title: Redox properties of structural Fe in clay minerals. 1. Electrochemical quantification of electron-donating and -accepting capacities of smectites.
abstract: 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.
authors: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland.
authors: Christopher A
authors: CA
authors: Gorski
authors: Michael
authors: M
authors: Aeschbacher
authors: Daniela
authors: D
authors: Soltermann
authors: Andreas
authors: A
authors: Voegelin
authors: Bart
authors: B
authors: Baeyens
authors: Maria
authors: M
authors: Marques Fernandes
authors: Thomas B
authors: TB
authors: Hofstetter
authors: Michael
authors: M
authors: Sander
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry; D000077215:Clay; D055664:Electrochemical Techniques; D004566:Electrodes; D004583:Electrons; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D017640:Silicates / Q000737:chemistry*
chemical_list: D000538:Aluminum Silicates; D017640:Silicates; D002244:Carbon; C033214:Smectite; D007501:Iron; D000077215:Clay