Uranium (Ⅵ) Reduction By Nanoscale Zero-valent Iron Under Anoxic Conditions: Kinetics And Mechanism

The extraction and processing of uranium for use in the nuclear weapons and in commercial nuclear energy production have led to extensive uranium contamination in the environment. This makes it imperative to understand the processes that affect the environmental mobility of uranium. The hexavalent oxidation state of uranium is mobile, and thus mostly susceptible to environmental transport and biological uptake. On the other hand, the reduced state uranium as U(IV) is sparely soluble, and hence its mobility, is limited. Since the mobility of uranium is largely determined by its valence, reduction-based technologies have been proposed to remediate the uranium contamination in the subsurface. Recently, nanoscale zerovalent iron (nano Fe0) particles have been proposed as a new generation of materials for environmental remediation. The material has been widely studied for the cleanup of organic contaminants such as TCE and PCBs. However, the potential application of nano Fe0 for uranium immobilization in the aquatic environment has not been well studied. The research described in this dissertation seeks to examine the kinetics and mechanism of the reactions between U(â…¥) and nano Fe0 in the subsurface setting.The aqueous environmental geochemistry of uranium and the application of nano Fe0 for site remediation are briefly reviewed in Chapter 2.The research presented in Chapters 3 through 5 monitors the reaction kinetics between U(â…¥) and nano Fe0, investigates the roles of Fe(â…¡) and Fe(â…¢) in the U(â…¥) reduction reactions, examines the temperature effect, identifies the reaction products, evaluates the extent of U(â…£) reoxidation, proposes the reaction scheme, and compares the efficiencies of three types of nano Fe0 on U(â…¥) removal and reduction. The research presented in this thesis collectively used several approaches including batch reactions, kinetics modeling and spectroscopic technique to investigate the interaction between nano Fe0 and uranium contaminant under anoxic conditions in order to identify the controlling factors and reaction pathways in the contaminated subsurface. The results of this research reveal that bicarbonate and Ca have notable impacts on uranium removal and reduction through formation of binary uranyl-carbonato complexes and ternary uranyl-calcium-carbonato complexes, which stabilized U(VI) in aqueous phase and decreased both the rates of U(VI) removal and reduction. The observed variability of reaction rate constants in the presence of Fe(II)/Fe(III) complexing agents also indicate the roles of Fe(II) and Fe(III) in U(VI) reduction. Fe0, Fe(â…¡) and Fe(â…¢) can mediate electron cycling and thus catalyze the redox process. The XPS analysis confirms U(â…¥) is reduced to U(â…£) by nano Fe0 under anoxic conditions, while the reduced U(â…£) can be reoxidized by air. In addition, the temperature and iron types also affect the rates of U(â…¥) removal and reduction by nano Fe0 under anoxic conditions. This research clearly demonstrates that the reductive immobilization of U(â…¥) by nano Fe0 in the groundwater is closely linked to geochemical conditions controlling uranium speciation as well as the anaerobic/aerobic conditions and iron types.In summary, the research presented in this dissertation evaluated and quantified the impacts of major controlling factors such as solution chemical components, temperature and iron types on uranium immobilization by nano Fe0. The study advanced our knowledge to use nano Fe0 for the remediation of uranium contamination in the complex subsurface environment.