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Thermodynamic controls on redox-driven kinetic stable isotope fractionation

C. Joe-Wong1,

1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305, USA

K.L. Weaver2,

2Department of Earth System Science, Stanford University, 473 Via Ortega, Stanford, CA 94305, USA

S.T. Brown3,

3Energy Geosciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA

K. Maher2

2Department of Earth System Science, Stanford University, 473 Via Ortega, Stanford, CA 94305, USA

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Joe-Wong, C., Weaver, K.L., Brown, S.T., Maher, K. (2019) Thermodynamic controls on redox-driven kinetic stable isotope fractionation. Geochem. Persp. Let. 10, 20–25.

National Science Foundation; U.S. Department of Defense; Stanford University; U.S. Department of Energy.

Geochemical Perspectives Letters v10  |  doi: 10.7185/geochemlet.1909
Received 30 October 2018  |  Accepted 26 February 2019  |  Published 29 March 2019
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 Isotope fractionation during Cr(VI) reduction by various aqueous Fe(II) species. Filled and open symbols in each plot show duplicate reactors. Rayleigh curves based on linear best fits are plotted as dashed lines (filled symbols) and dotted lines (open symbols). Vertical error bars (2 s.d.) are smaller than the symbols.
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Figure 2 Linear relationships of kinetic isotope fractionation factor for Cr(VI) reduction with (a) the Fe(II)-Fe(III) standard reduction potential and (b) rate constants. Solid line shows weighted linear regression, and dotted lines show 95 % confidence interval. Each symbol shows the average fractionation factor calculated for replicate experiments. Error bars are 2 s.d. The fractionation factor for Cr(VI) reduction by Fe(H2O)62+ is taken from Kitchen et al. (2012)

Kitchen, J.W., Johnson, T.M., Bullen, T.D., Zhu, J., Raddatz, A. (2012) Chromium isotope fractionation factors for reduction of Cr(VI) by aqueous Fe(II) and organic molecules. Geochimica et Cosmochimica Acta 89, 190–201.

, and the rate constants are taken from Buerge and Hug (1997

Buerge, I.J., Hug, S.J. (1997) Kinetics and pH Dependence of Chromium(VI) Reduction by Iron(II). Environmental Science & Technology 31, 1426–1432.

, 1998

Buerge, I.J., Hug, S.J. (1998) Influence of Organic Ligands on Chromium(VI) Reduction by Iron(II). Environmental Science & Technology 32, 2092–2099.

).
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Figure 3 Effects of pH on the kinetics and isotope fractionation of Cr(VI) reduction by aqueous Fe(II). In both parts, solid lines show the model based on the rate law of Pettine et al. (1998)

Pettine, M., D’Ottone, L., Campanella, L., Millero, F.J., Passino, R. (1998) The reduction of chromium (VI) by iron (II) in aqueous solutions. Geochimica et Cosmochimica Acta 62, 1509–1519.

; dashed lines show the model based on the rate law of Buerge and Hug (1997)

Buerge, I.J., Hug, S.J. (1997) Kinetics and pH Dependence of Chromium(VI) Reduction by Iron(II). Environmental Science & Technology 31, 1426–1432.

. (a) The fraction of Cr(VI) reduced by each Fe(II) species is contingent on pH-dependent Fe(II) speciation and the rate law of Cr(VI) reduction. (b) The effective fractionation factor is the weighted average of the fractionation factors for Cr(VI) reduction by each Fe(II) species. Error bars on the symbols in (b) are 2 s.d.
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Figure 4 Schematic of Cr(VI) reduction and variations in εkin with depth, groundwater age, and oxygen levels. The dominant reductants are ordered according to their relative locations on the redox tower. Kinetic fractionation factors are taken from this study; aqueous Fe(II); Kitchen et al. (2012)

Kitchen, J.W., Johnson, T.M., Bullen, T.D., Zhu, J., Raddatz, A. (2012) Chromium isotope fractionation factors for reduction of Cr(VI) by aqueous Fe(II) and organic molecules. Geochimica et Cosmochimica Acta 89, 190–201.

, aqueous Fe(II); Basu and Johnson (2012)

Basu, A., Johnson, T.M. (2012) Determination of Hexavalent Chromium Reduction Using Cr Stable Isotopes: Isotopic Fractionation Factors for Permeable Reactive Barrier Materials. Environmental Science & Technology 46, 5353–5360.

, Fe(II)-doped goethite and Fe(II) sulphide, and Ellis et al. (2002)

Ellis, A.S., Johnson, T.M., Bullen, T.D. (2002) Chromium Isotopes and the Fate of Hexavalent Chromium in the Environment. Science 295, 2060–2062.

, magnetite.
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