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

Abstract

Endocrine-disrupting chemical phthalate esters (PAEs) pose a serious threat to public health. Herein, a dual-defect g-C₃N₄ with strong unsaturated sites was successfully synthesized by thermal polymerization of freeze-dried tartaric acid-pretreated melamine precursor for photocatalytic peroxydisulfate (PS) activation, with the aim of solving the problem of difficulties in the treatment of wastewater with low concentration phthalate esters (PAEs). The nitrogen vacancies and –CN defects were 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 a DMP (0.1 mmol L⁻¹) degradation rate of 68.00%. 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 had a significant influence on DMP degradation with 42.11% and 76.72% removal efficiencies at pH 3.5 and 9.0, respectively. Moreover, the nitrogen vacancies and surface morphology changed little, maintaining a DMP removal rate of 65.00% in the fourth cycle. Furthermore, radical quenching experiments and EPR measurements revealed the dominant role of hydroxyl and sulfate radicals. The oxidative active species of hydroxyl (˙OH), superoxide (O₂˙⁻), sulfate (SO₄˙⁻) radicals and holes (h⁺) contributed to the oxidation, hydrolysis and hydrogen substitution of DMP. Ten intermediate degradation products were identified by GC-MS analysis and byproduct transformation pathways mainly included the decarboxylation reaction, hydroxylation reaction, carboxylation, and ring-opening reactions. 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 exhibited an optimal electronic structure, enhanced visible light response, promoted photogenerated electron hole separation, and achieved better photocatalytic activity towards peroxydisulfate (PS) for DMP removal. The potential environmental practicability was verified by wastewater simulation in a CN-3/PS/Vis system. The study demonstrated a promising candidate of defective g-C₃N₄ in the effective removal of trace phthalates from wastewater.