Deep eutectic solvent mediated synthesis of ZnO nanoparticles and their catalytic activity for the removal of hazardous organic dyes

Zinc oxide, ZnO, nanoparticles have been synthesized using a deep eutectic solvent (DES) consisting of choline chloride and D-Lactic acid. The presented method proved to be simple, efficient, cost-effective, and eco-friendly for the formation of ZnO nanoparticles with specific morphology and properties. A variety of techniques have been used to investigate the composition, size, shape, and crystallinity of the nanoparticles, including UV-Visible spectroscopy, FT-IR spectroscopy, XRD, FE-SEM, and EDX. The UV-Visible spectroscopy confirmed the synthesis of stable ZnO, with a band gap of 3.55 eV between 300 and 400 nm. The FT-IR spectroscopy revealed the presence of DES functional groups on ZnO nanoparticles. The average size of the ZnO nanoparticles was estimated to be 39 nm using XRD calculations, while the FE-SEM images showed that the particles are rod-like elongated and spherical in shape and have high surface activity causing their agglomeration. The synthesized ZnO nanoparticles were used as a catalyst to remove the organic dyes from the synthetic wastewater. Various factors were studied to achieve optimum conditions for the effective dye removal. The results showed that an adsorbent dose of 5 mg/50 mL removed 84 % and 99 % of Congo Red and Crystal Violet, respectively, within 70 min. The kinetic models used revealed that the adsorption followed the pseudo-first-order kinetics. Langmuir, Freundlich, and Temkin isotherms were used to explain the adsorption mechanism. Finally, the thermodynamic parameters for the removal of both dyes using ZnO nanoparticles demonstrated that adsorption was spontaneous.