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

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.