Reusable isotype heterojunction g-C₃N₄/alginate hydrogel spheres for photocatalytic wastewater treatment

Various visible-light-driven photocatalysts have been studied for practical applications in photocatalytic wastewater treatment via solar irradiation. Among them, g-C₃N₄ has attractive features, including its metal-free and environmentally friendly nature; however, it is prone to charge recombination and has low photocatalytic activity. To solve these problems, isotype heterojunction g-C₃N₄ was recently developed; however, the methods employed for synthesis suffered from limited reproducibility and efficiency. In this study, isotype heterojunction g-C₃N₄ was synthesized from various combinations of precursor materials using a planetary ball mill. The isotype heterojunction g-C₃N₄ synthesized from urea and thiourea showed the highest photocatalytic activity and completely decolorized Rhodamine B (RhB; 10 ppm) in 15 min under visible-light irradiation. Furthermore, to improve recyclability, isotype heterojunction g-C₃N₄ was immobilized in alginate hydrogel spheres. The isotype heterojunction g-C₃N₄/alginate hydrogel beads were used in 10 repeated RhB degradation experiments and were able to maintain their initial photocatalytic activity and mechanical strength. These achievements represent an advance towards practical, sustainable photocatalytic wastewater treatment.