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

Removal of Metronidazole (MNZ) and Oxytetracycline (OTC) from wastewater by the prepared (C, N codoped)-TiO/g-CN (Graphitic carbon nitride) was examined. l-Arginine (C, N codoped)-TiO and l-Arginine (C, N codoped)-TiO/g-CN photocatalysts were successfully synthesized through the sol-gel method, and optimal ratio of l-arginine:TiO, as well as l-arginine/TiO:g-CN, was determined by a kinetic study of photodegradation process. The maximum photocatalytic removal rate (0.062 min for MNZ removal) was observed using 1% l-Arginine-TiO/g-CN (1:1) under visible light illumination, 2.2 and 12.4 times greater than those of 1% l-Arginine-TiO and pure TiO, respectively. l-Arginine (1%)-TiO/g-CN (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 (BOD/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 •O> •OH > h>O. The effect of inorganic anions showed that all anions decreased the removal efficiency of both antibiotics in order of NO> Cl >SO>HPO >HCO for MNZ and NO> SO > Cl >HPO >HCO for OTC. Also, introducing different oxidants improved the photocatalytic removal efficiency with the order of HO>KSO> KBrO.