Probing the Redox Reactivity of a Reduced Nontronite: A Quick XAS Operando Study

Fe-bearing clay minerals contain structural iron that can be redox-active and can participate in electron transfer reactions with aqueous species. Although these redox properties have been studied extensively in the past decade, questions remain about the respective roles of kinetic and thermodynamic constraints in establishing steady-state redox conditions. In this study, the reduction kinetics of aqueous Cr(VI) to Cr(III) by Fe(II) contained in the structure of reduced ferruginous clay samples (reference Nontronite NAu-1) was monitored with quick-XAS (X-ray absorption spectroscopy). These measurements revealed the occurrence of at least two reaction processes with contrasting fast and slow kinetic rates. According to mass and electron balance calculations, Fe(II) located at the edge of the clay mineral particles alone cannot account for the fast reactivity of the samples, pointing out the presence of electron transfer from the inner part of the clay mineral layer structure to the reactive sites. The Fe(II)/Fe(III) ratio in the clay structure quickly reached a steady state after each Cr(VI) addition to the solution. These steady-state conditions were consistent with either a complete depletion of the Cr(VI) reactant for the first spikes of Cr(VI) or a thermodynamic equilibrium between the redox couples, i.e., between structural Fe(III)/Fe(II) and aqueous Cr(VI)/Cr(III), after the pool of fast-reacting Fe(II) was depleted. These results highlight the need to consider kinetic and thermodynamic controls of clay structural iron redox reactivity to predict the fate of redox-sensitive contaminants in the environment.