WITHDRAWN: Synthesis and characterization of nano-zinc oxide: adsorption of acid blue 92 dye, isotherms, thermodynamics and kinetics

Rapid industrialization, particularly textile/dyeing, causes substantial surface water contamination by various dyes and pigments globally. The export-oriented textile sector has flourished in Bangladesh in recent decades, but little attention has been paid to the proper treatment of industrial wastewater, which causes significant contamination of nearby water bodies. In this work, fiber-like nano-zinc oxide (ZnO) was synthesized by precipitation method and characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) techniques. The SEM image exhibited fiber-like nano ZnO while XRD and FTIR data confirmed a wurtzite structure perfectly and the purity of the nano-ZnO, respectively. The as-prepared nano-ZnO was used to remove an anionic dye, acid blue 92 (AB 92), from aqueous solution. To optimize the adsorption conditions, the effects of pH (5.0-9.0), adsorbent dosage (0.40-3.00 g/L), initial concentration of AB 92 (25.34-67.34 mg/L), and contact time (5-90 min) on the adsorption process were investigated. Furthermore, thermodynamic evaluation through studying equilibrium and kinetics of the adsorption process was also attempted. The coefficients of correlation (R2) for Langmuir, Freundlich and Temkin’s isotherm models were 0.9831, 0.9994 and 0.9972, respectively. These results suggested that the adsorption process was found to fit Freundlich followed by Temkin’s model. For adsorption thermodynamics, change in enthalpy (ΔH) was found to be +20.32 kJmol -1 , and the free energy (ΔG) was varied from -1.86 to -3.84 kJmol -1 as the temperature increased from 291 to 317 K. These findings revealed that the adsorption process was endothermic and spontaneous. The values of R 2 ranged from 0.9999 to 1.0000 for pseudo second order while that for pseudo first order was 0.9919-0.9996. On the basis of these kinetic data, the adsorption process was realized to follow pseudo second order. The reusability of the nano-ZnO for adsorption of organic pollutants suggests that it could be used as an alternative for designing an effective and recyclable removal technique. An adsorption mechanism of anionic dyes onto nano-ZnO is discussed.