Uranium isotope fractionation during adsorption to Mn-oxyhydroxides

verfasst von

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.

Organisationseinheit(en)

Institut für Mineralogie
AG Geochemie

Externe Organisation(en)

Arizona State University
Indiana University Bloomington
Stanford University

Typ

Artikel

Journal

Environmental Science and Technology

Band

45

Seiten

1370-1375

Anzahl der Seiten

6

ISSN

0013-936X

Publikationsdatum

15.02.2011

Publikationsstatus

Veröffentlicht

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Allgemeine Chemie, Umweltchemie

Ziele für nachhaltige Entwicklung

SDG 14 – Lebensraum Wasser

Elektronische Version(en)

https://doi.org/10.1021/es103061v (Zugang: Unbekannt)