Walnut shell powder as a low-cost adsorbent for methylene blue dye: isotherm, kinetics, thermodynamic, desorption and response surface methodology examinations

The low cost, eco-friendly and potential biomass, i.e. walnut (Juglans regia) shell powder was deployed for the removal of toxic methylene blue dye from contaminated water solution. The important characterization of the waste material was conducted by using several techniques, i.e. Scanning electron microscope, Fourier-transform infrared spectroscopy, Energy-dispersive X-ray spectroscopy, X-ray powder diffraction, Brunauer-Emmett-Teller surface area analysis, and Thermogravimetric analysis. The marked impact of various operating conditions, i.e. dose, concentration, time, pH and temperature on the adsorption process was investigated. Increasing pH resulted in an increase of percent dye adsorption, and the adsorption mechanism was occurred by electrostatic attraction between negative adsorbent surface and positive dye molecules. The equilibrium data suited with Langmuir isotherm model while the adsorption practice followed the pseudo-second-order kinetic model. Higher temperature reduced the adsorption of dye molecules. The adsorption process was spontaneous, exothermic and chemical. The critical statistical analysis of the experimental results was directed by forming the design of the experiment, which was further, optimized by ANOVA, 3D and perturbation plots. The error and predicted values of both the studied responses as derived from the statistical model showed the agreeable results. 0.1 N HCl was found to be effective in complete desorption. The results are very practical and prove the effectiveness of walnut shell powder in the usage of decolorization for methylene blue.