Uranium Isotope Fractionation during Adsorption to Mn-Oxyhydroxides

Abstract

Previous work has shown uranium (U) isotope fractionation between natural ferromanganese crusts and seawater. Understanding the mechanism that causes (238)U/(235)U fractionation during adsorption to ferromanganese oxides is a critical step in the utilization of (238)U/(235)U as a tracer of U adsorption reactions in groundwater as well as a potential marine paleoredox proxy. We conducted U adsorption experiments using synthetic K-birnessite and U-bearing solutions. These experiments revealed a fractionation matching that observed between seawater and natural ferromanganese sediments: adsorbed U is isotopically lighter by ∼0.2‰ (δ(238/235)U) than dissolved U. As the redox state of U does not change during adsorption, a difference in the coordination environment between dissolved and adsorbed U is likely responsible for this effect. To test this hypothesis, we analyzed U adsorbed to K-birnessite in our experimental study using extended X-ray absorption fine structure (EXAFS) spectroscopy, to obtain information about U coordination in the adsorbed complex. Comparison of our EXAFS spectra with those for aqueous U species reveals subtle, but important, differences in the U-O coordination shell between dissolved and adsorbed U. We hypothesize that these differences are responsible for the fractionation observed in our experiments as well as for some U isotope variations in nature.

Evidence of isotopic fractionation of natural uranium in cultured human cells

“…Stylo et al (35) recently demonstrated that the uranium isotopic fractionation found during the biologically mediated U(VI) reduction by Shewanella oneidensis was not significantly different from that obtained when the redox reaction was performed in the presence of cell-free extracts of the same bacteria, thus confirming that the redox reaction is responsible for the isotopic fractionation observed. Smaller isotopic fractionation of around 0.2‰ was found during U adsorption to mineral phases, which was attributed to a ZPE mechanism (37). In another framework, U(VI) complexation experiments gave rise to an enrichment of the light 235 U isotope in complexed U(VI) species (38).…”

Isotope fractionation of 238U and 235U during biologically-mediated uranium reduction

“…Contrary to expectations, initial studies by Stirling et al (2007) and Weyer et al (2008) documented permillevel 238 U/ 235 U isotopic fractionations in a wide range of low-temperature terrestrial environments. These unexpectedly large isotopic shifts appear to be primarily controlled by nuclear volume-dependent isotopic fractionation associated with the nuclear field shift effect (Bopp et al, 2009;Bopp et al, 2010;Brennecka et al, 2010a), although opposing, but smaller isotope shifts consistent with mass-dependent fractionation have also been observed (Brennecka et al, 2011b). Nuclear field shift fractionation arises because of varying nuclear volumes and electron density distributions between different isotopes of the same element (Bigeleisen, 1996;Schauble, 2007), and is predicted to have a particularly strong effect on the heavy masses with large nuclei, especially uranium.…”

Isotope Fractionation Processes of Selected Elements

“…The potential of uranium isotopes as a paleo-redox tracer has been investigated by Montoya-Pino et al (2010), Brennecka et al (2011), Kendall et al (2013 and Noordmann et al (2015). Montoya-Pino et al (2010) demonstrated that U isotope variations in black shales can be used to quantify the extent of marine anoxia.…”