Catch me if you can – understanding antimony geochemistry during Fe(II) oxidation

The mobility of antimony (Sb), a toxic metalloid of increasing concern, is closely linked to the biogeochemical cycling of iron (Fe). For example, the microbial production of soluble Fe(II) under oxygen-free conditions has been shown to result in the release of co-associated Sb (occurring as Sb(V) and/or Sb(III)). In redox-dynamic environments, Fe(II) may be re-oxidized and precipitate as Fe(III) oxides. The extent to which Fe(III) precipitation by Fe(II) oxidation immobilizes Sb, however, is still poorly understood and likely depends on an array of factors such as pH, Sb species and the nature of the newly formed Fe(III) oxides.

This study aims at investigating the effect of Fe(II) oxidation on Sb sequestration and on the mineralogy of the resulting Fe(III) precipitates in a pH range typical of Sb-contaminated systems (i.e. pH 6 – 7). To initiate the oxidation reaction, 0.5 mmol L-1 Fe(II) are added to an oxygen-saturated electrolyte solution containing 0 – 50 µmol L-1 Sb(V) or Sb(III). Changes in aqueous Sb and Fe(II) concentration are monitored during the experiment. Resulting solid phase samples are collected and characterized using a combination of spectroscopic, microscopic, and wet chemical extraction techniques.

First results show that Fe(II) oxidation kinetics and Sb sequestration are highly pH-dependent. At circumneutral pH and in the absence of Sb, X-ray diffractometry revealed lepidocrocite as the only solid-phase product. In contrast, the presence of Sb(V) partially inhibited lepidocrocite precipitation, and additionally resulted in formation of feroxyhyte – a rarely reported FeOOH polymorph. Sb EXAFS analysis indicated the incorporation of Sb(V) into the Fe oxide structure.

Our results are important for a robust understanding of Sb geochemistry in redox-dynamic environments as they demonstrate that Sb itself influences the pathways of secondary Fe oxide formation. This study also provides important information for the development of adequate remediation practices of Sb-contaminated soils under changing redox conditions.