Influence of Mg doping on ZnO NPs for enhanced adsorption activity of Congo Red dye

Abstract

A facile microwave-assisted combustion method was adopted to synthesize pure ZnO and Mg-doped ZnO nanoparticles (NPs), and characterization of the NPs and the sorption behavior with Congo Red (CR) Azo dye was extensively investigated. The structure, morphology and chemical assay of the synthesized NPs were studied with respect to the pure and 2% Mg-doped ZnO samples. The X-ray diffraction pattern confirmed the presence of crystalline, cubic phase with fluorite structure of both the NPs. The morphological differences between the pure and Mg-doped ZnO NPs were evaluated by FE-SEM, which revealed that the NPs were in the nanoscale regime with irregular crystalline morphology having a grain size of ~18–45 nm. N2 adsorption/desorption isotherms measured at −196 °C suggested a Type II structure with limited hysteresis loops, likely due to some interparticle porosity. The BET SSA ZnO NPs obtained was found to be 32 m2 g−1, indicating about its potential in sorption processes. Batch adsorption study for the removal of CR carried out showed that, the Mg-doped NPs showed higher loading efficiency (125 mg·g−1) than the ZnO NPs (25 mg·g−1) counterpart. This was corroborated by the isotherm results in which the CR dye sorption followed a Freundlich Isotherm model for both undoped ZnO and Mg-doped ZnO NPs. The sorption of CR dye followed a pseudo-second order kinetics. The negative ΔG° (−31.91 and −109.41 kJ mol−1) showed the feasibility of the adsorption process. The ΔH° was determined as +13.74 kJ mol−1 and 50.7 kJ mol−1 for ZnO NPs and Mg doped-ZnO NPs respectively, confirming the endothermic nature of the adsorption for removing CR dye from the solution.