Uranium isotope fractionation during adsorption to Mn-oxyhydroxides

Gregory A. Brennecka, Laura E. Wasylenki, John R. Bargar, Stefan Weyer, Ariel Anbar

Research output: Contribution to journalArticle

92 Citations (Scopus)

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‰ (δ238/235U) 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.

Original languageEnglish (US)
Pages (from-to)1370-1375
Number of pages6
JournalEnvironmental Science and Technology
Volume45
Issue number4
DOIs
StatePublished - Feb 15 2011

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uranium isotope
Uranium
Fractionation
Isotopes
Adsorption
fractionation
adsorption
birnessite
Seawater
Bearings (structural)
X-Rays
Extended X ray absorption fine structure spectroscopy
seawater
experiment
Experiments
Groundwater
X ray absorption
Proxy
Oxides
Oxidation-Reduction

ASJC Scopus subject areas

  • Chemistry(all)
  • Environmental Chemistry
  • Medicine(all)

Cite this

Uranium isotope fractionation during adsorption to Mn-oxyhydroxides. / Brennecka, Gregory A.; Wasylenki, Laura E.; Bargar, John R.; Weyer, Stefan; Anbar, Ariel.

In: Environmental Science and Technology, Vol. 45, No. 4, 15.02.2011, p. 1370-1375.

Research output: Contribution to journalArticle

Brennecka, Gregory A. ; Wasylenki, Laura E. ; Bargar, John R. ; Weyer, Stefan ; Anbar, Ariel. / Uranium isotope fractionation during adsorption to Mn-oxyhydroxides. In: Environmental Science and Technology. 2011 ; Vol. 45, No. 4. pp. 1370-1375.
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