VIRTUAL GOLDSCHMIDT 2020 PRESENTATION


Antimony mobility and influence on iron(II)-catalyzed ferrihydrite transformation pathways 


Kerstin Hockmann, Niloofar Karimian, Sara Schlagenhauff, Britta Planer-Friedrich & Ed Burton 

This work has been published as:

Hockmann, K, Karimian, N., Schlagenhauff, S., Planer-Friedrich, B., Burton, E.D., 2021. Impact of antimony(V) on iron(II)-catalyzed ferrihydrite transformation pathways: A novel mineral switch for feroxyhyte formation. Environmental Science and Technology.  DOI: 10.1021/acs.est.0c08660

Our motivation.

Antimony (Sb) is a toxic metalloid of increasing international significance as an environmental contaminant. Its environmental mobility is strongly influenced by interactions with Fe(III) oxides, such as ferrihydrite. While it is well established that Fe(II) may accelerate the transformation and crystallization of ferrihydrite to more stable phases in contaminated wetland soils and sediments, the effect of Sb on these geochemical processes is largely unknown. 

What we did.

We investigated the effect of sorbed Sb on the Fe(II)-induced transformation of Sb-bearing ferrihydrite across a range of Sb(V) loadings (molar ratios of Sb:Fe = 0.003, 0.016 and 0.08) at pH 7. Particular attention was made on the characterization of Fe mineral assemblages and Sb sorption mechanisms using Extended X-ray absorption fine structure (EXAFS) spectroscopy and powder X-ray diffraction (XRD).

What we found.

Our results from XAS and XRD analyses showed that addition of Fe(II) induced the rapid conversion of ferrihydrite via metastable lepidocrocite to goethite at lower Sb loadings. Interestingly, the highest Sb:Fe ratio favoured the formation of feroxyhite, a relatively rare FeOOH-polymorph, from precursor ferrihydrite. EXAFS shell-fit analyses revealed that this Sb immobilization was attributable to the incorporation of Sb into the structure of the neo-formed Fe oxides. 

Why it matters.

Our results show that Sb itself can influence – or even induce –  major changes in Fe oxide mineralogy. This suggests that Fe oxide transformation pathways 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 those of contaminants that co-occur with Sb (such as arsenic).
 

DETAILS

Fe(II) sorption to ferrihydrite was a function of Sb(V) loading.

  • The 5 mM Fe(II) that we added in our reactor experiments quickly equilibrated with the Sb(V)-bearing ferrihydrite. 
  • Steady-state Fe(II) concentrations decreased with Sb(V) loading, since the sorbed antimonate increased the negative charge of the ferrihydrite, making it more favourable or Fe(II) sorption.

Fe(II) catalyzed the transformation of ferrihydrite to more stable Fe oxides.

  • Results from X-ray diffractometry (XRD) confirm that the initial Fe phase was composed of 2-line-ferrihydrite.
  • XRD data collected over time show that the sorbed Fe(II) catalysed the transformation of ferrihydrite to more crystalline phases.
  •  In the zero-Sb, low-Sb, and medium-Sb treatments,  goethite (via  lepidocrocite) was the end product of this transformation.
  • In the high-Sb treatment, feroxyhite formed besides goethite.

Feroxyhite was a major Fe oxide phase at high Sb loading.

  • Linear combination fitting of Fe EXAFS data revealed that lepidocrocite as an intermediate phase was less important with increasing Sb loading. This was likely due to the faster transformation kinetics caused by higher Fe(II) sorption.
  • Feroxyhite formed immediately after Fe(II) addition and contributed up to 40% of Fe speciation in the high-Sb treatment, but was absent at all other Sb loadings.



Sb was "locked away" into the structure of the neo-formed Fe oxides.

  • 1 M HCl- extractable Sb concentrations sharply decreased within the first three days of the experiment.
  • This was attributed to Sb incorporation into the neo-formed Fe oxides. 

EXAFS data show 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 that the Fe(II)-catalysed ferrihydrite transformation led to increases in the coordination number (CN) for both edge-sharing and double-corner-sharing linkages between Sb(V)−O and Fe(III)−O octahedra.  
  • The coordination numbers for the two Sb-Fe shells in the Fe(II)-treatments at 21 days match those for  Fe(III) in goethite, which points to the incorporation of Sb(V) into the goethite structure via heterovalent substitution for Fe(III). 

Why did Sb(V) incorporation favour feroxyhite formation?

The results show that Sb(V) was incorporated into the structure of the neo-formed Fe oxides via heterovalent substitution for Fe(III). Such substitution is feasible since Sb(V) and Fe(III) have comparable ion sizes and similar bond distances in their octahedral coordination. This substitution caused a disorder of the Fe oxide structure, which favoured the formation of feroxyhite, an FeOOH polymorph of low crystallinity.