The molecular mechanism of Mo isotope fractionation during adsorption to birnessite

Wasylenki, Laura E.; Weeks, Colin L.; Bargar, John R.; Spiro, Thomas G.; Hein, James R.; Anbar, Ariel D.

doi:10.1016/j.gca.2011.06.020

Cited by 100 publications

(74 citation statements)

References 39 publications

(77 reference statements)

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“…The molecular mechanism responsible for the sign and size of metal isotope fractionation between solids and aqueous phases remain poorly understood. Wasylenki et al (2011) postulated that largest isotope effects occur when a trace solution species with different coordination than the major solution species absorb. Kashiwabara et al (2011) argued similarly by stating that small isotope fractionations are associated with little changes in local structures during absorption.…”

Section: (D) Sorptionmentioning

confidence: 99%

Theoretical and Experimental Principles

Hoefs

2015

Stable Isotope Geochemistry

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Isotopes are atoms whose nuclei contain the same number of protons but a different number of neutrons. The term "isotopes" is derived from Greek (meaning equal places) and indicates that isotopes occupy the same position in the periodic table.It is convenient to denote isotopes in the form m n E, where the super-script "m" denotes the mass number (i.e., sum of the number of protons and neutrons in the nucleus) and the subscript "n" denotes the atomic number of an element E. For example, 12 6 C is the isotope of carbon which has six protons and six neutrons in its nucleus. The atomic weight of each naturally occurring element is the average of the weights contributed by its various isotopes.Isotopes can be divided into two fundamental kinds, stable and unstable (radioactive) species. The number of stable isotopes is about 300; whilst over 1200 unstable ones have been discovered so far. The term "stable" is relative, depend-ing on the detection limits of radioactive decay times. In the range of atomic numbers from 1 (H) to 83 (Bi), stable nuclides of all masses except 5 and 8 are known. Only 21 elements are pure elements, in the sense that they have only one stable isotope. All other elements are mixtures of at least two isotopes. The relative abundance of different isotopes of an element may vary substantially. In copper, for example, 63 Cu accounts for 69 % and 65 Cu for 31 % of all copper nuclei. For the light elements, however, one isotope is predominant, the others being present only in trace amounts.The stability of nuclides is characterized by several important rules, two of which are briefly discussed here. The first is the so-called symmetry rule, which states that in a stable nuclide with low atomic number, the number of protons is approximately equal to the number of neutrons, or the neutron-to-proton ratio, N/Z, is approximately equal to unity. In stable nuclei with more than 20 protons or neutrons, the N/Z ratio is always greater than unity, with a maximum value of about 1.5 for the heaviest stable nuclei. The electrostatic Coulomb repulsion of the positively charged protons grows rapidly with increasing Z. To maintain the

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Section: (D) Sorptionmentioning

confidence: 99%

Theoretical and Experimental Principles

Hoefs

2015

Stable Isotope Geochemistry

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“…Although no resolvable U isotopic fractionation has been observed between modern aragonite and calcite samples and seawater (Weyer et al, 2008;Romaniello et al, 2013) (Juillot et al, 2008;Brennecka et al, 2011b;Wasylenki et al, 2011;Nielsen et al, 2013;Little et al, 2014;Bryan et al, 2015). Previous studies found that the predominant U(VI) species in seawater, UO 2 (CO 3 ) 3 4-, was directly incorporated as a unit into aragonite without coordination change or reduction (Reeder et al, 2000).…”

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Uranium isotope fractionation during coprecipitation with aragonite and calcite

Chen

Romaniello

Herrmann

et al. 2016

Geochimica et Cosmochimica Acta

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Natural variations in 238 U/ 235 U of marine carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~ 8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~ 8.5, with heavier U in the solid phase. 238 U/ 235 U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca 2 UO 2 (CO 3 ) 3 (aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca 2 UO 2 (CO 3 ) 3 (aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.ii These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, P CO2 , [Ca], or [Mg] concentrations. In general, these effects are likely to be small (<0.13 ‰), but are nevertheless potentially significant because of the small natural range of variation of 238 U/ 235 U.

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“…1B and C). The process responsible for this particular fractionation appears to be related to a change in coordination from tetrahedral to octahedral (e.g., Wasylenki et al, 2011;Kashiwabara et al, 2011).…”

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Molybdenum isotope fractionations observed under anoxic experimental conditions

2012

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Experiments were carried out to determine molybdenum isotope fractionation associated with adsorption to pyrite. Results show that the Mo isotope composition of the aqueous solution becomes progressively heavier as Mo is adsorbed, with a Mo isotope fractionation as large as 2.9‰. This fractionation is larger than observed for typical anoxic continental margin marine sediments (e.g., ~0.7‰), suggesting that Mo adsorption to pyrite is not the dominant process operating in these environments. However, our adsorption results do suggest the possibility that anoxic fractionation processes could impart an isotope signature within the geologic record similar to the isotope fractionation observed under oxygenated conditions. Additional experiments were conducted at high (~100 µM) dissolved Mo concentrations, but at varying pH and ∑H 2 S concentrations, and in each case these experiments promoted Mo-sulfide precipitation. Even though the experiments were conducted under differing conditions and produced different amounts of precipitate, the results suggest a constant fractionation of ~0.9‰ associated with this Mo removal process. This fractionation is more consistent with that inferred for anoxic continental margin marine sediments, suggesting that a process similar to Mo-sulfide precipitation, rather than an adsorption process, may be responsible for the Mo isotope compositions observed in these environments. The findings of this study suggest that, regardless of the geochemical mechanism employed, sediment Mo sequestration under anoxic conditions may impart a significant isotopic fractionation relative to parent seawater.

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The molecular mechanism of Mo isotope fractionation during adsorption to birnessite

Cited by 100 publications

References 39 publications

Theoretical and Experimental Principles

Theoretical and Experimental Principles

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Molybdenum isotope fractionations observed under anoxic experimental conditions

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