Facile synthesis of pure g-C₃N₄ materials for peroxymonosulfate activation to degrade bisphenol A: Effects of precursors and annealing ambience on catalytic oxidation

Previous studies have reported that metal-doped graphitic carbon nitride (g-C₃N₄) materials can activate peroxymonosulfate (PMS) to destruct organic pollutants. Herein, we interestingly found that pure g-C₃N₄ materials synthesized via a facile calcination protocol without any metal doping could serve as the efficient PMS activator for degrading bisphenol A (BPA). The influences of different precursors (i.e., melamine, dicyandiamide, and urea) and annealing ambience (i.e., air or N₂) on the catalytic activity were investigated. The resultant g-C₃N₄ materials exhibited different catalytic activities for PMS activation. The best performance was found in the case of g-C₃N₄ synthesized from melamine and annealed in N₂ (g-C₃N₄-Melamine-N₂); 100% BPA (10 μM) removal was achieved by using 0.5 g/L catalyst and 1 mM PMS at pH 7 in 15 min. The difference in catalytic activities of these g-C₃N₄ materials might be attributed to the fact that different preparation procedures impacted the chemical composition and structure of catalysts and thus changed the type and density of active sites. Important influencing factors such as PMS dosage, temperature, solution pH, and water matrices on degradation kinetics of BPA were evaluated. Results of quenching experiments and electron paramagnetic resonance spectroscopy revealed that sulfate radical and hydroxyl radical were main reactive species for BPA degradation by the g-C₃N₄/PMS system, which was also confirmed by the non-selectivity of this system towards various organics. Furthermore, possible transformation pathways of BPA by the g-C₃N₄/PMS system were proposed based on the identified oxidation products, primarily involving hydroxylation, bond cleavage, ring opening, as well as radical coupling. This study deepens mechanistic understanding of g-C₃N₄ materials driven PMS oxidation processes, and also provides a green and efficient strategy for eliminating BPA in water.