Electrochemical Deposition of U(IV): Immobilizing Aquatic Actinides

Remediation of nuclear contamination sites, such as Hanford in southeastern Washington, where an estimated 1 million tons of high-level nuclear waste has leaked from storage tanks,¹ requires a fundamental understanding of radionuclide transport in the environment. Using electrochemistry, the conditions that affect the reduction of uranium in aqueous environments can be determined, as well as how these conditions affect the speciation of the deposited product. The challenge of long-term storage of nuclear waste and the poorly understood fate and transport mechanisms of actinides, such as uranium, in the environment pose a major barrier to the consideration of nuclear energy as a viable alternative to fossil fuels. Previous work in characterizing waste forms at contamination sites have involved sampling stored waste and reporting what is present;² working in a laboratory setting, experimental conditions can be systematically controlled to understand what causes certain species to be present. Other studies have also looked at contamination sites, but merely characterize the waste and report what is present in complicated environmental matrices. Actinide reduction mechanisms, potentials and how they are systematically affected by different conditions cannot predictably be explored in the environment. This project seeks to develop scientific precedent for efficient environmental remediation using laboratory-based electrochemical techniques.<br/>It is well understood that U(VI) and U(IV) oxidation states are the most common forms of uranium on Earth.³ U(VI) is water soluble and highly mobile in the environment, while U(IV) is immobile, insoluble, and will precipitate out of aquatic systems.⁴ Developing scalable, low-cost electrochemical methods will allow for more widespread remediation of aquatic actinide contamination that can easily be applied to many sites of interest. Two electrochemical techniques are employed to identify reduction potentials and deposit films: cyclic voltammetry (CV) and chronoamperometry (CA). CV is used for identifying oxidation/reduction potentials of compounds, and in this case, it will be used to identify the U(VI) to U(IV) reduction potential. After the reduction potential is identified, CA is used to deposit a thin film of U(IV) on the working electrode. CA fixes the applied voltage at the desired potential and measures current over time. Changing the time course in CA allows for deposition of a U(IV) thin film on the working electrode as U(VI) is reduced. Electrochemistry allows for many environmental parameters to be tested in a time- and cost-efficient manner. Solution pH, supporting electrolyte, atmospheric conditions, and deposition time are adjusted to produce ideal reduction conditions for U(VI). Changing parameters in electrochemical experiments also produce changes in morphology of deposited films. This research helps to provide an understanding of the speciation of uranium in aqueous environments, as well as what environmental factors provide the best conditions for reducing U(VI) to U(IV).<br/><b>References</b><br/>[1] Depart. of Ecol, WA. “Ecology Tracking Hanford Waste Tank Leak.” (2021). [2] Tingey, et al. <i>PNNL-14832 Rev.1</i>. (2004). [3] O’Laughlin, et al. <i>ACS Symposium Series</i>, 1071, 477-517. (2011). [4] Wall and Krumholz. <i>Annu. Rev. Microbiol</i>. 60:149-66. (2006).