Adsorption of methylene blue by a high-efficiency adsorbent (polydopamine microspheres): Kinetics, isotherm, thermodynamics and mechanism analysis

Polydopamine (PDA) microspheres were synthesized by a facile oxidative polymerization method and used as a high-efficiency adsorbent for the removal of a cationic dye (methylene blue, MB) from aqueous solution. Characterizations of the as-synthesized PDA microspheres and PDA microspheres-MB (MB absorbed PDA microspheres) systems were performed using several techniques such as SEM, TEM, FTIR, N 2 adsorption/desorption isotherms, particle size and zeta potential analysis. The effects of initial solution pH, temperature, initial concentration, and contact time were systematically investigated. Results showed the adsorption capacity at 25   °C could reach up to 90.7   mg/g. Besides, adsorption kinetics showed that the adsorption behavior followed the pseudo-second-order kinetic model. The equilibrium adsorption data fitted the Langmuir isotherm well. Thermodynamic analyses showed that the adsorption was endothermic and spontaneous, and it was also a physisorption process. In addition, the possible adsorption mechanism was also proposed based on the experimental results.