ADSORPTION OF METHYLENE BLUE ONTO YEMENI CAPPARIS SPINOSA LEAVES: KINETIC, ISOTHERM, AND THERMODYNAMIC MODELING STUDIES FOR SUSTAINABLE DYE REMEDIATION

Abstract

The release of dye contaminated effluents from industrial activities poses a serious threat to aquatic environments due to their toxicity, persistence, and aesthetic impact. In this study, dried leaves of Capparis spinosa (CPL) were evaluated as a low‑cost and eco‑friendly bio‑adsorbent for the removal of methylene blue (MB) from aqueous solutions. Batch adsorption experiments were performed to investigate the effects of contact time, adsorbent dosage, initial dye concentration, solution pH, and temperature. Surface functional groups of CPL were identified using ATR‑FTIR spectroscopy. Adsorption equilibrium, kinetic behavior, and thermodynamic feasibility were analyzed using Langmuir and Freundlich isotherms, pseudo‑first‑order and pseudo‑second‑order kinetic models, and thermodynamic parameters (ΔG°, ΔH°, ΔS°), respectively. The adsorption process were determined to be at pH 8, with the highest removal efficiency observed at pH 12. A contact time of 40 minutes and the removal attained to 91.58%, with an adsorption capacity of 1.99 mg/g. The highest adsorption capacity, 12.469 mg/g, was attained under conditions of 20 g/L dye concentration, 0.5 g dose and the optimum removal efficiency 99.95%. The equilibrium data were best fitted by the Langmuir model (R2=0.9997) fit than Freundlich model (R2=0.894), confirming a monolayer adsorption process on a homogeneous surface and the separation factor (RL) 0.4332 confirming that the adsorption of MB onto CPL is a favorable. While the kinetic data followed PSO model exhibited (R2= 0.9995) compared to the PFO model (R2=0.9296) and the calculated qe,cal (2.068 mg/g) close with the experimental qe,exp (1.99 mg/g). Thermodynamic analysis, where the positive enthalpy (ΔH°) 80.041kJ/mol confirmed that MB adsorption onto CPL is endothermic, and negative Gibbs free energy (ΔG°) values (-6.687, -9.597, -12.508, and -15.418kJ/mol) confirmed that the process is spontaneous. These results demonstrate that Capparis spinosa leaves are a promising sustainable adsorbent for dye‑laden wastewater treatment.