Uranium isotope fractionation during adsorption to Mn-oxyhydroxides

authored by: Gregory A. Brennecka, Laura E. Wasylenki, John R. Bargar, Stefan Weyer, Ariel D. Anbar
Abstract: Previous work has shown uranium (U) isotope fractionation between natural ferromanganese crusts and seawater. Understanding the mechanism that causes ²³⁸U/²³⁵U fractionation during adsorption to ferromanganese oxides is a critical step in the utilization of ²³⁸U/²³⁵U 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^238/235U) 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.
Organisation(s): Institute of Mineralogy
Geochemistry
External Organisation(s): Arizona State University
Indiana University Bloomington
Stanford University
Type: Article
Journal: Environmental Science and Technology
Volume: 45
Pages: 1370-1375
No. of pages: 6
ISSN: 0013-936X
Publication date: 15.02.2011
Publication status: Published
Peer reviewed: Yes
ASJC Scopus subject areas: General Chemistry, Environmental Chemistry
Sustainable Development Goals: SDG 14 - Life Below Water
Electronic version(s): https://doi.org/10.1021/es103061v (Access: Unknown)