Isotope fractionation of 238U and 235U during biologically-mediated uranium reduction. (15th August 2015)

• Record Type: Journal Article
• Authors: Stirling, Claudine H.; Andersen, Morten B.; Warthmann, Rolf; Halliday, Alex N.
• Is Part Of: Geochimica et cosmochimica acta. Volume 163(2015:Aug. 15)
• Page Start: 200
• Page End: 218
• Publication Date: 2015-08-15
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.03.017
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: A series of laboratory-controlled microbial experiments using gram-negative sulphate-reducing bacteria ( Desulfovibrio brasiliensis ) inoculated with natural uranium were performed to investigate 238 U/ 235 U fractionation during bacterially-mediated U reduction. Control experiments, without bacteria to drive U reduction, were conducted in parallel. Paired measurements of 238 U/ 235 U and U concentration for both the residual growth medium solution and the accumulated biologically-mediated precipitate were obtained using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). The control experiments show that only minor (<0.1‰), if any 238 U/ 235 U fractionation occurs during co-precipitation with calcite. This implies that carbonate sediments are capable of faithfully recording the signature of the global ocean during Earth's major climatic events, including oxygenation and de-oxygenation transitions in the marine environment. The results for the microbial experiments demonstrate that the 238 U/ 235 U composition of the unreacted growth medium containing U(VI) is isotopically lighter than the composition of the U(IV)-bearing precipitate as U(VI) is consumed, in agreement with field-based observations of microbially-mediated U reduction. Uranium isotopic shifts of up to 0.8‰ were observed between the liquid and solid phases. These observations can be modelled using a Rayleigh distillation approach describing kinetic uptake in a closed system, which yields a fractionation factor α of 0.99923 ± 0.00004 ( ε = −0.77 ± 04‰) for U(VI)–U(IV) reduction mediated by the D. brasiliensis microbe. This fractionation behaviour is consistent with that observed in field-based redox environments, which give rise to similar α values. Competing processes such as U co-precipitation (e.g. adsorption) may act to lower the apparent value for α and possibly play a secondary role both in the microbial experiments of this study and in natural U reduction settings where variable α values are found. These results may suggest that microbes adept at inducing U(VI) reduction play a crucial role in facilitating significant 238 U/ 235 U isotope fractionation in nature.