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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-85
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-85
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  20 Mar 2020

20 Mar 2020

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A revised version of this preprint is currently under review for the journal BG.

Interpretation of kinetic isotope fractionation between aqueous Fe(II) and ferrihydrite under a high degree of microbial reduction

Lei Jiang1, Chuanjun Wu1, Mingqing Li2, Xuegong Li1, and Jiwei Li1 Lei Jiang et al.
  • 1CAS Key Laboratory for Experimental Study under Deep-sea Extreme Environment Conditions, Institute of Deep-sea Science and Engineering, Chinese Academy of Science, Sanya, 572000, China
  • 2University of Chinese Academy of Sciences, Beijing 100049, China

Abstract. Microbial dissimilatory iron reduction (DIR) often ceases when the degree of iron mineral reduction is low, at which point isotope fractionation occurs between an aqueous Fe(II) solution and a reactive Fe(III) phase on the surface of ferric (oxyhydro) oxides, forming an equilibrium fractionation factor (~ 3 ‰). Recent experimental abiotic studies suggest that Fe(II) adsorption onto the mineral surface may affect the isotope fractionation, which reminds us that the isotope exchange may be greatly inhibited during the DIR process. In this study, ferrihydrite is used as a terminal electron acceptor to conduct Shewanella piezotolerans WP3 and Shewanella oneidensis MR-1 experiments at 0.1 and 15 MPa to ensure a significant variation in the degree of reduction. During the 30-day experiment, the degree of ferrihydrite reduction by S. piezotolerans WP3 is 14 % (at 0.1 MPa) and 8 % (at 15 MPa), whereas the degree of ferrihydrite reduction by S. oneidensis MR-1 is 39 % (at 0.1 MPa) and 36 % (at 15 MPa). Based on the isotope mass balance, the estimated ranges of iron isotope fractionation for S. piezotolerans WP3 and S. oneidensis MR-1 are obtained. The former ranges between −3.58 ‰ and −0.88 ‰ (at 0.1 MPa) and between −2.37 ‰ and −0.66 ‰ (at 15 MPa), and the latter ranges between −0.39 ‰ and 0.10 ‰ (at 0.1 MPa) and between −0.6 ‰ and −0.16 ‰ (at 15 MPa). However, it is difficult to distinguish variations in the same bacteria at 0.1 and 15 MPa due to the large estimation ranges of isotope fractionation. In the S. oneidensis MR-1 experiment, the fractionation factor obtained is significantly different from that obtained in the S. piezotolerans WP3 experiment, indicating that kinetic fractionation occurred. In combination with previous studies, we propose a transient modified Fe(II) adsorption mechanism to explain the isotope fractionation between aqueous Fe(II) and ferrihydrite. When the adsorbed Fe(II) exceeds the surface saturation, the atom (isotope) exchange will be suppressed.

Lei Jiang et al.

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Lei Jiang et al.

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Short summary
We use ferrihydrite as a terminal electron acceptor to conduct microbial dissimilatory iron reduction experiments. The results show that Fe isotope equilibrium fractionation occurs due to rapid electron transfer and atom exchange (ETAE) between aqueous Fe(II) and Fe(III) on the surface of minerals at a low degree of reduction. At a high degree of reduction, the kinetic isotope fractionation occurs as the amount of absorbed Fe(II) increases and the driving force of ETAE decreases.
We use ferrihydrite as a terminal electron acceptor to conduct microbial dissimilatory iron...
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