Tunable mesoporous g-C3N4 nanosheets as a metal-free catalyst for enhanced visible-light-driven photocatalytic reduction of U(VI)

Although photocatalytic reduction has been proven to be a green, efficient, and economical strategy for uranium(VI) extraction from water, it faces several challenges, such as low quantum conversion efficiency and utilization of sunlight. In this work, the photocatalytic efficiency of g-C3N4 was tuned by introducing proportionally-adjustable mesoporous. The prepared mesoporous g-C3N4 samples (MCNr, r represents the initial silica/cyanamide mass ratios during synthesis) exhibited higher surface area and larger pore volume. Compared with pristine g-C3N4 (BCN), both light utilization and photo-generated carrier separation efficiency were significantly improved for MCNr. MCNr showed an enhanced photocatalytic performance for the reduction of U(VI) under visible light. The optimum photocatalytic performance was achieved for MCN1.0, which was 6.75 times higher than that of BCN. Both transmission electron microscopy (TEM) and X-ray diffraction (XRD) have confirmed the formation of UO2 on MCN1.0 surface after light irradiation. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) further revealed the re-oxidation of UO2 during the photocatalytic process. A high uranium extraction capacity (~2990 mg/g) could be achieved by MCN1.0, and the deposited UO2 could be easily eluted by 0.01 mol/L Na2CO3 solution after exposure to air, showing high reuse performance. In the presence of co-existing ions, the photocatalytic reduction of U(VI) remained a high selectivity.