Photogenic MnO2/Ag metal nanocomposites and their dye adsorbing activities

• Manganese dioxide/silver (MnO₂/Ag) nanoparticles were fabricated by using KMnO₄-NaBH₄ redox reaction at room temperature. The optical and structural properties of MnO₂/Ag were determined using UV–visible and Fourier transform infrared spectroscopies. The morphology was established with scanning and transmission electron microcopies, and X-ray diffraction. MnO₂/Ag showed excellent adsorbing activity to the removal of Congo red. The various kinetic models were used to determine the rate of dye removal. Congo red adsorption onto MnO₂Ag proceeds through the pseudo-second-order kinetic model. Langmuir adsorption capacity (Q⁰_max = 97.1 mg/g), and sorption intensity (n = 1.6) were estimated with Langmuir and Freundlich adsorption isotherm models for 250 mg/L Congo red. Elovich model suggest the adsorption of Congo red with the MnO₂Ag proceeds through the film diffusion. The positive values of enthalpy changes (ΔH⁰), entropy changes (ΔS⁰), and negative Gibbs free energy changes (ΔG⁰) showed that the Congo red adsorption process was endothermic, spontaneous, and chemisorption process followed with physical mechanism. The results showed that the removal efficiency decreases from 98% to 89% after the six consecutive experiments.