Removal of methylene blue from aqueous solutions using a solid residue of the apple juice industry: Full factorial design, equilibrium, thermodynamics and kinetics aspects

The use of adsorbent materials derived from agro-industrial residues represents a simple, easy and low-cost process for the removal of dyes from aqueous solutions. In this context, this study aimed to evaluate the use of a solid residue from the apple juice industry in the removal of methylene blue (MB) dye. Several different analytical techniques, including elemental analysis, Fourier transform infrared spectroscopy, solid-state ¹³C nuclear magnetic resonance, nitrogen adsorption/desorption and determination of pH at zero-point charge, were used to investigate the physico-chemical properties of the apple pomace. Conventional bath experiments were carried out by simultaneously optimizing five variables (initial MB concentration, amount of adsorbent, pH, stirring frequency and temperature) using a full 2^k factorial design with central point. In general, the characterization analyses evidenced the presence of several types of oxygenated functional groups in the adsorbent chemical structure and a favorable morphology for dye removal. Furthermore, the initial MB concentration, the amount of adsorbent and the pH of the medium had the strongest effect on the response variables, including their interactions and quadratic terms. In terms of equilibrium, it was found that the Langmuir model gave a good fit to the data, while the thermodynamic parameters indicated that the process was spontaneous and exothermic in nature. Moreover, kinetic studies revealed that the MB removal rate followed a pseudo-second-order model, suggesting an adsorption controlled by either diffusion on the stagnant film covering the adsorbent particles or intraparticle diffusion. Finally, the data acquired in this study also emphasized the interest to use materials derived from agro-industrial residues as potential adsorbents in the light of its properties for the removal of basic dyes, including MB.