Consecutive Fe redox cycles decrease bioreducible Fe(III) and Fe isotope fractionations by eliminating small clay particles

Abstract

Previous work has shown that about 10% of total clay-bound Fe(III) in unaltered nontronite NAu-1 is bioreducible, although it remains unclear how much of the bioreducible Fe pool persists after repeated oscillations between anoxic and oxic conditions. Here, we report on results from an experiment where we monitored the abundance of bioreducible Fe(III) in NAu-1 over three consecutive redox cycles using chemical extractions and Fe isotope analysis to document the changes in the nature and extent of Fe atom exchange. During each cycle, NAu-1 was reduced biotically by Shewanella oneidensis MR-1 and then re-oxidized abiotically by O2 via aeration. By the third reduction period (RP3), the bacteria were only able to reduce 5.7% of the total clay Fe, that is, 40% less than during the first reduction period (RP1). The decrease in bioreducible Fe(III) is attributed to preferential reductive dissolution of Fe(III) from the finest clay particles. Extrapolation of the observed trend implies that, once the reducible Fe of the finest clay particles is removed, around 4% of the total Fe of the clay remains permanently redox-active, presumably as Fe atoms within the octahedral mineral structure that are accessible to the bacteria. The proposed particle size-dependent evolution of bioreducible Fe(III) from RP1 to RP3 is supported by the observed increasing crystalline domain size, preferential Fe dissolution from the smallest aggregates, and decreasing Fe isotope fractionation factors between aqueous Fe(II) and structural Fe(III) and between solid-bound Fe(II) and structural Fe(III). Our results imply that, in redox dynamic environments, the fraction of insoluble clay-bound Fe that is potentially renewable for use by Fe-reducing bacteria is a function of the evolving size distribution of the clay particles.