Laura E. Wasylenki

Uranium Isotope Fractionation during Adsorption to Mn-Oxyhydroxides

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.

Isotope fractionation during microbial metal uptake measured by MC-ICP-MS

High-precision isotopic analyses by MC-ICP-MS were used to investigate the mass-dependent fractionation of Mo and Fe isotopes during bacterial metal assimilation in experiments with Azotobacter vinelandii. A. vinelandii is a diazotroph with high demand for both Mo and Fe during nitrogen fixation. Our results demonstrate that the growth medium became progressively enriched in heavier isotopes of Mo during bacterial growth, indicating preferential assimilation of lighter isotopes. In contrast, for Fe, the medium become isotopically lighter as Fe was removed from solution. The experimental data can be interpreted in terms of Rayleigh fractionation, yielding fractionation factors of 0.9997 and 1.0011 for Mo and Fe, respectively. Hence, we infer Δ97/95Mocells-medium = –0.3‰ and Δ56/54Fecells-medium = 1.1‰. Fractionation of Mo isotopes could result from simple kinetic effects during assimilation, but may also be affected by complexation with high-affinity metal binding ligands. Kinetic effects cannot easily account for the sense of Fe isotope fractionation, and so equilibrium effects, possibly between different Fe complexes, are implied. Adsorption of Mo and Fe onto cell surfaces may also play a role and requires further examination. Isotope fractionation studies using MC-ICP-MS may provide new constraints on the processes by which microbes extract metals from their surroundings, ultimately yielding insights into the mechanisms of metal assimilation into the metallome.

Fe isotope fractionation during equilibration of Fe-organic complexes.

Despite the importance of Fe-organic complexes in the environment, few studies have investigated Fe isotope effects driven by changes in Fe coordination that involve organic ligands. Previous experimental (Dideriksen et al., 2008, Earth Planet Sci. Lett. 269:280-290) and theoretical (Domagal-Goldman et al., 2009, Geochim. Cosmochim. Acta 73:1-12) studies disagreed on the sense of fractionation between Fe-desferrioxamine B (Fe-DFOB) and Fe(H(2)O)(6)(3+). Using a new experimental technique that employs a dialysis membrane to separate equilibrated Fe-ligand pools, we measured the equilibrium isotope fractionations between Fe-DFOB and (1) Fe bound to ethylenediaminetetraacetic acid (EDTA) and (2) Fe bound to oxalate. We observed no significant isotope fractionation between Fe-DFOB and Fe-EDTA (Delta(56/54)Fe(Fe-DFOB/Fe-EDTA) approximately 0.02 +/- 0.11 per thousand) and a small but significant fractionation between Fe-DFOB and Fe-oxalate (Delta(56/54)Fe(Fe-DFOB/Fe-Ox(3)) = 0.20 +/- 0.11 per thousand). Taken together, our results and those of Dideriksen et al. (2008) reveal a strong positive correlation between measured fractionation factors and the Fe-binding affinity of the ligands. This correlation supports the experimental results of Dideriksen et al. (2008). Further, it provides a simple empirical tool that may be used to predict fractionation factors for Fe-ligand complexes not yet studied experimentally.

Inter-calibration of a proposed new primary reference standard AA-ETH Zn for zinc isotopic analysis

We have prepared a large volume of pure, concentrated and homogenous zinc standard solution. This new standard solution is intended to be used as a primary reference standard for the zinc isotope community, and to serve as a replacement for the nearly exhausted current reference standard, the so-called JMC-Lyon Zn. The isotopic composition of this new zinc standard (AA-ETH Zn) has been determined through an inter-laboratory calibration exercise, calibrated against the existing JMC-Lyon standard, as well as the certified Zn reference standard IRMM-3702. The data show that the new standard is isotopically indistinguishable from the IRMM-3702 zinc standard, with a weighted δ66/64Zn value of 0.28 ± 0.02‰ relative to JMC-Lyon. We suggest that this new standard be assigned a δ66/64Zn value of +0.28‰ for reporting of future Zn isotope data, with the rationale that all existing published Zn isotope data are presented relative to the JMC-Lyon standard. Therefore our proposed presentation allows for a direct comparison with all previously published data, and that are directly traceable to a certified reference standard, IRMM-3702 Zn. This standard will be made freely available to all interested labs through contact with the corresponding author.