To decipher the mechanisms governing the PEC U extraction processes, the electrochemical interactions of aqueous U and the Ti electrode were studied using cyclic voltammetry (CV), and the results are shown in Figure and b. In the presence of U(VI), a reduction peak at around -0.85 V emerged. Meanwhile, an oxidation peak emerged at -0.36 V, in the presence of U(VI). Considering that no redox peaks were detected in the absence of U(VI), all these observed redox peaks can be assigned to the redox changes of U species on the Ti electrode. Further, when increasing scanning rate, the reduction peaks shifted to lower potentials, whereas the oxidation peaks moved to higher potentials (Figure ). These reflected the kinetics of the redox reactions . Linear relations between the reduction/reduction peak current values and the square root of scanning rates were identified (Figure ), suggesting that both the reductive conversion and the oxidative conversion were diffusion-controlled reactions that could be described by the Randles-Sevcik model . Fit with the Randles-Sevcik model, the CV data revealed a one-electron U(VI) reduction process on the Ti electrode (see Supplementary Note 8 for details). Therefore, the reduction peaks at around -0.85 V vs. SCE can be assigned to the one-electron reduction of U(VI) to pentavalent U (U(V)) , and the oxidation peaks at -0.36 V vs. SCE represents the oxidation of U(V) to U(VI) . The small variation of the positions of these peaks compared with those in the literature could be attributed to the differences in over potentials of the different applied electrodes and the scan rates.