Ficcus palmata leaves as a low-cost biosorbent for methylene blue: Thermodynamic and kinetic studies.

In the present study, methylene blue (MB) removal has been studied from its aqueous solution, using Ficcus palmata leaves (FPL)-based plant material. The effect of different parameters such as contact time (10-100) minutes, initial concentration (5-25) mg/L, pH (4-13), temperature (298-318 K), and adsorbent dosage (0.15-0.45 g/0.05 L) was investigated. The maximum removal efficiency was calculated to be 98% for sample having initial concentration 15 mg/L along with 0.45 g of adsorbent agitated for 80 min at 318 K and pH = 7. The data were fitted to adsorption isotherm models (Langmuir and Freundlich) and kinetic models (pseudo-first order, pseudo-second order, and intra-particle diffusion). The data were found to be best fitted with Freundlich adsorption isotherm (R2  = 0.99) and pseudo-second-order (R2  = 0.991). Thermodynamic parameters (free energy change, enthalpy change, and entropy change) were also estimated. The Gibbs free energy values were found to be -1.808, -5.139, and -5.991 kJ/mol at 298, 308, and 318 K, respectively. The decrease in free energy with increasing temperature has indicated spontaneity of adsorption process, and positive enthalpy change (35.75 kJ/mol) showed that the adsorption process was endothermic. 0.1 M HCl was found to be most effective desorbing agent with percent desorption 53.51%. PRACTITIONER POINTS: FP leaves are low cost and easily available biomass for removal of MB from aqueous solution. The adsorption capacity was obtained to be 6.89 mg/g at (15 mg/g dye concentration, pH = 7, and contact time 80 min). The maximum removal efficiency for MB was 98%. The thermodynamic studies indicated the endothermic adsorption process. The 0.1 M HCl was found as best desorbing agent for MB loaded on FP leaves.