Fe2+ catalyzed iron atom exchange and re-crystallization in a tropical soil

Aqueous ferrous iron (Fe2+(aq)) is known to transfer electrons and exchange structural positions with solid-phase ferric (FeIII) atoms in many Fe minerals. However, this process has not been demonstrated in soils or sediments. In a 28-day sterile experiment, we reacted 57Fe-enriched Fe2+(aq) (57/54Fe = 5.884 ± 0.003) with a tropical soil (natural abundance 57/54Fe = 0.363 ± 0.004) under anoxic conditions and tracked 57/54Fe in the aqueous phase and in sequential 0.5 M and 7 M HCl extractions targeting surface-adsorbed and bulk-soil Fe, respectively; we also analyzed the reacted soil with 57Fe Mössbauer spectroscopy. In 28 days, the aqueous and bulk pools both moved ∼7% toward the isotopic equilibrium (57/54Fe = 1.33). Using a kinetic model, we calculate final adsorption-corrected 57/54Fe ratios of 5.56 ± 0.05 and 0.43 ± 0.03 in the aqueous and bulk pools, respectively. The aqueous and surface/labile Fe initially exchanged atoms rapidly (10-80 mmol kg−1 d−1) decreasing to a near constant rate of 1 mmol kg−1 d−1 that was close to the 0.74 mmol kg−1 d−1 exchange-rate between the surface and bulk pools. Thus, after 28 days we calculate aqueous Fe has exchanged with 20.1 mmol kg−1 of bulk Fe atoms (1.9% of total Fe) in addition to the 17.0 mmol kg−1 of surface/labile Fe atoms (1.6% of total Fe), which have likely turned over several times during our experiment. Extrapolating these rates, we calculate a hypothetical whole-soil turnover time of ∼3.6 yrs. Furthermore, Mössbauer spectroscopy indicates the soil-incorporated 57Fe label re-crystallized as short-range-ordered (SRO) FeIII-oxyhydroxides: our model suggests this pool could turnover in less than seven months via Fe2+-catalyzed recrystallization. Thus, we conclude Fe atom exchange can occur in soils at rates fast enough to impact ecological processes reliant on Fe minerals, but sufficiently slow that complete Fe mineral turnover is unlikely, except perhaps in permanently anoxic environments.