ZnFe2O4/g-C3N4 S-scheme photocatalyst with enhanced adsorption and photocatalytic activity for uranium(VI) removal

The reduction of soluble hexavalent uranium (U(VI)) to insoluble tetravalent uranium (U(IV)) by photocatalytic method is an emerging and effective approach to remove U(VI) from aqueous solution. In this study, the ZnFe2O4/g-C3N4 (ZFOCN) step-scheme (S-scheme) heterojunction was synthesized and used as both adsorbent and photocatalyst for remove U(VI). The batch adsorption experiments showed that the adsorption process of U(VI) by ZFOCN matches the Langmuir isotherm and pseudo-second-order models, and its maximum Langmuir adsorption capacity (qmax) for U(VI) reached 699.3 mg/g at pH 5.0. In addition, ZFOCN exhibited superior photocatalysis properties for reduction of U(VI) under visible LED light irradiation, and its optimal removal capacity for U(VI) via simultaneous adsorption and photoreduction achieved up to 1892.4 mg/g, with the removal rate being 94.62%, which is significantly higher than the physicochemical adsorption. Furthermore, the photoreduction mechanism of U(VI) by ZFOCN is explored by FT-IR, XPS, XRD, EPR and DFT calculations, which indicated that the excellent photocatalysis performance of ZFOCN was mainly due to its strong visible light absorbability and narrow band gap. The U(VI) could be captured on the surface of ZFOCN, and then reduced to U(IV) under visible LED light illumination. Moreover, the removal capacity of U(VI) by ZFOCN remained over 90% after five cycles of tests, and it has negligible decrease in the presence of co-existing metal ions.