Experimental and theoretical identifications of durable Fe–Nx configurations embedded in graphitic carbon nitride for uranium photoreduction

Uranium extraction from seawater has attracted increasing attentions for nuclear raw material to support the sustainable development of nuclear power plants. However, it is still a grand challenge to overcome the high salinity background and ultra-low uranyl concentration. Herein, we successfully achieved Fe-Nx configurations embedded in graphitic carbon nitride (FeNx/g-C3N4) through one-step pyrolysis strategy. Our density function theory (DFT) calculations reveal that the site-isolated FeNx centers are the predominant binding sites for U(VI) species, and visible-light photoexcitation can effectively facilitate the electron escaping from FeNx/g-C3N4 surface relative to pure g-C3N4, which is consequently favorable for electron transfer in the photoreduction conversion of U(VI) to U(IV). Consistent with the theoretical results, the obtained FeNx/g-C3N4 (w(Fe-MOFs): w(g-C3N4) = 2:1) delivers remarkably higher reduction activity with the conversion efficiency of 99 % and rate constant of 0.091 min−1, almost 9.1 and 12.5 times faster than those of g-C3N4 and TiO2 catalysts, respectively. The results showed that the introduction of FeNx could effectively activate g-C3N4 catalysts for significantly broadening the absorption range of g-C3N4 to visible light, inhibiting recombination of the photogenerated e--h+ and further facilitating the reduction of U(VI). It is worthwhile to mention that FeNx/g-C3N4 maintains a high level for the extraction of uranium from seawater, endowing the metal-N configurations embedded g-C3N4 as a kind of promising candidate for the sustainable conversion of radionuclides.