Reductive transformation reactions involving mineral-bound Fe²⁺ species are of great relevance for the fate of groundwater contaminants. For clay minerals, which are ubiquitously present in soils and sediments, the factors determining the reactivity of structural Fe²⁺ and surface-bound Fe²⁺ are not well understood. We investigated the reactivity and availability of Fe²⁺ species in suspensions of chemically reduced montmorillonite (SAz-1) as well as in suspensions of oxidized and reduced nontronite (SWa-1, ferruginous smectite) using two acetylnitrobenzene isomers as reactive probe compounds. The analyses of the reduction kinetics of the two nitroaromatic compounds (NACs) suggested that Fe²⁺ bound in the octahedral layer of reduced smectites is the predominant reductant and that electron transfer presumably occurs via basal siloxane planes. In contrast, reduction of NACs by Fe²⁺ associated with oxidized nontronite is orders of magnitude slower than reduction by octahedral Fe²⁺. Reductive transformation and reversible, nonreactive electron donor−acceptor (EDA) complexation of NACs at basal smectite surfaces occur simultaneously at reduced montmorillonite exhibiting low structural iron content. In contrast, EDA complexation was not observed in suspensions of reduced iron-rich nontronite. Due to the similar reduction rate constants measured for the two NACs, we propose that the (re)generation of octahedral Fe²⁺ sites, e.g., by electron transfer and/or Fe rearrangement within the octahedral nontronite layers, partly limited the rate of contaminant transformation. Since iron in clay minerals is available for microbial reduction, our study suggests that octahedral Fe²⁺ can contribute to abiotic contaminant transformation in anoxic environments.

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