Dual defect-driven g-C3N4 photocatalytic peroxydisulfate activation for enhanced dimethyl phthalate removal: mechanism and toxicity assessment insights

Endocrine-disrupting chemical phthalate Esters (PAEs) pose a serious threat to public health. Herein, a dual-defect g-C3N4 with stronger unsaturated sites was successfully synthesized by thermal polymerization of freeze-dried Tartaric acid-pretreated melamine precursor for the photocatalytic peroxydisulfate (PS) activation, aiming at solving the problem on the treatment difficulty of wastewater with low concentration phthalate Esters (PAEs). The nitrogen vacancy and -C≡N defects was determined by a combination of spectroscopic techniques, XPS and electron spin resonance tests. Based on photoluminescence and electrochemical tests, electron traps generated at defect sites resulted in enhanced electron-hole separation. The CN-3/Vis system exhibited excellent PS catalytic activity with the 68.00% DMP (0.1 mmol L−1) degradation. The first-order kinetic constant (k) of DMP degradation in the CN-3/PS/VIS system reached 0.00576 min−1 with high PS utilization efficiency. The solution pH showed significant influence on DMP degradation with 42.11% and 76.72% removal efficiency at pH 3.5 and 9.0, respectively. Moreover, the nitrogen vacancy and surface morphology changed little and maintained the removal rate of 65.00% of the DMP in the fourth cycle. Furthermore, Radical quenching experiments and EPR measurements revealed the dominated role of hydroxyl and sulfate radicals. The oxidative active species of hydroxyl(•OH), superoxide(O2•-), sulfate (SO4•−) radical and hole(h+) contributed to the oxidation, hydrolysis and hydrogen substitution of DMP. The 10 intermediate degradation products were speculated by GC-MS analysis and byproducts transformation pathways mainly included decarboxylation reaction, hydroxylation reaction, carboxylation, and ring-opening reactions. The TEST and ECOSAR software revealed the production of relatively less toxic compounds in the CN-3/PS/VIS system. The mechanism analysis revealed that dual-defect CN-3 showed optimal electronic structure, enhanced visible light response and promoted photogenerated electron hole separation, achieved better photocatalytic activity towards peroxydisulfate (PS) for DMP removal. The potential environmental practicability was verified by wastewater simulation in CN-3/PS/Vis system. The study demonstrated a promising candidate of defective g-C3N4 in the effective removal of trace phthalates from wastewater.