Iron-Rich Clay Minerals Mediate Abiotic Methane Formation through Distinct Fe Coordination Environments

Area of Science:

• Geochemistry
• Environmental Science
• Mineralogy

Background:

• Methane (CH4) is a key greenhouse gas, but abiotic sources in sedimentary environments are not well understood.
• Iron-rich clays are common in sediments, yet their role in catalyzing CH4 formation is largely uninvestigated.

Purpose of the Study:

• To investigate the role of iron-rich clay minerals in catalyzing abiotic methane formation.
• To compare the catalytic activity of different iron-rich clays and understand the underlying mechanisms.

Main Methods:

• Experimental catalysis using Fe3+-exchanged montmorillonite (Fe3+-MMT) and ferric nontronite (NAu) with methylated organic substrates.
• Spectroscopic analyses (e.g., XAS) and density functional theory (DFT) calculations to elucidate reaction mechanisms.
• Varying environmental conditions (pH, temperature, cations, ligands) to assess their impact on CH4 yield.

Main Results:

• Both Fe3+-MMT and NAu catalyzed abiotic CH4 formation, outperforming dissolved Fe3+.
• Fe3+-MMT produced rapid CH4 pulses via interlayer redox cycling, while NAu yielded sustained CH4 through structural Fe activation.
• Mineral structure controlled iron speciation and methyl radical stabilization, reducing CO/CO2 by 42-62% compared to homogeneous systems.
• CH4 yields were sensitive to pH, temperature, cations, and organic ligands.

Conclusions:

• Iron-rich clays are significant catalysts for abiotic CH4 formation in sedimentary environments.
• Mineral structure dictates the kinetics and selectivity of CH4 production pathways.
• These findings highlight the importance of mineralogy in sedimentary carbon cycling and organic compound transformation.