Methylene blue removal by natural clay adsorbent: equilibrium, kinetic and thermodynamic analyses in batch and fixed-bed systems
摘要
We evaluated a locally sourced clay (Garmak clay) as a low-cost adsorbent for removing the cationic dye methylene blue (MB). Experiments included laboratory batch tests and continuous fixed-bed column runs to assess equilibrium, kinetics, thermodynamics and dynamic (breakthrough) behavior. Characterization showed a quartz aluminosilicate assemblage (high SiO2 and Al2O3) with kaolinite and mica, slit-shaped interparticle porosity, a moderate BET surface area (~ 12.8 m2 g−1) and thermal stability up to 500 °C. These characteristics yield a negatively charged, mesoporous surface conducive to cation uptake. In batch assessments, the clay attained an equilibrium adsorption capacity nearing 31 mg g−1 at 30 °C, successfully extracting around 62% of MB from a 50 mg L−1 solution. Kinetic evaluation was best characterized by the Elovich model, suggesting the presence of heterogeneous surface sites and a mechanism governed by a blend of chemisorption and diffusion, whereas equilibrium data were more aligned with the Freundlich isotherm, indicating multilayer adsorption on a non-uniform surface. Thermodynamic parameters suggest that the adsorption process is endothermic; at low MB loading, the reaction is spontaneous (ΔG° ≈ − 15.9 kJ mol−1), yet at elevated concentrations, it becomes thermodynamically less favorable, consistent with site saturation and a reduction in entropy gain. Fixed-bed trials involving granulated clay identified a calcination temperature of 550 °C as optimal for column efficacy; the Thomas model simulation for a flow rate of 5.0 mL−min−1 and a feed concentration of 50 mg L−1 estimated qe ≈ 3.74 mg g−1. Collectively, the mineralogical properties, mesoporosity, and surface chemistry of the material suggest a promising engineering potential for the continuous treatment of dye-laden wastewater.
Graphical abstract