Uranium(VI) spiked in natural seawater was photoelectrochemically reduced using porous TiO2 film electrodes at varying potential biases (E(bias)s) and pH values. Linear sweep voltammograms of TiO2 electrodes in seawater with U(VI) at pH 8.2 exhibited the characteristic reduction peaks at -0.95 V-SCE and anodic peaks at -0.75 V-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 = -1.0 V-SCE; AM 1.5 G) 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(bias)s and pH values, and was further confirmed by the Faradaic efficiency (epsilon), 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 similar to 95% of uranium in the PEC-treated experiment was adsorbed on TiO2 with 57% as U4.6+ and 14% as U4+, whereas similar to 98% of uranium in the EC-treated experiment remained in solution, containing 62% as U4.6+ and 12% as U4+. Under certain conditions (e.g., at a pH of 3.0 and/or with N-2-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. (C) 2014 Elsevier B.V. All rights reserved.