Efficient removal of organic dyes by Cr-doped ZnO nanoparticles

Organic dyes, especially Congo red, are utilized primarily in the textile industry and consequently discharged into water resources that pollute aquatic environments. This study aims to investigate the fabrication of Cr-doped ZnO nanoparticles by sol-gel method to eliminate Congo red dye from wastewater. The obtained Cr-doped ZnO was characterized by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The Cr-doped ZnO crystallizes within a hexagonal wurtzite structure with a BET surface area equal to 9.5 m(2).g(-1). It is found that the variation of dye concentration and pH influence the removal of Congo red by Cr-doped ZnO. Excellent efficiency of 155.52 mg.g(-1) is achieved under optimum operating conditions, i.e., the adsorbent dosage of Cr-doped ZnO (0.400 g/L), contact time of 110 min, and solution pH (7.00). Pseudo-second-order adsorption kinetics and Langmuir isotherm models best fitted Congo red adsorption onto Cr-doped ZnO. The Congo red adsorption mechanism is associated with the chemisorption and hydrogen bond, as indicated by the pH, isotherms, and Fourier transforms infrared spectroscopy studies. The examination of Cr-doped ZnO with other dyes (malachite green, crystal violet, basic fuchsin, methylene blue, and eriochrome black T) suggests the high adsorption capacity of Cr-doped ZnO towards malachite green, crystal violet, and basic fuchsin dyes compared with methylene blue and Eriochrome black T dyes. The findings demonstrate that Cr-doped ZnO nanostructures manifest excellent adsorption capability to remove organic dyes from aqueous solutions.