Kinetic Adsorption Studies of Cationic Dyes onto Molecular Sieve and Activated Carbons

Background: Dye-containing wastewater causes irreparable damage to the ecological water system. Although adsorbents are widely used for treating wastewater containing dyes, the comparative investigation on these materials is still insufficient for their wide applications in the industries. Objective: With the aim of comparing efficient and fast adsorbent materials for cationic dyes, we analyzed and evaluated the adsorbents of the MCM-41 molecular sieve and activated carbons. Methods: The adsorption performance was studied on the common colored organics, such as cationic dyes of rhodamine B (RhB) and methylene blue (MB) dyes. The present work examined the impact of experimental variables, including initial dye concentration, adsorption time, and pH, on the adsorption process and performance, as well as the adsorption kinetics of the diverse adsorbents towards two cationic dyes. Results: MCM-41 molecular sieves showed relatively high adsorption capacity for RhB and AC-2, which made their adsorption capacity for MB much higher than that of MCM- 41 molecular sieves. A comprehensive analysis was conducted using pseudo-first-order and pseudo-second-order to decipher the mechanism of dye adsorption. The heterogeneous adsorption mechanism could explain the dye adsorption behavior of MCM-41 molecular sieve and activated carbons. Conclusion: The results demonstrated the influence of the pore structure and surface properties of the adsorbents on the adsorption capacity of dye molecules in an aqueous solution. For the initial concentration of cationic dye solutions of 20 mg/L, the MCM-41 molecular sieve had a MB adsorption capacity of 130.8 mg/g under alkaline conditions at pH=10, while the activated carbon adsorbents showed a stable MB adsorption capacity of 266.6 mg/g under different pH conditions, proving their applicability in treating wastewater containing dyes under different acid/base environments.