Geochemical control on the reduction of U(VI) to mononuclear U(IV) species in lacustrine sediments. (1st February 2018)

• Record Type: Journal Article
• Authors: Stetten, L.; Mangeret, A.; Brest, J.; Seder-Colomina, M.; Le Pape, P.; Ikogou, M.; Zeyen, N.; Thouvenot, A.; Julien, A.; Alcalde, G.; Reyss, J.L.; Bombled, B.; Rabouille, C.; Olivi, L.; Proux, O.; Cazala, C.; Morin, G.
• Is Part Of: Geochimica et cosmochimica acta. Volume 222(2018)
• Page Start: 171
• Page End: 186
• Publication Date: 2018-02-01
• Subjects: Uranium -- Sediment -- Lake -- Speciation -- Redox -- Iron -- EXAFS -- XANES -- Mononuclear; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2017.10.026
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Contaminated systems in which uranium (U) concentrations slightly exceed the geochemical background are of particular interest to identify natural processes governing U trapping and accumulation in Earth's surface environments. For this purpose, we examined the role of early diagenesis on the evolution of U speciation and mobility in sediments from an artificial lake located downstream from a former mining site. Sediment and pore water chemistry together with U and Fe solid state speciation were analyzed in sediment cores sampled down to 50 cm depth at four locations in the lake. These organic-rich sediments (∼12% organic C) exhibited U concentrations in the 40–80 mg kg −1 range. The sediment columns were anoxic 2–3 mm below the sediment-water interface and pore waters pH was circumneutral. Pore water chemistry profiles showed that organic carbon mineralization was associated with Fe and Mn reduction and was correlated with a decrease in dissolved U concentration with depth. Immobilization of U in the sediment was correlated with the reduction of U(VI) to U(IV) at depth, as shown by U LIII -edge XANES spectroscopic analysis. XANES and EXAFS spectroscopy at the Fe K-edge showed the reduction of structural Fe(III) to Fe(II) in phyllosilicate minerals with depth, coincident with U(VI) to U(IV) reduction. Thermodynamic modeling suggests that Fe(II) could act as a major reducing agent for U(VI) during early diagenesis of these sediments, leading to complete U reduction below ∼30 cm depth. Shell-by-shell and Cauchy-Wavelet analysis of U LIII -EXAFS spectra indicates that U(VI) and U(IV) are mainly present as mononuclear species bound to C, P or Si ligands. Chemical extractions confirmed that ∼60–80% of U was present as non-crystalline species, which emphasizes that such species should be considered when evaluating the fate of U in lacustrine environments and the efficiency of sediment remediation strategies.