Solar conversion of seawater uranium (VI) using TiO₂ electrodes

Uranium(VI) spiked in natural seawater was photoelectrochemically reduced using porous TiO₂ film electrodes at varying potential biases (E_biass) and pH values. Linear sweep voltammograms of TiO₂ electrodes in seawater with U(VI) at pH 8.2 exhibited the characteristic reduction peaks at -0.95V_SCE and anodic peaks at -0.75V_SCE. These peaks shifted anodically and became less pronounced with decreasing pH values from 8.2 to 3. At the natural seawater pH of 8.2, photoelectrochemical (PEC) treatment (E_bias=-1.0V_SCE; AM 1.5G) was found to be considerably more effective in reducing U(VI) than photocatalytic (PC) treatment (without E_bias) and electrochemical (EC) treatment (without irradiation), and even the combination of the two treatment processes. The synergistic effect of the PEC U(VI) reduction varied depending on the E_biass and pH values, and was further confirmed by the Faradaic efficiency (ε), which was close to 100%. Inductively coupled plasma spectroscopy (ICP) and detailed surface analyses of the TiO₂ using various techniques (TEM/EDX, SEM/EDX, and XPS) indicated that ~95% of uranium in the PEC-treated experiment was adsorbed on TiO₂ with 57% as U^4.6+ and 14% as U⁴⁺, whereas ~98% of uranium in the EC-treated experiment remained in solution, containing 62% as U^4.6+ and 12% as U⁴⁺_. Under certain conditions (e.g., at a pH of 3.0 and/or with N₂-purging), no reduction of U(VI) was observed irrespective of the E_bias and irradiation. To further investigate the observed lack of U(VI) reduction, surface chemistry and energetics between TiO₂ and U(VI) were studied as a function of pH. Finally, a role of superoxide radicals as an electron shuttle between TiO₂ and U(VI) was established as a primary U(VI) reduction mechanism.