochim. Acta, 58 (1994) 2829. (authors)