Homogeneous photoconversion of seawater uranium using copper and iron mixed-oxide semiconductor electrodes

- Sunlight-driven conversion of U(VI) in seawater is achieved with mixed CuO and CuFeO2.
- U(VI) is photoelectrochemically reduced with a Faradaic efficiency of ∼98%.
- Virtually all conversion reactions of U(VI) occur in the bulk solution.
- Induction times are present in uranium conversion.
- O2 acts as an electron shuttle between electrodes and U(VI).

Sunlight-driven conversion of hexavalent uranium (U(VI)) in seawater is achieved with mixed p-type CuO and CuFeO2 (CuO/CuFeO2) photocatalyst film electrodes synthesized via electrodeposition (ED) of Cu(II) and Fe(III), followed by annealing in air. The mixed photocatalysts exhibit a double-layer configuration with crystalline structures of CuO and CuFeO2. On irradiation of the CuO/CuFeO2 electrodes (held at −0.5 V vs. SCE) with solar simulated light (air mass 1.5; 100 mW cm−2), the U(VI) concentration decreases with time, while the total amount of uranium in solution does not change. This indicates that virtually all conversion reactions of U(VI) occur in the bulk solution, while surface reactions are limited due to insignificant adsorption of U(VI). U(VI) conversion leads to the mixed production of lower oxidation states U4+, U14/3+, and U16/3+ at a ratio of 42:28:30, with an overall Faradaic efficiency of ∼98%. The kinetics and induction time for U(VI) conversion are significantly influenced by the conditions of photocatalyst synthesis (CuO/CuFeO2, CuO, and CuFeO2; ED times of 2-4 h), the applied potential value (−0.4, −0.5, and −0.6 V vs. SCE), and the seawater condition (air-equilibrated vs. N2-purged; pH 3-10.4). Based on the obtained results, O2 is proposed to play a key role in shuttling photogenerated electrons between the electrodes and U(VI). In addition, the existence of an induction time is discussed in terms of material and reaction pathway.

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