Characterization and Adsorption Mechanism of Methylene Blue Dye by Mesoporous Activated Carbon Prepared from Rice Husks

Environmental contamination due to synthetic dyes is a severe problem due to their adverse eco-toxicological effects. This study prepared activated carbon from H3PO4-activated rice husks (AC-RH) to adsorb methylene blue (MB) and predicted the adsorption mechanism. The AC-RH was characterized for N2 adsorption, surface functional groups, chemical compositions, and surface morphology. The activated carbon was classified to be a mesoporous material because 87% of its pore volume diameters are 3-50 nm. MB adsorption was studied under different conditions. Optimal MB adsorption occurred at pH 8, and the ideal equilibrium time was 360 min. The equilibrium adsorption was evaluated at concentrations of MB between 25 and 200 mg/L at 30°C. The Freundlich isotherm model matched the equilibrium data, and the greatest adsorption capacity of the Langmuir isotherm was 26.31 mg/g. The kinetic analysis revealed that the adsorption was pseudo-second-order, and its rate constant (k2) was higher at higher temperatures. For the thermodynamic adsorption study at 20 to 40°C, the Gibbs free energy (G) values were -6.291 to -9.197 kJ/mol, and the activation energy (Ea) was 26.248 kJ/mol: therefore, the methylene blue adsorption was spontaneous and physical. This study also revealed that the adsorption mechanisms were H-bonding, pore-filling, Yoshida H-bonding, n- interactions, electrostatic, and cation exchange.