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 (Q0max = 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.