Optimization of metal dopant effect on ZnO nanoparticles for enhanced visible LED photocatalytic degradation of citalopram: comparative study and application to pharmaceutical cleaning validation

Abstract Metal doping is an effective method to tune the optical and chemical properties of nanoparticles. Herein, a comparative study was conducted to assess the effect of metal dopant (Mg, Cu and Sn) on ZnO nanoparticles for visible LED photocatalysis. The photocatalysts were synthesized via a facile co-precipitation method. Doped ZnO nanoparticles were employed for photodegradation of citalopram; a commonly used antidepressant drug. The structural, morphological and optical properties of the nanoparticles were analyzed using high resolution transmission electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller measurements and diffuse reflectance spectroscopy. A decrease in band gap energy was obtained for Mg (3.21 eV), Cu (3.15 eV) and Sn (3.05 eV) compared to undoped ZnO (3.34 eV). Results showed that the photocatalytic activity of ZnO nanoparticles towards citalopram degradation under visible light was enhanced by doping with Sn which showed superior photocatalytic performance compared to Cu. Whereas, Mg doped ZnO demonstrated the lowest photocatalytic activity. Full factorial design (2 4 ) was conducted to investigate the effect of dopant, pH, catalyst loading and initial citalopram concentration on the efficiency of the treatment process. The interaction between the metal dopant and pH had significant impact on photodegradation efficiency. At optimum conditions, 80% degradation of 25 µg mL −1 citalopram was obtained in 2 h using commercially available LED light using 0.5 mg mL −1 Sn doped ZnO. Kinetics of citalopram degradation was also investigated and was found to follow pseudo-first order kinetics. The optimized photocatalytic protocol was successfully applied for treatment of water samples obtained from production lines during the cleaning validation cycles of citalopram. Sn and Cu doped ZnO nanoparticles had great sustainability for wastewater treatment as it kept its catalytic behavior up to three cycles without significant decrease in photocatalytic activity. The integration of such an approach into the currently employed cleaning validation protocols would offer an economical advantage for pharmaceutical wastewater treatment. Graphical Abstract