Synergistic effect of KCl mixing and melamine/urea mixture in the synthesis of g-C3N4 for photocatalytic removal of tetracycline

To prepare g-C3N4 for photocatalytic tetracycline (TC) removal, KCl was employed to mix with or cover the top of the precursors, namely, melamine and urea. The mixing of KCl with the precursor will result in the incorporation of KCl within the layer structure of g-C3N4, whereas KCl covering the top might not have such an effect. Different precursor ratios contributed to the formation of heptazine-rich or triazine-rich units in the g-C3N4 structure. Melamine applied alone as a precursor will undergo a phase transformation into melam and triazine-rich g-C3N4, whereas with the addition of urea, the mixture will polymerize to form melem and heptazine-rich g-C3N4. The KCl-incorporated, heptazine-rich g-C3N4 (KCN80m) exhibited an improved photocatalytic activity for TC removal (greater than 80% during a 120-min period for 50 mL of a 20-ppm TC solution). The enhanced activity can be attributed to the improved charge separation through an electron and hole transfer through the K+ and Cl- sites, respectively; the formation of a nanojunction between the triazine and heptazine units of g-C3N4; an increased number of photoexcited electrons, indicated by the electron paramagnetic resonance spectroscopy results. We varied the mixing conditions of KCl and precursor ratio to synthesize different KCl-incorporated heptazine-rich g-C3N4 samples for effective removal of TC from water through photocatalysis.