Adsorptive removal of methylene blue using activated carbon derived from Jatropha curcas husk

Synthetic dyes such as methylene blue (MB) are widely used in industries, leading to environmental contamination due to their persistence and toxicity. In this study, activated carbon derived from Jatropha curcas husk (JCH-AC) was prepared through pyrolysis and synthesized using potassium hydroxide (KOH) as the activating agent. JCH-AC serves as a sustainable and low-cost adsorbent for the removal of MB from aqueous solutions. Batch adsorption experiments examined the effects of adsorbent dosage, initial concentration of MB, contact time, pH, and temperature. Furthermore, adsorption isotherms were evaluated using Langmuir, Freundlich, and Temkin models. The Langmuir model provided the best fit, indicating monolayer adsorption with an adsorption capacity of 226.89 mg/g and a correlation coefficient of 0.9940. Kinetic modelling was conducted using pseudofirst- order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) models. PSO model showed an excellent fit with R2 values ranging from 0.9997 to 0.9990 and predicted adsorption capacities (qe) closely matching the experimental values, suggesting chemisorption as the dominant mechanism. Meanwhile, IPD indicated a multi-stage process involving surface diffusion and intraparticle diffusion with R2 values between 0.2706 to 0.3422. Thermodynamic analysis was performed at temperatures ranging from 25 to 33℃, showing negative Gibbs free energy changes (ΔG° ranging from -18.18 to -22.22 kJ/mol) and positive enthalpy change (ΔH° = 132.31 kJ/mol) values which confirm that the process was spontaneous and endothermic nature of the process. Additionally, the positive entropy change (ΔS° 504.99 J/mol·K) indicated increased randomness at the solidsolution interface and strong affinity of MB molecules and JCH-AC. In conclusion, JCH-AC demonstrates high adsorption efficiency, favourable thermodynamic behaviour, and environmental sustainability, making it an effective and eco-friendly adsorbent for MB removal, supporting sustainable wastewater treatment and valorisation of agricultural waste within circular economy initiatives.