Synthesis and comparison of different metal-doped ZnO nanoparticles for catalytic degradation and adsorptive removal of direct orange: kinetics, isotherms, and thermodynamics

Commercial dyes from industrial sector contribute significantly to environmental contamination. This research work focuses on the elimination of synthetic anionic dye through batch investigation employing adsorption technique. Chemical Sol–gel synthesis approach was employed to synthesize the adsorbents. The novelty of this work involves the utilization of the following chemicals Ni(NO₃)₂, Cu(NO₃)₂, Fe(NO₃)₃, and Co(NO₃)₂ used as reducing metals to prepare metal-doped zinc oxide nanoparticles (Co/ZnO, Ni/ZnO, Fe/ZnO, and Cu/ZnO). The synthesized metal-doped ZnO nanoparticles were characterized by using FTIR, BET, SEM and XRD, zeta potential analysis. The optimum pH for direct orange (RDO) dye was detected in an acidic range which was 7 for Co/ZnO (87.40 mg/g), 3 for Ni/ZnO (83.13 mg/g), 5 for Fe/ZnO (79.31 mg/g), and 5 for Cu/ZnO (69.41 mg/g). The optimum dosage for all nanoparticle was found 0.05 g/50 mL and showed best adsorption for Co/ZnO 86.78 mg/g, Ni/ZnO 78.54 mg/g, Fe/ZnO 70/mg/g, and Cu/ZnO 59.64 mg/g. The initial concentration of dye from 25 to 250 mg/L was determined to be optimal for the maximum remediation of anionic dye using several adsorbents. The best removal was achieved at 100 ppm by Co/ZnO (84.61 mg/g) and (Ni/ZnO), while Fe/ZnO (72.51 mg/g), and Cu/ZnO (68.65 mgg/g) achieved maximum adsorption capacity at 150 ppm. The highest adsorption capacity was found by Co/ZnO (90.31 mg/g) at 43 min, Ni/ZnO (81.31 mg/g) at 49 min, Fe/ZnO (78.75 mg/g) at 75 min, and by Cu/ZnO (71.75 mg/g) at 90 min with optimal temperature at 37 °C. A decrease in temperature resulted in decline of adsorption capacity of the adsorbents. It demonstrated the release of heat in the complex during the adsorptive removal of negatively charged RDO dye molecules. The Langmuir adsorption isotherms revealed the highest correlation with the equilibrium data, while the pseudo 2nd order model demonstrated an acceptable fit with the adsorption kinetic data. Various thermodynamic factors like enthalpy free energy and entropy were investigated.