Date of Award
Doctor of Philosophy in Environmental Engineering - (Ph.D.)
Civil and Environmental Engineering
Donna Elaine Fennell
James A. Dyer
Reactive iron mineral coatings are found throughout reduction-oxidation (redox) transition zones and play a significant role in contaminant transformation processes. In this study, an 18.3-meter core is collected, subsampled, and preserved under anoxic conditions to maintain its original redox state. Screening analyses are conducted at sampling increments of 5.08 cm in depth for the following: elemental concentrations with X-ray fluorescence (XRF), sediment pH, sediment oxidation-reduction potential (ORP), total volatile organic carbon (TVOC) in the sample headspace, and abundant bacteria (16S rRNA sequencing). Using the Fe and S gradients correlated with microbial data, five RTZs are delineated. To characterize iron mineral speciation, a six-step sequential extraction is applied in four out of the five RTZs. Based on extraction results, amorphous Fe sulfide minerals, mackinawite and greigite, increase with depth in the Upper Zone, the shallowest RTZ. Because of the abundance of these amorphous minerals and given the extent of contamination at the site, the absence of volatile organic compounds in the sediment headspace suggests (a)biotic attenuation may be significant. In Zone 1, crystalline Fe sulfide mineral nano-coatings are abundant in the presence of sulfate-reducing bacteria Desulfosporosinus. In Zone 2, the Fe(II/III) mineral magnetite is dominant, suggesting a biogenic pathway as the iron-reducing bacteria, Geobacter, is abundant. Fe mineral coatings in Zone 3 reveal significant variability between each subsample, indicating active Fe cycling with biotic processes based on the abundance of Desulfosporosinus bacteria in the clay lenses. Reactive iron mineral coatings in RTZs supports evidence of (a)biotic processes in natural attenuation.
To understand the contribution of these Fe reactive coatings to attenuation of chlorinated solvents, a bench study is designed for reductive 1,4-dichlorobenzene (1,4-DCB), tetrachloroethylene (PCE), and trichloroethylene (TCE) degradation. Control groups included pure pyrite and siderite minerals. For 1,4-DCB treatment, although dechlorination is not observed over the time period of the study in the control groups, reaction kinetics with RTZ sediments followed second order rate expressions. Chlorobenzene and benzene are detected as byproducts, suggesting hydrogenolysis reduction. The second-order rate constants for the Fe(II) mineral nano-coatings in 1,4-DCB degradation are 1.73x 10-3Lg-1h-1 for pyrite (FeS2), 1.24x 10-3Lg-1h-1 for mackinawite (FeS), 1.89x 10-4Lg-1h-1 for siderite (FeCO3), and 1.79x 10-4Lg-1h-1 for magnetite (Fe3O4). The high reactivity of these nano-Fe mineral coatings is due to their large surface areas. PCE and TCE reduction are observed in the control and sediment groups, also following second-order rate expressions. Rate constants are of the same order of magnitude for the mineral coating contributions ranging from (2.45 ± 0.41) x 10-3 to (4.00 ± 0.74) x 10-3h-1. Given the rate constants found, 90% degradation of these COCs occurs over 24 to 39 days demonstrating the importance of these abiotic processes. For these three chlorinated solvents, the trend for abiotic processes with Fe(II) mineral nano-coatings follows: Fe(II) sulfide minerals > magnetite > siderite. As a result, reactive Fe mineral nano-coatings are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments.
Yin, Xin, "Reactive iron mineral coatings in redox transition zones of a site with historical contamination: abiotic attenuation" (2022). Dissertations. 1710.