Eco-Friendly Synthesis of ZnO for Efficient Photodegradation of Pharmaceutical Drug Removal by Photocatalysis

In the present work, a comparative study on eco-friendly synthesis of zinc oxide (ZnO) sample 1 and sample 2 with 3.17 and 4.17 M NaOH, respectively, is reported. Sample 2 with 4.17 M NaOH is applied in the photocatalytic degradation of paracetamol (pure and raw both) using the ultraviolet (UV, 280-400 nm) and UV/H₂O₂ reaction systems. Pure paracetamol (PCM1) and raw paracetamol (PCM2) from tablets are used for photocatalytic degradation by photocatalysis. Our experimental evidence show that ZnO sample 2 was more active in the UV/H₂O₂ reaction system than under ultraviolet (UV, 280-400 nm) irradiation only in the photocatalytic degradation process. Field emission scanning electron microscopy (FE-SEM) confirms the homogeneous growth of a rod-like structure for sample 1 and brittle and randomly aggregated rod-like and wire-like nanostructures for sample 2. The peaks observed in the region around 440 to 900 cm^-1 in the FTIR spectra for sample 1 and sample 2 annealed at 250 °C confirms the presence of ZnO bonds. UV absorption spectroscopy indicates a red shift in the absorption spectra due to the increase in the molar concentration of NaOH to 4.17 M for sample 2. In this study, the band gap values are found to be 3.33 and 3.01 eV for the synthesized ZnO sample 1 and sample 2, respectively, which are 40 and 360 meV less as compared to that of bulk ZnO (3.37 eV). The oxidation rate is increased in the UV/H₂O₂ reaction system, producing the highest rate for PCM1 drug removal with rate constant 9.7 × 10^-3 min^-1 and half-life 71.5 min. The kinetic study results for the removal of PCM1 and PCM2 show good results and follow the pseudo-first-order kinetic model with correlation coefficients 0.69556 and 0.90851, respectively, whereas PCM2 follows the pseudo-second-order kinetic model with correlation coefficient 0.9993. The experimental and calculated values of removal capacity (q _e) at equilibrium is found close to those of the pseudo-second order kinetic model for the removal of both the paracetamol forms PCM1 and PCM2 with the catalyst ZnO nanostructure. The photostability of ZnO sample 2 is also tested with a reusability test in photocatalytic degradation of paracetamol at least four times. The absence of a maxima peak at 243 of PCM1 in the UV/H₂O₂ reaction system indicates nearly 100% successful conversion of 20 ppm PCM1 by using synthesized catalyst ZnO sample 2. The comparative results of both reaction systems, i.e., UV and UV/H₂O₂, show that the hydroxyl radicals, as the active species, are responsible for major degradation of both paracetamol forms (PCM1 and PCM2).