Iron Isotope Fractionations Reveal a Finite Bioavailable Fe Pool for Structural Fe(III) Reduction in Nontronite.
We report on stable Fe isotope fractionation during microbial and chemical reduction of structural Fe(III) in nontronite NAu-1. (56)Fe/(54)Fe fractionation factors between aqueous Fe(II) and structural Fe(III) ranged from -1.2 to +0.8‰. Microbial (Shewanella oneidensis and Geobacter sulfurreducens) and chemical (dithionite) reduction experiments revealed a two-stage process. Stage 1 was characterized by rapid reduction of a finite Fe(III) pool along the edges of the clay particles, accompanied by a limited release to solution of Fe(II), which partially adsorbed onto basal planes. Stable Fe isotope compositions revealed that electron transfer and atom exchange (ETAE) occurred between edge-bound Fe(II) and octahedral (structural) Fe(III) within the clay lattice, as well as between aqueous Fe(II) and structural Fe(III) via a transient sorbed phase. The isotopic fractionation factors decreased with increasing extent of reduction as a result of the depletion of the finite bioavailable Fe(III) pool. During stage 2, microbial reduction was inhibited while chemical reduction continued. However, further ETAE between aqueous Fe(II) and structural Fe(III) was not observed. Our results imply that the pool of bioavailable Fe(III) is restricted to structural Fe sites located near the edges of the clay particles. Blockage of ETAE distinguishes Fe(III) reduction of layered clay minerals from that of Fe oxyhydroxides, where accumulation of structural Fe(II) is much more limited.
Department of Earth and Environmental Sciences, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.|Water Institute, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
Bingjie
B
Shi
Department of Earth and Environmental Sciences, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.|Water Institute, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
Kai
K
Liu
Department of Earth and Environmental Sciences, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.|Water Institute, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
Lingling
L
Wu
Department of Geoscience, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.|NASA Astrobiology Institute, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.
Weiqiang
W
Li
Department of Earth and Environmental Sciences, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.|Water Institute, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
Christina M
CM
Smeaton
Department of Geoscience, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.|NASA Astrobiology Institute, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.
Brian L
BL
Beard
Department of Geoscience, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.|NASA Astrobiology Institute, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.
Clark M
CM
Johnson
Department of Geoscience, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.|NASA Astrobiology Institute, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.
Eric E
EE
Roden
Department of Earth and Environmental Sciences, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.|Water Institute, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
Philippe
P
Van Cappellen
D000538:Aluminum Silicates / Q000737:chemistry; D005591:Chemical Fractionation; D000077215:Clay; D005290:Ferric Compounds / Q000737:chemistry*; D045302:Geobacter / Q000378:metabolism; D007501:Iron / Q000737:chemistry*; D007503:Iron Isotopes / Q000737:chemistry; D008903:Minerals; D010084:Oxidation-Reduction; D020592:Shewanella / Q000378:metabolism
D000538:Aluminum Silicates; D005290:Ferric Compounds; D007503:Iron Isotopes; D008903:Minerals; D007501:Iron; D000077215:Clay