Synthesis, Thermal Activation, and Thermodynamic Evaluation of Hydrotalcite for Methylene Blue Adsorption

This study evaluated the synthesis, pelletization, thermal activation and adsorptive performance of a layered double hydroxide (LDH) Mg-Al in the removal of methylene blue (MB) from aqueous solution. The material was synthesized by coprecipitation at pH 10 and 80 °C, subjected to aging for 24 h, pelletized using starch as a binding agent and then calcined at 500 °C. The structural, morphological and chemical characterization was performed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled to energy dispersive spectroscopy (SEM/EDS), elemental analysis by complexometry and determination of the zero-charge point (pHPZC). The batch adsorption assays were conducted at pH 7, at temperatures of 25, 40 and 50 °C, using 0.050 g of HT500 in 100 mL of MB solution. The adsorption capacity at equilibrium increased from 64.29 to 87.65 mgg-1 with the increase in temperature. The Langmuir model presented the best fit to the experimental data (R2 = 0.9934-0.9971), with maximum estimated capacities between 83.89 and 112.33 mgg-1. The kinetics were adequately described by the pseudo-second-order model (R2 = 0.9984-0.9985), while the intraparticle diffusion model indicated that this mechanism is not the only limiting step of the process. The thermodynamic parameters showed endothermic (ΔH° = 43.48 kJmol-1) and spontaneous (ΔG° = -3.12 to -7.03 kJmol-1) behavior. The results demonstrate the potential of thermally activated and pelletized LDH Mg-Al for MB removal, although additional studies are needed in continuous flow and adsorbent regeneration systems.