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+).

Incorporation of Mo6+ in Ferrihydrite, Goethite, and Hematite

“…The fitting results for octahedral Mo 6+ at the Fe(1) site suggested Mo-O distances of between 1.788(2) and 2.293(3) Å (corrected for phase shift, Table 2), which are consistent with the strongly distorted MoO 6 octahedron in sardignaite (Orlandi et al 2010) used by Bolanz et al (2017) to fit the first shell of Mo-substituted akaganéite. While the next Fe neighbors coordinated with Mo within the akaganéite structure of Bolanz et al (2017) are at 3.311, 3.510, and 3.640 Å (two atoms for each path), the fitting results of the ferrihydrite model showed two additional groups of two Fe atoms at 2.860(2) Å and 3.068(2) Å. To compensate for the strongly distorted MoO 6 octahedron, the six remaining Fe atoms split into four Mo-Fe paths with different interatomic distances ranging from 3.204(5) to 3.663(7) Å.…”

“…All paths of the Fe shell shared one value for the Debye-Waller factor (σ 2 ), but had different Δr values except two groups of Fe atoms at distances of 2.860(2), 3.068(2), and 3.224(2) Å, as well as 3.569(7) and 3.663(7) Å, respectively. The formation of Fe vacancies as described for akaganéite by Bolanz et al (2017) was not observed.…”

“…Comparing the Mo L 3 -edge XANES spectra of the six-line ferrihydrite phases with spectra of Mo-bearing akaganéite of Bolanz et al (2017) showed that the peak-splitting values of the synthesized samples (2.96 to 3.01 eV) were considerably greater than the 2.7 eV measured for akaganéite (Bolanz et al 2017). Mo 6+ was, therefore, considered to be octahedrally coordinated in six-line ferrihydrite.…”

“…Despite their low Mo concentrations, the goethite samples were included in this study because crystallographic changes in the samples could still be observed. For hematite with larger Mo concentrations, the molar loss of Fe for each mole of Mo (Fe loss ) was calculated by (Bolanz et al 2017):…”

“…All spectra are uncorrected for phase shift Table 2 Scattering paths for octahedrally coordinated Mo 6+ in six-line ferrihydrite (6L-Feh_0.1) at the Fe(1) site ΔE 0 = 0, N var = 14, N ind = 28, R = 0.007, k-range = 3-12, R-range = 1.000-6.000 Michel et al (2007). The formation of Fe 3+ vacancies as described by Bolanz et al (2017) for akaganéite or as for hematite (this study) was not observed. Possible ways to balance the excess charge of Mo 6+ could be the release of three H + atoms or the incorporation of three additional hydroxide ions (OH − ) in the structure of ferrihydrite, which still is not fully understood.…”