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.

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

“…60 Similar observations have been reported for iron-bearing clays at different Fe(II) to Fe(III) ratio, where a distribution of redox potentials was modelled. 61 Therefore, a wide range of redox potentials for magnetite at neutral pH have been quoted in the literature including +0.27 V, 53 -0.314 V, 55 -0.38 V 62 when measured against the standard hydrogen electrode (SHE). The reason for this discrepancy in redox potentials for magnetite has been suggested to be due to variations in the Fe(II)/Fe(III) ratio of the samples under investigation.…”

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

“…Evidence for redox potentials ranging over a continuum of values has been seen in electrochemical characterization of iron minerals and in electron donating capacities of microbially reduced humic substances. 83,104,105 Some of those studies modeled the larger than expected range over which electrons are accepted and donated with a Nernst equation that contained a factor (b) to account for the non-ideal behavior. When b is 1 behavior is Nernstian and when b is less than 1 the potential range is widened.…”

“…By using these data (change in E H , and the fraction of reduced to oxidized species) in the Nernst equation, a coefficient can be extracted and used to calculate a theoretical potential. 83,104,105 The latter two applications of MER/MEO are indirect measures of the target properties and therefore require careful attention to the assumptions of the method in order to avoid misleading results. 106 Aprotic solvents have long been used in organic electrochemistry, [107][108][109][110][111][112][113][114][115] and also have only recently been shown to be useful for characterization of NOM redox properties.…”

Electrochemical activity of iron in acid treated bentonite and influence of added nickel

“…Therefore, the transfer of electrons to and from iron cations is hindered [9]. This limitation has been overcome by using electrontransfer mediated compounds, which facilitate electron transfer between structural iron in clay minerals and conductive substrate [9,[13][14][15][16][17]. On the other hand, using such approach it is difficult to distinguish the electroactive performance of iron inherent in clays.…”