Highly uranium elimination by crab shells-derived porous graphitic carbon nitride: Batch, EXAFS and theoretical calculations

The natural crab shell with the unique structure consisting of CaCO₃ was employed as the biotemplate encapsulating urea to form porous g-C₃N₄ for U(VI) elimination. A multiscale characterization of porous g-C₃N₄ provided evidences for successfully introducing porous structure with increased surface area as well as multi-sorption sites (e.g. C–N[dbnd]C, N–(C)₃ and C–N–H groups). The interaction of U(VI) with porous g-C₃N₄ was investigated by batch experiments, spectroscopic analyses and theoretical calculations. The sorption experiments indicated that g-C₃N₄-550 with the highest polymerization exhibited rapidly sequestering U(VI) within 120 min and superior uptake performance (149.70 mg/g) at pH = 5.0. The presence of U-N shell (R = 2.80 Å) calculated from EXAFS spectra revealed the U(VI) immobilization on g-C₃N₄-550 at pH = 5.0 was ascribed to the inner-sphere surface complexation. The DFT calculations further evidenced the strong interaction between uranyl and g-C₃N₄ (E_ad = 156.83 kcal/mol), and the most effective sorption site was inside the holes of g-C₃N₄. The superior uranium sorption ability along with low-cost and environmental friendly raw materials pointed to the high potential of porous g-C₃N₄ for uranium preconcentration. The findings will expand an exciting direction towards the effective removal of actinides in environmental cleanup.