VIRTUAL GOLDSCHMIDT 2021 PRESENTATION


 Fe(II) oxidation in the presence of Sb(V): Mutual effects on Fe(III) precipitates and Sb sorption 


Kerstin Hockmann, Laura Wegner, Catherine McCammon, Stefan Peiffer, Britta Planer-Friedrich  & Ed Burton 

This data will be part of the publication:

Wegner, L., Burton, E.D., McCammon, C., Scheinost, A., Peiffer, S., Planer-Friedrich, B., 

Hockmann, K.: "Fe(II) oxidation in the presence of Sb(V): Mutual effects on Fe(III) oxide formation and Sb sorption". (manuscript in preparation)

Our motivation.

In redox-variable environments, the mobility of antimony (Sb), a toxic metalloid of increasing concern, is closely linked to the biogeochemical cycling of Fe. Antimony has a high affinity for Fe(III) oxides, and microbial production of soluble Fe(II) has been shown to release co-associated Sb under anaerobic conditions. In contrast, the Sb-Fe interactions during aerobic Fe(II) oxidation and subsequent Fe(III) precipitation have received only marginal attention.

What we did.

Here, we investigated the effect of Fe(II) oxidation in the presence of a range of environmentally relevant Sb(V) concentrations on the nature of the resulting Fe(III) precipitates and on coupled sequestration of Sb. All oxidation experiments were carried out in oxygen-saturated solutions (pH 7) containing an initial Fe(II) concentration of 1 mM and Sb(V) at Sb:Fe molar ratios of 0, 1:100, 1:10, and 1:4 (no-, low-, medium-, and high-Sb treatments). Aqueous Sb and Fe(II) concentrations were monitored during oxidation and precipitates were characterized by a combination of spectroscopic and microscopic techniques.

What we found.

In the absence of Sb(V) and at low Sb(V) levels, X-ray diffractometry and electron microscopy revealed lepidocrocite as the only solid-phase Fe(II) oxidation product. In contrast, higher Sb:Fe molar ratios (1:10 and 1:4) inhibited lepidocrocite precipitation, and instead resulted in increased formation of feroxyhyte – a rarely reported FeOOH polymorph. Fe-57 Mössbauer spectroscopy showed a strong asymmetric line broadening and a reduction of the hyperfine magnetic field of feroxyhyte, suggesting incorporation of diamagnetic Sb(V) into the Fe(III) mineral.  EXAFS shell-fit analyses confirmed that Sb substituted for Fe in the structure of the precipitated lepidocrocite and feroxyhyte. 

Why it matters.

Our results are important for a robust understanding of Sb geochemistry in redox-dynamic environments as they demonstrate that Sb itself can steer pathways of secondary Fe(III) oxide formation. This study also suggests that Fe oxide formation in Sb-contaminated systems may differ strongly from the well-known pathways that occur under Sb-free conditions. This is significant as the composition of Fe mineral assemblages does not only alters Sb’s environmental mobility, but also that of contaminants that co-occur with Sb (such as arsenic).
 

DETAILS

Sb sequestration during Fe(II) oxidation was a function of the Sb:Fe ratio.

  • Aqeous Fe(II) quickly oxidized (within ~10 min) to Fe(III) after it had been added to the aerated Sb-containing solutions.
  • Aqueous Sb concentrations decreased in parallel with Fe(II).
  • Steady-state Sb concentrations after completion of Fe(II) oxidation were a function of the initial Sb(V) concentration.







High Sb(V) concentrations promoted the formation of feroxyhyte.

  • Scanning and transmission electron microscopy revealed lepidocrocite as the only solid-phase Fe(II) oxidation product in no- and low-Sb treatments.
  • In contrast, higher Sb:Fe molar ratios (1:10 and 1:4) inhibited lepidocrocite precipitation, and instead resulted in the formation of feroxyhyte.

Mössbauer results suggest incorporation of diamagnetic Sb(V) into feroxyhyte

  • Mössbauer spectroscopy confirmed a gradual increase of Fe incorporated in feroxyhyte, with 0%, 67%, and 100% of Fe in feroxyhyte in low-, medium-, and high-Sb treatments, respectively.
  • The hyperfine magnetic field of feroxyhyte (33 T) was substantially less than the magnetic field of pure feroxyhyte (46 T) and showed a strong asymmetric line broadening, which hints at the incorporation of diamagnetic Sb(V). 






XRD results suggest incorporation of Sb(V) into lepidocrocite

  • XRD results showed a shift of the (020) peak to lower 2-theta values (i.e. from 19° to 15°).
  • This peak shift reflects an increase of the interlayer d-space from 6.31 to 8.21 Angström and can be attributed to the heterovalent substitution of Fe(III) by Sb(V). 
  • Sb(V)-for Fe(III) substitution resulted in a charge imbalance, which was compensated for by a release of hydrogen from the interlayer regions. 
  • Hence, structural Sb(V) incorporation weakened the hydrogen bonding between the dominating faces (020) of lepidocrocite, thus leading to the observed increase in d-spacing.


EXAFS data confirm that Sb(V) substituted for Fe(III) in the Fe oxide structure.

  • Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Sb K-edge shows the incorporation of Sb(V) into the octahedral structure via heterovalent substitution for Fe(III).   
  • The coordination numbers for the two Sb-Fe shells in the high-Sb treatment are consistent with those previously obtained for Sb(V) incorporation into feroxyhyte. 
  • The coordination number for the Sb-Fe shell in the low-Sb treatment matches the one for Fe(III) in lepidocrocite.


Our results in a nutshell

Here, we showed that the oxidation of Fe(II) by molecular oxygen caused the incorporation of substantial amounts of Sb(V) into the structure of the precipitated Fe(III) oxides. In particular, our study provides direct evidence for the incorporation of Sb(V) into lepidocrocite. We also showed that Sb(V) itself can drive major changes in Fe oxide mineralogy by causing a shift of the Fe(III) mineral products from lepidocrocite to feroxyhyte. 
It is clear that more research is necessary to resolve the specific structural mechanisms driving this shift to feroxyhyte formation at high Sb(V) concentrations. Nevertheless, the observed formation of feroxyhyte implies that in situ Fe mineralogy in Sb-contaminated systems may not reflect what would be expected based on our understanding of non-contaminated environments.