From the lab to the real world : the redox reactivity of Fe-bearing clay minerals in complex biogeochemical environments

Abstract

Microbial and chemical processes drive reduction of structural iron in clay minerals, which, in turn, can facilitate the reduction of a range of contaminants including nitroaromatic compounds. Fe-bearing clay minerals are largely resistant to reductive dissolution and can thus undergo redox cycling, rendering them an important and renewable source of electron equivalents in natural environments. However, their reactivity outside controlled laboratory environments is currently not well understood. Here, we used a series of batch experiments of increasing complexity to assess how components representative of natural systems influence the reactivity of Fe-bearing clay minerals. We investigated the degradation of a nitroaromatic compound (NAC, 50 2-acetylnitrobenzene) in suspensions of 2.0 g/L nontronite NAu-1, at a range of Fe(II)/Fe(tot) ratios (pH 7.5) in the presence of various naturally occurring redox active phases in order to develop a mechanistic understanding of how iron-rich clay minerals behave in complex natural environments. Specifically, we investigated the influence of: i The naturally ubiquitous reductant aqueous Fe(II) that reduces clay mineral Fe while forming a potentially reactive Fe-oxidation product; ii Fe(II)-organic ligand complexes, using small acids representative of natural organic matter; iii Electron transfer mediators (ETMs) representative of microbial exudates. Our results show that Fe(II)-reduced nontronite, that may occur in natural systems facilitates the degradation of NACs more rapidly than microbially reduced clay minerals of comparable reduction extents. Fe(II)-reduced nontronite exhibits biphasic reduction kinetics, which are unique to abiotically reduced nontronite, due to the presence two distinct reactive Fe(II)- entities. We found that these were both contained within the mineral structure for NAu-1 reduced using dithionite whereas for Fe(II)-reduced NAu-1, although the highly reactive site was likely to comprise the same structural Fe(II) as the dithionite-reduced mineral, the site with lower reactivity likely comprised Fe(II) associated with the secondary Fe-bearing phase that forms as a result of the mineral reduction. We show that carboxylate Fe(II)-organic ligand complexes may rapidly reduce both clay mineral Fe(II) and NACs. In the presence of the Fe(II)-organic complexes, the nontronite acts as a redox buffer providing a sink of electrons in its native form but regenerating the reactive organic Fe(II) pool in its reduced form. We generally observed more rapid NAC reduction in the presence of quinone and flavin-containing ETMs and the magnitude of this effect depended on both ETM structure and clay mineral Fe reduction extent. Overall, we highlight the importance of considering all phases in complex biogeochemical systems when assessing contaminant fate in natural and engineered systems.