Phosphorous-modified porous graphitic carbon nitride activated by phytic acid for efficient, selective extraction and photoreduction of uranium from aqueous solutions

Abstract

Graphitic carbon nitrides (CNs) have attracted tremendous attention because of their response to visible light, facile synthesis, low precursor cost and good stability. However, bulk CNs suffer from low sunlight utilization, easy recombination of photo-generated electron–hole pairs and weak reactivity. Given the strong coordination ability of the phosphorous group with U(VI), phosphorous-modified graphitic carbon nitrides (PCNs) with improved porosity (24.5–108.5 m² g⁻¹, 3.4–5.8 times enhancement compared to bulk CNs) and sufficient P-modified groups (10.3–7.7 wt% of P) were fabricated by co-thermolysis of phytic acid and melamine. Multiple techniques, including scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis, were applied to analyse the microstructure, composition and elemental chemical states of PCNs. PCNs exhibited narrowed band gaps (2.02–2.37 eV), decreased valence band energies (1.68–1.79 eV) and improved absorption of visible light. The sorption behavior of U(VI) on PCNs as functions of contact time, solution pH, ionic strength, selectivity and thermodynamics was evaluated. A quick equilibrium (4.0 h), high maximum capacity (220.3–368.1 mg g⁻¹) and excellent selectivity for extracting U(VI) from simulated nuclide wastewater and seawater were presented. Moreover, photocatalytic reduction of U(VI) was carried out. U(VI) could be photo-reduced at an enhanced rate (0.034–0.067 min⁻¹) with favorable photoreduction stability (80.3% of the initial rate remaining after five cycles). This work provides a valuable reference for guiding the design and modification of carbon nitrides with enhanced uptake and reduction activity for aquatic U(VI).