title: Redox properties of structural Fe in clay minerals. 2. Electrochemical and spectroscopic characterization of electron transfer irreversibility in ferruginous smectite, SWa-1.
abstract: Structural Fe in clay minerals is an important, albeit poorly characterized, redox-active phase found in many natural and engineered environments. This work develops an experimental approach to directly assess the redox properties of a natural Fe-bearing smectite (ferruginous smectite, SWa-1, 12.6 wt % Fe) with mediated electrochemical reduction (MER) and oxidation (MEO). By utilizing a suite of one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in SWa-1 and a working electrode, we show that the Fe2+/Fe3+ couple in SWa-1 is redox-active over a large range of potentials (from E(H) = -0.63 V to +0.61 V vs SHE). Electrochemical and spectroscopic analyses of SWa-1 samples that were subject to reduction and re-oxidation cycling revealed both reversible and irreversible structural Fe rearrangements that altered the observed apparent standard reduction potential (E(H)(ø)) of structural Fe. E(H)(ø)-values vary by as much as 0.56 V between SWa-1 samples with different redox histories. The wide range of E(H)-values over which SWa-1 is redox-active and redox history-dependent E(H)(ø)-values underscore the importance of Fe-bearing clay minerals as redox-active phases in a wide range of redox regimes.
authors: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland.
authors: Christopher A
authors: CA
authors: Gorski
authors: Laura
authors: L
authors: Klüpfel
authors: Andreas
authors: A
authors: Voegelin
authors: Michael
authors: M
authors: Sander
authors: Thomas B
authors: TB
authors: Hofstetter
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D000077215:Clay; D055664:Electrochemical Techniques; D004566:Electrodes; D004579:Electron Transport; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D017640:Silicates / Q000737:chemistry*; D015204:Spectroscopy, Mossbauer; D056928:X-Ray Absorption Spectroscopy
chemical_list: D000538:Aluminum Silicates; D017640:Silicates; C033214:Smectite; D007501:Iron; D000077215:Clay