Assessing Reactive Iron Mineral Coatings in Redox Transition Zones with Sequential Extraction

Abstract

In reduction−oxidation (redox) transition zones (RTZs), reactive iron minerals play an important role in electron transfer between bacteria and contaminants. To better understand their contributions, this study focuses on characterizing iron mineral speciation using sequential extraction. Sediment samples were collected from an anoxic core where the redox condition was preserved. Based on previous analyses, four RTZs were of focus: the Upper Zone [3.96−4.52 m depth below the surface (DBS)], Zone 1 (6.35−6.91 m DBS), Zone 2 (9.45−10.46 m DBS), and Zone 3 (14.63−15.24 m DBS). A six-step sequential extraction (SE) was applied to discern reduced and oxidized forms of Fe mineral coatings in these four RTZs. Based on extraction results, in the Upper Zone, the amorphous Fe sulfide minerals, mackinawite and greigite, increased with depth, while the crystalline Fe sulfide, pyrite, decreased. Because metastable mackinawite was dominant in the Upper Zone and given historical contamination at the site, the absence of volatile organic compounds in the sediment headspace suggests (a)biotic attenuation may be significant. In the Zone 1, the highest concentrations of crystalline Fe sulfide mineral nano-coatings were observed when compared to other three RTZs; importantly, sulfate-reducing bacteria, Desulfosporosinus, was abundant. Fe concentrations in the sediment dramatically decreased in Zone 2, where the Fe(II/III) mineral magnetite was dominant, suggesting a biogenic pathway as iron-reducing bacteria, Geobacter, was abundant. In the aquifer to aquitard interface Zone 3, Fe mineral coatings revealed significant variability between each subsample, suggesting active Fe cycling with biotic processes based on the abundance ofDesulfosporosinusin the clay lenses. Iron speciation with respect to mineralogy [and therefore Fe(II) and Fe(III) forms] in RTZs further supports evidence of (a)biotic contributions in natural attenuation.