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

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-arginine:TiO2, as well as l-arginine/TiO2:g-C3N4, was determined by a kinetic study of photodegradation process. The maximum photocatalytic removal rate (0.065 min−1 for MNZ removal) was observed using 1% l-Arginine-TiO2/g-C3N4 (1:1) under visible light illumination, 2.2 and 6.5 times greater than those of 1% l-Arginine-TiO2 and pure TiO2, respectively. l-Arginine (1%)-TiO2/g-C3N4 (1:1) (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.