Kinetics and mechanisms of Fe2+-catalyzed recrystallization of iron oxides

Restricted (Penn State Only)
Author:
Joshi, Prachi
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 30, 2018
Committee Members:
  • Christopher Aaron Gorski, Dissertation Advisor
  • Christopher Aaron Gorski, Committee Chair
  • Matthew S Fantle, Committee Member
  • William D Burgos, Committee Member
  • Peter J Heaney, Outside Member
Keywords:
  • Iron oxides
  • Iron chemistry
  • Geochemistry
  • Mineral processes
  • Nanoparticles
Abstract:
Iron oxides and oxyhydroxides are coupled to the biogeochemical cycles of macronutrients such as carbon, trace metals such as Ni, and radionuclides such as U. Recent work has found that iron oxides such as goethite and magnetite, previously thought to be stable, may recrystallize in reducing redox conditions without undergoing overt changes in morphology, structure, and reactivity as measured by chemical dissolution rates. This process, referred to as Fe2+-catalyzed recrystallization, can result in the release/uptake of ions (e.g, Cu or U) into/from the surrounding water, thus affecting water quality. Recrystallization may also affect the use of iron oxides as paleoenvironmental proxies due to the alteration of the oxides’ isotopic and elemental composition post formation. The goal of this dissertation was to elucidate the kinetics and mechanisms of recrystallization in order to account for this process in the fields of water quality and paleoenvironmental reconstructions. In Chapter 2 of this dissertation, I examined the morphological changes in goethite nanoparticles over the course of recrystallization. Previous studies observed no substantial changes between goethite before and after recrystallization. Here, I investigated if changes were occurring in the morphology and aggregation of goethite nanoparticles at intermediate time points during the goethite-aqueous Fe2+ reaction using transmission electron microscopy (TEM) and cryogenic-TEM. The goethite particle morphology changed anisotropically over 30 days, suggesting that multiple transformation mechanisms may be occurring. In chapters 3 and 4, I investigated the kinetics of recrystallization. Based on a multiple tracer addition experiment, goethite was observed to become less susceptible to recrystallization over time. Box models were constructed to infer the kinetics of recrystal- lization; the results of the fitting suggested that <20% of goethite recrystallized over 60 days, which was a much lower estimate that previous studies. These two studies together suggested that Fe2+-catalyzed recrystallization may be important only over short time pe- riods. Using box models, a better constrained approach to quantification of the extent and rate was developed. Chapters 5 and 6 focused on the mechanisms of recrystallization. In Chapter 5, I proposed that recrystallization was driven by mineral crystallinity. To test this hypothesis, I reinterpreted the isotopic data reported by previous studies that varied mineral structure on the basis of aging, metal substitution, and the isotopic data reported for hematite and magnetite. The results of fitting indicated that the kinetics of recrystallization were inversely related to the crystallinity of the unrecrystallized mineral. In the final chapter of this dis- sertation, halite was developed as a model system to further investigate the mechanisms and controls on stable mineral recrystallization using a combined isotopic and microscopic approach.