Assessing the redox reactivity of structural iron in smectites using nitroaromatic compounds as kinetic probes

Structural Fe(II) in clay minerals is an important source of electron equivalents for the reductive transformation of contaminants in anoxic environments. We investigated which factors control the reactivity of Fe(II) in smectites including total Fe content Fe(II)/total Fe ratio, and excess negative charge localization using 10 nitroaromatic compounds (NACs) as reactive probe molecules. Based on evidence from this work and previous spectroscopic studies on Fe redox reactions in iron-rich smectites, we propose a kinetic model for quantifying the reactivity, abundance, and interconversion rates of two distinct Fe(II) sites in the minerals' octahedral sheet. Excellent agreement between observed biphasic NAC reduction kinetics and model fits points toward existence of two types of Fe(II) sites exhibiting reactivities that differ by 3 orders of magnitude in iron-rich ferruginous smectite (SWa-1) and Olberg montmorillonite. Low structural Fe content, as found in Wyoming montmorillonite (SWy-2), impedes the formation of highly reactive Fe sites and results in pseudo-first order kinetics of NAC reduction that originate from the presence of a single type of Fe(II) species of even lower reactivity. Similar correlations of one-electron reduction potentials of the NACs vs their second order reduction rate constants for all smectite suspensions suggest that contaminant-Fe(II) interactions were identical in all smectite minerals.