Redox\nProperties\nof Structural Fe in Clay Minerals:\n4. Reinterpreting Redox Curves by Accounting for Electron Transfer\nand Structural Rearrangement Kinetics

Iron-bearing\nsmectite clay minerals can act as electron\nsources\nand sinks in the environment. Previous studies using mediated electrochemical\nanalyses to determine the reduction potential (<i>E</i><sub>H</sub>) values of smectites observed that the relationship between\nthe structural Fe<sup>2+</sup><sub>(s)</sub>/Fe<sub>Total</sub> ratio\nin the smectite and <i>E</i><sub>H</sub> varied based on\nthe redox history of the smectite. We hypothesize that this behavior,\nreferred to as redox hysteresis, results from the smectite particles\nnot equilibrating with the applied <i>E</i><sub>H</sub> over\nthe course of the experiment (∼30 min). To test this hypothesis,\nwe developed a model incorporating interfacial electron transfer kinetics\nand charge redistribution within the particle to simulate the mediated\nelectrochemical experiments from previous studies. The simulated redox\ncurves accurately matched the previously reported experimental redox\ncurves of the smectite SWa-1, demonstrating that longer equilibration\nperiods led to a decrease in redox hysteresis. We validated this experimentally\nby measuring the redox curve of SWa-1 after an equilibration period\nof at least 12 h. Furthermore, we extended the simulations to three\nother smectites (NAu-1, NAu-2, and SWy-2) and extracted their respective\nthermodynamic and kinetic parameters. This work offers a framework\nfor interpreting and modeling redox reactions on clay surfaces, along\nwith key parameters for four commonly studied smectites.