Antimony(V) Retention by Lepidocrocite: Sorption, Coprecipitation, and Extractability.

Area of Science:

• Geochemistry
• Environmental Science
• Mineralogy

Background:

• Antimony (Sb) is a metalloid with significant environmental implications, often found in aquatic systems associated with iron (Fe) oxides.
• Understanding antimony sorption and coprecipitation mechanisms with mineral phases like lepidocrocite is crucial for predicting its environmental fate.
• Assessing the effectiveness of common extraction schemes is vital for accurate geochemical speciation and risk assessment.

Purpose of the Study:

• To elucidate the structural mechanisms controlling antimony(V) sorption and coprecipitation with lepidocrocite.
• To evaluate the performance of Wenzel and BCR sequential extraction schemes in recovering sorbed and coprecipitated antimony(V) from lepidocrocite.
• To provide insights into the reliability of common analytical methods for antimony speciation in environmental samples.

Main Methods:

• Antimony K-edge EXAFS spectroscopy was employed to determine the structural environment of Sb(V) sorbed and coprecipitated with lepidocrocite.
• Sequential extraction procedures, including Wenzel and BCR schemes, were utilized to assess antimony recovery.
• Dissolution experiments in 1 M HCl were conducted to compare the release behavior of Sb and Fe under different retention mechanisms.

Main Results:

• EXAFS analysis indicated Sb(V) coprecipitation involves Sb(V)-for-Fe(III) substitution within the lepidocrocite structure.
• Sorption was characterized by edge and double-corner sharing of SbO6 and FeO6 octahedra at the mineral surface.
• Sb(V) sorption led to faster Sb dissolution than Fe in HCl, while coprecipitation resulted in congruent dissolution.
• The Wenzel scheme underestimated surface sorption but recovered sorbed and coprecipitated Sb(V) in later steps.
• The BCR scheme poorly recovered Sb(V), misinterpreting its association with lepidocrocite.

Conclusions:

• Coprecipitation and sorption represent distinct Sb(V) retention mechanisms by lepidocrocite, with differing structural linkages and dissolution behaviors.
• Sequential extraction schemes exhibit significant limitations in accurately quantifying Sb(V) associated with lepidocrocite, particularly the BCR method.
• This research highlights the importance of understanding mineral-surface interactions and critically evaluating analytical methodologies for accurate environmental assessment of antimony.