Redox Properties of Structural Fe in Clay Minerals: 4. Reinterpreting Redox Curves by Accounting for Electron Transfer and Structural Rearrangement Kinetics

Iron-bearing smectite clay minerals can act as electron sources and sinks in the environment. Previous studies using mediated electrochemical analyses to determine the reduction potential (E_H) values of smectites observed that the relationship between the structural Fe²⁺_(s)/Fe_Total ratio in the smectite and E_H varied based on the redox history of the smectite. We hypothesize that this behavior, referred to as redox hysteresis, results from the smectite particles not equilibrating with the applied E_H over the course of the experiment (∼30 min). To test this hypothesis, we developed a model incorporating interfacial electron transfer kinetics and charge redistribution within the particle to simulate the mediated electrochemical experiments from previous studies. The simulated redox curves accurately matched the previously reported experimental redox curves of the smectite SWa-1, demonstrating that longer equilibration periods led to a decrease in redox hysteresis. We validated this experimentally by measuring the redox curve of SWa-1 after an equilibration period of at least 12 h. Furthermore, we extended the simulations to three other smectites (NAu-1, NAu-2, and SWy-2) and extracted their respective thermodynamic and kinetic parameters. This work offers a framework for interpreting and modeling redox reactions on clay surfaces, along with key parameters for four commonly studied smectites.