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

•This is the first report of the changes in Fe isotope fractionation during the microbially mediated redox cycling of a Fe-rich clay mineral.•Iron isotope fractionation between aqueous Fe(II) and structural Fe(III) in nontronite NAu-1 decreased with consecutive redox cycles.•Our results support a two-Fe(III)-pool conceptual model for bioreducible Fe(III), which consists of an insoluble Fe pool that is predicted to be permanently reversible after the other soluble Fe pool is exhausted after repeated redox cycles.