Electrochemical remediation of uranium(VI) from aqueous media using the boron-doped diamond electrodes: mechanisms, products and applications

Abstract

The advancement of nuclear science and technology has ushered in a mounting predicament in the form of radioactive waste, accompanied by its consequential contamination with nuclear activity, thereby raising significant apprehensions about its impact on human and environmental health. Among all radionuclides employed in various applications, uranium (U) is the principal contributor to nuclear contamination. This study proposes the use of different electrochemical processes to facilitate the reduction of uranium (VI) [U (VI)] to uranium (IV) [U (VI)]to remove this radionuclide from aqueous media.<br /> <br /> First, we developed the electrochemical bioremediation of uranium (VI) using the bacterium Geobacter sulfurreducens immobilized on a boron-doped diamond electrode (BDD). Using the G. sulfurreducens/BDD coupled system and applying a potential of -0.600 V vs. Ag/AgCl (3M NaCl) in a solution of 2.0 mM of uranyl acetate in the bacterium growth media, the U (VI) ion was removed from the solution. Techniques such as cyclic voltammetry, scanning electron microscopy (SEM), energy dispersive X-ray fluorescence spectroscopy (EDS), and Raman spectroscopy corroborated the removal of U (VI) from solution and the presence of U (IV) on the electrode surface.<br /> <br /> On the other hand, other electrochemical remediation work of the uranyl ion was carried out using a BDD electrode modified with nano zero-valent iron particles (nZVIs) on its surface. This study used the nZVIs/BDD assembly system to remove the U (VI) present in a solution of 2.0 mM of uranyl acetate in 0.10 M of KClO₄. The electrodeposition process applied a reduction potential of -0.800 V vs. reversible hydrogen electrode (RHE) through the chronoamperometry technique. Characterization studies such as cyclic voltammetry, SEM/EDS, and Raman spectroscopy corroborated the U (VI) electrodeposition or remediation process and the presence of a blend of uranium oxides on the BDD electrode surface, including UO₂, UO₃, and U₃O₈.<br /> <br /> Finally, a uranium electrodeposition tendency was studied on the BDD electrode surface. Reduction potentials between -0.600 V to -2.00 V vs. RHE were applied to make the electrodeposition of uranium. Techniques such as chronoamperometry, SEM/EDS, atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy were applied to study the tendency for electrodeposition.<br /> <br /> This work found that a blend of uranium oxide species, including UO₂, UO₃, and U₃O₈, were electrodeposited after the electrodeposition experiment at each applied potential with a significant presence of U₃O₈ on each sample studied. Also, it was found that -1.75 V vs. RHE was the optimal potential to obtain a more homogeneous layer on the BDD surface without crystal growth. Those studies contribute to the search for cost-effective and eco-friendly methods to resolve radionuclide contamination from different aqueous phases and its possible use in future emerging nuclear technologies.