Comparative Study on Enhanced Photocatalytic Activity of Visible Light-Active Nanostructures for Degradation of Oxytetracycline and COD Removal of Licorice Extraction Plant Wastewater

Abstract

This study evaluates the effects of carbon, nitrogen, and sulfur dopants on the photocatalytic activity of TiO2 for degradation of oxytetracycline (OTC) and chemical oxygen demand (COD) removal from licorice extraction plant wastewater (LEPW). Three novel visible-light-responsive nanostructures, including L-Histidine-TiO2, L-Methionine-TiO2 and L-Asparagine-TiO2, were successfully synthesized. The results showed that the modification of TiO2 with these three amino acids made the catalyst active in the visible light region and reduced the recombination rate of e−/h+ pairs according to PL analysis. The photodegradation efficiency of L-Histidine (2 wt.%)-TiO2 was 100% and 94% for OTC and COD, respectively. It showed the highest photocatalytic activity under illumination, compared to L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2. Synthesized composites were characterized with SEM, XRD, FTIR, DRS, and PL analyses. The biological oxygen demand to COD (BOD5/COD) ratio for treated LEPW was determined to be 0.5–0.6, confirming the enhanced biodegradability of the treated effluent. The effect of the independent variables, namely, initial concentration of OTC and COD, catalyst dosage, irradiation time, pH of solution, and light intensity, on the photocatalytic process was evaluated by Response Surface Methodology (RSM), and the optimum value of each independent parameter for maximum degradation of OTC and COD by L-Histidine (2 wt.%)-TiO2 was determined. The radical trapping experiment was performed with various scavengers in order to propose a photocatalytic mechanism, showing that hydroxyl radicals were the main active species. L-Histidine (2 wt.%)-TiO2 showed a stable and reusable structure even after four cycles of COD removal under the following optimal conditions of [COD]: 300 mg/L, [catalyst]: 1 g/L, light intensity: 25 W/cm2 at pH = 4 after 180 min irradiation.