Probing the Redox Reactivity of a Reduced Nontronite: A Quick XAS Operando Study.

Area of Science:

• Environmental Geochemistry
• Mineralogy
• Environmental Science

Background:

• Fe-bearing clay minerals possess redox-active structural iron crucial for electron transfer reactions with aqueous species.
• Understanding the interplay of kinetic and thermodynamic controls on clay structural iron's redox activity is vital for environmental applications.
• Previous research extensively studied clay redox properties, yet precise mechanisms governing steady-state conditions remain debated.

Purpose of the Study:

• To investigate the reduction kinetics of aqueous Cr(VI) to Cr(III) mediated by structural Fe(II) in ferruginous clay (Nontronite NAu-1).
• To elucidate the roles of kinetic and thermodynamic constraints in establishing redox steady-state conditions.
• To determine the contribution of different iron locations within clay structures to redox reactivity.

Main Methods:

• Utilized quick X-ray absorption spectroscopy (XAS) to monitor the reduction kinetics of Cr(VI) by Fe(II) in Nontronite NAu-1.
• Performed mass and electron balance calculations to assess the contribution of different iron pools.
• Analyzed the Fe(II)/Fe(III) ratio evolution to understand steady-state attainment.

Main Results:

• Observed at least two distinct reaction processes with contrasting fast and slow kinetic rates during Cr(VI) reduction.
• Calculations indicated that edge Fe(II) alone could not explain the observed fast reactivity, suggesting electron transfer from inner clay structures.
• The Fe(II)/Fe(III) ratio rapidly reached steady state, consistent with either Cr(VI) depletion or thermodynamic equilibrium between Fe and Cr redox couples.

Conclusions:

• Clay structural iron exhibits complex redox behavior involving both fast and slow kinetic pathways.
• Electron transfer from the clay mineral's interior significantly contributes to the overall reaction rate.
• Both kinetic limitations and thermodynamic equilibrium govern the redox state of structural iron and influence contaminant transformation.