Adsorption of methylene blue by lemon verbena leaf waste: valorization, decontamination and modeling

Wasted lemon verbena leaves have been repurposed as an eco-friendly, cost-effective, and sustainable biosorbent for the removal of Methylene Blue (MB), a hazardous dye, from aqueous environments. The structure of the produced biosorbent, functional groups, morphology and surface charge could all be clearly understood by XRD, FTIR, SEM, pH_pzc and Boehm titration analysis. Batch sorption studies were then used to look at the effects of pH (2-10), dosage of biosorbent (0.05 g to 0.15 g), agitation speed (0-300 rpm), time (0-120 min), beginning MB concentration (20-80 mg.L^-1), as well as temperature (20-50°C) and ionic force (via NaCl). At a pH of 5.13, 0.15 g of biosorbent, an agitation speed of 200 rpm and 20°C, MB uptake was determined to be 37.16 mg.g^-1, indicating a significant removal. The equilibrium data were best described by the Freundlich adsorption model (R²: 0.953, Adj. R²: 0.945 and χ²: 2.988) and the kinetic studies were well-fitted to the pseudo-second order model (R² > 0.992, Adj. R² > 0.990 and χ² < 1.221). Thermodynamic analysis revealed that the adsorption process is spontaneous (ΔG° < -3.536 kJ.mol^-1), endothermic (ΔH°: 8.184 kJ.mol^-1), and physical in nature (Ea: 12.12 kJ.mol^-1). This study highlights the potential of waste lemon verbena leaves as an effective biosorbent for removing toxic dyes from wastewater, offering an environmentally friendly and economical alternative for water treatment.