Removal of Methylene Blue from Aqueous Solutions Using Rumex Nepalensis Leaf and Roots as a Low-cost Adsorbents: Isotherm, Kinetics, and Thermodynamic Studies

The objective of this study was to investigate the effectiveness of Rumex nepalensis leaf (RNL) and root (RNR) powders as adsorbents for the removal of methylene blue (MB). The functional structure of the adsorbents was characterized using Fourier transform infrared (FTIR) spectroscopy before and after the adsorption process. The optimization of operational parameters such as pH, contact time, initial dye concentration, adsorbent dose, and temperature was conducted using a batch adsorption experiment. The adsorption isotherm was modeled using Langmuir, Freundlich, and Temkin isotherms. The highest dye removal efficiencies for RNL and RNR and the adsorption capacity of MB were observed at pH 3 and 5, respectively. The equilibrium contact time for both parts of the plant was determined to be 30 minutes at 35 °C. The Langmuir isotherm model showed the best fit with high correlation coefficients (R2 > 0.99) and a low nonlinear error function, as indicated by the nonlinear chi-square test (χ2) for both adsorbents. The maximum adsorption capacities of MB as estimated from the Langmuir isotherm model were 119 mg/g for RNL and 333.3 mg/g for RNR at 35 °C. The kinetic model revealed that the adsorption of MB on RNL and RNR followed pseudo-second-order kinetics. Thermodynamic analysis showed that the adsorption of MB on RNL was endothermic and spontaneous, whereas it was exothermic for adsorption of MB onto RNR and effective in the temperature range of 25–35 °C. These results indicated that the leaf and roots of Rumex nepalensis are promising and novel adsorbents for the removal of methylene blue from aqueous solutions.