Iron Adsorption on Clays Inferred from Atomistic Simulations and X-ray Absorption Spectroscopy

Area of Science:

• Environmental Geochemistry
• Materials Science
• Computational Chemistry

Background:

• Iron speciation and oxidation in clay minerals are critical for understanding environmental redox reactions.
• Iron adsorbs onto clay mineral edge surfaces at distinct strong and weak affinity sites.

Purpose of the Study:

• To investigate the atomistic structure and stability of iron complexes on clay mineral surfaces.
• To elucidate the Fe-complexation mechanism and quantify iron partitioning between different surface sites.
• To evaluate the energetic favorability of ferrous iron oxidation and sorption on clay surfaces.

Main Methods:

• Applied ab initio molecular dynamics simulations to study clay mineral edge surfaces.
• Calculated surface complexation energies and ab initio X-ray absorption spectra (XAS).
• Utilized ab initio structure relaxations for Fe2+/Fe3+ substituted clay structures.

Main Results:

• Revealed the Fe-complexation mechanism and quantified Fe partitioning based on oxidation state and loading.
• Observed increasing co-adsorption of Fe2+ with increasing iron loadings, despite Fe3+ being dominant.
• Demonstrated that oxidative sorption of ferrous iron is energetically favored at several edge surfaces.

Conclusions:

• The study provides fundamental insights into iron speciation and oxidative behavior at the atomistic level in clay minerals.
• Ab initio simulations combined with XAS are powerful tools for characterizing iron-clay interactions.
• Findings are crucial for environmental geochemistry, particularly concerning redox reactions in geological systems.