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

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