In situ redox flexibility of FeII-III oxyhydroxycarbonate green rust and fougerite

摘要

Bacterial activity is commonly thought to be directly responsible for denitrification in soils and groundwater. However, nitrate reduction in low organic sediments occurs abiotically by Fe-II ions within the fougerite mineral (IMA 2003-057), giving the bluish-green color of gleysols. Fougerite, the mineral counterpart of FeII-III oxyhydroxycarbonate, (Fe6(1-x)Fe6xO12H2(7-3x)CO3)-Fe-II-O-III, provides a unique in situ redox flexibility, which can adapt x = {[Fe-III]/[Fe-total]} between 1/3 and 2/3 as shown using Mossbauer spectroscopy. Chemical potential and E-h-pH diagrams for this system were determined from electrode potential monitored during deprotonation of hydroxycarbonate (Fe4Fe2III)-Fe-II(OH)(12)CO3 to assess the possibility of reducing pollutants in the field. Bioreduction of ferric oxyhydroxides in anoxic groundwater yields dissolved Fe-II, whereas HCO3- anions produced from organic matter are incorporated into fougerite layered double oxyhydroxide structure. Thus, fougerite is the solid-state redox mediator acting as electron shuttle that helps bacterial activity for reducing nitrate by coupling dissimilatory Fe-III reduction and oxidation of Fe-II with reduction of NO3-. It is proposed that this system could be used in the remediation of soils and nitrified waters.