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Probing Adsorption of Uranium Species at Electrochemical Interfaces in Support of Environmental Radiochemistry

Special nuclear material, like enriched uranium and plutonium, can contaminate environmental soils and water because of weapons development and testing or improper waste storage. To remediate affected lands and design long-term waste storage solutions, it is crucial to understand uranium speciation in the environment which depends on its complexation, oxidation state, and associated adsorption mechanisms. These phenomena are complex and poorly understood because they depend on many variables including the environmental pH, water or soil composition, uranium concentration, and surface chemistry of the minerals or soils. In previous studies, the proposed mechanism for adsorption of the hexavalent uranyl species, UVIO2
 2+, on metal-oxide surfaces follows these general steps: 1) adsorption of UVIO2
 2+ on the mineral, followed by 2) reduction to UVIO2
 2+ due to the coupled reaction of a redox active species within the mineral. To gain better mechanistic insight into coupled electron transfer and adsorption phenomena associated with uranyl species, we demonstrate here a novel technique to probe an electrochemical interface to detect adsorbed uranium species under reducing conditions. In this technique, we monitor the transient response of the electrode potential immediately after electrochemical reduction of UVIO2
 2+ on a rotating disc electrode is halted. This potential relaxation rate, when analyzed, contains information on presence and coverage of surface adsorbed species. In combination with electrochemical quartz crystal microgravimetry (e-QCM), these experiments confirm the presence of an adsorbed layer of uranium ions during the reduction of UVIO2
 2+. To simulate uranyl adsorption on minerals under environmental conditions, future work will aim to probe adsorption onto a metal-oxide film prepared on an e-QCM electrode under varying redox and pH conditions.