Redox properties of structural Fe in clay minerals. 2. Electrochemical and spectroscopic characterization of electron transfer irreversibility in ferruginous smectite, SWa-1

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^φ) of structural Fe. E_H^φ-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^φ-values underscore the importance of Fe-bearing clay minerals as redox-active phases in a wide range of redox regimes.