Towards a profound understanding of methyl orange removal from industrial wastewater using a raw walnut shell: Kinetics, equilibrium, thermodynamics, and statistical physics calculations

The present study investigates the adsorption of the anionic dye methyl orange (MO) using raw walnut shells (RWS). Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy techniques were used to evaluate RWS collected in the Moroccan High Atlas (Tounfite) highlands. The batch mode adsorption experiments showed a remarkable removal during the first ten minutes, suggesting that the adsorption capacity is controlled by several parameters related to both the medium and the adsorbent. Based on the experimental results, 98 % of the MO dye was adsorbed on the walnut shell at pH >5 and room temperature (25±2 °C). The general order best explained the adsorption of MO on the walnut shells, suggesting that the number of active sites required changes with temperature. The two-layer model was the most appropriate physical-statistical model to explain the L-2 isotherms regarding the adsorption equilibrium. According to the models, methyl orange tends to be adsorbed on the surface of the monolayer, with individual molecules then adsorbing on top of each other. At all temperatures studied, the adsorption is endothermic and tends to be spontaneous, according to simulations of thermodynamic functions of RWS.