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 Fe(2+)/Fe(total) fractions to fundamental redox properties, such as reduction potentials (EH). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe(2+)/Fe(total)) in smectites over a wide range of applied EH-values (-0.6 V to +0.6 V). We examined Fe(2+)/Fe(total )- EH 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 Fe(2+)/Fe(total) - EH relationships, were redox active over wide EH ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe(2+)/Fe(total) - EH 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 Fe(2+)/Fe(total) - EH 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 EH-values.
Department of Civil and Environmental Engineering, Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Christopher A
CA
Gorski
Laura E
LE
Klüpfel
Andreas
A
Voegelin
Michael
M
Sander
Thomas B
TB
Hofstetter
D000538:Aluminum Silicates / Q000737:chemistry*; D000077215:Clay; D055664:Electrochemical Techniques; D004579:Electron Transport; D007501:Iron / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D017640:Silicates / Q000737:chemistry*; D015204:Spectroscopy, Mossbauer; D056928:X-Ray Absorption Spectroscopy
D000538:Aluminum Silicates; D017640:Silicates; C033214:Smectite; D007501:Iron; D000077215:Clay