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 utilization rate of sunlight, easy recombination of photo-generated electron-hole pairs and weak reactivity. Given the strong coordination ability of phosphorous group with U(VI), phosphorous graphitic carbon nitrides (PCNs) with improved porosity (24.5-108.5 m2/g, 3.4-5.8 times enhancement compared to bulk ones) 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 microscopies, 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 bands (1.68-1.79 eV) and improved absorption of visible light. Sorption behaviors of U(VI) on PCNs as functions of contact time, solution pH, ionic strength, selectivity and thermodynamics were evaluated. A quick equilibrium (4.0 h), high maximum capacity (220.3-368.1 mg/g) and excellent selectivity for extracting U(VI) from simulate nuclide wastewater and seawater were presented. Moreover, photocatalysis reduction of U(VI) was carried out. U(VI) could be photo-reduced with enhanced rate (0.034-0.067 min-1) and favorable photoreduction stability (80.3% remained rate after five cycles). This work provided a valuable reference to guide the design and modification of carbon nitrides with enhanced uptake and reduction activity for aquatic U(VI).