Charge reconstruction from simultaneous Fe coordination and P/O co-doping in g-C₃N₄ for efficient photo-reductive recovery of uranium(VI)

The risk of radionuclide leakage always threats ecological security and human health, making the recovery of uranium from nuclear wastewater a meaningful and urgent issue. However, the low-cost and efficient capture of uranium remains a challenge at present. In this study, a homemade photocatalyst of iron coordinated graphitic carbon nitride (g-C₃N₄) co-doped with phosphorus and oxygen (OPFCN), which was facilely synthesized through a novel one-step thermal polymerization method, exhibited superior photoelectric efficiency and photocatalytic activity towards the photo-reductive capture of uranium(VI). The simultaneous Fe coordination and P/O co-doping in framework of g-C₃N₄ played a synergistic effect in charge reconstruction, in which electrons migration driven by coordinated Fe and lone electron pairs delocalization from doped P/O enormously increased electron density and enhanced charge transfer, thus effectively promoting the separation of photogenerated charge carriers. The specific surface area, pore volume and average pore size of OPFCN reached 70.2 m²·g⁻¹, 0.157 cm³·g⁻¹ and 7.78 nm, respectively, exhibiting a trend increasingly beneficial for photocatalytic adsorption and mass transfer with successive introduction of Fe, P and O into original g-C₃N₄. With the simultaneous introduction of Fe, P and O, the optical absorption edge underwent a significant redshift and fluorescence emission intensity dropped by 75 % compared to that of CN, demonstrating the enhanced visible light capturing ability and significantly inhibited recombination of photogenerated charge carriers in OPFCN. As a result, a uranium capture rate of over 98 % was achieved with catalyst dosage of only 5:1 at pH 5 with OPFCN, demonstrating its competitive photo-reductive activity towards uranium(VI) uptake compared with previously reported g-C₃N₄-based photocatalysts. Meanwhile, OPFCN exhibited remarkable reusability and stability with even over 65 % uranium recovery rate and almost unchanged peak patterns in XRD and FT-IR in fifth reuse recycle. Additionally, it was demonstrated that e⁻ and O₂^•− were the direct active species to realize the photoreduction of uranium(VI) over OPFCN, in which e⁻ played the dominant role. The modification method of simultaneous Fe coordination and P/O co-doping would be expected to effectively promote the utilization of g-C₃N₄-based photocatalysts towards photo-reductive recovery of uranium(VI) from radioactive wastewater.