Photocatalytic degradation of sulfamethazine by graphitic carbon nitride-modified zinc molybdate: Effects of synthesis method on performance, degradation kinetics, and mechanism

Abstract

In the present study, zinc molybdate (β-ZnMoO4) and graphitic carbon nitride (g-C3N4)-modified β-ZnMoO4 (β-ZnMoO4/g-C3N4) were prepared to decontaminate aqueous solutions from the antibiotic sulfamethazine (SMZ). Our results revealed that the hydrothermal synthesis method greatly influenced the photocatalytic activity of the resultant catalysts. The pristine β-ZnMoO4 samples obtained under more intensive synthesis conditions (24 h at 280 °C) showed higher photocatalytic activity than that prepared for 12 h at 180 °C (denoted β-ZnMoO4-180). In the case of in situ hydrothermal synthesis of β-ZnMoO4/g-C3N4, a surface-modified sample was only obtained under the reaction conditions of 180 °C for 12 h. Compared with the sheet-like β-ZnMoO4-180 sample, the β-ZnMoO4-180/g-C3N4 composite showed enhanced photocatalytic activity for the degradation of SMZ. By contrast, the hydrothermal reaction at 280 °C caused the gradual decomposition of g-C3N4. It is believed that the structural incorporation of g-C3N4 into β-ZnMoO4 at 280 °C might disrupt the crystal growth, thereby deteriorating the performance of the composite catalysts formed at this temperature. For the composite catalysts prepared by the ultrasonic method, a remarkable increase in the degradation rate of SMZ was only observed at a high g-C3N4 content of 8 mol%. The photocatalytic degradation of SMZ by β-ZnMoO4-180/g-C3N4 composite catalysts followed pseudo-first-order kinetics. Further study of the photocatalytic mechanism revealed that holes and superoxide radicals were the dominant oxidative species in the photodegradation process. The enhanced photocatalytic performance of the composites was attributed to the higher separation efficiency of the photogenerated electron-hole pairs at heterogeneous junctions. The degradation intermediates of SMZ were detected by liquid chromatography-mass spectrometry, from which plausible reaction pathways for the photodegradation of SMZ were proposed. Our results indicated that the synthesis method for g-C3N4 composites should be carefully selected to achieve superior photocatalytic performance.