Removal of Methylene Blue Dye from Aqueous Solution Using Marrubium vulgare Leaf Powder as a Low-Cost Biosorbent

Abstract

The present study evaluates the adsorption capacity of raw Marrubium vulgare leaf powder as a biosorbent material for eliminating methylene blue (MB) dye in industrial wastewater. The results demonstrated a high removal efficiency of approximately 93.7% at neutral pH (pH = 7), highlighting the material’s effectiveness under ambient conditions without the need for chemical adjustment. The maximum adsorption capacity (qₘₐₓ) of the monolayer reached 15.06 mg/g. Equilibrium observation showed strong agreement with the Langmuir isotherm model (R² = 0.9742), indicating monolayer adsorption on a homogeneous surface. The Freundlich model further supported these findings, with an intensity parameter (n = 1.516), suggesting favorable adsorption behavior. The analytical method exhibited high sensitivity, with LOD and LOQ values of 0.008758 and 0.026539, respectively. The adsorption efficiency at neutral pH is attributed to electrostatic interactions between cationic dye molecules and the negatively charged adsorbent surface, consistent with the point of zero charge (PZC) concept. Thermodynamic analysis confirmed the spontaneous and endothermic nature of the process, as indicated by negative ΔG° values and a positive ΔH°, whereas the positive ΔS° indicates enhanced disorder at the solid–liquid interface. Overall, raw marrubium vulgare leaf powder shows great promise as an affordable, environmentally friendly adsorbent for wastewater treatment applications.