Assessing the sustainability of Iron-bearing clay mineral redox reactions for application in engineered systems

Abstract

Iron is abundant in natural sediments, and the Fe(II)-Fe(III) redox couple is crucial in the cycling of nutrients and for the environmental fate of contaminants. Clay minerals contain structural Fe thought to be resistant to reductive dissolution, and which after reduction, can reductively degrade a range of organic compounds. In the natural environment, clay mineral Fe can be reduced via different pathways, most importantly direct reduction by micro-organisms and interactions with dissolved reductants such as Fe(II). Although sediments and soils are likely to experience natural fluctuations in redox chemistry over time, most research has focused on exploring clay mineral redox reactivity upon initial Fe reduction, disregarding how multiple reduction-oxidation cycles might impact the long-term efficacy of Fe-bearing clay minerals. This project investigated how reductionoxidation cycling via various reduction pathways impacts the structure and reactivity of Fe-bearing clay minerals. Reduction pathways included dithionite, Fe(II)aq, and electron shuttles (as bioreduction proxy), and hydrogren peroxide as oxidant. Effects of redox cycling on mineral structure were monitored with techniques including Mössbauer spectroscopy, scanning electron microscopy and X-ray diffraction analysis. Redox reactivity was assessed by measuring the reductive transformation of probe compound and model contaminant, 3-chloronitrobenzene. Results showed that redox cycling using dithionite did not affect reactivity, but caused structural alterations at high reduction extents. Using electron-shuttles as reductants was largely sustainable, with negligible impact on structure and reactivity. Using aqueous Fe(II) as reductant led to irreversible structural changes, formation of secondary minerals, and significant increases to system reactivity.