Reactivities of Fe(II) on calcite: selenium reduction

Area of Science:

• Environmental Chemistry
• Geochemistry
• Mineralogy

Background:

• Selenium(IV) is a mobile and toxic species in subsurface environments.
• Calcite is a common mineral in soils and geological formations.
• Iron(II) is known to be a reductant for various environmental contaminants.

Purpose of the Study:

• To investigate the reductive immobilization of selenium(IV) by calcite containing iron(II).
• To determine the influence of iron(II) location (sorbed vs. coprecipitated) on selenium reduction.
• To understand the transformation products and kinetics of selenium reduction.

Main Methods:

• X-ray absorption near-edge structure (XANES) spectroscopy to identify selenium speciation.
• Field emission scanning electron microscopy (FESEM) for surface morphology.
• X-ray diffraction (XRD) for mineral phase identification.

Main Results:

• Selenium(IV) sorption increased on calcite with sorbed iron(II) compared to pure calcite.
• Nearly 50% of sorbed selenium(IV) was reduced to elemental selenium(0) within 24 hours by sorbed iron(II).
• Elemental selenium precipitated as needle-shaped red monoclinic crystals (30-50 nm diameter, up to 100 nm length).
• Iron(II) coprecipitated within calcite did not reduce selenium(IV) within 72 hours.
• Reduction extent decreased with longer pre-equilibration times of calcite with iron(II) solution.

Conclusions:

• The reduction capacity of iron(II) for selenium(IV) is highly dependent on its surface-bound location on calcite.
• Surface-bound iron(II) on calcite facilitates rapid reductive immobilization of selenium(IV) to elemental selenium.
• Understanding iron(II)-calcite interactions is crucial for predicting selenium fate in subsurface and waste repositories.