Commercial Charcoal as an Effective Adsorbent for Methylene Green Dye Removal: Optimization, Mechanistic Insights, and Performance Evaluation

Abstract

This study investigates the adsorption of methylene green (MG) dye onto lemon charcoal (LC), a biomass-derived adsorbent, under varying physicochemical conditions. LC was characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analysis, revealing a predominantly amorphous structure with a surface area of 91.62 m² g⁻¹, morphology characterized by visible macropores, and thermal stability (65.93% residual mass at 1000°C). Batch adsorption experiments optimized key parameters: a 0.1000 g dose achieved 97.29% uptake, while pH 3.00 maximized electrostatic attraction between protonated LC surfaces and cationic MG. Kinetic studies aligned with the pseudo-second-order model (R² > 0.999), confirming chemisorption as rate-limiting. Langmuir isotherms (R² = 0.997–0.999) indicated monolayer adsorption with a maximum capacity (q_m) of 10.72 mg g⁻¹ at 45°C, while Freundlich parameters (n > 1) highlighted heterogeneous multilayer contributions. Thermodynamic analysis revealed an endothermic (ΔH° = +55.88 kJ mol⁻¹), spontaneous (ΔG° = –22.41 kJ mol⁻¹) at 318 K), and entropy-driven process (ΔS° = +0.25 kJ (mol·K)⁻¹), consistent with chemisorption dominance. LC’s performance surpasses many biochar-based adsorbents, positioning it as a viable, low-cost solution for wastewater treatment.