Kinetics and Thermodynamics Study of Methylene Blue Adsorption to Sucrose- and Urea-Derived Nitrogen-Enriched, Hierarchically Porous Carbon Activated by KOH and H3PO4.

Hierarchically porous nitrogen-enriched carbon materials synthesized by polymerization of sucrose and urea (SU) were activated by KOH and H3PO4 (SU-KOH and SU-H3PO4, respectively). Characterization was undertaken and the synthesized materials were tested for their ability to adsorb methylene blue (MB). Scanning electron microscopic images along with the Brunauer-Emmett-Teller (BET) surface area analysis revealed the presence of a hierarchically porous system. X-ray photoelectron spectroscopy (XPS) confirms the surface oxidation of SU upon activation with KOH and H3PO4. The best conditions for removing dyes utilizing both activated adsorbents were determined by varying the pH, contact time, adsorbent dosage, and dye concentration. Adsorption kinetics were evaluated, and the adsorption of MB followed second-order kinetics, suggesting the chemisorption of MB to both SU-KOH and SU-H3PO4. Times taken to reach the equilibrium by SU-KOH and SU-H3PO4 were 180 and 30 min, respectively. The adsorption isotherm data were fitted to the Langmuir, Freundlich, Temkin, and Dubinin models. Data were best described by the Temkin isotherm model for SU-KOH and the Freundlich isotherm model for SU-H3PO4. Thermodynamics of the adsorption of MB to the adsorbent was determined by varying the temperature in the range of 25-55 °C. Adsorption of MB increased with increasing temperature, suggesting that the adsorption process is endothermic. The highest adsorption capacities of SU-KOH and SU-H3PO4 (1268 and 897 mg g-1, respectively) were obtained at 55 °C. Synthesized adsorbents were effective in removing MB for five cycles with some loss in activity. The results of this study show that SU activated by KOH and H3PO4 are environmentally benign, favorable, and effective adsorbents for MB adsorption.