Solar irradiation driven catalytic dye degradation by novel biosynthesized zinc oxide nanoparticles (ZnO–NPs) from Barleria mysorensis: Kinetics, reusability and mineralization studies

•ZnONPs were biosynthesized from Barleria mysorensis by hydrothermal process.•Biosynthesized ZnONPs were of high quality, hexagonal shape with structural stability.•Biosynthesized ZnONPs efficiently degraded MB dye with potential reusability.•The photodegradation of MB dye by ZnONPs followed the pseudo-first-order kinetics.•Mineralization studies proved ZnONPs didn't produce non-toxic end products. During the study, Barleria mysorensis B.Heyne ex Roth was selected for biosynthesizing zinc oxide nanoparticles (ZnONPs) and evaluating its photocatalytic properties. The structural and morphological characterization of the synthesized particles were performed by UV–Vis diffusion reflectance spectroscopy (UV–Vis DRS), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), Fourier-Transform Infrared (FT-IR) spectroscopy and Braunauer–Emmett–Teller (BET) studies. The results indicate high-quality ZnONPs of 34.36 nm crystallite size with 92.32 % elemental purity (ZnO) and an energy band gap of 3.18 eV. According to the BET study, the synthesized particles had a surface area of 21.626 m2 g–1 with 0.001354 cm3g−1 (p/po=0.908) total pore volume of 0.001354 cm3g−1 (p/po=0.908) and 7.34 nm of mean pore diameter. The electron resonance spectroscopy (ESR) revealed that upon interaction of ZnONPs revealed the formation superoxide radical ions. The photocatalytic degradation of methylene blue (MB) dye using the synthesized nanoparticles revealed a degradation efficiency of approximately 91.32 % within 120 min of exposure at a rate constant of 18.34 × 10−3 min−1 and it was also noted that the dye degradation process followed pseudo-first-order kinetics. Besides, reusability and structural stability studies of ZnONPs as photocatalysts revealed a stable performance up to five cycles. In addition, mineralization studies revealed a steady decrease in COD and TOC content with an increase in reaction time, indicating an improvement in mineralization. In conclusion, biosynthesized ZnONPs get rid of the dye and lower the solution's COD and TOC, ensuring that the dye in question breaks down and non-toxic end products are made. [Display omitted]