Effect of nonreactive kaolinite on 4-chloronitrobenzene reduction by Fe(ii) in goethite-kaolinite heterogeneous suspensions

The kinetics of model contaminant 4-chloronitrobenzene (4-ClNB) reduction by Fe(ii) in aqueous suspensions containing either or both goethite (α-FeOOH) nanoparticles and kaolinite (Al₂Si₂O₅(OH)₄) were quantified to elucidate the effects of nonreactive clay minerals on the attenuation of nitroaromatic groundwater contaminants by iron oxide nanoparticles. Increasing the amount of kaolinite in the presence of goethite decreased the reduction rate of 4-ClNB and competitive Fe(ii) adsorption on kaolinite occurred. Cryogenic transmission and scanning electron microscopy (cryo-TEM and cryo-SEM) images did not reveal significant loss of accessible reactive surface area as a result of heteroaggregation. Sequential-spike batch reactors revealed that in the presence of kaolinite, 4-ClNB reduction rate decreased by more than a factor of three with extended reaction as a result of kaolinite dissolution and subsequent incorporation of Al and Si in goethite or on the goethite surface. The reactive sites residing on the {110} faces were comparatively more reactive in the presence of a large loading of kaolinite, resulting in shorter and wider goethite particles after reaction. These results elucidate the mechanisms by which nonreactive clays affect the reactions of Fe(ii)/iron oxides in groundwater systems and indicate that nonreactive clays are not passive components.