Redox properties of structural Fe in clay minerals: 3. Relationships between smectite redox and structural properties

Structural Fe in clay minerals is an important redox-active species in many pristine and contaminated environments as well as in engineered systems. Understanding the extent and kinetics of redox reactions involving Fe-bearing clay minerals has been challenging due to the inability to relate structural Fe²⁺/Fe_total fractions to fundamental redox properties, such as reduction potentials (E_H). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe²⁺/Fe _total) in smectites over a wide range of applied E_H-values (-0.6 V to +0.6 V). We examined Fe²⁺/Fe_total - E _H relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2) in their native, reduced, and reoxidized states and compared our measurements with spectroscopic observations and a suite of mineralogical properties. All smectites exhibited unique Fe²⁺/Fe_total - E_H relationships, were redox active over wide E_H ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe²⁺/Fe_total - E_H relationships correlated well with both bulk and molecular-scale properties, including Fe _total content, layer charge, and quadrupole splitting values, suggesting that multiple structural parameters determined the redox properties of smectites. The Fe²⁺/Fe_total - E_H relationships developed for these four commonly studied clay minerals may be applied to future studies interested in relating the extent of structural Fe reduction or oxidation to E_H-values.