Green synthesis of 2-hydroxypropyl-β-cyclodextrin polymers crosslinked by citric acid for highly efficient removal of methylene blue

Abstract

Herein, a new type of citric-acid-crosslinked 2-hydroxypropyl-β-cyclodextrin (HPCD-CA) adsorbents were synthesized, and their applications to the removal of the cationic dye—methylene blue (MB) from the water were thoroughly investigated. The HPCD-CA materials were fully characterized and analyzed by SEM, FT-IR, XRD, TGA, BET and XPS. Influence factors affecting the adsorption efficiency of the HPCD-CA materials were investigated separately, including contact time, adsorbent dosage, the concentration of MB, initial pH of the solution, and temperature. The Langmuir adsorption isotherm and pseudo-second-order kinetic model performed better, indicating that the adsorption process is more consistent with chemisorption and monomolecular layer adsorption. The maximum theoretical absorption capacity of HPCD-CA-0.25 for MB was 497.72 mg g⁻¹, much better than most of the previously reported materials. After four adsorption-desorption cycles, HPCD-CA-0.25 maintained a high removal efficiency of MB with good recoverability and stability. In addition, FT-IR and XPS analyses showed that electrostatic interaction, host-guest and hydrogen bonding interactions existed between HPCD-CA-0.25 and MB. Density functional theory (DFT) calculations further verified the above adsorption mechanism, indicating that the synergistic interaction between CA and HPCD enhanced the removal ability of MB by HPCD-CA-0.25. We believe that the current study here provides a feasible way to design wastewater treatment purification materials.