Phosphorus doping to enhance the peroxymonosulfate activation efficiency of carbon nitride for degrading tetracycline

Herein, a new type of peroxymonosulfate (PMS) activator—phosphorus-doped carbon nitride (P-C3N4)—was prepared using a one-step thermal shrinkage method. Material characterisation confirmed the successful doping of phosphorus in graphite-phase carbon nitride (g-C3N4) by replacing the carbon atoms in its triazine ring structure to form PN bonds. Phosphorus doping can regulate and re-organise the surface electronic structure of g-C3N4 without changing its structure. Moreover, phosphorus doping promotes varying degrees of optimisation of the g-C3N4 pore structure and electronic transport between the g-C3N4 sites. Batch experiments on tetracycline (TC) degradation revealed that 0.2 P-C3N4 showed the best physicochemical properties, the constructed P-C3N4/PMS system exhibited high TC removal efficiency and P-C3N4 exhibited good reusability and structural stability. Quenching experiments and electron paramagnetic resonance tests revealed that singlet oxygen was the primary active species in the P-C3N4/PMS system. The mechanism of PMS activation using P-C3N4 was then proposed. The analysis of TC and its intermediate products using liquid chromatography–mass spectroscopy demonstrated that TC was primarily degraded via ring-opening, hydroxylation, deamination and dealkylation reactions. Analysis using the toxicity estimation software tool demonstrated that most intermediate products were less toxic than TC. Thus, this study provides a feasible method for removing TC from aquatic environments.