Incorporation of molybdenum(vi) in akaganéite (β-FeOOH) and the microbial reduction of Mo–akaganéite by Shewanella loihica PV-4

Abstract

Among all highly-crystalline iron oxides present in the environment, akaganéite (β-FeO(OH, Cl)) possesses one of the most unconventional structural setups and is a known scavenger for large quantitates of molybdenum (Mo⁶⁺). The factors controlling the exact mechanism for Mo⁶⁺ incorporation into the akaganéite crystal structure are poorly understood and the ability of dissimilatory Fe(III)-reducing microorganisms to reduce pure akaganéite or Mo-carrying iron oxides is not well characterized. In the current study, we investigated the short-range order around Mo⁶⁺ in akaganéite and the fate of Mo⁶⁺ under microbially-mediated Fe(III)-reducing conditions. We found that akaganéite can incorporate up to 14.11 ± 0.22 wt% Mo, while the Fe content decreases from 59.70 ± 0.31 to 40.40 ± 0.24 wt%, which indicates a loss of 2–3 Fe atoms for each Mo incorporated. Simultaneously, the crystal structure unit cell parameters a, b and c decrease, while β increases with increasing Mo content. Surprisingly, dissolution of akaganéite by Shewanella loihica PV-4 showed higher dissolution rates of Mo-bearing akaganéite compared to Mo-free akaganéite. Moreover, these results suggest the reduction of Mo⁶⁺ is most likely microbially-induced (Fe³⁺ → Fe²⁺, Mo⁶⁺ + 2Fe²⁺ → Mo⁴⁺ + 2Fe³⁺). Furthermore, X-ray absorption spectra collected at the Mo L₃-edge show a peak-splitting of the white line with a splitting gap of 2.7 eV and an increased amplitude for the first peak. This observation indicates Mo⁶⁺ is octahedrally coordinated by oxygen, assuming a strongly distorted MoO₆-octhaedron. Fitting of the short-range order around Mo⁶⁺ in akaganéite supports the presence of a strongly distorted MoO₆-octahedron in a coordination environment similar to the Fe position in akaganéite and the formation of Fe-vacancies close to the newly incorporated Mo⁶⁺.