Redox properties of structural Fe in clay minerals. 1. Electrochemical quantification of electron-donating and -accepting capacities of smectites

Clay minerals often contain redox-active structural iron that participates in electron transfer reactions with environmental pollutants, bacteria, and biological nutrients. Measuring the redox properties of structural Fe in clay minerals using electrochemical approaches, however, has proven to be difficult due to a lack of reactivity between clay minerals and electrodes. Here, we overcome this limitation by using one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in clay minerals and a vitreous carbon working electrode in an electrochemical cell. Using this approach, the electron-accepting and -donating capacities (Q_EAC and Q_EDC) were quantified at applied potentials (E_H) of -0.60 V and +0.61 V (vs SHE), respectively, for four natural Fe-bearing smectites (i.e., SWa-1, SWy-2, NAu-1, and NAu-2) having different total Fe contents (Fe_total = 2.3 to 21.2 wt % Fe) and varied initial Fe ²⁺/Fe_total states. For every SWa-1 and SWy-2 sample, all the structural Fe was redox-active over the tested E_H range, demonstrating reliable quantification of Fe content and redox state. Yet for NAu-1 and NAu-2, a significant fraction of the structural Fe was redox-inactive, which was attributed to Fe-rich smectites requiring more extreme E _H-values to achieve complete Fe reduction and/or oxidation. The Q_EAC and Q_EDC values provided here can be used as benchmarks in future studies examining the extent of reduction and oxidation of Fe-bearing smectites.