Photocatalytic degradation of metronidazole and oxytetracycline by novel l-Arginine (C, N codoped)-TiO2/g-C3N4: RSM optimization, photodegradation mechanism, biodegradability evaluation.

ABSTRACT

Removal of Metronidazole (MNZ) and Oxytetracycline (OTC) from wastewater by the prepared (C, N codoped)-TiO2/g-C3N4 (Graphitic carbon nitride) was examined. l-Arginine (C, N codoped)-TiO2 and l-Arginine (C, N codoped)-TiO2/g-C3N4 photocatalysts were successfully synthesized through the sol-gel method, and optimal ratio of l-arginineTiO2, as well as l-arginine/TiO2g-C3N4, was determined by a kinetic study of photodegradation process. The maximum photocatalytic removal rate (0.062 min-1 for MNZ removal) was observed using 1% l-Arginine-TiO2/g-C3N4 (11) under visible light illumination, 2.2 and 12.4 times greater than those of 1% l-Arginine-TiO2 and pure TiO2, respectively. l-Arginine (1%)-TiO2/g-C3N4 (11) (co-doped-TCN) was investigated using X-ray diffraction analysis (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX), Photo-luminescence (PL), and Differential Reflectance Spectroscopy (DRS) as the best-performing photocatalyst. Response surface methodology (RSM) was used to study the effect of co-doped-TCN dosage (0.5-1.0 g/L), pH of simulated wastewater (4-10), initial concentration of MNZ and OTC (50-100 mg/L), and irradiation time (30-90 min for MNZ and 20-40 min for OTC) on removal efficiency of the antibiotics. Also, their optimum values were determined by RSM. The treated pharmaceutical wastewater showed high biodegradability features with 5-day biological oxygen demand/chemical oxygen demand (BOD5/COD) of 0.51 and 0.46 after 40 and 100 min reaction for OTC and MNZ, respectively. The order of reactive species responsible for the photodegradation of pollutants was •O2─> •OH > h+>1O2. The effect of inorganic anions showed that all anions decreased the removal efficiency of both antibiotics in order of NO3─> Cl─ >SO42─>HPO42─ >HCO3─ for MNZ and NO3─> SO42─ > Cl─ >HPO42─ >HCO3─ for OTC. Also, introducing different oxidants improved the photocatalytic removal efficiency with the order of H2O2>K2S2O8> KBrO3.