Document Type

Dissertation

Date of Award

12-31-2020

Degree Name

Doctor of Philosophy in Environmental Engineering - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Lisa Axe

Second Advisor

James A. Dyer

Third Advisor

Donna Elaine Fennell

Fourth Advisor

Wen Zhang

Fifth Advisor

Lucia Rodriguez-Freire

Sixth Advisor

Michel Boufadel

Abstract

Reactive iron mineral coatings in redox transition zones play an important role in contaminant attenuation. These mineral coatings include poorly crystalline to crystalline iron sulfides, carbonates, and oxyhydroxides, and are a signature of the biogeochemical processes occurring. To better understand these processes, reactive iron mineral coatings are characterized in an 18-m Anaerobic Core collected from a contaminated industrial site. This study targets redox transition zones uncovered in the core. A suite of complementary analyses is applied to distinguish the surface coating mineralogy using X-ray Diffraction, X-ray fluorescence, and field-emission scanning electron microscopy (FESEM) with energy dispersive X-ray analyzer (EDX). In the shallowest redox transition zones, framboidal pyrite and greigite are observed in the clay lenses, while iron (III) phases in the aquifer include goethite, ferrihydrite, lepidocrocite, and hematite. In the transition zone in aquitard, iron sulfides are found as flaky aggregates of mackinawite, pyrite, and pyrrhotite. In addition, the iron (II)/(III) mineral magnetite is also observed in this same area. Other related data such as groundwater chemistry and microbial genera are also collected. Possible cycling pathways for Fe and S mineral coatings are proposed and compared between transition zones. Using multiple lines of evidence, the shallowest two redox transition zones are expected to play a significant role in the degradation of site contaminants. Reactions in other redox transition zones may be slower where iron mineral coatings are not dominant.

The identified reactive iron mineral coatings in the Anaerobic Core are compared with a Cryo Core which has been collected with the cryogenic technique applying liquid nitrogen. After thawing the Cryo Core in an oxygen-free glovebox, the same suite of analyses is applied. Among the iron minerals identified, crystalline pyrite is found throughout the Cryo Core sediment samples, which contrasts with that observed for the Anaerobic Core. Moreover, mackinawite and greigite which are ubiquitous in the Anaerobic Core were not observed in Cryo Core samples. Meanwhile, a freeze/thaw process is simulated on Anaerobic Core samples using a liquid-nitrogen quench with surface coatings characterized by FESEM/EDX. In these quenched samples, mackinawite is no longer observed, and in its place was pyrite. In addition, both greigite and pyrite are found to be unique morphologically after quenching. Dissolution and re-precipitation of iron sulfide coatings during the freeze/thaw process appears to affect the geochemistry of the pore water through two main mechanisms of freeze-concentration and freezing potential.

Overall, reactive mineral coatings characterized with multiple chains of evidence are important contributor to the natural attenuation processes of contaminants of concern in redox transition zones.

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