Direct Visualization of Arsenic Binding on Green Rust Sulfate

Area of Science:

• Environmental Science
• Geochemistry
• Mineralogy

Background:

• Green rust (GR) is a redox-active mineral with potential for arsenic (As) sequestration.
• Understanding As interaction mechanisms with GR is crucial for subsurface remediation.
• GR exhibits high As uptake at circum-neutral pH, but binding mechanisms remain unclear.

Purpose of the Study:

• To elucidate the bonding and interaction mechanisms of As(III) and As(V) with GR sulfate.
• To investigate As sequestration pathways in reduced, anoxic subsurface environments.
• To combine advanced microscopy and spectroscopy for detailed surface complex analysis.

Main Methods:

• Scanning transmission electron microscopy (STEM) with energy-dispersive X-ray (EDX) spectroscopy.
• Synchrotron-based X-ray total scattering and pair distribution function (PDF) analysis.
• As K-edge X-ray absorption spectroscopy (XAS) including extended X-ray absorption fine structure (EXAFS).

Main Results:

• As(III) and As(V) preferentially adsorb at GR crystal edges.
• Arsenic binds primarily as bidentate binuclear inner-sphere surface complexes.
• Authigenic parasymplesite (ferrous arsenate nanophase) formed and immobilized ~87% of As(V).

Conclusions:

• Combining STEM-EDX, PDF, and XAS is essential for determining As binding mechanisms on GR.
• GR phases significantly impact As sequestration in anoxic subsurface environments.
• GR acts as a vital substrate for arsenic immobilization, particularly As(V).