Uranium extraction experiment

U extraction experiments were carried out using a single-compartment three-electrode reactor, which has an effective volume of 0.06 L. A 3 cm × 3 cm TiO₂ nanotube array (TNA) was used as the anode (Figure S1), as TiO₂ is the most developed and most commonly used model photocatalyst (Yang et al., 2008), and its conduction band edge potential is suitable for U(VI) reduction (Li et al., 2017). A 3 cm × 3.5 cm Ti foil was used as the cathode, because its capability of serving as the U deposition site has been demonstrated in previous electrochemical uranium extraction studies (Liu et al., 2019). A saturated calomel electrode (SCE) was used as the reference electrode. An electrochemical workstation (CHI660E) was used to provide the designated bias on the TNA anode, and a UV-LED array with 60 mW/cm² irradiation was used as the light source. In the case of the PC method, the reactor was operated at open-cell conditions without circuit connection between the TNA anode and the Ti electrode under illumination. In the case of the EC method, the system was operated in dark at designated bias potentials. Synthetic U-containing water was made by dissolving UO₂(NO₃)₂·6H₂O into ultra-pure water to reach desired concentrations. Meanwhile, 20 mM NaCl was also added as the supporting electrolyte, unless otherwise stated. For the real U-mine tailings seepage water experiments, seepage water collected from a decommissioned granite-related U mine tailings deposit was used after filtration by 0.2 μm filter membrane to remove suspended solids and microorganisms which may interfere the measurements. Detailed characterizations of the seepage water can be seen in Table S2. Upon each cycle (12 h) of the real U-mine tailings seepage water experiment, fresh Ti cathode was used while the TNA was continuously used without any means of regeneration. All experiments were conducted in the open-air. The U concentration was measured by a well-established spectrophotometric method as described in the literature (Kim et al., 2015). The extracted mass of the U was calculated by comparing the difference between the remaining and the initial U concentration in the reaction solution.

For the uranium recovery from the Ti electrode, a two-chamber (40 mL: 40 mL) two-electrode electrochemical reactor was used (Figure S16). The desorption experiments were carried out under open-circuit conditions (physicochemical desorption), 1.0 V bias voltage, and 1.5 V bias voltage. For each desorption experiment, samples were taken from the anodic chamber to examine the uranium concentration in the electrolyte (0.1 M HCl).